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Subsea Pipeline Installation Calculations Training
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Pipeline, riser and subsea engineering Installation calculations for subsea pipelines 2 All information contained in this document has been prepared solely to illustrate engineering principles for a training course, and is not suitable for use for engineering purposes. Use for any purpose other than general engineering design training constitutes infringement of copyright and is strictly forbidden. No liability can be accepted for any loss or damage of whatever nature, for whatever reason, arising from use of this information for purposes other than general engineering design training. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means whether electronic, mechanical, photographic or otherwise, or stored in any retrieval system of any nature without the written permission of the copyright holder. Copyright of this book remains the sole property of: Jee Limited Hildenbrook House The Slade Tonbridge Kent TN9 1HR England © Jee Limited 2010 Notes created: January 2010 Table of contents Volume one CATENARIES 7 Expectation 9 Catenary curves 10 Pipe lay catenary equations 13 Pipe stresses 17 Worked example 26 Exercise 32 Control by tension 33 General equations for catenaries Thick wall formulae 37 41 BENDING 43 Expectation 45 Bending during S-lay 48 Worked example 55 Exercise 64 Break-over length 65 Concrete-coated pipe 76 Horizontal lay radii 81 REELING 87 Expectation 89 Reeling criteria 90 Worked example 105 Exercise 108 4 Installation calculations for subsea pipelines SAGBEND BUCKLING 111 Expectation 113 Local buckling criteria 114 Prediction of buckling in catenary 122 Worked Example 127 Exercise 133 Buckle propagation 136 ROPES, WINCHES AND CHAINS 143 Expectation 145 Pulley usage 146 Wires and ropes 153 Fatigue life 170 Winches 177 Worked example 183 Exercise 187 Chains 190 PIPE PULL FORCES 199 Expectation 201 Landfall Setup 202 Worked Example 216 Exercise 225 Alternative Pull Setups 226 STEELWORK 237 Expectation 239 Analysis and design process 241 Visualisation 247 Analysis 250 Analysis exercise 258 Design code check 260 Worked Example 273 Strut design exercise 285 Worked Example 286 Fatigue 297 Connections 301 Volume two PIPE LIFTING 311 Expectation 313 Attachments 314 2, 3 and 4 point lifts 320 Strop exercise 330 Strop sizing 331 Spreader beams 337 Worked Example 341 Beam Exercise 350 Deep water resonance 353 LUGS AND SEA FASTENINGS 369 Expectation 371 Lugs and stiffeners 372 Worked Example 386 Exercise 393 Foundations and deck fixings 394 Sea fasteners 401 6 Installation calculations for subsea pipelines BARGE STABILITY 411 Expectation 413 Stable floating bodies 414 Determining vessel stability 418 Free liquid surfaces and suspended loads 425 Modular craft and local barges 434 Worked Example 438 Exercise 442 Background information 445 Determination of the metacentre Reduction due to free surfaces of tanks 445 450 ANCHORS AND PILES 451 Expectation 453 Anchors 454 Exercise 465 Piles 466 Sheet pile anchorage 478 Worked Example 484 Exercise 491 Background information 493 Survey methods Soil types and properties 493 500 ABANDONMENT AND RECOVERY 505 Expectation 507 A&R overview 508 Case study 516 Case study 523 A&R analysis 530 Exercise 535 PROFILES 539 ACRONYMS AND ABBREVIATIONS 547 ACKNOWLEDGEMENTS AND REFERENCES 561 Pipe lifting 313 Pipe lifting EXPECTATION EXPECTATION Develop an understanding of the processes involved in lifting pipelines offshore Attachment to crane Lift configurations Static and dynamic lift forces Sizing the strops Checking the pipe will not buckle when lifted In this module we will be looking at the process of lifting a pipeline offshore using a crane(s). This will include consideration of the type and number of lift points, the forces involved (both static and dynamic) during the installation process, how to size the strops based on DNV guidelines and also checking that the pipe will not buckle when lifted, due to self-weight. 314 Installation calculations for subsea pipelines ATTACHMENTS ATTACHMENTS Attaching pipe to strops Straps / slings Hooks Clamp arrangement or permanent lugs Considerations Does the attachment need to be ROV friendly? Can the attachment slip? (friction, restraints) Crane selection Main components / dimensions Capacity Straps or slings are the most common form of attachments between the crane and the pipeline being lifted. However, there are various configurations in which these can be installed. The configuration chosen will affect the load which the sling can support, and also its resistance to slippage along the pipeline’s axis. Pipes being transferred from the transport barge to the laybarge may use special hooks in the ends. For lifts other than pipes or spools, the item may require clamps or permanent lugs may be fitted. In particular, we need to know whether the attachment method can be released (or cut) using an ROV. It is important that the attachment does not slip and change its position. Should this happen then the load on other lift points may increase and cause catastrophic failure. At the other end of the strop, we need to know how the crane geometry affects the load capacity and reach during installation. Where the barge has a permanent crane of fixed 315 Pipe lifting capacity, then it is important that the lift does not exceed the specified radius for safe operation. STROPS Safe working load (SWL) of slings Varies depending on the configuration 1 SWLstrop = 1 x SWLwire 4 SWLstrop = 2.1 x SWLwire 2 SWLstrop = 1.4 x SWLwire 5 SWLstrop = 2.1 x SWLwire Included angle <90° 3 SWLstrop = 1.4 x SWLwire 6 SWLstrop = 2 x SWLwire Above are some examples of sling configurations, using either single or multiple legs. The safe working load of these slings varies, depending on the configuration and number of straps. Refer to BS 6210 CP for the Safe Use of Wire Rope Slings for General Lifting Purposes; and BS 1290 . A summary of the safe working loads and description of the configurations is shown below: 1. Single straight lift: SWL of the sling is 1 x SWL of wire and fittings 2. Two single legs: SWL of the sling is 1.4 x SWL of wire and fittings 3. Basket hitch single sling: SWL of the sling is 1.4 x SWL of wire and fittings 4. Double wrap basket hitch: SWL of the sling is 2.1 x SWL of wire and fittings 5. Double basket hitch: SWL of the sling is 2.1 x SWL of wire and fittings 6. Doubled and choked sling: SWL of the sling is 2 x SWL of wire and fittings It can be seen from the above data that if two single legs are used instead of one (i.e. configuration 2 instead of 1), the SWL of the sling lift only increases by 40%, not 100%. This is because the included angle may be up to 90°. Also, the double wrap basket hitch (4) has the same SWL as the double basket hitch (5). Therefore, wrapping the strap around the lifted object twice has no effect on the amount that can be lifted. However, the additional wrap will reduce slippage on the attachment, as the frictional constraint will increase. When determining the length of the sling, the included angle must be limited to a maximum of 90 for the above values to apply. 316 Installation calculations for subsea pipelines SAFE WORKING LOADS Factor of safety – typically: SWL Where: MBL FoS MBL = Minimum breaking load (covered in strop sizing – design load for wire strop) FoS = Factor of safety – typically 3 to 8 FoS depends on usage – multiple use requires higher FoS Allows for dynamic effects, such as wave / current loads, splash zone loads as object enters water etc When calculating the safe working load, the minimum breaking load (MBL) must be divided by a safety factor (minimum breaking loads are covered later in this section). This safety factor is typically 5, but can be client specified depending on a case-by-case analysis of the risk involved in the job. This safety factor then covers dynamic load variations, and also the potential for an unevenly distributed load through the strops. In many cases, strops for heavy lifts are single usage and then the strop is disposed of. Where multiple use is permitted, a higher factor of safety is normally demanded. CRANES Main parts of a crane: Reach Height Brothers Boom (or stick or jib) Angle of boom 317 Pipe lifting Above are some examples of the main parts of a crane. When two or more strops are attached to the lifted object, they are commonly referred to as brothers. CRANE SELECTION Dependent on load weight and boom length Load tonne (kip) As boom length increases, load capacity decreases but maximum reach increases Minimum reach Reach m (ft) When selecting an appropriate crane for the load to be lifted, the reach and weight of the load must be considered. For a particular crane, e.g. one with a 60 tonne load capacity, a range of boom lengths can be selected. By varying the boom length, the desired reach can be obtained. However, as the reach is increased, the maximum load capacity of the crane will decrease. This is due to the angle at which the crane boom is positioned when at maximum reach. As the angle increases (from horizontal), the bending moment acting on the crane boom decreases, and hence the load capacity is greater than when horizontal. Another consideration when selecting an appropriate crane is the height required between the top of the boom and the object to be lifted. This will be dependent on the lift configuration (e.g. are there spreader beams / multiple lift points etc) and the water depth (if lowering a pipeline to the seabed). 318 Installation calculations for subsea pipelines TYPICAL CRANE SELECTION TABLE Select mobile crane capacity (in tonnes) based on load and reach required Boom radius/reach Load tonnes (kip) 2 (4.4) 4 (8.8) 10 m (33 ft) 20 m (66 ft) 30 m (98 ft) 40 m (131 ft) 50 m (164 ft) 20 (44) 35 (77) 60 (132) 90 (198) 20 (44) 50 (110) 90 (198) 120 (265) 200 (441) 6 (13.2) 25 (55) 70 (154) 100 (220) 160 (353) 225 (496) 8 (17.6) 35 (77) 90 (198) 160 (353) 225 (496) 300 (661) 10 (22.0) 50 (110) 100 (220) 160 (353) 250 (551) 800 (1764) Required 160 (353) crane rating tonnes (kip) Numbers given are for illustrative purposes, and vary according to crane manufacturer The table shown illustrates the relationship between the boom radius and the load weight of a mobile crane, which might be used at a quayside. It can be seen from this that to lift a load of a certain weight, the crane must have a considerably larger rated capacity (e.g. to lift 10 tonnes (22 kip) needs a minimum crane load capacity of 50 tonnes (110 kip) when using a 10 m (33 ft) boom radius). Lifts need to be carefully controlled with regard to boom reach (radius), otherwise extremely large cranes will be required for even modest weights. USE OF MOBILE CRANES AT QUAYSIDE Courtesy of Barth Crane Inspections (http://www.craneoperator.com) 319 Pipe lifting If the safe working chart (shown top right) is ignored, damage will result. Here, we can see two examples of failures collected by Barth Crane Inspections. The upper two photographs show bending and buckling of the extendable boom of a mobile crane. At the bottom, there are two examples of an over-toppling incident. ATTACHMENTS – SUMMARY Straps are more common offshore Clamps more suitable for 1 point lift – short loads Strop configuration affects SWL Typical factors of safety of 3 to 8 is used Higher values when strop is used more than once Crane selection depends on boom length and weight of load As reach increases, maximum load decreases Any questions? We have examined how the method of release needs to be considered when lowering spool pieces to the seabed. Other items may have permanent lugs or releasable clamps. The method of slinging affects the safe working load of strops and higher safety factors are required for multiple use. The stated capacity of the crane may be much greater than the object being lifted. This is due to the reach of the boom. With permanent barge cranes, the limiting envelopes of their lift capacity need to be checked to prevent overloading. 320 Installation calculations for subsea pipelines 2, 3 AND 4 POINT LIFTS STROP CONFIGURATION How do we choose the number of lift points? Commonly 2, 3 or 4 attachments Factors to consider Pipeline / spoolpiece geometry Is it symmetrical? 1D (straight pipe section) or 2D (L, Z or U shaped spools) 3D items may need extension strops or shackles to make up Typically chain brothers are of equal length Pipeline / spoolpiece material properties Yield strength (SMYS) Density / Self-weight in and out of water Position of centre of gravity and centre of buoyancy Keep spoolpiece level - crane boom above CoG The most common lift configurations have either 2, 3 or 4 attachments to the object being lifted. One point lifts can be used, if the object is sufficiently small to avoid instability problems caused by the moment from its self-weight acting at either end. Therefore, when lifting pipelines, this method is generally only used for short lengths, using a clamp attachment to prevent slippage. Of the remaining options, the selected number of lift points will depend on the shape of the pipeline, its weight and also material properties. For example, a straight pipeline may be lifted using two lift points, but if it is long and thin-walled (flexible), the bending moments and compressive forces induced during lifting may lead to a requirement for further lift points for additional support, to avoid plastically deforming or buckling it. When configuring the strops, the top of the crane boom must be positioned directly above the centres of gravity and buoyancy of the pipeline, otherwise it will twist when lifted until they are aligned. If the centres of gravity and buoyancy are not in alignment, the rigging will twist again if lowering into water (this is discussed later in this section). To assist in handling, the spool is normally kept level as it is lifted from the deck and 321 Pipe lifting placed onto the seabed. This means that the end of the crane boom must be above the centre of gravity of the spool, otherwise it will swing into that position. 2, 3 AND 4 POINT LIFTS Example lift configurations: 2 point lift on short straight pipe 3 point lift on U-shaped spoolpiece 4 point lift on Z-shaped spoolpiece Extension pieces for 3D spoolpiece Ideally, make up strops to exact lengths Ship with 3D spool The simplest lift is for a straight section of pipe. Here the chain brothers can be fitted equidistant from both ends. With an L or U-shaped spool piece, care must be taken to equalise the load between the chain legs. An alternative for the U-shape might be a four point lift but this is only possible with long leg lengths. The vertical attachment to the crane must be directly above the centre of gravity of the spool. With a Z-shaped (or dogleg) spool piece, the arrangement shown above is preferable to four equal length chains. With the latter, it is not possible to find four points on the spool equidistant from the centre point ring. It would mean that the length of two of the chains would need to be either lengthened or shortened. However, it is possible to attach the above system and the shorter lengths of chain will adjust to suit. Note that the loads on each of the shorter chains will not be equal and that the position of the attachments needs to be carefully chosen to prevent loose legs. With a true 3D spool, there is no option but to add extension strops (shown in red) to the chain brothers. This can be done with wire strops or shackles. However, as before, the arrangement shown will provide some self adjustment, and is preferable to four separate chains joining in a ring. The length of four chains would have to be very carefully controlled to avoid all load being distributed well. For very complicated shapes, it may be necessary to provide a lifting frame with multiple, individually lengths of strops. However, costs and time constraints when fabricating strops to tight tolerances on individual lengths must be considered. Perhaps it might be better to split the spool into two simpler legs able to make use of standard lifting equipment. 322 Installation calculations for subsea pipelines COMPARISON OF LIFT CONFIGURATIONS How many lift points are required? Type Straight pipe U spool Z spool 3D 1 point x x x 2 point (short length only) x x x 3 point x x 4 point (with extensions) The table above represents the most commonly installed spoolpiece geometries. When considering the lifting configurations for other shapes or objects, the rotational constraints and load weight limitations must be considered. Environmental conditions should also be considered, e.g. wave and current loads on the object once lowered into the sea. This may have implications on the stability of the system, and may lead to additional strops being required. LOAD TRANSFER Strop lengths must be accurate, otherwise: Lifted object will not be level (2 or 3 point lift) Load may be unevenly distributed (4 point lift) Object rotates until both brothers are taut - unable to align horizontally Potential for one or two brothers to remain slack on 4-point lift - increases load on others It is crucial to ensure that the strop lengths are measured accurately when setting up the lifting gear. If one or more of the strops is too short or too long, there is a potential Pipe lifting 323 problem for the 2, 3 and 4 point lifts. It is normal to use equal lengths of chain brothers, the hook heights of which can be checked as being level when hanging from the crane. The use of unequal legs should be avoided whenever possible. In the case of a one-point lift, the strop length is less important, as the discrepancy can be accounted for by changing the boom angle slightly, and this will be negligible in comparison to the tolerances on the strops. However, with 2 and 3 point lifts, if one of the brothers is longer than the other(s), the lifted object will rotate until the tension is taken up in all of the brothers. This will lead to the object being skewed from the horizontal axis, and may cause problems if it is being aligned with another object, e.g. a spoolpiece being aligned with a pipeline, with the flange hubs requiring accurate alignment. With the four-point lift, the situation is slightly different. If the strops are configured in the arrangement shown above, then potentially two of the brothers can be tensioned with the others remaining slack. This will result in the load being transmitted through these brothers alone, effectively doubling the load in each brother. This problem occurs due to the constraint of having four brothers. The Z-spool can rotate in one direction to remove the slack from one of the two brothers. However, this will increase the slack in the remaining brother, and hence equilibrium can not be obtained. FOUR POINT LIFT OF SUBSEA STRUCTURE Here a subsea protection structure is being installed using a four point lift in a calm sea. The structure is designed to be overtrawlable by typical North Sea fishing vessels. The end flaps of this roof structure will be lowered over the base unit which holds the valves and pigging loop pipework. Following flume tank model trials carried out by Jee Ltd, the design was modified to reduce the height of the main section of the unit. 324 Installation calculations for subsea pipelines USE OF FOUR-POINT LIFTS WITH RIGID STRUCTURES Minor discrepancies in strop length or fixings Statically indeterminate Most of load may be lifted on just one pair of strops Size for this eventuality Lifting points Around circumference Centred at centroid At same level Valve skid It is important to assess the stresses in individual strops when using four-point lifts. If the lifting points on a rigid structure are not aligned and equidistant from the centroid, then load may be attracted unevenly between the lines. These may fail due to overloading. Some extension occurs in the individual wires. Where the frame can also flex slightly, the loads in individual strops may be able to equalise. The design codes make allowance for this in skew load factors. When locating the lifting points, ideally, they should all be at the same level, the same horizontal distance away from the centre of gravity of the structure (that is, on the circumference of a circle). The centroid is not necessarily at the middle of the frame. In this way, the item being lifted remains horizontal. The lower left figure shows (diagrammatically) the optimum location of four lugs on a valve skid. The weight of the valve (pink) dominates. The pictures show how this may result in handling problems. This skid used a spreader bar, which distributes loads better and ensures the lugs remain in plane with the web of the side members. However, the lug points were selected before all items weights were known (thus enabling the centroid to be calculated). The diving contractors had requested equal length strops to avoid rigging confusion, and then trimmed the skid by adding shackles at the connection points. Pipe lifting 325 COMMON CONNECTION DETAILS Determinate Indeterminate more challenging The lefthand arrangement (with a spreader bar) can be analysed determinately when the centroid is known, providing the lugs and strops are close to their true size. Any discrepancies will be taken up by movement of the end of the beam to suit. However, the four strop arrangement shown on the right is not determinate because it is unable to accommodate minor differences in strop length or lug position. The strops will stretch to some extent but it may be insufficient to fully equalise their loads. Typical details using shackle attachments to tubular members are shown. The lug plates are connected to the swage ends of the strops using shackles. A ring or pear shaped link is used at the top end to bring the strops together. The top of the post on the lower detail is used to provide stiffness to the tubular. Additional hand-held ropes should be used to prevent swinging and rotation on deck. Once submerged, the seawater dampens such motion. 326 Installation calculations for subsea pipelines MENTOR DESIGNED WYE-PIECE AND PROTECTION STRUCTURE This wye-piece protection structure was designed by Mentor Subsea Technology Services of Dubai – a leading consultant in subsea equipment design and installation (www.jraymcdermott.com/mentor). It has just such a lug detail. The lifting points are carefully protected within the hinged cage lids to avoid risk of snagging by fishing gear. There are a series of ‘skirts’ beneath to prevent the unit sliding over the seabed. CENTRE OF GRAVITY AND BUOYANCY CoG and CoB may not be aligned Partially coated riser - splash zone transition CoB and CoG will be offset, due to variation in densities Splash zone corrosion protection coating CoG CoB Not a problem for lifts in air Problem occurs when riser becomes submerged Pipe will rotate due to misalignment of CoG and CoB 327 Pipe lifting If a crane is lowering an object into water, the centres of buoyancy and gravity must be directly aligned, with the top of the crane boom aligned directly above both of them. If the crane strops are aligned to the centre of gravity, but the centre of buoyancy is offset, the crane will be balanced until the pipeline starts to enter the water. Once the buoyancy forces begin to act on the system, it will become out-of-balance, and hence the pipe will start to skew towards one side. Therefore it is essential that the CoB and CoG are aligned, by use of one of the methods shown on the next slide. In the above example, one section of a riser is coated, for protection against corrosion in the splash zone. As the coating density is different to that of the steel pipeline, the centres of buoyancy and gravity will be misaligned. Another example of this problem occurs when lowering a plough into the sea. A plough is made up of many components, including buoyancy tanks and control modules. Some sections may be flooded whilst others are airtight and hence add to the buoyancy when lowered into water. Therefore calculations must be completed before the lowering operation, to ensure the centres of buoyancy and gravity are aligned vertically. CENTRE OF GRAVITY AND BUOYANCY Correct by re-aligning CoB and CoG Adding buoyancy - move CoB towards CoG CoG Air-filled or buoyant container Lift force CoB Add lump mass - move CoG towards CoB Down force Lump mass CoG CoB The above diagrams show how the centres of buoyancy and gravity can be re-aligned for an unsymmetrical pipe section. There are two options: ■ Buoyancy is added to counteract the additional weight on one side. This shifts the centre of buoyancy towards the centre of gravity; ■ A mass is added on the opposite end of the riser to counteract the additional weight of the flange / connector etc. This shifts the centre of gravity back into alignment with the centre of buoyancy. 328 Installation calculations for subsea pipelines CODES AND STANDARDS General DNV Marine Operations Part 2: RP5 – Lifting PM 20 (HSE withdrawn publication – but still used) HSE L113 – Lifting Operations and Lifting Equipment Regulations (LOLER) 1998 Cranes, hoists and winches ASCE Engineering Practice No. 93 – Crane safety API Spec 2C – Offshore cranes API RP 2D – Operation of offshore cranes BS 7121-11 – Safe use of offshore cranes BS EN 13852-1 – Offshore Cranes BS 7121-11 – Safe use of cranes The above list is not intended to be exhaustive, but does cover a wide range of codes used in the UK, USA and internationally when considering the use of handling and lifting equipment offshore. CODES AND STANDARDS Lifting gear EN 13414-3 - Steel wire rope slings BS 1290:1983 and BS 6210:1983 - Wire rope slings BS 3481 Part 3 - Web slings ISO 7531 and ISO 8792 - Wire rope slings ASME B30.9 - Slings: web, steel wire and fibre rope 329 Pipe lifting 2, 3 AND 4 POINT LIFTS – SUMMARY 2, 3 or 4 point lifts Depends on pipe geometry, material properties and dynamic conditions of installation Strop lengths – must be accurate or: 2, 3 point lifts will be skewed – rotate until taut 4 point lifts may have unequal loading on each strop Align CoG and CoB with top of crane boom Otherwise lifted object will twist Tension in strop is related to included angle Self-weight induces bending moment in pipe May cause plastic deformation or buckling Any questions? The lifting chains commonly have either two, three or four hooks – but normally hang level from the crane when not in use. The selection of which option to use depends on the shape of the spool and any changes in section along its length. The aim is to keep the spoolpiece level as it is lifted from the deck and keeping all chains in tension. This can be achieved by ensuring the crane boom is directly above the centre of gravity of the spool. This must remain true as the spool is immersed in water. Any tension in the chain or strop can be calculated by trigonometry and is related to their included angle. Remember too that the spoolpiece is normally subjected to higher bending stresses during installation than it is likely to experience during operation. This is particularly true for long slender spools. 330 Installation calculations for subsea pipelines STROP EXERCISE EXERCISE Determine the forces in the sling Resolve forces horizontally and vertically algebraically in terms of Draw BM, compression and shear diagrams for pipe Use w = unit pipe weight = W / L (W = total weight) T Included angle, Strop tension, Ts Strop length, Ls Ls Pipe compression, Cp W m g a a Pipe L The above slide shows a simple two point lift on a straight pipe. The forces present are the self weight of the pipe and the tension in the sling strops. Using a force balance, the forces in the individual strops can be determined, and also any forces present in the pipe being lifted. Hint: The free body diagram can be resolved by balancing the forces firstly in the vertical direction, and then horizontally. Shear load and bending moment diagrams are constructed by firstly integrating the distributed load to get the shear diagram. The shear diagram is then integrated to get the bending moment diagram. For the shear diagram, point loads will result in a step in the diagram. Similarly, there will be steps in the moment diagram for point couples. The diagram should show that for a simple two point lift of a straight pipeline, the maximum bending moment will be either at the centre of the pipe or at the points of attachment to the sling. Optimum design might try to equalise these moments by moving the points of attachment. 331 Pipe lifting STROP SIZING STROP SIZING Determine the minimum breaking load (MBL) for the crane strops Design using DNV Standard for Insurance Warranty Surveys in Marine Operations. Part 2: RP5 Lifting This section covers the methodology used by DNV to determine the minimum breaking load (MBL) to be specified when sourcing suitable steel rope for the strops on a crane. The method incorporates a number of safety factors to account for conditions such as skew loads, hydrodynamic effects, weight tolerances and rigging configuration. 332 Installation calculations for subsea pipelines WEIGHT OF OBJECT AND RIGGING Weight of object to be lifted Normally determine by weighing (if > 95% complete) May be determined using specific weights and volumes (if estimated then multiply by 1.1) Also used to determine centre of gravity Weight of rigging Total weight of rigging equipment Shackles, slings, spreader bars, frames etc When considering the weight to be lifted by the crane, this should include both the object being lifted and the rigging attached to it. Ideally the object’s weight will be determined by weighing, with the CoG being determined at the same time. This may be impractical, in which case an approximation can be made based on specific weights and volumes of all components in the object. 10% should be added to this estimate to allow for any inaccuracies. If the object is weighed, there should be a consideration of inaccuracies in the weighing equipment. It is recommended that equipment less than 97% accurate should not be used, and that at least 95% of the object is fabricated before it is weighed. The weight of the rigging should include all equipment being supported by the crane’s boom, i.e. spreader bars, slings, shackles etc. 333 Pipe lifting DYNAMIC AMPLIFICATION FACTOR Dynamic amplification factor (DAF) Accounts for global dynamic effects Influenced by: Environmental conditions Type of crane Stiffness of crane boom and lifting appliances Weight of lifted object Type of cargo vessel Lift in air or water Consider hydrodynamic and hydrostatic effects in water The dynamic amplification factor (DAF) accounts for global dynamic effects experienced by the system. There are a number of parameters affecting the DAF, the main ones of which are listed above. When considering lifts in water, consideration should be made of the hydrostatic and hydrodynamic effects present. DYNAMIC AMPLIFICATION FACTOR Typical values of DAF Weight of lifted object in tonnes (kip) DAF Offshore DAF Inshore < 100 100 (220) to 1000 (2205) to >2500 (< 220) 1000 (2205) 2500 (5512) (>5512) 1.3 1.15 1.2 1.1 1.15 1.05 1.1 1.05 Above factors only apply if: Not in adverse conditions Lifting in air – no hydrodynamic or hydrostatic effects Why do factors reduce as load increases? 334 Installation calculations for subsea pipelines MINIMUM BREAKING LOAD De-rating factor – applies if one or more of the following conditions occur Minimum bend radii compared to that of cable laid rope nominal diameter (Dnom): Eye of single part sling: < Dnom Other part of sling: < 6 · Dnom Eye or other part of grommet: < 6 · Dnom De-rating factor if above conditions are not met: 0 .5 f b 0.8 1 0.5 / D / d D = diameter of bend d = nominal diameter of cable laid rope or single part grommet The de-rating factor is generally not required, as the strength reduction of a sling or grommet due to bending is normally within allowable limits. However, in cases where de-rating is required, i.e. when the MBL is reduced by more than 20%, a de-rating factor is required. The strength reduction is directly related to the ratio of bend radius to the diameter of the cable laid rope or grommet. If this ratio falls outside the limits specified in the above slide, the de-rating factor should be used. This will increase the MBL specified when selecting an appropriate steel rope for the sling, i.e the de-rating factor (fb) > 1. OTHER FACTORS Tugger or guide lines - special loads (SPL) Tugger lines are used to control the spool during lift Wind and hydrodynamic/hydrostatic loads Dynamic hook loads (DHL) Includes effects from global dynamic amplification Skew load factor (SKL) Allowance for load not being in-line (vertical) Of particular concern with multi-crane lifts Usage factor Will the strops be used only once? 335 Pipe lifting A number of other factors are used to multiply the basic load and allow for dynamic and other effects, such as above. It is necessary to strictly follow the approach set out in DNV, Lloyds or other applicable code. MINIMUM BREAKING LOAD Minimum breaking load (MBL) Specifies the strength required for the sling MBL calculated force SKL FoS f b Additional safety factors Nominal safety factor (FoS) De-rating factor – applies if MBL is reduced by > 20% due to splicing or bending (fb) Nominal safety factor (FoS) W eig h t (W ) o f o b je ct FoS W > 5 0 to n n es (1 1 0 kip ) 3 .3 W < 4 0 to n n es (88 kip ) 4 .0 The minimum breaking load is used to specify the guaranteed minimum load at which the steel rope breaks. This will normally be determined by the steel rope fabricator by testing the entire rope, or part of it, to destruction. If the MBL is determined from a part of the rope, a spinning loss coefficient should be applied. This is typically 0.85. If a grommet has an unspliced core then the core strength should not be included in the calculation of the MBL. There are two safety factors to consider when calculating the MBL to be specified for a particular sling. These are the nominal safety factor and the de-rating factor. Nominal safety factor – this is dependent on the weight of the object being lifted. If the weight is greater than 50 tonnes (110 kip), a factor of 3.3 should be used. If less than 40 tonnes (88 kip), a safety factor of 4.0 should be used. For all intermediate weights, the factor should be linearly interpolated between the two values. 336 Installation calculations for subsea pipelines STROP SIZING – SUMMARY MBL of strops depends on: Weight of rigging and lifted object Global dynamic effects (DAF) Special loads such as hydrodynamic (SPL) Dynamic hook load (DHL) Determined from the above factors Skew load factor (SKL) Allows for uneven loading on strops De-rating factor to account for tight bend radii of strops and terminations Strop sizing is dependent on a number of multipliers applied to the self-weight of the object and slinging equipment. These allow for dynamic lifting/lowering through the air and water. The factors are set out in the applicable codes, but each code’s approach has slight differences. 337 Pipe lifting SPREADER BEAMS PIPE BUCKLING Compressive forces related to: Angle of sling strops Number of strops Weight of object Geometry of pipe spool Compressive forces in spool (L-shaped) With direct two point lifts, the limitation on the length of pipeline / spoolpiece that a single crane can lift is usually governed by pipe buckling. We have shown how the angle of the lifting wires can result in axial compressive loads being applied to the spool. If the spool is too long or the angles too shallow, these compressive loads (in combination with its self weight) may become large enough to cause Euler buckling of the spool. The above diagram is designed to clearly demonstrate strut buckling with a L-shaped spoolpiece. However, it would be more common to have a three or four leg lift with an L or Z spool. Nevertheless, these can also induce compressive forces in the spool if a spreader bar is not used. Larger DSVs and MSVs have twin cranes allowing longer spools to be installed. If using twin cranes, the operators should always work in unison to ensure lift forces are always acting in the vertical direction and not transferred axially into spool compression. 338 Installation calculations for subsea pipelines SPREADER BEAMS Advantages Alleviate compressive forces by transferring from pipe to spreader beam Reduce the risk of buckling the pipe Spool need not be designed for temporary installation condition Disadvantages Add weight to the rigging Additional crane height required to accommodate beam Compressive forces in spreader beam Spreader beams can be used to alleviate this problem. However, they need to be very stiff compared to the pipe, so are usually large I-beams. They add their own weight and also increase the height of the lifting configuration. Note that the figure shows the same L-shaped spool again being lifted with only two strops from the spreader beam. This would not be an optimum solution due to the difficulty in keeping the spool level. A better solution would have three or four strops. SPREADER BEAMS Transfer of compressive load (P) to beam Increased weight and height W m g ( pipe) weight of beam ( strops etc) Can more than double W the lifted load Crane jib height increased by H P P H W Vertical strops attached to pipe, no compressive force in pipe 339 Pipe lifting All buckling forces are now taken by the beam But this can be sized to withstand these forces. This contrasts with having to size the spool for the temporary condition during installation instead of the permanent stresses during operation. However, the increased weight of the beam and extra strops can more than double the lifting load needed. We also need to allow for the increased height of lift due to the strops. USE OF CHS OR UB AS SPREADERS Circular hollow section Optimum for compression Universal I beam or column Good for bending and axial Single plate welded into slot in spreader Handling rope on deck Spool can support self weight Small pipe needs extra support Intermediate lugs slotconnected using welds to web stiffeners It is common on vessels to have a selection of only CHS (circular hollow sections) or UB (universal I beams) and pipes. The former provide excellent stiffness against Euler buckling. The shape provides the maximum radius of gyration for the amount of steel. It is the best solution when the spoolpiece can support the bending stresses due to self weight. The combined lugs at either end are made from a single piece of metal welded into a slot in the spreader beam pipe. Note that the spreader will need to be able to support its own self-weight so true Euler buckling analysis is not appropriate. I beams are used where there are intermediate slings to the object being lifted and stiffness in bending is needed as well as axial compression resistance. It is used for long flimsy spools which need intermediate support to withstand their self weight. The load is then transferred to the spreader beam. The intermediate lugs are slotted in replacing part of the web. They are also welded to the web stiffener plates: this provides additional resistance to pull out. 340 Installation calculations for subsea pipelines SPREADER BEAMS – SUMMARY Lifting induces compressive force in pipe Increases as strop deviates from vertical orientation Spreader beams Alleviate compressive stress in spool Multiple attachments can reduce bending But increases weight and height of rigging Designed for: Compressive buckling Bending due to self weight Any questions? If the pipe spool is lifted by itself, it will tend to buckle due to the self weight and the compressive forces induced by the strops. Spreader beams can be used to lift spools into position safely. They need to be designed to resist the same compressive force and bending due the self weight of the beam. But the spool can then be economically designed for the permanent stresses during operation. 341 Pipe lifting WORKED EXAMPLE PIPE LIFT WORKED EXAMPLE Make use of equations you developed earlier Two-point lift on spreader beam Straight pipeline spoolpiece Calculate for beam: Compressive force and bending moment Limiting stress conditions Assume: Static analysis – doesn’t account for crane acceleration, hydrodynamic loads etc We want to lift a double butt pipe using a lifting beam. It is 168.3 mm diameter with heavy weight concrete. On board the vessel we have a BS4 UC which we can use. Are the stresses acceptable? 342 Installation calculations for subsea pipelines WORKED EXAMPLE - INPUT DATA Double-header pipe spool 168.3 mm by 12.7 mm wall (6⅝ in by ½ in) Length 24.3 m (79.7 ft) Grade X42 (y = 289 MPa) Lb 51 mm (2 in) heavyweight concrete Density 3044 kg/m³ (190 lb/ft³) Beam (universal column) Length 14.3 m (46.9 ft) End overhang 5 m (16.4 ft) UC 254 x 254 x 89 kg/m (10.2 x 10.1 x 59.7 lb/ft) Steel grade S275 (39.9 ksi) Lp Total weight of pipe & beam, Strop angle with beam = 45° Wtot = 67.5 kN (15.1 kip) For the simple straight pipe lift, we want to size a suitable spreader beam using AISC code and a non-US steel section. Column sections can provide the best resistance to pure compression since they have similar stiffness in the x and y axes. Struts’ strength is determined by their minimum stiffness, so by equalising these, their capacity is maximised. A check is also needed for built-up beams to ensure that they are deemed ‘compact sections’. Possible sections that might be used are: ■ European HE wide flange beam section in accordance with Euronorm 53-62 ■ European IPE narrow flange beams to Euronorm 89 (although these may not be not stiff enough in the lateral direction) ■ BS 4 : Part 1 : 1993 UC or UB sections to BS EN10056 : 1999 ■ USA W (wide flange) shapes (metric versions are available in the Far East) The enclosed angle of 90° for the sling is the maximum that we would really want to use. The equations developed earlier should be used to calculate the compression in the beam. In this instance, we are only going to check the chosen beam for over-stress conditions. For this reason, we are providing the total mass of pipe, beam and strops. An allowance of 105% has been made for the weight of the lugs, stiffeners and strops attached to the beam. The additional items are concentrated at the ends, so for the unit self weight, we can assume the listed value. The pipe stresses have been checked separately. approximately half that of yield. The bending stress in pipe is Note that the orientation of the beam (and therefore pipe) is controlled using additional ropes to the ends of the beam. If they were attached to the pipe, there is a risk that additional unwanted loads could be applied to the system. 343 Pipe lifting MOMENT, SHEAR AND AXIAL COMPRESSION DIAGRAMS UC spreader beam Moment Shear Axial compression Slope = self weight of spreader beam Pipe spool Shear Moment Slope = self weight of pipe spool This is a fully determinate system. BEAM FORCES Unit weight of beam w 88.9 kg/m g 0.872 kN/m 59.7 lbf/ft – no need for g Compression in beam compression Pr Wtot 2 tan 67.5 kN 15.2 kip 2 tan 45 33.7 kN7.6 kip Bending at centre sag M r w g L2 8 88.9 kg m 9.81 m s2 14.3 m 8 22.3 kN m 2 59.7 lb ft 46.9 ft 8 16.4 kip ft – no need for g Shear at ends 2 Vr w g L 2 88.9 kg m 9.81 m s 2 14.3 m 2 6.5 kN 59.7 lb ft 46.9 ft 2 1.5 kip – no need for g Where: = angle of slings with horizontal ■ ■ g = acceleration due to gravity (9.80665 m/s²) ■ L = length of lifting beam ■ Mr = required moment strength in beam at mid-length due to self weight ■ Pr = required compressive strength of beam ■ Vr = required shear strength in beam at ends due to self weight 344 ■ ■ Installation calculations for subsea pipelines Wtot = total weight of pipe, concrete coating and beam (including an allowance for lugs etc) w = unit weight of beam (ignore allowance for end lugs etc) In most instances of a spreader beam, the beam acts as a ‘long column’ subject to moment at middle and shear at the ends (due to self weight of the beam). However, the combination of moment and axial compressive force is the main design consideration. By ‘long’, we mean that the beam is likely to fail by Euler elastic buckling rather than inelastic buckling (short or intermediate beams). We need to check that the beam does not bend sideways (yy axis) under the compression since the beam is less stiff in this direction. Remember that when working in imperial units (lb and lbf - or kip), there is no need to multiply by g. CRITICAL DESIGN CONDITION IN BEAM The critical design condition is Compression and bending at mid-span of beam Bending stress is much greater than compression We will ignore other checks in exercise Shear at ends Values are small Generally not critical for lifting beams Full design should include these checks Support lugs covered elsewhere in course 345 Pipe lifting BEAM STRESS DUE TO COMPRESSION Effective length, I K Lb 14.3 m 46.9 ft K = 1.0 for a pin-pin free-ended lifting beam Section data from tables Depth of section, d = 260.3 mm (10.2 in) Width of flange, bf = 256.3 mm (10.1 in) Thickness of flange, tf =17.3 mm (0.7 in) Weak axis moment of inertia, Iy = 4857 cm4 (116.7 in4) Torsional constant, J = 102 cm4 (2.5 in4) Elastic modulus, Sx = 1096 cm 3 (66.9 in3) Plastic modulus, Zx = 1224 cm 3 (74.7 in3) Minimum radius of gyration, ry = 6.55 cm (2.6 in) Cross-sectional area, Ag = 113 cm² (17.5 in²) Yield stress of beam, Fy = 275 MPa (39.9 ksi) Ultimate stress, Fu = 430 MPa (62.4 ksi) Be aware that different section data sources use different symbols for these variables. For example, the new EuroNorms have the x axis along the centreline of the beam between supports. This means that what used to be the x and y directions are now the y and z directions. Note too that many tables use centimetres to reduce the number of digits printed. Remember to factor values into common units (multiply ft by 12 to get inches and metres by 100 to get centimetres). SLENDERNESS OF MEMBER Effective length of member Effective length factor for pinned end, K=1.0 Limiting slenderness ratio for member BC sr K Lb 1.0 14.3 m 1.0 563 .0 in 218.3 ry 65.5 mm 2.6 in Value over 200 ! slender – use with caution Elastic critical buckling stress Fe 2 E sr 2 2 200 10 9 41.4 MPa 218 .32 2 29 10 6 6.0 ksi 2 218 .3 346 Installation calculations for subsea pipelines Following the method outlined in the Steelwork module for tubular members in compression. Normally, we would like to limit the value of sr to less than 200. However, we will see later that the beam is very lightly loaded so we will use this section with caution. Alternatively, we could reanalyse using a beam with a greater value of ry. Remember also that the strops to the lugs are not actually fitted at the end of the beam, so the supported length is shorter than Lb. NON-COMPACT COLUMN FORMULA Critical slenderness ratio 200 GPa E 4.71 127 .0 275 MPa Fy 29 10 3 ksi 4.71 127 .0 39.9 ksi Cc 4.71 sr = 218 & Cc = 127 Intermediate or slender Flexural buckling stress Fcr 0.877 Fy 0.877 275 241 MPa 0.877 39.9 35.0 ksi This time, we have an intermediate stiffness so the alternative formula applies for intermediate or slender members. Where: ■ Fe = elastic critical buckling stress ■ Fcr = flexural buckling stress Other terms as before. 347 Pipe lifting ACTUAL AND ALLOWABLE COMPRESSION STRENGTH Compressive load in member, Pr Pc 33.7 kN 7.6 kip Allowable compressive strength, Pc Pn Ωc Nominal axial strength, Pn Fcr Ag and c=1.67 Pc Fcr Ag Ωc 241 MPa 11.3 10 3 mm 2 1632 kN 1.67 2 35.0 ksi 17.5 in 612.7 kip 1.67 Ratio, Pr Pc 33.7 1632 7.6 367 2.1% OK (100%) Where: ■ Ag = gross area of section ■ Pn = nominal compressive strength c = Safety factor for compressive members (Refer to p16.1-32) ■ Other terms as before. PURE BENDING (FLEXURE) AT MID-SPAN Yielding (plastic moment) – nominal strength M n M p Fy Z x 275 MPa 1224 10 3 mm 3 337 kN m 39.9 ksi 74.7 in3 248 kip ft Lateral-torsional buckling Determine whether UC is compact or non-compact Length between braced points, Lb I 14.3 m 46.9 ft Limiting length for yielding L p 1.76 ry E 200 GPa 1.76 65.5 mm 3.109 m Fy 275 MPa 2.6 in 30 000 ksi 10.2 ft 1.76 12 39.9 ksi Limiting length for lateral buckling, Lr = 13.6 m (44.7 ft) See equation over page 348 Installation calculations for subsea pipelines LIMITING LENGTH FOR LATERAL-TORSIONAL BUCKLING 0.7 Fy S x ho E J c Lr 1.95 rts 1 1 6.76 Fy S x ho J c E 200 GPa 1.020 106 mm 4 1.0 1.95 73.3 mm 275 MPa 1096 103 mm 3 243 mm 2 0.7 275 MPa 1096 103 mm 3 243 mm 1 1 6.76 1.020 106 mm 4 1.0 200 GPa 13.6 m 30 000 kip 2.5 in4 1.0 1.95 2.9 in 3 66.9 in 9.6 in 39.9 kip 2 3 0.7 39.9 kip 66.9 in 9.6 in 1 1 6 . 76 44 . 7 ft 4 30 000 kip 2 . 5 in 1 . 0 2 For conservatism, c may be taken as 1.0 (Section F1.2 p16.1-47). Where: ■ ho = distance between centroids of flanges = d – tf = 243 mm (9.6 in) LATERAL-TORSIONAL BUCKLING LIMITING LENGTHS If Lb ≤ Lp Lateral-torsional buckling would not apply If Lp<Lb ≤ Lr Strength would be determined by Lb L p M p M n Cb M p M p 0.7 Fy S x Lr L p 14.3 m 3.1 m 1.0 337 MPa 337 MPa 0.7 275 MPa 1096 103 mm3 337 MPa 3.109 m 13.6 m 3.1 m 66.9 in3 46.9 ft 10.2 ft 1.0 248 kip ft 248 kip ft 0.7 39.9 ksi 12 44.7 ft 10.2 ft 248 kip ft 10.2 ft Else if Lb>Lr – true for this beam M n Fcr S x M n 241 MPa 1096 103 mm3 337 kN m 200kN m 35 ksi 66.9 in3 248 kip ft 147.5 kip ft See F2.2 p16.1-47. 349 Pipe lifting PURE BENDING (FLEXURAL) STRENGTH Required moment strength Mr = 22.3 kN m (16.4 kip ft) Nominal strength of member in bending For yielding Mn = 336.6 kN m (248.3 kip ft) For flexural-torsional buckling – lesser dominates Mn = 200 kN m (147.5 kip ft) Permitted strength in bending Mc = Mn/b = 200/1.67 = 133.7 kN m (147.5/1.67 = 98.6 kip ft) Ratio, Mr/Mc = 16.7% OK (100%) COMBINING TENSION AND BENDING Combined loading for UC beam For Pr 0.2 Pc Pr 8 M rx M ry 1.0 Pc 9 M cx M cy Or for Pr 0.2 Pr M rx M ry 1.0 Pc 2 Pc M cx M cy For this section Pr Pc 0.021 so second equation applies M 33.7 kN Pr 22.3 MPa r 0 0 0.177 2 Pc M c 2 1632 kN 133.7 MPa 7.6 kip 16.4 kip ft 0 0.177 2 367 kip 98.6 kip ft OK – less than unity AISC provides a means of assessing combined flexure and axial force. See equations H1-1a and H1-1b in Section H1.1 p16.1-70. 350 Installation calculations for subsea pipelines BEAM EXERCISE EXERCISE What if the spool were 36.5 m (120 ft) long? Use a longer spreader beam of 24.5 m (80.4 ft) Pipe ends overhang beam by 6 m (19.7 ft) Intermediate slings to prevent overstress in pipe Calculate for beam: Compressive force and bending moment Add weight of pipe to the beam as an uniform load Other parameters and assumptions as before Omit pipe calculations Lb Lp Weight of pipe, W tot = 81.7 kN (18.4kip) For simplicity, assume that the weight of the pipe is added to that of the beam as a uniformly distributed load (UDL) rather than evaluating the two intermediate equidistant support straps. The 5% additional allowance for the lugs and stiffeners on the beam should also be distributed along the beam rather than concentrated at just the ends. In actual fact, the system is statically indeterminate. We need to ignore the stiffness of the pipe (assuming it is essentially a flexible load) to simplify the design of the beam. Nevertheless, this is a reasonable assumption in this instance. 351 Pipe lifting MOMENT, SHEAR AND AXIAL COMPRESSION DIAGRAMS Slope = self weight of spreader beam UC spreader beam Moment Axial compression Shear Moment Pipe spool Shear Slope = self weight of pipe spool This is an indeterminate system. However, if the stiffness of the beam is significantly greater than that of the pipe spool, then the latter can be deemed to be simply slung beneath the beam contributing little to the stiffness of the system. It also depends on the accuracy of the length of the strops. Where one strop is much longer than the others, then the pipe may span between the adjacent supports. In this case, no uplift will occur at that point, so the shear diagram for the spreader beam (as well as the pipe spool) will have different step heights at each of the intermediate points. The moment diagrams for both the beam and spool will also change. BEAM STRESS DUE TO COMPRESSION We need a longer and deeper section beam Try UB 914 x 419 x 343 kg/m (35.9 x 16.5 x 230.5 lb/ft) Section data from tables Depth of section, d = 911.4 mm (35.9 in) Width of flange, bf = 418.5 mm (16.5 in) Thickness of web, tw = 19.4 mm (0.8 in) Root radius (fillet), tf =24.1 mm (0.9 in) Second moment of area, Iy = 39160 cm4 (941 in4) Torsional constant, J = 1193 cm4 (28.7 in4) Radii of gyration, rx = 37.8 cm (14.9 in) and ry = 9.11 cm (3.6 in) Elastic modulus, Sx = 13 691 cm3 (835.5 in3) Plastic modulus, Zx = 15 480 cm3 (944.6 in3) Cross-sectional area, Ag = 437 cm² (67.7 in²) Yield stress of beam, Fy = 275 MPa (39.9 ksi) 352 Installation calculations for subsea pipelines A UB provides better resistance to bending about the strong (horizontal) xx axis than the UC we used previously. 353 Pipe lifting DEEP WATER RESONANCE INTERFACE WITH WATER Lowering through water interface Most risky part of operation Surge and wave motion Out of phase effects Object and vessel Swinging Short arc movement Proximity with side of vessel Swings more in air than in water Less resistance Lowering (or lifting out) an object through the air-water interface can be the most risky part of the operation. In addition to the surge and wave motion of the vessel - which is transferred to the end of the lifting derrick and thence to the rope - we have the swinging of the object itself. The derrick must be stiff enough to resist any dynamic amplification effects from the rope and object being lifted, as well as any out of phase effects between the lifting system and the vessel. Swinging of the weight is worst when the rope is shortest. This is often at the water interface and close to the side of the vessel. We will also not have the damping effect of the object as it moves through the water. 354 Installation calculations for subsea pipelines CURRENT EFFECTS Currents are combination of Tidal current Calculable for each phase of moon (springs and neaps) Wind generated current at still water level vB Vc VT Vw h0 z h0 Surface Vw 0.17 VR Adjust for wave crests and troughs to maintain constant current flow volume ho = 50 m ' Vc Vc h z h vw ho h vB Current profile An assessment of currents may be needed for suspended loads beneath a barge. Where: ■ Vc = current profile at depth z ■ VT = tidal current ■ Vw = wind-generated current at the still water level ■ VR = reference wind velocity (defined earlier) ■ h = water depth ■ ho = reference depth for the wind generated current (ho=50 m) ■ z = distance from the still water level (positive downward) Currents can be calculated as the combination of tidal currents and wind generated currents. Tidal currents are often greatest near to shore on the continental shelf. Mid-ocean, they are considerably less due to the greater water depth. They can be assessed knowing the location and time of year. The strongest tides are found at spring tides, which occur a few days either side of full and new moons (once a fortnight). Smaller tidal currents are associated with neap tides, which occur one week either side of the springs. At the spring and autumn equinoxes, spring tides are particularly high and currents run stronger. The DNV RP2 code provides a method of assessing wind-generated currents. Their effects are effectively restricted to the upper 50 m (164 ft) of the water column. The method also permits adjustments to be made, allowing constant current flow volume through the wave crests and troughs. 355 Pipe lifting DEEP WATER RESONANCE Waves RAO of vessel Phase angle and response Weight of object Response near seabed Current Current effects on wire Variation with depth Vertical out of phase Over-stress in wire Varies with depth We often assume sinusoidal waves of fixed height. In practice at sea, they are random both in period and height. The vessel operating characteristics need to be described in all six degrees of freedom (yaw, pitch, roll and heave, sway, surge). The responses or RAOs vary with wave frequency. We need to take account of both the phase angle and percentage of the initiating wave movement is felt at the fixed point. It is normal to lift from near the centre of the vessel to minimise movement. However, some movement is inevitable. We often need to set an object gently down in a specified location on the seabed. Close to the bottom, the object does not feel the effects of the waves directly but may rise and fall slightly as it is being held by the wire. Its inertial mass must be allowed in the analysis. Allowance for swinging motions must also be made. The current may vary in both speed and direction with depth. Often, either the 7th power law or a parabolic distribution of velocity is used, based on readings just above the seabed, at mid-point and just below the surface. However, in some instances, the bed current can flow in a different or even reverse direction compared with that at the surface. Where it is known that there are significant changes in current velocity and direction through the water depth, then this must be taken into account. It may be that because of resonance, the weight tries to go down just as the vessel rises in the water. This effectively stretches the wire, increasing tension forces in it. Different wire lengths tend to have different responses as the object is lowered to depth. 356 Installation calculations for subsea pipelines ORCAFLEX ANIMATION OF SWINGING AND RESONANCE This animation shows the alarming swinging of the manifold in air and then its behaviour near the seabed. An umbilical is to be laid away from the vessel from a drum at the stern, which means that the manifold has to be lowered from the side. However, we have shown it being installed with beam seas. In practice, the vessel would be orientated heading directly into the waves to limit its motion. The video speed is twice normal. Motion multiplier DAF DYNAMIC AMPLIFICATION OF A LOAD DURING INSTALLATION 1+ K·L/Static load 6 5 4 3 2 1 0 0.0 0.5 1.0 Hardware displacement Excitation amplitude 1.5 /n f Ldeployed 2.0 n K M + Ma C K n Eequiv Acirc L K 2 mw M Ma 5 2 Tnat 2.5 3.0 Pipe lifting 357 During installation in a swell, the natural response of the system may be excited, and dynamic loading occurs. A number of factors play a part in the oscillating system, such as the mass of the equipment, the rope, and the added mass due to water, the stiffness of the wire, and the damping caused by the seawater. It can be seen on the graph that when the frequency of the swell matches the natural frequency of the system, n = 1) there is the greatest dynamic amplification of the load. ■ The red line (wire load) indicates the factor by which the wire load increases due to the dynamic effects. In this instance, at ω/ωn = 1, the peak load (dynamic amplification factor) is 5.3 times the static load. ■ The blue line (amplitude) shows by how much the object is moving relative to the displacement of the vessel. In this instance, at ω/ωn = 1, the peak vertical displacement of the object is 2.5 times that of the vessel. For a full description of the analysis, refer to Petrobras AMAE 2006 Hamburg Conference Paper "The need for the pendulous installation method" by Maxwell B de Cerqueria et al. Where: ■ Acirc = area of circumscribed circle of wire ■ C = seawater damping ■ DAF = dynamic amplification factor ■ Eequiv = equivalent or apparent Young’s modulus of the wire ■ f = function ■ K = axial stiffness of the wire ■ L = length of wire ■ Ldeployed = deployed length of wire L = change in length due to dynamic effects ■ ■ M = mass of lowered equipment ■ Ma = added mass of seawater ■ mw = mass of wire ■ Tnat = natural time-period of the system = frequency ratio ■ = frequency of the waves ■ = natural frequency of the system ■ 358 Installation calculations for subsea pipelines PETROBRAS ANALYSIS FOR MANIFOLD TRANSPORTATION The above animation shows the natural frequency of the line matching the wave period and vessel response, resulting in the development of large oscillations. This work was undertaken by Petrobras using the Anflex package. The vessel is heading into a current resulting in a total manifold-water speed of 3 m/s (5.8 knots). The two streamers show how the manifold is moving relative to the water particles. As might be expected, the length of this combination of lifting/lowering gear has been adjusted to show the effect most strongly. However, when lowering from the surface to the seabed, it is always possible to encounter a particular length of wire that causes such oscillations. It can be seen that at some stages, the lifting strops go completely slack. When load is taken by them again, the resulting snatch forces are likely to cause these to break. 359 Pipe lifting ACTIVE HEAVE COMPENSATION SYSTEM Waveriding launch system for ROV Gas/oil accumulators act as spring Fitted into winch hydraulic circuit Operable in 14 m (46 ft) waves Problems Localised cable wear Rope fatigue Reduced life To overcome problems at the air-water interface a number of systems have been developed to either guide or push objects below the surface. An additional problem with ROVs is that their thrusters cannot operate near the surface - they suffer from cavitation. To avoid the deep-water resonance, we can install a heave compensation system. The examples shown were fitted to the Beryl Alpha and Beryl platforms, courtesy of Subsea 7. The system minimises heave forces developing in the wire or cable. Problems were encountered due to wear on a localised length of the cable when positioned at a set depth. This same section of cable would be repeatedly pulled back and forth over the sheaves as the waves passed through and dynamic resonant effects of the ROV were felt. In addition, other effects such as fatigue contributed to a reduced life. 360 Installation calculations for subsea pipelines CRANE HEAVE COMPENSATOR Hydraulic accumulator The crane heave compensator is controlled by the unit reaching the full height to the side of the cab as shown on the photo. This hydraulic accumulator essentially smoothes out any power surges in the systems. The crane wire winches are driven by hydraulic motors hence the need a damping device. The vessel and object load are controlled by a computer system in the crane driver cab. The accumulator is only active in ‘heavy active heave mode’. This particular crane has a number of operational modes: ■ Manual mode – no heave compensation is used; ■ Constant tension – this mode is used when a payload is supported by something other than the lift line. In this mode, the winch will haul or render as the vessel rises and falls, and will maintain the line tension within limits pre-set by the operator; ■ Active heave compensation (heavy / passive) – active heave compensation is used to maintain the vertical load within pre-defined limits. A joystick control may control the vertical position of the payload from a motion reference unit. This mode is commonly used when landing a load onto the seabed. The 3D spool is part of the valve unit covered by the overtrawling protection structure we saw earlier. 361 Pipe lifting ROPE RESONANCE AND FATIGUE Suspended load in deep water Multiple sheave system Bending and tension over the pulleys Resonance of rope - fluctuation loads in pulleys Wear and fatigue failures of the wires and strands Severely limits the life of the rope Life assessment Interaction of wear, bending, tension and fatigue Not directly calculable analytically Feyrer’s empirical formula used Endurance bending stresses - testing to failure If an active heave compensator is used, then the effects of wear and fatigue must be assessed for dynamic ropes working on a multiple sheave system. Single sections of the wire will be repeatedly passed back and forward across the pulley system used to compensate for the resonance. Each time, the wire will be subjected to bending and tension. Areas will be worn and damaged by fatigue. This all contributes to a much reduced life of the rope. The combined effects of wear, bending, tension and fatigue interact together. Professor Feyrer’s empirical formula can be used to estimate the life of a wire rope subjected to cycles of tension and bending. The formula includes several parameters, specific to a given rope design, that must be derived from a large number of bending fatigue tests. Feyrer’s empirical lifetime formula can then give a guide to the life of a rope and so an appropriate discard time can be obtained when the service cycles are known. 362 Installation calculations for subsea pipelines ADDED MASS OF WATER WITH TUBULARS AND PLATES Wave/barge motion Through wave zone Assess drag and inertia of associated water Wave winch Bitumen mattress and frame and jib Updrag – selfweight – slack wire motions Downdrag – overload winch/wire Factors for pipe much lower than for plates Weight of water much more than the frame itself Ensure mud-mats have holes to let water pass Mattress recovery – degradation of rope When lifting or lowering objects through the water, it is important to allow for any associated water. Slow lifting through still water produces a drag in itself, but if the barge is rocking in the waves or even the motion of the wave itself near the surface, thus drag is increased. It is common to apply a multiplier onto the weight of the object, although because the water is linked to the shape, it would be more logical to use volume based on the lateral dimension. In the case of pipe-shaped sections, values for the added mass of water are similar to the volume of water displaced. When lowering flat plates, however, the volume is much more. This may result in the load being five or ten times that of the object itself. Inertia is assessed, giving an effective dynamic amplification factor (DAF). It is also necessary to assess the drag. When lowering through the waves, water particles will at some phase be moving upwards. The object itself will have a lowering velocity due to the crane releasing wire, and also the motion of the vessel will be moving the top of the crane jib. The total flow of water relative to the object may result in the wire slackening unless the total submerged weight is greater than the total drag. When the vessel rolls back and the wave drops away again, the load on the wire may become excessive as the drag is then in the opposite direction. The photograph shows installation of bitumen mattresses for pipe or umbilical pipe protection. We need to include the water associated with both the mattress (assume a flat plate) and the framework supporting it (tubular). If a skid or other object is designed to sit on the mudline, it may be necessary to cut holes in the plate to permit the water to pass through. Care should also be taken when lifting concrete block mattresses for inspection after a few years on the seabed. Movement of the polypropylene rope linking the sections may have resulted in wear of the rope resulting in it not being as strong as during installation. The added mass of water may cause it to disintegrate and fall on the pipeline. 363 Pipe lifting FOUR-POINT LOWERING EXAMPLE 6400 mm (21ft) 2000 mm x 24 mm (79 in x 0.95 in) Umbilical termination unit and support stand 40° 25° 2000 mm x 19 mm (79 in x ¾ in) UTA Stand UTA This lowering analysis was undertaken by Jee Ltd for a client installing an electrical umbilical termination pod in Australia in around 120 m (400 ft) of water depth. It is installed affixed to the top of a stand to provide height. A pair of clump weights (shown dotted blue) for stability against the current forces are added to the base of the stand in a separate lowering operation. They are located over loops and are supported by plates on the base. Pairs of additional lugs are fixed to the front and back of the unit to enable it to be drawn into position on the seabed. During installation, the stand and umbilical termination assembly (UTA) are lowered using four equal length strops gathered together at an oval ring with a single central wire. A case could be made for the use of a short spreader beam across the width of the UTA at the top of the four strops, to ensure that the loading remains in-plane with the lugs. This would also assist in equalising the loading in the strops as much as possible. 364 Installation calculations for subsea pipelines DESIGN PARAMETERS Mass of UTA and stand – 2.0 tonne Dynamic amplification factor – 1.8 Mass of entrained water – 2.2 tonne Box, UTA & skid plates Mass of attached umbilical – 1.0 tonne Diameter – 0.1 m Added mass coefficient – 1.1 Water associated with umbilical during lowering – 0.1 tonne Dynamic amplification factor – 1.1 Suspended length – 10% of 125 m (410 ft) Contributing to dynamics of lift Minor differences in length of strops or positioning of lug attachments can also cause a perfectly balanced lift to be predominantly held by two opposite strops. The added mass of water (shown blue on figure) was taken as that filling the UTA box and RHS framework itself (although there are slots and holes to aid drainage), along with semi-cylindrical volumes top and bottom of the UTA plates and clump weight support plates on the stand. Although all the weight of the umbilical is allowed for in the lowering operation, most of the length will be supported by the barge. For this reason, the added mass of water associated with just 10% of the umbilical’s length has been added to determine the design loading. 365 Pipe lifting SUMMARY OF DESIGN VALUES Distribute through just two rear lugs Skew load factor – 1.1 (for uncertainties in lift) Total vertical load – 47 kN (10.6 kip) M UTA M stand M add DAF M umb M addumb DAFumb g SKL Allow for angle of vertical lug to inclined strops Tensile load in each lug – 50.5 kN (11.4 kip) Shackle pin diameter 25.4 mm (1 in) Safe working load 63.7 kN (13 kip) DNV safety factor for lug design – 1.7 Lugs sized to suit shackle SWL and moment Fillet welded to RHS frame Because of the weight and eccentricity of the attached umbilical, the whole load was conservatively assumed to act on just two of the four strops. The total vertical load at each of these two lifting points (including all dynamic loading and added mass of water) is given above. This needs to be corrected for the angle of the strops relative to the lugs. From this, a suitable shackle can be selected from supplier’s tables to provide safe lowering operations through the air and water. Where: ■ DAF = dynamic amplification factor for UTA and stand – Client’s specified value = 1.8 ■ DAFumb = dynamic amplification factor for umbilical = 1.1 ■ g = acceleration due to gravity ■ Madd = mass of added mass of water for stand and UTA ■ Maddumb = mass of added mass of water associated with umbilical ■ Mstand = mass of stand (without clump weights) ■ MUTA = mass of umbilical termination assembly during lowering operations ■ SKL = skew load factor = 1.1 The lugs are matched to suit the safe working load (SWL) of the shackle with an additional safety factor of 1.7. The strength of the attaching weld and the rectangular hollow section (RHS) framework are also checked. Similarly, strops must be made with safe working loads to suit the strength of the selected shackle. 366 Installation calculations for subsea pipelines DEEPWATER RESONANCE – SUMMARY Air-water interface Swinging and wave effects Deep water effects during lowering to bed Out of phase effects change with wire length Heave compensators Wire fatigue over sheaves Added mass of water Any questions? There are two phases in lowering spoolpieces or other objects to the seabed. The air-water interface presents risks due to swinging of objects as they are acted on by waves. The crane is also subjected to surges as the spool is lowered into and out of the water - due to changes in waterlevel arising from waves. Once the object is below the surface, any heave movement on the barge will cause sympathetic movement in the object. These may be out of phase with the vessel. With different lengths of wire let out as the object is lowered, these movements may develop into resonant action, increasing peak wire loading. Damping due to water needs to be allowed for. The use of heave compensators can help once the object is through the water surface. All wire is subject to fatigue as we have discussed earlier. But heave compensators are particularly vulnerable because the same section of wire will be passed repeatedly over sheaves with each wave. If there are a lot of spools to be lowered to the water depth, wire may have a very short life. It is important to allow for what can be a significant drag of added mass of water especially for plates. Pipe lifting 367 PIPE LIFTING – SUMMARY Attachments 2, 3 and 4 point lifts Strop sizing Spreader beams Deep water resonance Any questions? In this module, we have examined some of the problems with lifting spool pieces down to the seabed. Different lifting arrangements are needed depending upon the shape of the spool. The spool should remain level both in air and underwater. This may need either weights or buoyancy to be attached. We have calculated the forces induced into the wire and the spoolpiece during lifting. We have examined the safety factors used with strops. Spreader beams reduce the stresses in long spools. The AISC method of designing I beams to resist both axial compression and bending has been described. This complements the earlier work for tubular members. Some of the methods and considerations needed to minimise resonance have been described. Lugs and sea fastenings Lugs and sea fastenings 371 EXPECTATION EXPECTATION Considerations in designing lugs Single and double shear Doubler plates Out-of-plane resistance Fixing plates AISC 9th edition permitted stresses Similar analysis needed Pulling for pulling heads head lug Securing plate to suitable foundation Seafasteners Lugs are commonly designed and fabricated on board pipelay vessels. They are used for permanent and temporary uses and the installation engineer should be able to size one. They are commonly highly overdesigned to allow for misuse. We will examine in detail the stresses that different areas of lugs must resist. The stress levels will be compared with the AISC permitted values. A similar analysis can be used when designing pulling heads for landfalls and A&R. The lug must be fixed to a safe location on deck. For ease of removal in the future, this is commonly achieved by the use of a base plate bolted or welded to a suitable foundation. Finally, we will determine the forces on lugs as components of seafastener systems, used to transport equipment such as pipes, spoolpieces or valve structures to an offshore field. 372 Installation calculations for subsea pipelines LUGS AND STIFFENERS TYPICAL LUG USAGE Strut T ie Restraining struts and ties Lugs The figure shows an A-frame handling structure located at the stern of a vessel. The lugs used in the design have been highlighted. In this structure, the lugs at the ends of the hydraulic cylinders (that act as struts) will be designed primarily for carrying compressive loads. The lugs upon which the frame rotates will then be designed for significant tensile loading. 373 Lugs and sea fastenings LUGS AND STIFFENERS Point of fixity on a flat surface Loads applied via pin through the lug-hole Single principal plane of loading Translational restraint in the x-z plane No rotational restraint about y axis z x y Pin joint Lugs are a simple means of providing a point of fixity on a flat surface. They are usually temporary fixtures that are either welded or bolted on to the surface. They provide a pinned-joint type of restraint with a principal plane of loading. For the figure shown above, the principal plane of loading is a translational restraint in the x-z plane. A small (5%) translational restraint is provided in other directions, but these loads are secondary in the design of the lug. There is also no rotational restraint provided about the y-axis. Minimal rotational restraint in other planes should also be considered as secondary loading during the lug design. 374 Installation calculations for subsea pipelines STIFFENED LUGS Designed to accommodate significant secondary loads Translational restraint in the y-z plane Minor bending restraint z About the x axis About the z axis x y Stiffener It is likely that lugs will not always experience loading solely in the principal axis (x-z plane). Therefore, it may be necessary to reinforce the lugs against bending loads applied in other planes. The reinforcement can be achieved by welding stiffeners to sides of the lug. With a stiffened configuration as shown in the above diagram, there will be an increased bending stiffness about the x axis that restrains translations in the y-z plane. SINGLE AND DOUBLE SHEAR Two common configurations for applying loads to lugs P/2 P P/2 Tie Pin Single-shear Double-shear The loading configuration on the pin will either be a single or double shear. 375 Lugs and sea fastenings The above figure shows a lug attached to a tie via a pin. A tensile load is applied to the tie, which is transmitted to the lug via the pin. For single shear, the tie is attached to one side of the lug. For double shear, the tie is attached to each side of the lug. Each half of the tie carries half the applied tensile load, P. SINGLE-SHEAR Forces acting on close-fitting pin Shear diameter 4 P D2 P Tie t Nut Nut Bearing diameter x thickness P bearing D t D Pin Forces acting on the lug Tension or tearing cross-section Bending moment, P · t t Lug P Fillet welds Resisted by section moment at hole Single-shear lugs are those where the load is applied to one side of the lug. The main difference between this and the double-shear type lug is that the shear stresses in the pin are twice as large for the single-shear configuration. There is also a bending moment that should be considered during the lug design. This is resisted by the section moment at the hole location. The pin is sized for the maximum applied load, P. The pin diameter is sized from the allowable shear stress and the thickness of the lug is sized from the allowable bearing stress. Lug dimensions should also account for tearing and bending as detailed in the relevant design code. With this design, it is normal for the tie and lug to have the same thickness plate. Note that these equations are for close-fitting pins. Loose pins will have a stress concentration at the point of bearing. With single shear and loose pins, there is also a tendency to rotate the pin. This can be avoided by using friction grip bolts which transfer loads in a different way using the roughness on the faces of the plates. 376 Installation calculations for subsea pipelines DOUBLE-SHEAR Forces acting on close-fit pin P Shear diameter 2 P D2 P/2 Fillet welds P/2 T Bearing diameter x thickness P bearing D T D Forces acting on the lug Tension or tearing cross-section Zero bending moment in lug t P t Fillet welds But check for bending in long pins The double-shear configuration applies the load equally to each side of the lug. This configuration effectively halves the shear force applied to the pin and so allows for smaller pin or bolt diameters than required for the single-shear configuration. It makes sense to have tie plates thinner (t) than the lug itself (T). This configuration also has the advantage that no bending moment is applied to the lug. As with the single-shear configuration, the lug should be sized with account for possible tearing of the lug due to the applied tension. Again, we assume a close-fitting pin. Whereas there is no tendency for the bolt to rotate, we need to check for bearing stress. But now, we cannot use friction-grip bolts. However, we may find double shear with a shackle used in an oversized hole. In this instance, the stress concentration at the point of bearing must be assessed. The material (lug and shackle pin) will deform slightly until sufficient bearing area is provided by local yielding. Many lugs are essentially overdesigned to permit such misuse. Because the material and welding costs are relatively small, there is often little advantage with temporary hold points in designing them close to the maximum permitted stresses. Often they have a very large safety factor. What we wish to achieve is a balanced design with all failure mechanisms having a similar FoS. The strength of a lug is like that of a chain: it is only as good as the weakest link. 377 Lugs and sea fastenings DOUBLER-PLATES Welded onto lug Doubler plate One or both sides Increased area for bearing load application Increasing thickness Reduces bearing stress P bearing D (T 2 t ) Applied load Doubler plate weld - assume all load taken by doubler plate Bearing surface D t T t Plan view Reduced bending in long pins Doubler-plates (also known as ‘cheek plates’) are relatively thick rings welded onto the outside of the lug. They are an inexpensive way of increasing the allowable applied load by reducing the bearing stress applied to the lug by the pin. The bearing stress is inversely proportional to the thickness of the lug. The doublerplates increase this thickness and so reduces the pin bearing stress. However, care should be taken when the lug is being manufactured using a flame-cut hole or where the force may not truly be in-plane. The former may be the case for a temporary fixing fabricated on board the vessel. If it is possible for all the load to come onto one doubler plate for any reason, the fixing weld needs to be assessed to ensure that it does not become overloaded. If this weld fails, then the doubler plate would become ineffective and the bearing surface of the lug would be reduced. A ‘domino’ failure may take place. They are also used where the pin is relatively long and bending moments in the pin itself may cause concern. Examples are for shackles, which are generally loose fitting for ease of use. 378 Installation calculations for subsea pipelines LUGS AND SHACKLES – PADEYES Bow shackle Doubler-plate Bolted padeye Lug Pin Stiffeners Welded padeye Fastener plate If a purpose made tie rod is not used, shackles can fix the end of rope or chain to the lug to simulate a padeye. The above figure shows a bow-shackle fixed to a lug by a threaded nut and bolt fastener. This is often used to take out of plane loads, so the lug needs to be fitted with stiffeners. If the load direction is always in-plane with the lug, a D-shackle may be more appropriate. The section of the bolt in contact with the lug will not be threaded and acts as the pin through the lug-hole. The doubler-plates prevent the over-sized shackle from moving along the axis of the lughole and reduces the bearing loads applied to the pin. Lugs are sometimes mistakenly called padeyes. However, true padeyes - shown above are rings with very high safety factors so that they can withstand sideways forces from any direction. They may have a ring fitted to aid tying off of ropes and minimise wear. The important difference between a padeye and lug is that in the former, the loading is a point-to-point contact with extreme stress concentrations. In this module, the vector forces acting on the lugs are accurately known, so factors of safety can be kept close to the code limits. Lugs and sea fastenings 379 LUG FAILURE MODES Shear pull-out The lug and doubler plate fail in shear Tension pull-out The lug and doubler plate fail in tension Tear-out The lug fails in tension behind the doubler plate The above modes of failure for the lug will result from bearing stresses greater than the yield stress for the lug material. The three different failure modes will be dependant on the dimensions of the lug and the doubler-plate. The three failure modes are: ■ Shear pull-out occurs when there is insubstantial lug and doubler-plate thickness causes significant shear stresses and resulting in the pin shearing through the lug and doubler-plate; ■ Tension pull-out is a result of insufficient lug and doubler-plate thickness and too small of a radius for the lug around the pin; ■ Tear-out occurs when there is an insufficient lug thickness and a small lug radius but the thickness of the doubler-plate is sufficient. 380 Installation calculations for subsea pipelines BENDING FAILURE MODES In-plane bending Moment due to lever arm La and force P Resisted by section b 2 modulus, z b d 6 At each section through lug La d Allow min ±5% transverse load Out-of-plane bending Moment resisted by combined I value of the stiffeners At each section through lug In-plane and out-of-plane bending of the lug is resisted by the combined section modulus of the plate and stiffener at each level down to the base plate. Even when the load is assessed as purely in-plane, good practice will always consider a small transverse load equivalent to ±5% of the pull. Out-of-plane bending resistance can be increased by the use of a pair of stiffeners. Ensure that they leave enough space for welds and fitting of the shackle. Not only should the steel of the lug or the stiffeners be checked, but also the welds which connect to the base plate. 381 Lugs and sea fastenings PIN FAILURE MODES Bending Loose shackle Slipping P/2 A P/2 Shear P P Bearing A P Shear force Section A-A Local Bearing failure stress Original Flame-cut pin profile holes Original lug hole profile Deformed (weaker) lug Undeformed (stronger) pin The modes of failure for the pin (or bolt) used in a lug and shackle design are bending, bearing and shear. The pin (especially on a loose fitting shackle) may bend due to the moment induced in it. Long pins (relative to lug) should be checked for bending. However, it is not good practice to use pins which are much longer than the lug width. The tie can slip sideways and more load come onto one plate than the other. Later, we will examine the use of doubler plates to maintain the pin in the correct position. Bearing failure occurs when the lug or pin plastically deform as they are pushed together by the applied load. In the majority of designs, the pin will often be made from a stronger material than the lug, which will result in the lug-hole being deformed by the pin. The bearing check needs to be made on the weaker material. In some instances, especially when the hole is flame-cut rather than drilled for a close fit, the lug may deform at the hole. A small amount of local yielding due to stress concentration is not necessarily a cause of failure. The AISC code recognises that this may take place, providing that the surrounding material is able to safely accept the distributed load. Shearing of the pin may occur along the plane of interface between the lug and shackle. 382 Installation calculations for subsea pipelines TOLERANCES Shackles Looser fit Ability to fit quickly Cleated pins Pin and cleat Close tolerance fit AISC tol 0.8 mm (1/32 in) Shouldered bolts Maximum hole diameter Shoulder AISC specifies 1.6 mm (1/16 in) larger than bolt Europe uses 2.0 mm Castellated nut, Friction-grip bolts and hot rivets Not usually appropriate split pin, washer and shouldered bolt Care should be taken to define the required tolerance of pin and hole; and also for double-shear connections, any gap between the doubler plates and side plates. If the hole diameter is too big relative to the pin, then secondary deformation of the steel may occur. For shackles, there is a large tolerance on manufacture but factors of safety within the metal are higher than for simple tie connections. Refer to BS 3032 Higher Tensile Steel Dee and Bow Shackles for dimensions. Larger diameter (100 mm and above), purpose-made pins are normally sized with close tolerances, better than 1 mm. The holes through the lug and tie are assembled and bored in a single operation to ensure concentricity in all members. The figure shows a pin with two grooves either end. Into these fits a steel plate, which is bolted to the outside of the connection. Even if the pin rotates, there is no risk of the pin coming loose. It is normal to provide four plates (two at either end). Where the pin is not purpose-made for the hole, slightly larger tolerances may be used, perhaps up to 3 mm. Holes for threaded bolts with shoulders used for lug-tie connections generally have close tolerances. In many cases, the codes require standard dimensions based on the diameter of the bolt or pin. US and European practice is given above. In general, lugs do not use friction grip bolts properties, but remain slightly loose. Friction grip bolts grip the steel plate tightly together and hold in a different manner than we have discussed. Tolerances for friction grip bolts depend on their diameter and oversize holes may be used to make steelwork connections. For nominal sizes of bolts up to 24 mm, the standard hole clearance is 2 mm; for 27 mm and larger, the clearance is 3 mm. With 27 mm bolts in oversize holes, the clearances are up to 8 mm. Really, their use with lugs is limited to single-shear rather than double-shear cases. Similar considerations apply to hot rivets. These are rarely used today but it must be remembered that during riveting, the rivet steel deforms (expands) to fill the hole completely. It grips the plates tightly in a similar manner to friction grip bolts. Lugs and sea fastenings 383 AISC 9th EDITION PERMITTED STRESS LEVELS Bearing Bearing of pin against hole of lug bear 90% y Local yielding in FEA >100% y Bending Bending of pin bend 66% y Shear Shearing of pin or lug (pullout) shear 30% y Tensile Axial pull of lugs tens 45% y Welds Fillet welds weld 30% y In this module, we are considering a design to the AISC code Manual of Steel Construction – 9th Edition Allowable Stress Design. ■ For bearing, refer to section J8 p5-79. Note that the code recognises stresses determined by finite element analysis may locally exceed yield providing that the stresses are blunted by confined yielding. Refer to A5.1 p5-127 ■ For bending, refer to section F3.1 p5-48 ■ For shear, refer to section J4.1 p5-77 ■ For axial tensile, refer to section D3.1 p5-41 ■ For fillet welds, refer to table J2.5 p5-70 Other steelwork codes provide similar permitted limits on the steel and weld stresses. We have chosen to use the 9th edition rather than its ASD replacement, the 13th edition because it shows the permitted stress levels more clearly. Essentially, the AISC approach in both editions is an empirical one, based on an extensive test programme conducted in the 1930s – B G Johnston (1939) Pin-connected plate links, transactions of the American Society of Civil Engineers 104, 314 to 339. The code also provides rules for waisted eyebars. 384 Installation calculations for subsea pipelines STRESS CONCENTRATIONS AND FAILURE MECHANISMS Duerr – ASCE Pinned connection behaviour Comparison of other lug test studies Different lug shapes Square, cut corners, rounded, tapered sides Pin-in-hole stress concentration Equation as function of ‘snugness of fit’ Six limit state failures for plane lugs and pins A B C D E F A comparison of methods for designing lugs may be found in David Duerr’s Pinned Connection Strength and Behaviour from the ASCE Journal of Structural Engineering February 2006 pp 182 to 194. It examines the result of many tests undertaken by different published experimenters since the original AISC testing but using its own consistent notation. Many different lug shapes have been used. The fit of the pin in the hole is shown to have little effect on the stress concentration unless the pin is greater than 98% of the hole diameter. An equation is given to determine stress concentration of looser pins. The six limit states identified are: ■ A tension on net section through the hole ■ B splitting on single plane beyond the hole ■ C shear on two planes beyond the hole ■ D out-of-plane instability or dishing with thin lugs ■ E shear of the pin ■ F bending of the pin Of these, we have discounted D because we do not normally use thin lugs offshore. Comparisons of equations proposed by the various experimenters with actual testing results are included. Where: ■ ASCE = American Society of Civil Engineers Lugs and sea fastenings 385 LUGS AND STIFFENERS – SUMMARY Single and double shear connections Failure mechanisms Failure of pin and plates Stiffeners and doubler plates Shackles Tolerances and fit AISC permitted stress levels Other experimental results Any questions? We have examined the main failure mechanisms associated with lugs. The two main approaches are single or double shear connections. Checks need to be made for both the pin and the plates. Where significant out-of-plane forces may be encountered, the lug is normally strengthened with stiffeners. To spread the load or centralise a shackle, doubler plates may be needed. We have contrasted the close fitting pins with considerations needed for looser fitting shackles. Finally, we have examined typical tolerances in fitting pins into lugs. The loose fitting shackles demand higher safety factors. Where temporary lugs are provided and fabricated with flame-cut holes, then the much greater tolerances and safety factors are normally considered. Permitted stress levels have been given to the AISC code. This and other codes normally assess the different types of stresses acting within the lug - bearing, bending, shear, tensile and the welds - and then sets limits for each relative to the yield. 386 Installation calculations for subsea pipelines WORKED EXAMPLE AISC LUG CAPACITY ASSESSMENT Parallel sided lug with semicircular ends Doubler plates either side Welded to base plate Bolted to deck In-plane loading only No need for angles Rl Rt Dd Check to AISC stress levels Bending of pin Bearing of pin, lug, doubler plate and tie cheek plate Shear/tensile failure of lug, doubler plate and tie Fillet welds at doubler plate and base plate Let us design a double-shear lug and compare the stresses to those allowed in the AISC code. We are going to simplify the design by considering only a true vertical upward loading can be applied. No out-of-plane forces need be considered. This may be the case when there is a tie to a fixed point above. The lug will be attached to a plate bolted to the deck of the barge. Because of the fixed pull direction, the lug and tie plates can have simple radiussed ends. Note the plates are dimensioned as radii but the doubler plate is defined by its diameter. 387 Lugs and sea fastenings WORKED EXAMPLE – INPUT PARAMETERS Pull force, P = 100 kN (22.5 kip) Pin diameter, Dp = 50 mm (1.969 in) P Tie bar Hole tolerance, ht = 0.8 mm (1/32 in) Jaw tolerance, jt = 1 mm (0.039 in) Lug thickness, tl = 20 mm (0.787 in) tl jt Tie thickness, tt = 22 mm (0.866 in) Lug radius, Rl = 52 mm (2.969 in) D p Tie radius, Rt = 40 mm (1.575 in) Doubler plates, td = 8 mm (0.315 in) Dd tt td Doubler dia, Dd = 80 mm (3.150 in) Fillet weld Fillet weld leg lengths Doubler plate, Ld = 8 mm (0.236 in) Lug to base, Ll = 10 mm (0.394 in) P/2 Fillet welds Pin Doubler plates P Base plate Deck The other items are as shown. The welds securing the tie bar to its twin cheek-plates will not be assessed in this example, nor will the cross-section of tie bar or the size of base plate and bolts. PRACTICALITY OF FILLET WELDING Cannot form fillet weld without melting base metal Rounded off edges Too close to edge results in loss of shape Allow a proximity tolerance Plate sizes slightly thicker and longer fwt Typically at least 3 mm (⅛ in) Function of heat input and size Chamfer off inside face Partial penetration at doublers If a fillet weld is too close to the edge of a plate, it is inevitable that the edge will be melted and the shape lost. In order to avoid this, it is necessary to allow a fillet weld proximity tolerance, resulting in a rounding off of the fillet. 388 Installation calculations for subsea pipelines Typically, this should around 3 mm (⅛ in) although it may be more for a large fillet weld with high heat input. Edition 13 of AISC suggests in the commentary section that this can be reduced to 2 mm (1/16 in) [sic] for plates thicker than 6 mm (¼ in) – see p16.1332. WORKED EXAMPLE – PIN Pin bending – assume force acts at tie cheek centres Moment in pin, M P 2 t1 2 td jt 2 tt 2 1000 kN 2 20 2 8 1 2 22 2 mm 1475 N m 22.5 kip 2 0.787 2 0.315 0.039 2 0.866 2 in 1088 lbf ft 3 Section modulus of pin, z D p 32 3 50 mm 32 12 272 mm3 3 1.969 in 32 0.749 in3 Bending stress in pin M z 1475 N m 12 272 mm3 120.2 MPa 1088 lbf ft 0.749 in3 17.4 ksi Pin shear (double shear stress) 2 A D p 4 50 mm 2 4 1963 mm 2 1.969 in 4 3.043 in 1 P A 1 100 kN 1964 mm 2 25.5 MPa Shear 2 2 12 22.5 kip 3.043 in2 3.7 ksi 2 2 WORKED EXAMPLE – PIN Pin bearing from lug and doubler plate Close-fitting pin (not flame-cut) Bearing area, A D p t1 2 t d 50 mm 20 mm 2 8 mm 1800 mm 2 1.969 in 0.785 in 2 0.315 in 2.8 in2 Bearing stress on pin P A 100 kN/ 1800 mm2 55.6 MPa 22.5 kip / 2.8 in2 8.1 ksi Pin bearing at tie cheek plates Bearing area, A 2 tt D p 2 22 mm 50 mm 2200 mm 2 2 0.866 in 1.969 in 3.4 in2 Bearing stress on pin P A 100 kN/ 2200 mm2 45.5 MPa 22.5 kip / 3.4 in2 6.6 ksi 389 Lugs and sea fastenings WORKED EXAMPLE – LUG Bearing from pin on lug Equals stress onto pin = 55.6 MPa (8.1 ksi) Tension through lug behind doubler plates Plate cross-sectional area, A = 2 · Rl · tl = 2 · 52 mm · 20 mm = 2080 mm² (2 · 2.047 in · 0.785 in = 3.224 in²) Tensile stress in lug = P / A = 100 kN / 2080 mm² = 48.1 MPa (22.5 kip / 3.224 in² = 7.0 ksi) Fillet weld around lug at baseplate Circumference around lug, C = 2 · (2 · Rl + tl) = 2 · (2 · 52 mm + 20 mm) = 248 mm (2 · (2 · 2.047 in + 0.785 in) = 9.8 in) Throat Stress at throat of weld = P / (C · Ll ·½) = 100 kN /(248 mm · 10 mm · ½) = 57.0 MPa (22.5 kip / (9.8 in · 0.394 in · ½) = 8.3 ksi) Leg Again, we should check bearing stresses in lug and pin because they are not necessarily made from the same grade of steel. The figure shows the two alternative methods of defining the size of a fillet weld. Some codes define the throat, others define the leg length. We will use the latter. The strength of a fillet weld is determined by the minimum thickness - the throat. WORKED EXAMPLE – DOUBLER PLATE Tensile stress through doublers and lug at pin CL Net cross-section, A = 2 · (Dd – Dp – ht) · td + (2 · Rl – Dp – ht) · tl = 2 · (80 mm – 50 mm – 0.8 mm) · 8 mm + (2 · 52 mm – 50 mm – 0.8 mm ) · 20 mm = 1531 mm² 2 · (3.150 in – 1.969 in – 0.031 in) · 0.315 in + (2 · 2.047 in – 1.969 in – 0.031 in) · 0.787 in = 2.373 in² Tensile stress in lug = P / A = 100 kN / 1531 mm² = 65.3 MPa (22.5 kip / 2.373 in² = 9.5 ksi) Fillet weld around doubler plates Circumference around doubler, C = · Dd = · 80 mm = 251 mm (· 3.150 in = 9.9 in) Assume all of the load acts on just one doubler Stress at throat of weld = P / (C · Ld ·½) = 100 kN /(251 mm · 8 mm · ½) = 70.3 MPa (22.5 kip / (9.9 in · 0.236 in · ½) = 10.2 ksi) For a safe design, we must assume that the load does not act truly in the plane of the weld. We design as if all the load is applied to one of the doubler plates and is then transferred to the weld. 390 Installation calculations for subsea pipelines In actual fact, this fillet weld proves to have a high stress. We could improve the design by using a different weld design. This is often achieved by increasing the root thickness by chamfering the inside face of the doubler plate. WORKED EXAMPLE – LUG/DOUBLER SHEAR Shear (tear rupture) through lug and doubler Surface, A = tl · (2 · Rl – Dp – ht) + 2 · td · (Dd – Dp – ht) = 20 mm · (2 · 52 mm – 50 mm – 0.8 mm) + 2 · 8 mm · (80 mm – 50 mm – 0.8 mm ) = 1531 mm² 0.787 in · (2 · 2.047 in – 1.969 in – 0.031 in) + 2 · 0.315 in · (3.150 in – 1.969 in – 0.031 in) = 2.373 in² Shear stress in lug = P / A = 100 kN / 1531 mm² = 65.3 MPa (22.5 kip / 2.373 in² = 9.4 ksi) Shear (tear rupture) in lug around back of doubler Failure surface, A = (/2 · Dd + 2 · Rl – Dd) · tl = (/2 · 80 mm + 2 · 52 mm – 80 mm) · 20 mm = 2993 mm² (/2 · 3.150 in + 2 · 2.047 in – 3.150 in) · 0.787 in = 4.640 in² Shear stress in lug = P / A = 100 kN / 2993 mm² = 33.4 MPa (22.5 kip / 4.640 in² = 4.8 ksi) The length of the shear failure planes is approximated by assuming the minimum distance from the pin hole to the outside of the lug and doubler or the cheek plate. That for the lug itself assumes tearing at the back of the doubler plate. It means that the rupture path distance is slightly longer than for simple tension. However, for a more normal shaped, sloping-sided lug (rather than the parallel sided one shown here), this may be the critical length when determining the stress. It should be measured emerging normal to the slope of the lug. Remember that the permitted stress for shear is considerably less than for tension. Lugs and sea fastenings 391 WORKED EXAMPLE – TIE CHEEK PLATES Bearing from pin Equals stress onto pin = 45.5 MPa (6.6 ksi) Tensile stress through tie cheek plates at pin centreline level Cross-section, A = tt · (2 · Rt Dp – ht) = 22 mm · (2 · 40 mm 50 mm – 0.8 mm) = 642 mm² (0.866 in · (2 · 1.575 in – 1.969 in – 0.031 in) = 0.996 in²) Tensile stress in tie cheek plates =½·P/A = ½ · 100 kN / 642 mm² = 77.8 MPa (½ · 22.5 kip / 0.996 in² = 11.3 ksi) May be double – depends on tie far-end restraint Shear in tie cheek plates - same area Shear stress – as for tensile = 77.8 MPa (11.3 ksi) Again, we may find the tie cheek plates are a different material from that of the lug or pin. Depending on the design of the tie cheeks and the way that the tie is held, it may be prudent to assume that all the load is acting on only one of the cheek plates. Again, although the tensile area and shear area are the same in this instance, the critical value is for shear since this has the lower permitted stress. STRESS LEVEL SUMMARY AND AISC YIELD Pin Bearing at tie = 45.5 MPa (7.3 ksi) / 90% y = 50.5 MPa (7.3 ksi) Bearing at lug = 55.6 MPa (8.1 ksi) / 90% y = 61.7 MPa (9.0 ksi) Bending = 120.2 MPa (17.4 ksi) / 66% y = 182.1 MPa (26.4 ksi) Shear (double) = 25.5 MPa (3.7 ksi) / 30% y = 84.9 MPa (12.3 ksi) Lug and doubler plates Bearing = 55.6 MPa (8.1 ksi) / 90% y = 61.7 MPa (9.0 ksi) Axial in lug = 48.1 MPa (7.0 ksi) / 45% y = 106.8 MPa (15.5 ksi) Thro’ doubler = 65.3 MPa (9.5 ksi) / 45% y = 145.1 MPa (21.0 ksi) Shear in lug = 33.4 MPa (4.8 ksi) / 30% y = 111.4 MPa (16.2 ksi) Thro’ lug & doubler = 65.3 MPa (9.5 ksi) / 30% y = 217.7 MPa (31.6 ksi) Tie cheek plates Bearing = 45.5 MPa (6.6 ksi) / 90% y = 50.5 MPa (7.3 ksi) Axial tensile = 77.8 MPa (11.3 ksi) / 45% y = 173.0 MPa (25.1 ksi) Shear = 77.8 MPa (11.3 ksi) / 30% y = 259.4 MPa (37.6 ksi) Welds on lug Doublers = 70.3 MPa (10.2 ksi) / 30% y = 234.5 MPa (34.0 ksi) Base plate = 57.0 MPa (8.3 ksi) / 30% y = 190.1 MPa (27.6 ksi) Use steel grade higher than 248.2 MPa (36 ksi) or thicker cheek plate The highlighted values indicate the critical stresses. 392 Installation calculations for subsea pipelines It means that the minimum yield stress needed for the pin is 182 MPa (26 ksi). The lug and doubler plates require a 218 MPa (32 ksi) steel yield. The doubler plate welds on the lug require 234 (34 ksi) grade steel. The tie cheek plates require a 259 MPa (38 ksi) steel yield. The welding rods will match this. However, the critical stresses are those for the tie cheek plates. This demands a yield stress in the plates equivalent to 259 MPa (38 ksi). This means that we can not use the lesser of the two grades of steel from the AISC code, as designed. Note that AISC steel has two grades : 248.2 MPa (36 ksi) and 344.7 MPa (50 ksi). We either use the higher grade steel or increase the tie cheek plate thickness. GEOMETRIC CHECKS Base plate weld leg lengths Not more than half lug thickness Doubler plate welds Not more than plate thickness Not more than half lug thickness Not more than diameter of lug Not more than ring thickness Pin diameter & hole tolerance Not more than lug diameter Not more than doubler plate Not more than cheek plate diameter Until the lug is drawn up, it is easy to specify an unworkable design due to geometrical constraints. In particular, checks are needed to ensure that the welds will fit onto the plates. For this reason, care should be taken when writing software (such as an Excel spreadsheet) to size lugs. 393 Lugs and sea fastenings EXERCISE EXERCISE Pull force, P = 200 kN (45.0 kip) Pin diameter, Dp = 68 mm (2.677 in) P Tie bar Hole tolerance, ht = 0.8 mm (1/32 in) Jaw tolerance, jt = 1 mm (0.039 in) Lug thickness, tl = 30 mm (1.181 in) tl jt Tie thickness, tt = 40 mm (1.575 in) Lug radius, Rl = 74 mm (2.913 in) D p Tie radius, Rt = 52 mm (2.047 in) Doubler plates, td = 14 mm (0.551 in)Dd tt td Doubler dia, Dd = 98 mm (3.858 in) Fillet weld Fillet weld leg lengths Doubler plate, Ld = 14 mm (0.551 in) Lug to base, Ll = 14 mm (0.551 in) Calculate stresses in the larger lug for double the load. Deck P/2 Fillet welds Pin Doubler plates P Base plate 394 Installation calculations for subsea pipelines FOUNDATIONS AND DECK FIXINGS SUITABLE FOUNDATION Design of base plate Lug welded to a base plate Thick, stiff plate to distribute forces and moments Connection details Bolts or welded Transfer load to deck or member Find suitable foundation Follow load path beneath the deck Structural analysis Local strengthening of existing structure A base plate is normally welded to the bottom of the lug. This then provides an easy means of transferring the forces and moments to the foundation. The bolted or welded connection between the deck or structural member needs to be designed like any other. But a check needs to be made that the underlying load path is able to withstand the load. It may be necessary to undertake a structural analysis to assess whether the existing structure will need local strengthening. Lugs and sea fastenings 395 FIXING BASE PLATE – CONNECTION DESIGN Steel base plates Base plate stiff enough to prevent it bending Fixings distribute moment and load combinations Prevent tearing of deck plate Pullout, punching shear or bending Fixings Friction-grip bolts, nuts and washers Welds Rivets It is normal to provide a stiff base plate that can withstand any moments and forces transmitted from the lug welded to it, without it bending during operation. Compression forces are transmitted through the plate at an angle of 45°. Bearing stresses need to be checked at the deck plate or foundation level. It is essential that the deck plate can withstand the forces transmitted to it by the base plate without tearing (such as shown in the photograph). The AISC code provides guidance in designing bearing plates for beams on pp2-141 to 145 and for columns on pp3-106 to 110. However, these need to be adjusted for lugs, having an upward pull rather than a down force. A number of different fixings of the base plate will be considered. These are usually bolted or welded connections, although rivets can be used. Preference may be given to bolted connections on decks since future removal of the lug often leaves the stub of the lug and base plate in place. Trip hazards are associated with uneven decks. Part 4 of the AISC code gives information regarding fixings. 396 Installation calculations for subsea pipelines PRYING FAILURE OF BASE PLATE Check bending stiffness of baseplate Associated with uplift forces and bolted plates Also with weld-filled slotted holes Deck Because lugs are frequently used for uplift applications, a check should be made that it does not fail by prying forces. This is more true with bolted baseplates rather than construction fully welded around the edges. Sometimes, slotted holes are left in plates to be filled by welding - effectively acting as permanent ‘bolts’ to the deck. These too may tend to bend. BASE PLATE TO LUG Simple four bolt at corners – or more detailed Check plate for edge proximity Access for the wrench or spanner Deck Weld Check that the bolt can be tightened using a torque wrench without hitting welds or a stiffener Check for edge distance to plate to prevent tearing The simple example given above shows a base plate with the lug welded onto it. 397 Lugs and sea fastenings The usual minimum number of bolts will be four - set at the corners. However, to limit the bolt diameter or improve the moment-resistant capability, it may be necessary to use patterns of additional bolts along the edges of the plates. A check needs to be made with proximity of the bolt hole to the edges of the plate. Checks should also ensure sufficient access to torque up the bolts using the wrench or spanner, avoiding stiffeners, adjacent welds or even other bolt heads. Bolt groups must be sized to withstand tensile, shear and bending in both the in-plane and out-of-plane directions. BOLTS, NUTS AND WASHERS Preloading (stretching) bolts Places parts in compression resistance to tension Creates friction between faces of plates resistance to shear Torque requirement estimation d tan ψ μ sec α T m 0.625 μc Fi d 2 d 1 μ tan ψ sec α T = torque to give required preload = helix angle = tan-1(1/( ·d)) Fi = preload = nut friction dm = mean thread diameter c = thrust collar friction d = major thread diameter = half angle of thread (see figure) Bolted connections are commonly used when the two adjoining parts need to be disassembled without using destructive methods. They are suitable where the connection requires a resistance to both external tensile loads and shear loads, or a combination of the two. The effect of applying a preload to the bolt is to place the parts in compression, hence providing resistance against the tensile load. It also creates friction between the parts which resists any shear load applied. One method for measuring the preload in a bolt is to measure the change in its overall length. By relating this to its spring stiffness, the resultant force, or preload can be determined. However, it is often impractical to measure the change in length of the bolt, so other methods are commonly used, such as the equation shown in the above slide. The expression in square brackets is sometimes referred to as k and bolt manufacturers sometimes give relevant values for their products. Typical values range from k = 0.3 for standard oxide finish down to k = 0.18 for well lubricated bolts. Because we want to stretch the bolt to provide good grip between the plates, we should use well-lubricated bolts, nuts and washer faces. See Mechanical Engineering Design third edition by Joseph E Shigley ISBN 07-085722-9 McGraw Hill 1981. 398 Installation calculations for subsea pipelines BOLTED CONNECTIONS General and high-strength friction-grip bolts Bolt sizes from M16 to M30 BS 4395 and BS 4604 – EN 24014 and EN 24016 US – ASTM A307, A325 or A490 bolts Lack of fit Due to bolt hole misalignment Due to tolerances in member dimensions Bolt tolerances Varies with bolt size – 2 mm or 3 mm (1/16 in or 1/8 in) Slotted holes up to 8 mm (5/16 in) clearance Require large washers Alternative use of welded plug slots Typically, general grade or high strength friction grip bolts will be used to connect the lug base plate to the foundation member. These should be matched to the grade of steel plate. Typical standard sizes are range between 16 mm and 30 mm nominal diameter. The British and European ISO standards for metric bolts are given above. A major consideration with assembling frame structures is lack of fit either due to measurement of the bolt hole locations or the accuracy of manufacture of the members. These can be partly accommodated by using relatively loose fitting holes (depending on the bolt diameter – 2 mm for diameters up to 24 mm). It may be necessary to increase this further using a slotted hole and a larger washer. Slightly tighter fitting holes are specified in the US. AISC permits the use of slots filled with weld as an alternative to bolts. This provides a permanent means of fixture to the deck. Combined with a fillet weld around the base plate, it can provide extra fixity. 399 Lugs and sea fastenings WELD TYPES Butt welds Best practice uses full penetration welds Included angle Weld face Excess weld metal Throat thickness Root face Fusion face Sealing run Fillet welds Leg length Fusion face must be less Leg Included length angle than plate thickness Root Crack initiator at root Weld face Butt is better weld detail Throat thickness Fillet and butt welds are the most commonly types used for structural steelwork. Butt welds The size of a butt weld is specified by throat thickness. When plate thicknesses are equal, the throat thickness is equal to the plate thickness. If the plates vary in thickness, the thinner plate thickness is taken as the specified throat thickness. The welds are reinforced with excess material, but this additional thickness is not accounted for when determining the strength of the weld. If it is not possible to provide this additional sealing run, the throat thickness is taken as 5/8 of the thinner plate thickness. The allowable stress in mild-steel butt welds must not exceed the allowable tensile, compressive and shear stresses of the parent material. Therefore it is normally not necessary to make calculations on complete penetration welds. Fillet welds It is always preferable to use full penetration butt welds. Fillet welds should be reserved for when they are not possible. Gaps between the plates can gather moisture, resulting in hidden corrosion. The gap between plates at the weld root effectively acts as a potential crack initiator. Fillet welds are sized based on their throat thickness and leg length. The latter cannot be larger then the thickness of the thinner plate. The allowable stress in a fillet weld must not exceed the allowable shear stress in the parent material. Some key considerations for fillet welds are as follows: ■ For fillet welds connecting parts, if the fusion faces form an angle of more than 120° or less than 60°, the weld should not be relied upon to transmit loads at the full working stresses unless permitted to by the appropriate standard for the particular application; ■ For fillet welds ending at the side or end or parts, they should be returned continuously around the corners for a distance of twice the size of the weld or greater. 400 Installation calculations for subsea pipelines FOUNDATIONS AND DECK FIXINGS – SUMMARY Selection of foundations Base plates Design of connections Bolts Welds – fillet or butt Any questions? Once the lug is designed, a suitable point of fixture on the structure must be selected. The load must be transferred though the deck or plate to the underlying steelwork. Failure of baseplates may be due to the stiffness of the plate itself or the means of connecting to the deck. We have looked at bolted plates and welded plates, comparing fillet and butt welds. Lugs and sea fastenings 401 SEA FASTENERS SEA FASTENING ‘Fixed points’ on vessel deck or in hold Provide longitudinal and transverse restraint Used to fasten items to vessel Spoolpieces, SSIVs, piles, pipe, spreader beams and containers Loose fixings cause higher impact loads It is important that cargo transported on vessels is restrained from movement during transportation. For ‘one off’ items of equipment it may be necessary to construct fixed points on the vessel deck that can provide a restraint against the movement of equipment. These fixed points will generally be simple steel structures that are welded to the vessel deck. If fixings work loose, then the object being held may start to slide. The fastener then has to withstand the additional shock or impact loading at the end of its travel. 402 Installation calculations for subsea pipelines SEA FASTENING Layout of spoolpieces and sea fasteners DSV Bar Protector Plan of DSV Bar Protector deck Stern Spoolpieces SSIV Fasteners The schematic shows the layout of spoolpieces and an SSIV being transported for the Goldeneye project on board the DSV Bar Protector. The spoolpieces are fastened to strongbacks by strapping. The strongbacks are then restrained by the sea fasteners that are welded to the vessel deck. The location of the sea fasteners restraining the spoolpieces is also highlighted (in colour) on the schematic. FASTENER DESIGNS Pad plate or wooden spacer Spoolpiece Fastener Fastener raises spool off deck Deck Lateral restraint Cable reel Ratchet Webbing strap D-shackle Bottlescrew / Turnbuckle Lug Vertical restraint Wire Sea fastener designs may be made from cheap and widely available standard steel sections, such as I-section or angle-sections. It may be possible to utilise scrap steel and offcuts. More specialist fasteners may be custom built for the specific object being fastened. The fastener designs may utilise steel pad plates or wooden spacers to provide 403 Lugs and sea fastenings a tight enough fit for individual items of equipment. Wooden spacers prevent damage to the equipment during installation and removal from the fastening. This is particularly important for coated pipe and spoolpieces. Sea fastening designs may combine lugs and webbing straps for vertical restraint along with fixed lateral restraints. It maybe necessary to raise equipment above the deck level for complex spool geometry or to accommodate larger equipment. For larger items of equipment, such as cable reels, fastenings may require additional support through wire cables attached to lugs. Turnbuckles (or Bottlescrews) are used to put tension into the wire. FASTENER DESIGN Movement of vessel applies inertial forces to equipment on deck Inertial force = mass · acceleration Inertial force applied at centre of gravity of the body Fastener designed to withstand inertial Angled face to forces and allow easy release prevent snagging CofG FT /2 Bow Stern FT /2 FT /2 Weld FT is the transverse inertial force A moving vessel will undergo movements, such as pitch, heave and yaw that will impart inertial forces on the cargo it carries. The inertial forces act at the centre of gravity of each item of equipment. Inertial forces are calculated from the known mass of each item and the maximum accelerations it is expected to undergo during transport. The prediction of the accelerations is a complex process requiring knowledge of the vessel dynamics. These are related to the vessel’s RAOs and the maximum seastate permitted for shipping and operations. Software packages such as the DnV spreadsheet LASHCON (based on the International Maritime Code for Cargo Stowage and Securing), allow determination of the accelerations for different vessels. Typical conservative values, which are independent of sea conditions, but which are used by classification societies are: a characteristic roll of 30° for barges and 20° for ships; and a pitch of 7° to 10°. When the inertial forces have been estimated, they can be used to design the sea fasteners. The principle of designing fasteners is similar to designing the supports for beam structures. Inertial loads are often applied as point loads at the centre of gravity of bulky, compact objects. Long objects (such as spools) normally apply them as a uniformly-distributed load (UDL) along their length. Such long items need to be checked to see if they can withstand shear and bending between the fastener locations. 404 Installation calculations for subsea pipelines It is normal to angle the face of restraints to permit the object to be lifted out during installation without snagging. Snagging would impart forces onto the crane additional to the object’s self weight. In the figure above, the force is distributed equally between the two lateral fastenings only because the centre of gravity of the spool is midway between. No lateral load is attracted to the two fastenings at the stern. These, however, must resist all fore-aft loads by themselves. SEA FASTENING CODE DNV Marine Operations Part 2 Recommended Practices RP2 Sea transportation Assesses wind, waves and currents Towing speeds and hawser arrangements Vessel accelerations and motions in 6DF Surge and gravity acceleration, sway, amplification factors Assess requirements for fixing Loads during transit Clearances (tolerances) chocks for easy removal Prevent snagging during lifting operations One such seafastening code is DNV Part 2. Conformance with this will be needed to achieve certification. Other codes are available, such as: Lloyds Register of Shipping, Norsok standard J003 (Marine Operations), ISO 19902 (fixed steel offshore structures) or API RP2A (Recommended Practice for Planning, Designing and Construction of Fixed Offshore Installations). DNV first assesses the wind, waves and currents acting on the vessel. The vessel towing arrangements are also considered in finally assessing the accelerations and motion of equipment on board. From this, the naval architect can assess the fixings needed to withstand the generated forces, without compromising the integrity of spool piece etc on board. It is important to prevent snatching of the item as it is removed from the fastening blocks at sea. A slight chamfer or bevel aids removal. 405 Lugs and sea fastenings WIND EFFECTS Wind speeds Quoted at zo = 10 m (32.8 ft) above sea surface Corrected for near surface effects (or higher structures) 0.09 V VR z / z0 Height Maximum of 55 m/s (123 mph) Wind and waves are normally taken as in same direction Relative to vessel heading Wind pressures qref ρ 2 vR 2 vR zo = 10 m Wind profile Air density, = 1.25 kg/m³ (0.078 lb/ft³) Where ■ qref = reference wind pressure = density of air (in UK waters, can reduce to 1.225 kg/m³) ■ ■ vR = reference wind velocity The reference height for steady wind speeds is taken at 10 m above the surface of the sea or land. This figure needs to be integrated over the height of the structure to assess the true wind force acting. The relationship between wind speeds at different heights is given above. It is normally not necessary to apply wind speeds acting on vessels greater than 55 m/s. For most calculations, the direction of the wind and waves are deemed to be the identical. However, it may be necessary to examine different directions relative to the vessel; for example, from bow, stern, sides or quarters. Using the reference wind velocity, the pressure may be obtained. A simple trapezoidal method can be used to summate the different heights above the sea surface. This may be modified for shorter (stronger) gusts for items of smaller diagonal dimensions. 406 Installation calculations for subsea pipelines WIND FORCE ON ITEMS Wind pressures Adapt for small items subject to shorter period gusts Depends on diagonal length exposed to wind Guyed masts/towers or latticed structures may flutter Check for suction forces for flat objects on deck Wind force F qref area Cd Drag coefficients Cylindrical Cd = 0.95 to 1.2 Rectangular Cd = 0.90 to 1.10 Thin rectangular Cd = 1.2 to 2.0 Where ■ F = force on item ■ qref = reference wind pressure ■ area = area of item facing wind ■ Cd = coefficient of drag The reference wind speeds are customarily quoted as ten minute or one hour mean speeds. Small items can be subjected to short period gusts which are greater than this mean. Flutter of guyed masts and towers or latticed structures should be checked for flutter. Again this is at a higher velocity than the reference wind speed. The flutter is caused by vortex shedding and if not controlled may develop into galloping. Any seafastenings or fixings can then be subject to vastly increased forces. Additional checks should be made for suction effects for flat, wide objects held on deck. These present little cross sectional area to the wind but may be subject to a differential pressure beneath the plate which could be lifted off the deck if not secured well. Forces are a function of the area presented to the wind and the shape of the item (streamlined shapes use lower coefficients). If no other information is available, the default air density and maximum drag coefficients should be used. The DNV offshore standard OS-C301 Stability and Watertight Integrity gives values for Cd for a range of marine structures in table B1 on page 13. Eurocode 1 (EN 1991-1-4 Actions on structures - wind actions) gives a fuller explanation of wind actions on structures, higher gust speeds on smaller items and shape functions. See also A. J. Adams, N. D. Barltrop, M. G. Hallam Dynamics of Marine Structures ISBN 0750610468. Lugs and sea fastenings 407 WAVE EFFECTS Waves and wave loads Long period waves (swell) Locally wind-generated waves (‘seas’) Calculated using four alternative methods Wave data from statistics 10% fractile of highest waves Predict characteristic height Hk from Hs and Hmax Weibull parameters for regions around the world Design wave method Design spectrum method Assessment of vessel motion due to waves Two types of seas need to be considered: these are long period or local waves. The former are caused by a distant storm that have travelled to the vessel. These are generally attenuated to some extent and may not be associated with any winds locally. The term swell is often used for these waves. Steeper waves are caused by local winds. These are often termed ‘seas’. A number of methods are suggested by the DNV RP2 code to derive suitable wave data. The aim is to quantify a characteristic wave height which acts on the vessel. In the simplest case, a chart and table can be used to quantify the wave height for coastal regions around the world. Finally, the naval architect can determine how the particular vessel will behave in certain wave conditions. From this, the motions can be determined and thus an assessment can be made of the forces exported by the object being held by the seafastening. The position of the seafastener relative to the centre of vessel motion affect the felt acceleration of the object. 408 Installation calculations for subsea pipelines SEA FASTENERS – SUMMARY Examples of seafastenings Fully restrained during transportation Easy release until clear of the barge DNV RP2 Wind, waves and currents Assess vessel accelerations and motions Determine forces on fixing equipment Any questions? Seafasteners are used for transportation of numerous items to the field. These include pipes, spool pieces and subsea valve structures. The intention is to provide full restraint to prevent high impact loads which might be generated if the items slide about on deck. However, the fasteners should not snag the items when the crane finally lifts them from the deck. Any snagging would increase the crane lift and possibly loss of control of the item by the crew when it finally breaks clear. We show the main considerations for loads during transportation as stated in the DNV codes. These are mainly due to wind and the movement of the vessel due to waves. However the code also provides information on currents (discussed in the pipe lifting module). The combined accelerations and the location of the item relative to the centre of motion of the vessel provides a means of determining the forces on the sea fastenings. Design of the seafasteners themselves should be to the appropriate code recognised by the insurers. DNV RP2 does not give stresses in steel used for seafasteners. However, the AISC code examined earlier may be used for this. Lugs and sea fastenings 409 LUGS AND SEA FASTENINGS – SUMMARY Lugs and stiffener design Foundations and deck fixings Sea fasteners Any questions? We have examined how to design lugs, which are used for many purposes offshore. It is important to ensure that the lug is provided with a load path through the deck to the main structure of the vessel. A common use of lugs is for sea fastenings. Barge stability Barge stability 413 EXPECTATION EXPECTATION Examine stability of floating objects Determine whether a vessel is unstable Incorporation of cranes and free surfaces Tend to reduce stability In this section we will examine why floating objects adopt stable configurations. In particular, we will look at box-shaped barges and other craft used for pipelay support. However, the methods are general and are applicable to all marine vessels including laybarges. The calculations will show how swinging loads from cranes and free liquid surfaces of tanks tend to reduce stability. A full discussion of stability may be found in DNV offshore standard OS-C301 Stability and Watertight Integrity. 414 Installation calculations for subsea pipelines STABLE FLOATING BODIES FLOTATION For equilibrium Force upwards (Fb) equals Force downwards (m · g) Or: Weight of fluid displaced = Weight of body in air For flotation Buoyancy force = weight of body Fb ρwater V g m g m·g Fb Where V is the volume of the submerged section of the body Buoyancy force acts through centre of gravity of displaced fluid Where ■ Fb = buoyancy force ■ g = acceleration due to gravity (=9.80665 m/s²) ■ m = mass of object ■ V = volume of the submerged section of the body water =density of seawater ■ Note that the gravitational acceleration term ‘g’ can be omitted in imperial calculations using lb for mass and lbf for force. A body partly submerged in water will float if the force generated by its buoyancy, Fb is greater than the weight of the body (in air). The uplift or buoyant force is equal to the weight of fluid displaced. This is Archemedes’ principle. So if we know the mass of a floating body, we can determine how much water will be displaced. The mass of the body acts through its centre of gravity (CofG) or centroid. The uplift acts through the centre of buoyancy (CofB) or centroid of the displaced water. 415 Barge stability CENTRE OF BUOYANCY G is the centre of mass (centroid) = Combined centre of mass of ice and stone B is the centre of buoyancy = Centroid of the displaced seawater C B A m·g B Fb Fb B G m·g Unstable B G G Fb m·g Stable Stable An iceberg with its embedded stone at the base has a centre of gravity slightly below the centroid of the ice itself. The centre of buoyancy is always at the centroid of the volume of displaced water. As icebergs melt (A), their stability changes. The berg will rotate to find a new stable position (B). The centre of gravity remains fixed but the centre of buoyancy moves to the centroid of the new displaced volume of water. It is easy to see that with the centre of buoyancy above the centre of gravity that the berg remains stable. But what of a homogeneous floating solid. That can never rotate to achieve this state. It will always have the centre of gravity above the centre of buoyancy because the centre of buoyancy will keep shifting. If the berg loses its embedded stone, then it will rotate until it achieves stability with CofG above CofB (C). What is happening? 416 Installation calculations for subsea pipelines STABILITY OF VESSELS M is ‘Metacentre’ (MB is metacentric radius) LM is ‘Metacentric height’ Block is stable when M is above B M Overturning moment P x p Righting moment m g Lm Thus L P x L M m g M xp P P m·g G B Fb G m·g x Fb B Instead of an iceberg, think of a block of wood. This will float in a stable position and have CofB below CofG. But it will always float on its flattest side. Let us imagine a small force, P applied for an instant which is trying to rotate the block a small angle . By small, we mean that tends to sin Any destabilisation that we apply will tend to be righted by the moment generated by the forces at G and B and the lever arm x. STABLE FLOATING BODIES – SUMMARY Centre of mass, G Centre of buoyancy, B Vertically in line It is not necessary for the centre of mass to be below the centre of buoyancy for a stable body Metacentre, M M must be above B With ideal level deck, M is above G Any questions? We have defined the centres of mass, G and buoyancy, B. Barge stability 417 These will be vertically above one another. It is simple to see how a body is stable when B is above G. But for vessels (or homogenous blocks of ice or wood), it is common to find stability with the forces reversed. For barges, their stability can be assessed by finding the metacentre. No movement will occur when M is above B. With a level deck, M is above G. 418 Installation calculations for subsea pipelines DETERMINING VESSEL STABILITY CRAFT DEFINITIONS Bow Forward Port Aft Beam Starboard Heave Surge Yaw Draught Moulded side Stern Free boar d Sway el Ke Roll Pitch Vessel motion We will be using some of these terms for the barge. For a stable vessel, the draught and the freeboard should be known as well as the metacentric height. If the water ships over the deck at one corner (or air gets under the bottom of the vessel), then the restoring effect will be lost. With a dinghy as shown above, these effects are counteracted by the action of the sailors and adjustment of the sails. Our vessels normally operate with a level deck. We will see why this is easier to obtain in the fore-and-aft direction than from side-toside. (It is due to the much greater stability due to increased I value along the vessel length as compared with the breadth.) 419 Barge stability DETERMINING VESSEL METACENTRE Assumptions Plan view Vertical sides to vessel at free surface Small angle of rotation, M Metacentric radius BM I oo V Metacentric height, LM = GM = BM – BG BG = difference in height between CofB and CofG LM For zero O O G x m·g Fb B Where: ■ B = centre of buoyancy ■ BM = metacentric radius (distance from B to M) ■ G = centroid (centre of mass or centre of gravity) ■ Ioo = moment of inertia (second moment of area) about axis OO ■ LM = metacentric height ■ M = metacentre ■ V = volume of displaced water For the solution of the problem, the forces acting on a small segment of the vessel are considered. Once determined, the forces are summed for all segments. At the end of this section, we give the derivation of the formula for BM. 420 Installation calculations for subsea pipelines RECTANGULAR BARGES V = Total mass ÷ water density Draught = V ÷ water plane area I = second moment of area For a rectangle Plan view X O (vessel centreline) O d X y Water plane area A bd Moment about midpoint I XX b d 3 12 Moment about any other axis, such as centreline of vessel I OO I XX A y 2 b Many small vessels can be approximated to rectangles. The above equations provide the information for calculating the plan area, A, and the second moments of area, I, of such shapes about any axis (OO) that is parallel to the midline of the rectangle (XX). DETERMINE ANGLE OF HEEL Aim for a level deck < ±0.5° Trimming tanks to restore vessel trim M Assess heel as masses are moved Use of crane or derrick to move loads may not be 0° for all conditions L M tan 1 P x p W LM Spreadsheet evaluates destabilising moment xp G m·g x P Fb B Note that when loads are not arranged symmetrically about the centreline, the deck will not be level. To solve this problem, trimming or ballasting tanks are used to achieve a deck within ±0.5°. Note that the equations given assume an almost level deck. 421 Barge stability Even a small angle can cause problems onboard. Some ships’ decks are cambered slightly to help water drain away. Any additional angle of heel needs to be added to this on the seaward side for operations. For comparison, roads with an incline of 4% are deemed steep enough to require warning signs at the top of the hill. They correspond to an angle of just 2.3°. Notwithstanding this, when derricks are used to move large weights overboard from the deck of small craft, it may be that greater angles will occur temporarily. With larger craft, the overwhelming mass of the barge and ballast helps to provide enough stability to the system. That is: don’t use too small a craft when lifting objects. Even small changes in angle can be alarming for crew on board. Commonly, a spreadsheet or dedicated programme is used to assess trim of vessels. The net destabilising moment is the sum of all masses times their leverarm for items not evenly distributed about the centreline. In the above equation: ■ P = a destabilising load ■ x = its offset from centreline of vessel ■ W = total mass of vessel, ballasting, trim and items on board ■ LM = metacentric height FREEBOARD AND DRAUGHT For larger angles of rotation, Lighter vessel lifts out of water Heavier vessel ships water onto deck Either rapidly reduces I at free surface Barge may become unstable Check for freeboard and draught Lighter craft Heavier craft We made the assumptions that the angle of heel was small and that the sides of the vessel vertical at the free water surface. The first of these may not be true for all configurations of the vessel. If the angle becomes too large, then either the bottom of the barge may lift out of the water or the deck of the vessel becomes submerged on one side. In either instance, the second moment of area will reduce. Since I is related to the third power of the width of the free surface, the stability can be quickly dramatically reduced and the vessel become unstable. 422 Installation calculations for subsea pipelines For this reason, we also need to check that freeboard and draught are sufficient. When water has been able to enter the vessel through openings in the deck of heavier craft, this has often proved catastrophic. We will examine the effects of flooded compartments later. SMALL BARGES With smaller ‘square’ craft Check both metacentric heights Port-starboard Fore-aft Stability may be critical in either direction Check for freeboard and draught at corners Metacentric heights are code-dependent Vary from 0.5 m (20 in) upwards depending on usage Wind & wave effects – righting arm (see later) For small rectangular barges with eccentric loading, both fore-aft and port-starboard stability is needed. What is a normal value of metacentric height? The maximum values for metacentric heights are given in the relevant codes published by Lloyds or DNV etc. Different craft usage (for example, for inshore waters or for high seas) will have different requirements. Codes also specify minimum freeboards for each operating condition. Normal merchant vessels and laybarges have metacentric heights in the order of 1 m. Naval vessels want to be able to fire guns accurately and have a very stable platform. They may have values of 8 m. However, they give a very uncomfortable ride for their crew. A low value for metacentric height is not necessarily a bad thing. Some reel, flexible or J-laybarges have heavy reels, towers, cranes or lay towers high above the deck. This has the effect of reducing their metacentric height. They lose some rigidity but do not become unstable. The righting moment or arm should also be checked against code. It particularly affects larger vessels with high sides that are liable to be affected by wind pressure. We will discuss this and the response of craft to waves (seas) later. 423 Barge stability TYPICAL INPUT TABLE OF DECK LAYOUT ITEMS y 11 z 3 2 12 Case 1 load out Item Description 9 10 5 4 8 x 1 7 x y z Fluid Free surfaces I values from stern from CL above keel density Transverse Axial Mass N° 6 1 Derrick lift 2 300 1.50 0.00 4.80 - - - 2 Spoolpiece A 1 700 2.35 0.90 3.05 - - - 3 Spoolpiece B 1 500 2.55 0.75 3.05 - - - 4 Stores half container 20 000 9.20 -2.10 3.95 - - - 5 Workshop 17 000 5.30 -2.10 3.50 - - - 6 Reel drum 2 500 12.85 0.00 4.20 - - - 1 500 11.70 -2.50 - - - 70 10.75 2.10 - - - 2 500 5.95 2.10 - - - 200 5.12 2.10 0.850 1200 800 7 Generator 8 Lighting column 9 Client office Portakabin 10 Diesel tank 11 Port ballast tank (empty) 12 Starboard ballast tank 5.90 4.7 0 3.52 2.05 1.05 1.025 0 0 1 800 3.52 -2.05 1.05 1.025 2500 2500 It is necessary to tabulate (using Excel or another spreadsheet package) all the items on the deck and any free surfaces of tanks. This simplifies the analysis in both directions for a number of load cases. The above fictitious table shows the typical inputs required. The other load cases, as items are installed on the seabed – and which may require different ballasting of the trim tanks – can be determined from modifying the masses in the tables. DETERMINING VESSEL STABILITY – SUMMARY Definitions of vessel terms Method of determining metacentric radius I BM V Rectangular barge properties V, A and I Values for stability Metacentric heights = 0.5 m (20 in) upward Check for heel or trim angle < ±0.5° Assess draught and freeboard (critical at corners) Any questions? 424 Installation calculations for subsea pipelines We have provided the key equations for determining the metacentric height. For rectangular barges - often used for construction work - their properties have been given. Typical values for metacentric heights and trim angles have been given. The need to assess draught and freeboard (especially at corners of ‘square’ barges) has been highlighted. 425 Barge stability FREE LIQUID SURFACES AND SUSPENDED LOADS FREE LIQUID SURFACES Vessel carrying liquid in tanks with a free liquid surface Free-liquids adversely affect stability Reduce metacentric height from M to N M N A vessel carrying liquid in tanks where the liquids have a free liquid surface will be affected adversely by the movement of the CofG of the liquid in the tanks as the whole vessel heels. Changes in position of the CofG of the liquid in the tanks causes a change in position of the CofG of the whole vessel. This change in the vessel CofG, as it heels, causes a reduction in the metacentric height of the vessel and so a reduction in the vessel’s stability. In vessels carrying liquids, such as oil tankers, they will usually be loaded in such a way as to ensure that individual tanks are filled. Thus eliminating, where possible, the free liquid surfaces. If the hull of a vessel is breached then the problem of reduced stability becomes a particular problem and is usually a key factor in the loss of vessels at sea. An example of this is the capsizing of the Herald of Free Enterprise ferry near the port of Zeebrugge in 1987. After departure, the bow doors of the car ferry had not been properly shut and water leaked into the car decks. The partially flooded decks then acted as a tank with a free liquid surface which reduced the metacentric height and thus the vessel stability. This led to the vessel capsizing and the loss of 187 people. 426 Installation calculations for subsea pipelines A particular problem with the Herald of Free Enterprise ferry design was that it only had a full width hold that acted as the car deck (for ease of parking). Therefore, when it flooded, there was a greater second moment of area of the free liquid surface than later car ferry designs, which have the hold divided into two by a bulkhead running along the keel. Oil tankers and other vessels designed to carry liquids have their hull divided into many smaller tanks. FREE LIQUID SURFACES Metacentric height reduces by NM I t ρt V ρ Dependent on density of liquid in tank Free water surface dimensions of tank Width of tank relative to vessel trim And thus the tank’s second moment of area Therefore Ensure tanks are either completely full or empty Split into smaller individually-operated tanks Where: ■ I = 2nd moment of area of the vessel in the waterline plane ■ It = 2nd moment of area of the tank in the waterline plane ■ NM = change in metacentric height ■ V = volume of water displaced by vessel = density of the water in which the vessel is floating ■ t = density of the fluid carried in the tanks ■ For each tank WITH A FREE SURFACE OF LIQUID, there is a reduction in the metacentric height. This is also a function on the relative densities of the seawater that the barge is floating in and that of the contained liquid. If there is no free surface of liquid then there is no contribution to the effect. Therefore tanks should be kept either completely filled or totally emptied. It is common to split larger tanks into smaller compartments, each being operated (flooded or emptied) independently. By simply splitting a tank into two across the beam of the barge, the combined I value (which is dependent upon the third power of its width) of the pair of tanks is reduced by a quarter. 427 Barge stability SUSPENDED LOADS Loads act at top of crane jib Comparison of metacentric heights Barge is 12 m long by 8 m wide by 2.5 m side (39.4 ft by 26.2 ft by 8.2 ft) Crane cab is 2.59 m (8.5 ft) high Weight incl jib = 24 tonnes (26.5 tons) Jib height = 6 m (19.7 ft) Object 53 tonnes (58.4 tons) At deck level Item lifted just off deck 6 m (19.7 ft) Ballasted weight = 80 tonnes (88.2 US tons) CofGc CofGM CofGb CofB Combined CofG MG drops from 2.1 m (6.1 ft) down to 0.1 m (0.2 ft) We are about to work through an example of a very similar barge in a moment. Instead of the crane, we will examine a similar size and weight of container However, for this example above, the barge is stable with the object sitting on the deck of the barge. The metacentric height is an acceptable 2.1 m (6.1 ft). As soon as the crane lifts the object even 1 mm off the deck, the metacentric height drops to less than 0.1 m (0.2 ft). This is close to being unstable. A very slightly longer jib or a slightly increased load would make the vessel capsize. Essentially, the weight of the object when it is sitting on the deck is adding to the ballasting of the barge. As soon as it is lifted by the crane, that stability will be lost. The object will tend to swing towards the crane and the barge will heel significantly. The craft is somewhat poorly balanced to begin with, with the relatively large weight of the crane always trying to tip it over. We really need a larger craft, or the crane should be more centrally located. 428 Installation calculations for subsea pipelines DISCHARGE CRANE – HEAVY LIFT INSTABILITY Courtesy of Barth Crane Inspections (http://www.craneoperator.com) Stability can suddenly be lost. Barth Crane Inspections are a leading investigating company when lifting systems fail. Here, the normal lifting on board of a railway locomotive caused the vessel to roll over. This may have been because of a heavier locamotive than shown in the upper photograph, or perhaps the ballasting of the vessel on the subsequent operation was different. CRANES AND EXCAVATOR ARMS Check for crane arm swinging to port and starboard Horizontal pull forces from bucket Acts at top of jib Allow for suction of soil in bucket Pulling arm downwards and sidewards Barge stability 429 It is normal to be able to swing the crane forward to lift an object off the deck and into the water. All possible positions for the crane jib must be checked. Other destabilising effects can occur when excavators are digging beneath the barge. These tend to induce both vertical and horizontal forces onto the arm. COMBINED EFFECTS Use of trim tanks to counteract crane lift Avoid free surface area Fill a number of tanks completely Where ballast tanks are being used to counteract the effect of the crane lifting destabilisation, it is better to fill a number of tanks completely to avoid the effects of the free surface. If tanks are partly filled, as shown in the diagram, then this reduces the barge stability. 430 Installation calculations for subsea pipelines EFFECT OF WIND ON STABILITY Comparison of semi-sub and conventional Righting arm in m (ft) Righting Wind 0 30 Heel angle 40 60 90 120 150 180 With larger vessels, an assessment must be made of the effect of wind on the vessel. The graph shows the change in righting arm as a vessel is heeled completely over. However, codes such as DNV Marine Operations RP2 usually only consider acceptable angles of heel up to 40. Sailing vessels (yachts) are an exception. The righting arm is the righting moment divided by its displacement. Some codes refer to the righting moment curve rather than the righting arm curve. We need to also take account of the destabilising effect of cross-winds (shown dotted). The graph shows the righting and wind traces for both semi-sub and conventional vessels. Each vessel will adopt the stable angle where the destabilising wind and righting lines cross. Semi-submersibles (shown in yellow) are extremely stable up to a few degrees. Conventional vessels (pink) have less initial stability though they will self right up to about 130. It may be that as the vessel rolls, there is a bigger area of side to be affected by the wind. For stability, the codes generally demand that the area beneath the righting curve should be at least 1.4 times the area of the heeling curve for wind sustained for a 1 minute. 431 Barge stability RESPONSE OF CRAFT TO SEAS Natural frequency of roll Lateral motion Natural frequency of pitch Fore-aft motion Natural frequency of heave Up-down motion c g GM f4 4 b 0.5 g f 5 c5 d 0.5 g f 3 c3 d 0.5 Critical waves frequency 0.71 to 1.2 of vessel Roll amplitude > twice that of swell Resonance reduced by bilge keels or stabilising fins Where: ■ b = beam (width) of the vessel ■ c3 = typically around 0.13 for many ships ■ c4 = typically between 0.35 and 0.4 for most ships ■ c5 = typically around 0.13 for many ships ■ d = depth of vessel hull ■ f3 = natural heave frequency (Hz) ■ f4 = natural roll frequency (Hz), typically between one cycle every 4 to 30 seconds ■ f5 = natural pitch frequency (Hz) ■ g = acceleration due to gravity, 9.80665 m/s² (32.174 ft/s²) ■ GM = Metacentric height, typically around 1 m (3ft) for ships or laybarges Since the equations for pitch and heave frequencies are similar, these motions tend to be intercoupled. For ship-shaped vessels the bow and stern have different shapes. This means that the pitching moment when the bow is down tends to be different from that when the stern is down. This asymmetry results in a net vertical force applied to the vessel as the ship pitches: this then causes the ship to heave. When the frequency of waves approaches the natural frequency of the vessel, then the amplitude of the roll increases sharply. In the critical range, where the waves have a frequency 0.71 to 1.2 that of the ship, the amount of rolling can be more than twice that of low frequency waves such as swells. Ships are generally fitted either with passive dampers such as bilge keels to increase viscous damping or active devices such as hydraulic stabiliser fins to reduce the resonant roll. For a more detailed discussion, refer to Chapter 11 of Flow-Induced Vibration by Robert D Blevins ISBN 0-89874-891-7 Robert E Krieger Publishing Company, 1986. An up-to-date summary of roll damping of vessels underway can be found in Oceanic Engineering International, Vol 9 No 1, 2005 pp 1 to 10, “Roll damping: a review” by M R Haddara. The following paper, pp 11 to 27, “Design criteria for parametric rolling” by K J Spyrou provides guidance on roll avoidance. 432 Installation calculations for subsea pipelines SOFTWARE Orcaflex and Anflex General purpose analysis for lifting and for mooring Moses Determination of RAOs Lifting of modules in air and in water Gmoor and Arianne Mooring analysis for laybarges – anchor legs Liftsim Lifts in air and water Typical packages available for barge stability analysis are shown. FREE LIQUID SURFACES AND SUSPENDED LOADS – SUMMARY Free liquid surfaces Suspended loads act at top of jib Stability comparison when lifting weights Digger forces due to soil Use of cranes to lift weights Combined effects of cranes and trim tanks Response of craft to wind and waves Any questions? The reduction of metacentric height due to free liquid surfaces means that wherever possible, tanks should be completely full or empty. Loads act at the top of crane jibs. We have shown how the apparently stable craft can topple when relatively large loads are lifted from the deck of vessels. Barge stability 433 Additional destabilising forces on floating diggers can be caused by soil suction. Full stability analysis includes examining how the craft responds to the effect of wind and waves. 434 Installation calculations for subsea pipelines MODULAR CRAFT AND LOCAL BARGES MODULAR CRAFT Uniflote barge - modular system Assembled manually Standard dimensions 5.3 m (17.4 ft) long by 2.4 m (8 ft) wide by 1.2 m (4 ft) deep Ideal for work in shallow water It is unlikely that field engineers will be expected to analyse pipelay vessel stability. However, use of small pontoons or assessment/adaptation of small local barges may be required. These are used for nearshore operations where laybarges cannot operate. Examples of such uses include wire laying for landfalls and dredging or excavation. With modular pontoons, the main two components are shown. Most rafts consist of a number of the main box sections, as shown on the left. Where access is needed a ramp section may be incorporated or a bow section for movement close to shore. All these can be coupled manually by rocking the vessel using the self weight of the workers. Other equipment such as guard rails, bollards, anchor winches can be bolted on. 435 Barge stability TYPICAL UNIFLOTE CONFIGURATION 20 tonne (22 US ton) excavator 7 Uniflote raft 15.0 m (49 ft) 10.7 m (35 ft) 1 3 2 4 5 10.4 m (34 ft) Ramp Ramp 6 7 This is a typical small raft suitable for shallow dredging work. Other larger configurations are possible. These include ‘moon pool’ where the central section of the craft is left open for working. INSHORE WORK CRAFT Landlines in swamps Rivers, estuaries and sheltered waters Landfalls Pull wire lay operations Excavation and piling ‘Landlines’ laid across swamps may make use of small work craft such as shown to prevent damage to the soils. However, such barges are frequently used in rivers, estuaries and sheltered waters where heavy wave action is not encountered. 436 Installation calculations for subsea pipelines Landfalls make use of such units to help install pull wires. Standard small units like these are regularly used for excavation or piling in sheltered waters nearshore. ADAPTING LOCAL BARGES Often an existing barge is found operating locally to the worksite. This may be adapted for use in order to saves mobilisation from other parts of the world with consequential cost savings. Its suitability for use will need to be assessed. MODULAR CRAFT AND LOCAL BARGES – SUMMARY Modular craft Uniflote configurations Adapting existing local barges Any questions? Barge stability 437 We have looked at modular craft such as the Uniflote and shown a typical arrangement with a tracked digger on board. Typical use of these calculations may be needed when assessing local craft for nearshore use. 438 Installation calculations for subsea pipelines WORKED EXAMPLE WORKED EXAMPLE Length of barge = 12 m (39.4 ft) Mass of barge = 80 tonne (88.2 ton) 2.59 m (8.5 ft) 2.44 m (8 ft) CofG Cargo container Mass = 24 tonne (26.5 US short ton) located in centre 2.75 m (9 ft) Barge deck Port CofG 1.25 m (4.1 ft) z x Side = 2.5 m (8.2 ft) Starboard Freeboard Draught Keel (origin) 4.0 m (13.1 ft) Beam = 8.0 m (26.2 ft) Let us consider a simple flat bottom ‘brick-shaped’ barge with a cargo container positioned to the starboard side. The container is located midway between the bow and the stern along its length, so we only need to consider the port-starboard axis shown above. Earlier we made use of the same mass and location of the container for a crane. We examined what happens when a weight was lifted off the deck. 439 Barge stability WORKED EXAMPLE Weight mass g Ballasted barge 80 tonnes 9.81 m s 784.5 kN 88.2 US tons 176 kip 2 Cargo container 24 tonnes 9.81 m s 235.4 kN 2 26.5 US tons 53 kip Total weight 784.5 235.4 1019.9 kN 176 53 229 kip Displaced volume of seawater 3 2 3 1019.9 kN 1025 kg m 9.81 m s 101.5 m 229 kip 64 lb ft 3583 ft 3 3 It is common practice to use a value for seawater density of 1.025 tonne/m³ (64 lb/ft³) and value for gravitational acceleration, g = 9.81 m/s² (9.80665 m/s², exactly). US practice using imperial units (pounds and pounds force) does not need to multiply by gravity to obtain weights (or the volume of displaced water). WORKED EXAMPLE Vessel draught volume breadth length 8 m 12 m 1.057 m 3583 ft 26.2 ft 39.4 ft 3.5 ft 101.5 m 3 3 Mean freeboard moulded side draught 2.5 m - 1.057 m 1.443 m 8.2 ft - 3.5 ft 4.7 ft Deck Freeboard Draught Keel This is the draught and freeboard at the centreline. Once we have calculated the vessel trim angle, we can derive minimum freeboard values at both sides of the barge. 440 Installation calculations for subsea pipelines WORKED EXAMPLE Find centroid of mass (moments about keel) Horizontal Lxb container weight lever arm total weight xcb Wc xcb Wb Wc 235.5 kN 2.75 m 1019.9 kN 0.635 m Combined CofG Side 52.9 kip 9 ft 229 kip 2.1 ft Wb Lxb Wbc Wc Hc Hkb Vertical Keel (origin) H kb Wb side 2 Wc side hc 2 Wbc 784.5 2.5 2 235.4 2.5 2.59 2 1019.9 1.837 m 176 8.2 2 53 8.2 8.5 2 229 6 ft This gives us a position for the combined centre of gravity of the barge and container relative to the keel. WORKED EXAMPLE Metacentric radius BM 2 nd moment of area displaced volume 3 4 I length breadth 12 12 m 8 m 12 512.0 m 3 39.4 ft 26.2 ft 12 59 10 ft 3 4 V 101.5 m 3583 ft3 BM 512.0 101.5 5.046 m 3 3 59 10 3583 16.6 ft Metacentric height Metacentre MG 3 Combined CofG MK MG BM BK GK BK draught 2 GK H kb 1.837 m 6 ft MG 5.046 1.057 2 1.837 3.737 m 16.6 4.7 2 6 12.3 ft BM GK Kee l BK New CofB As a comparison, the metacentric height without the container was 5.717 m (18.8 ft). This and other diagrams tend to exaggerate the angle of the vessel. 441 Barge stability WORKED EXAMPLE Check for vessel trim Angle of heel, Wc xcb Wbc MG 235.4 kN 2.75 m tan 1 1019.9 kN 3.737 m 53 kip 9 ft tan 1 229 kip 12.3 ft tan 1 tan 1 (0.170) 0.168 radians or 9.6 Metacentre MG Combined CofG MK BM GK Kee BK l New CofB Trim by adding more ballast to port side Check minimum draught and minimum freeboard At 9.6, this is far too steep a slope for operational use. It will be necessary to fill balancing or trimming tanks on the opposite side from the container to restore a level deck. Simple trigonometry can be used to check the minimum freeboard and draught at the sides of the barge. 442 Installation calculations for subsea pipelines EXERCISE EXERCISE Find mean draught and freeboard Check for combined centroid of vessel, crane and object (acts at jib) Find metacentric height Weight acts at top and trim CofG of jib c CofGM CofGb CofB 443 Barge stability EXERCISE – INPUT DATA Barge Length = 40 m, beam = 18 m, side = 3.5 m (131.2 ft 59.1 ft 11.5 ft) Ballasted mass = 600 tonne (661.4 US tons) Crane and jib Weight = 24 tonne (26.5 US tons) Height above deck = 2.59 m (8.5 ft) Distance from keel centreline = 6.25 m (20.5 ft) Top of jib (suspension point of object) Weight of object in air 100 tonne (110.2 US tons) Height of jib above deck = 30 m (98.4 ft) Distance top of jib from centreline = 15 m (49.2 ft) BARGE STABILITY – SUMMARY Stable floating bodies Centres of buoyancy and mass Determining vessel stability Metacentric radius Metacentric height Vessel trim angle Minimum freeboard and draught Free liquid surfaces and suspended loads Modular craft and adaptation of local barges Any questions? We examined why floating bodies are stable. We have shown how the metacentric height (that is the height of the metacentre above the centre of buoyancy) can be derived. The reduction in stability associated with free liquid surfaces in tanks or due to loads suspended from cranes has been demonstrated. 444 Installation calculations for subsea pipelines The use of these type of calculations are generally associated with nearshore work, pontoons, modular craft or when adapting local barges for operations associated with pipelaying. However, they are general principles that also apply to all ships and laybarges. 445 Barge stability BACKGROUND INFORMATION Determination of the metacentre DETERMINING THE METACENTRE Vessel heels through small angle Shape of displaced water changes Removal of wedge AOA’ Addition of wedge COC’ CofB moves from B to B’ Wedge of emersion M For small angles of BM BB' A O C’ G C Wedge of immersion B’ A’ B Cross-section view The following explanation details the method of predicting the metacentric height relative to the centre of buoyancy (CofB). The method assumes the vessel is heeling (also known as listing or tilting) by a small angle and considers the changes in the buoyancy forces that result from this heel. The example shown above considers a cross-section through a vessel which has an original waterline plane of AC and location of the original CofB at B, directly below the vessel's centre of gravity, located at the midpoint along the vessel beam or breadth. When the vessel heels the shape of the volume of fluid displaced by the vessel will change. A portion of the vessel (wedge AOA’) becomes emersed from the water by the rotation and a portion equal in volume (wedge COC’) will become immersed in the water. The total weight of displaced fluid must remain the same as the vessel weight does not change. Therefore, the area of volume of wedge AOA’ must be equal to the volume of wedge COC’. 446 Installation calculations for subsea pipelines The change in shape of the cross-section of displaced water results in movement of the CofB from B to B'. For small angles of heel, the location of the metacentre can be predicted in terms of the change in location of the CofB. The method for determining the metacentre is detailed in the following slides. DETERMINING THE METACENTRE Consider a small area offset from the axis of rotation Swept-out volume when vessel heels through an angle Va DD'a x a Weight of swept-out volume Wa g x a Plan view O Area, a x M O D O C’ G C B’ D’ A A’ B Cross-section view For the solution of the problem, the forces acting on a small segment of the vessel are considered. Once determined, the forces are summed for all segments. Considering a small area (a) that is offset a distance (x) from the axis of rotation (OO), the volume (Va) that is swept-out by the area when the vessel heels, is determined. The weight of this swept-out volume (Wa) is then predicted based on the specific gravity of the water and the swept-out volume. 447 Barge stability DETERMINING THE METACENTRE Summation of the weights of small areas Weight of wedges AOA’ and COC’ x AO x AO x 0 x CO x 0 x CO x 0 x 0 WAOA' g a x g a x WCOC ' g a x g a x Axis OO will pass through centroid of waterline plane x AO x CO x 0 x 0 Therefore a x a x, or a x 0 The weight of each small swept-out volume (Wa) that forms each wedge can then be summed to give the total weight of each wedge (WAOA’ and WCOC’). The summation of the area (A) times the distance from centroid (x) for each small area making up the wedge gives the first moment of area of the waterline plane (ax) about the axis OO. It is known that the first moment of area for the emersed wedge will be equal to the first moment of area for the immersed wedge. Therefore, the total first moment of area for the total swept volume (both emersed and immersed) will be zero. DETERMINING THE METACENTRE Moments about centroid OO Moment of weight of water swept out by area a M a Wa x g a x x g a x 2 Total moment due to displaced water by addition of wedge COC’ M COC ' g a x C’ M COC ' g I 2 Where I is the 2nd moment of area C A O G B’ A’ x Wa B Cross-section view The weight of each small swept-out volume that is offset from the centroid (OO) will result in a moment about the centroid. When summing the weight of each swept-out 448 Installation calculations for subsea pipelines volume resulting in a displacement of the water, the moment can be given in terms of the second moment of area of the waterline plane about OO. DETERMINING THE METACENTRE Moments about centroid OO Moment when moving buoyancy force from B to B’ M b FB BB ' g V BB ' Where V is the total volume of water displaced A O C’ G C B’ A’ Wa B FB Cross-section view The heeling of the vessel and the subsequent displacement of a volume of fluid resulting from the addition of the wedge COC' results in movement of the CofG of the displaced fluid. This corresponds to movement of the CofB from position B to B'. The buoyancy force (Fb) is equal to the weight of the displaced volume of fluid. This buoyancy force will provide a righting moment that, for equilibrium, will oppose the weight of the wedge AOA' that is emersed from the water. 449 Barge stability DETERMINING THE METACENTRE Equating moments about OO, Ma = Mb g I g V BB' I BB ' V From previous M BB ' BM O Therefore I BM V G B’ B Cross-section view The moments caused by the weight of the wedge emersed from the water (Ma) and the buoyancy force acting to right the vessel (Mb) will be equal and opposite and can be equated. The equation can be rearranged to give the new position of the CofB (BB’) in terms of the angle of heel, the second moment of area of the waterline plane and the volume of water displaced. If the angle of heel is unknown and assumed to be small then the previously defined equation for the metacentric height, relative to the CofB (BM) can be used. Substituting in this equation gives the metacentric height in terms of the second moment of area of the waterline plane and the volume of displaced water. 450 Installation calculations for subsea pipelines Reduction due to free surfaces of tanks FREE LIQUID SURFACES From resolution of moments g V GG ' t g V1 G1G1 ' t g V2 G2G2 ' 1 Therefore, GG ' t I 1 I 2 V M Effective metacentric height NM Z B BM Z G GM where ZG Z B BM I V and 1 GN GG ' t I 1 I 2 V Therefore NM Z B Z G N G1 G G’ B’ B G 1’ G2 G2 ’ 1 I ρt ρ I 1 I 2 V Where: = density of the water in which the vessel is floating (kg/m3) ■ ■ g = acceleration due to gravity constant (m/s2) ■ V = volume of water displaced by the vessel (m3) ■ GG' = change in position of the Centre of Gravity of the vessel (m) ■ = density of the fluid carried in the tanks (assuming same fluid in both tanks) (kg/m3) ■ V1 = volume of the first tank (m3) ■ V1 = volume of the second tank (m3) ■ I = 2nd moment of area of the vessel in the waterline plane (m4) ■ I1 = 2nd moment of area of the first tank in the waterline plane (m4) ■ I2 = 2nd moment of area of the second tank in the waterline plane (m4) ■ G1G1' = change in position of the CofG of the first tank (m) ■ G2G2' = change in position of the CofG of the second tank (m) ■ NM = change in position of the metacentric height (m) ■ ZB = height of the CofB above a datum, the vessel keel (m) ■ ZG = height of the CofG above a datum, the vessel keel (m) ■ BM = metacentric height above the CofB (m) ■ GM = metacentric height above the CofG (m) Anchors and piles 453 Anchors and piles EXPECTATION EXPECTATION Anchors and piles provide a point of fixity Temporary Laybarge anchorage, pipelay startup Small anchor movements can be accommodated by winch Permanent Riser buoy for flexible pipes to a barge Cannot be allowed to move during life of facility Limiting this study to larger forces (>10 T) Determination of soils resistance force Requires knowledge – c, & permeability/porosity Phreatic level for land-based anchorages We will be looking at piles and anchors which are two methods used to provide a point of fixity on the seabed or beach. These may be either temporary or permanent structures. Here we are looking at how to design soil resistance to withstand larger forces such as those used by laybarges or buoys. We need to know what types of soils there are, and how their engineering properties can be assessed and used. Clays tend to be dominated by their cohesion (c). Sands are primarily categorised by angle of friction (). Knowledge of the density () of all soils is required to design anchors and piles, as is their permeability/porosity, which is a function of the particle size distribution and grain shape. For land-based anchorages the level of the ground water (phreatic surface) is important since dry soils are able to withstand greater loads than when waterlogged. Note that this module works solely in SI units because of the universality of these in soils reports. 454 Installation calculations for subsea pipelines ANCHORS TYPES OF ANCHOR Use US Navy information Developed by Europeans on newer anchor types Based on ship mooring requirements Multiplier depending on shape and soil type Based on mass of soil ‘cone’ above Added effect of chain or wire Increases fixity over time Shallow anchor in sand Deeper anchor in clay A lot of work was undertaken by the US Navy using their standard pattern of anchors for ships’ moorings in different seabed materials. The Europeans further developed this using more modern and efficient anchor types. It must be remembered, however, that moorings for ships are often for short periods and some small movement of the anchor is not a problem. Anchors holding SPMs are essentially permanent and cannot be allowed to move when subject to wave-induced dynamic forces. In general, the mass of the anchor is multiplied by a factor which is dependent upon the pattern of the fluke and the type of soil. The mass of the anchor for each type is an easy way of determining fluke size because they are essentially scaled up from the same pattern. We will see that more modern anchors such as the StevManta explicitly account for the fluke area in a more rigorous equation. Effectively, the normal method assesses the mass of the cone of soil above the anchor and applies a factor to account for the cohesion or friction around the perimeter. For example, sands tend to have high friction and a shallow anchor. Clay requires a deeper anchor because it relies on particle cohesion. The depth that the anchor adopts can be adjusted by altering the angle of the shank to the fluke. This is achieved by inserting steel wedges at the hinge. 455 Anchors and piles A significant contribution to the holding power of anchors may be provided by the chain cutting through the seabed from the shackle connection. With permanent anchors in cohesive soils, particularly, fixity or holding power may increase over a number of months as the soil reforms. TRADITIONAL VESSEL ANCHORING Vessel anchor wires 48 mm to 76 mm (2 in to 3 in) in diameter Minimum breaking loads (MBL) 150 to 375 tonnes Allow 2.5 FoS for breaking over allowable strength Wire lengths 1000 m to 4000 m (3000 ft to 13 000 ft) New deep water vessels exceed these values Anchor winches Braking capacities 120 tonnes to 400 tonnes Pulling capacities 80 tonnes to 280 tonnes Brake is usually 150% more than pull Limited also by breaking capacity of wire Anchoring of traditional laybarges in a ten or twelve mooring configuration uses up to 76 mm (3 in) diameter steel-core wire. This has a minimum breaking load (when new) of 375 tonnes. The working strength is typically defined as 40% of the minimum breaking load MBL. Its reciprocal allows us to think of a factor of safety of 2.5. The length of wire is determined by the winch capacity and the water depth needed. Typically, 1000 m to 4000 m (3300 ft to 13 000 ft)of wire is fitted although some newer barges have more when operating in deeper water. For example, Saipem’s 813 mm (32 in) diameter Greenstream project between Libya and Sicily had 3 km (1.9 miles) long wire extensions fitted, in water depths up to 1130 m (3707 ft). The anchor winches have capacities of up to 400 tonnes (440 US tons) braking load when locked off. However, the limiting value may be that of the wire fitted. Their pulling capacity is two-thirds of this figure. The braking load is the ability of the winch to hold a stationary load whereas the pulling load is able to be applied whilst the winch is rotating. (Note that the breaking capacity of the wire is greater than the brake capacity of the drum.) Winches are drum type and are automatically controlled (paying in or out) using software from the bridge. This means that the forward movement of the barge can be defined when laying pipeline, and then all winches moved simultaneously the correct amount. 456 Installation calculations for subsea pipelines TRADITIONAL VESSEL ANCHORING Anchor types and mass (weight in air) Baldt Stockless (20 tonnes to 30 tonnes) USN Stockless (10 tonnes to 14 tonnes) Flipper Delta (10 tonnes to 15 tonnes) Embedment Groundleg Stevin (10 tonnes)drag Bruce (6 tonnes) Holding capacity Anchor Wire Chain Multiplier depending on pattern and soil properties Also known as ‘efficiency factor’ Additional fixity from groundleg (typical 100 m) Drag distance up to 40 times fluke length Different types of vessels use different weights and patterns of anchors. The holding power is dependant on their pattern, mass and the properties of the soil. Additional holding capacity is developed by the groundleg chain and wire, which are dragged into the seabed during the anchor embedment. A typical groundleg chain for pipelay operations is 100 m long and 64 to 90 mm diameter. This ensures that the wire does not come into contact with the seabed. Typical breakout friction factors for chain are around 1.25 for clay, 1.0 for sand and 0.90 for soft mud. These may reduce to 75% for sliding friction. For comparison, wire rope breakout friction around 0.6 for clay, 1.0 for sand and 0.45 for soft mud. Factors may reduce to just 50% or 25% for sliding friction. Typical masses of anchor used for offshore pipeline installation are shown above. The US Navy stockless anchor originally used has typically been replaced by lighter and more efficient patterns. More efficient, lighter patterns are now available. Anchors do not develop their full holding power until they have been dragged into the seabed. As they are recovered by pulling on the pennant attached to their rear, damage to the bed may result. However, for pipelay operations, it is important that anchor scarring should be minimised. This should be kept to no more than a 50 m long scar under extreme weather (or down to 10 m in favourable conditions). Drag distances may be up to 40 times the fluke length for full holding capacity although typically they may be between 5 and 20 times. Penetration is generally around one fluke length in sand and clay. In muds and soft bottoms, the depth may be between three and six fluke lengths for all patterns of anchor. The angle between shank and fluke has an effect. A 50° angle may cause the anchor to dig twice as deep as one with 30°. 457 Anchors and piles VESSEL ANCHOR TYPES Flipper delta Stevin Danforth Bruce Shank Flukes Palm Stock Stockless Anchors are to a number of patterns. Some are better in certain types of ground than others. ■ The Danforth has hinged flukes and a separate stock to help it position itself flat on the seabed. The angle between the flukes and the shank may be adjusted for use in different ground. The angle is narrower for hard rock (~ 28°), the standard ~36° for granular and mixed bottom (used in around 80% of the seabed worldwide) and greatest (~50°) for soft clay anchorage. In hard ground a large fluke angle may cause the anchor to keel over as it lifts up. If the Danforth is installed in very soft seabeds then it may initially sink into the mud with the flukes pointing upward because of the weight of the head. It is then necessary to pull horizontally, using the drag from the palms to help to right it and gain good holding. For a 5.5 tonne (6 US ton) anchor, the shank is 3.7 m (12.1 ft) long, the flukes are 2.2 m (7.2 ft) long, the palm depth is 0.9 m (12 ft) and the combined stock width is 3.5 m (11.5 ft). For an 18 tonne (20 US ton) anchor, the shank is 5.5 m (18 ft) long, the flukes are 3.1 m (10.2 ft) long, the palm depth is 1.4 m (4.6 ft) and the combined stock width is 5.2 m (17.1 ft). ■ The Stevin is similar to the Danforth but has curved blades and two stock stubs at either side. Again the angle may be adjusted. For a 10 tonne (11 US ton) anchor, the shank is 4.6 m (15.1 ft) long, the flukes are 3.2 m (10.5 ft) long and the combined stock width is 5.7 m (18.7 ft). For a 25 tonne (27.5 US ton) anchor, the shank is 6.3 m (20.7 ft) long, the flukes are 4.3 m (14.1 ft) long and the combined stock width is 7.7 m (25.3 ft). ■ The Flipper Delta has open flukes but larger palms than the Danforth. The latter help it right quicker in soft seabeds and also provide additional holding area. In soft soil it has a holding of 2 times anchor weight. For a 10 tonne (11 US ton) anchor, the shank is 5.0 m (16.4 ft) long, the flukes are 3.4 m (11.2 ft) long, the palm depth is 1.6 m (5.2 ft) and the width is 4.3 m (14.1 ft). For a 40 tonne (44 US ton) anchor, the shank is 7.9 m (25.9 ft) long, the flukes are 5.3 m (17.4 ft) long, the palm depth is 2.5 m (8.2 ft) and the width is 6.7 m (22 ft). ■ The Bruce has a fixed triform fluke beneath the stock. For a 9 tonne (10 US ton) anchor, the shank is 5.0 m (16.4 ft) long, the depth is 2.8 m (9.2 ft) and the combined fluke width is 3.4 m (11.2 ft). For a 20 tonne (22 US ton) anchor, the shank is 6.9 m (22.6 ft) long, the depth is 2.9 m (9.5 ft) and the combined fluke width is 5.5 m (18 ft). 458 ■ Installation calculations for subsea pipelines There are a number of different patterns of Stockless shackle including the US Navy pattern and the Baldt. They are easier to stow in the draft tube on a vessel whilst underway. The fluke makes an angle of 45° with the shank. For a 9 tonne (10 US ton) anchor, the shank is 3.0 m (9.8 ft) long, the flukes are 2.1 m (6.9 ft) long, the depth is 1.6 m (5.2 ft) and the combined fluke width is 2.3 m (7.5 ft). For a 27 tonne (30 US ton) anchor, the shank is 4.8 m (15.7 ft) long, the flukes are 3.4 m (11.2 ft) long, the depth is 2.2 m (7.2 ft) and the combined fluke width is 3.2 m (10.5 ft). The code DNV-RP-E301, ‘Design and installation of fluke anchors in clay’ was published in 2000. ANCHOR HANDLING Recovery hook Ring or J chaser Direction of chaser Marker buoy Pennant swivel shackle Hinge link Wire rope to vessel (76 mm x 3 km) Groundleg anchor chain Swivel shackle Pennant (or pendant) wire rope or chain Anchor Safety shackle Stud link chain is fitted between the swivel shackle at the anchor and the end of the wire rope line to the vessel, in order to absorb the effect of waves. A direct line will transmit wave movement at the vessel to the anchor, causing it to move. Typically, the anchor holding capacity is 25% less for anchors with wire or if they are used in hard soil. Although the pull for the fore and aft anchors remains along the original line of the anchor, the quarter or side anchors will experience rotation as the laybarge moves forward during pipelay. The plan angle may be up to 30° different between installation and removal. They therefore may lose some of their tested holding power. Nautical terminology for the wire rope to anchor is the ‘anchor line’. However, we will avoid the use of ‘line’ for rope in order to avoid confusion with the pipe line. Typically, operators prefer to use pennants in calmer water but chasers may be needed in rougher waters (such as the North Sea) where losses of buoys have been experienced. Saipem used such chasers for the work in the West of Shetlands fields. Note that the pennant line digs into the ground behind the anchor as it is dragged forward during embedment. Enough slack in the pennant should be provided for movement in soft clay to prevent the marker buoy from submerging. 459 Anchors and piles If a pennant wire is not attached, the alternative method of removal is to use a hook or hoop chaser to travel down the wire and chain back down along the stock of the anchor. The anchor can then be lifted in a similar manner. Care needs to be taken to maintain tension on the wire whilst the chaser is travelling along it. This will prevent kinking damage. ROVs are used to confirm this has not happened. 12 POINT MOORING ~3000 m (10 000 ft) Breast anchor wire ~2300 m (7500 ft) Quarter anchor wire Lay direction Stern anchor wire Bow anchor wire ~30 m (100 ft) Pipeline tension 0m H SB SB S B SB S B SB S B SB S B S B S B SB SB SB SB SB SB SB 130 m (430 ft) c/c 180 m (600 ft) c/c SB SB SB SB SB SB SB SB SB SB SB SB SB SB SB SB 380 m (1250 ft) c/c SB SB There are two breast anchors located on either side of the vessel to maintain the lay along the correct pipeline route unaffected by the effects of current, wind and waves on the vessel. Four anchors fore and aft ensure that the correct pipeline tension is maintained. This is critical in the touchdown sagbend to prevent overstressing of the pipeline. Smaller vessels used in sheltered waters may require only 10 small anchors to maintain position. Conversely, laying larger diameter pipelines such as 609.6 mm (24 in) in heavy weather, individual wire forces may reach 90 tonnes. Each anchor leg is moved by the anchor handling tug separately, so that there are always 11 lines under tension at one time. With typical S-laybarge progress, each anchor is lifted and reset perhaps six to twenty times a day. Breast anchors can accommodate more lateral movement and are moved furthest but least often. They lie further away from the barge than the quarter anchors. Fore and aft anchors are moved most frequently to allow the lay rate to progress. The distance an anchor moves is dependent upon its location and the water depth but in the 600 m (1000 ft) of water shown above, the breast anchors may move 380 m (1250 ft) each step, the quarter anchors may be moved 180 m and the bow and stern anchor may be moved 130 m. In shallower water, the offset distances of the breast and quarter anchors will be less; perhaps down to 600 m to 1200 m (1000ft to 2000ft). For the vessel shown, two tugs are used but only one anchor at a time is relocated. At each operation, the tug has to recover the positioning buoy and lift the anchor out of the seabed. It then moves to the new location to reset the anchor by lowering it carefully onto the seabed. The whole operation takes perhaps 2½ hours for each anchor move. The anchor is not dropped as this could damage the wire. The barge winch recovers the wire and prove the anchor’s holding capacity at the new location. 460 Installation calculations for subsea pipelines Recovery of the anchor held fast in the seabed requires that the pennant cable is held off vertical (~15°) and hauled in. A vertical pull of the heavy anchor would cause the wire to tend to untwist and snap as the tug is lifted in the waves. If the anchor will not pull out using this method, it may be possible to use the combined pull of the tug and the laybarge: both pull in the same direction to loosen the anchor before it can be hauled in (not possible with a chaser). A short chain pigtail may be used to ensure that the pennant wire does not wear and is not kinked over the back of the anchor palm.. Care is needed to avoid other pipelines, wrecks or protruding rocks between the laybarge and anchors. To this end, careful surveying of the whole pipe route is required for a width either side of the line equal to the wire length. ANCHOR EFFICIENCIES Anchor Danforth Flipper delta Stevin Bruce Stockless Soft silt and clay 7 to 8 9 to 10 11 to 12 9 to 17 1.8 to 4.5 Sand 7 to 9 11 to 18 20 to 22 8 to 10 3 to 9 Multiplier applies to weight of anchor in air The US Navy undertook a comprehensive test programme for different types of anchor. Other work specifically for the oil industry has been undertaken in Norway and France. Anchor efficiencies refer to the horizontal pull at the vessel divided by the anchor weight (in air). This varied from between 2 and 60. Whereas these give a comparison of efficiencies, it is best to use the figures for each soil as recommended by the manufacturers of each anchor. In some instances, these may vary substantially from that given above: for example, in the 2005 edition of the Vryhof Anchor Manual, the range for Danforth is quoted as 8 to 15 and the Stevin range is given as 33 to 55. Anchors and piles 461 MAJOR SUPPLIERS OF ANCHORS Vryhof Anchors bv, Netherlands Anker Advies Bureau bv, Netherlands Baldt Inc, USA Bruce International, UK These major manufacturers provide the larger sizes of anchors used in the oil and gas industry. Note Dutch spelling of Vryhof Anchors – Vrijhof Ankers STEVMANTA VLA SYSTEM Efficiency factors (33 to 55) not applicable D 1.5 k 0 .6 d 0 .7 A0 .3 tan1.7 α UPC N c Su A Resists vertical pullout Use of tugger line at rear to remove The slide shows the Vryhof Stevmanta anchor. With this, the normal practice of multiplying the mass of the anchor by an efficiency factor is not applicable. These anchors are essentially thin steel plates which cut through the seabed at one angle, and rely on their area and a different angle of pullout (nearly 90°) to resist loads. This anchor can be either a temporary or permanent fixture point. 462 Installation calculations for subsea pipelines Where ■ D = penetration depth (m) ■ k = quotient of the undrained shear strength (kPa) for clay and depth (m) ■ d = mooring line or installation line diameter (m) ■ A = fluke area (m²) = fluke shank angle (°) ■ ■ UPC = ultimate pull-out capacity (kN) ■ Nc = bearing capacity factor ■ Su = (k · D) undrained shear strength clay (kPa) VLA type anchors such as the StevManta shown above or the Bruce Denla can resist much more pull. They depend on their fluke area rather than their weight so their high efficiency factor gives a poor indication of their ultimate holding capacity. The equations shown above provide a better estimation of their capability. They have been specifically developed for the offshore oil industry for use in deep water (eg SPM, FPSO, etc). The StevManta makes use of a hinged system to install the anchor. Three methods can be employed: in the first using the single mooring line, a shear pin breaks as the anchor reaches its designed pullout force and the point of loading switches to the permanent position. In the second (shown in diagram), two lines can be used to first pull the anchor to depth then the permanent mooring line located at the back takes over. By use of special tensioners in the final method, a pair of such anchors can be installed and proven at depth. A permanent tail line and submerged buoy near the seabed can be used to recover these anchors. The code DNV-RP-E302, ‘Design and installation of plate anchors in clay’ was published in 2002. CLUMP ANCHOR Used for permanent buoyancy at risers Simple mass weight Heavy-weight concrete and steel pan Circular or rectangular in plan Check for short and longterm stability Shallow deformation and dynamic loading failures Use of skirt to provide lateral fixity (sliding) Installation Limited by crane Possible use of additional weight sections This is used to fix the position of permanent buoys at risers. But other uses include maintaining marker buoys on location or for weighting down an anchor chain permitting free draught for shipping channels. 463 Anchors and piles Essentially they act as a simple weight. They are usually made of a steel basket or pan filled with concrete or stone ballast. Simple rectangular or circular shapes are common. Because concrete has a relative density of just 2.4 and seawater is 1.025, it is common to use heavy weight concrete of RD 3.05. These can fail either over the short or long term. This is dependent upon the ability of the soil to relieve pore water pressures. Dynamic loading caused by seismic or wave effects on the buoy may also cause problems over time. Soil strength may reduce with repeated application and relaxation. The risk of sliding failure on silty seabeds is reduced by the installation of skirts into the mudline. These are typically made of thin plate about 1 m (3 ft) deep. Installation of the clump is normally carried out using a barge mounted crane. This may have limited lifting capability and a number of techniques are used to help reduce weight. The buoy may be installed attached to the clump and additional weights are added in separate operations to achieve the final anchor mass. Alternative techniques of attaching the buoy may be to flood and purge one chamber of the buoy, or to fit a sheave to the clump once installed and then pull the buoy down from the surface and attach the chain. Guidance on the design of clump foundations can be found in API RP2A and in DNV classification notes 30.4. CLUMP ANCHORS Buoy Additional weight blocks Lifting lugs Guideposts Initial anchorage of heavyweight concrete and steel Skirt driven into seabed Anodes This shows a permanent anchor for a flexible riser buoy. It is installed using the help of the buoy to reduce the total submerged weight and then a pair of additional weight blocks are located between the guideposts. The skirt drives itself into the seabed once the full weight is installed. Sometimes, these anchors are installed over a pile to improve lateral stability. Long term settlement is important because buoy height is critical for the dynamic response of risers. 464 Installation calculations for subsea pipelines ANCHORS – SUMMARY Types of ships’ anchor Mass times efficiency for different soils Typical values of multiplier factor Laybarge anchor handling Modern plate type anchors Used for permanent moorings Clump anchors Any questions? We have looked at the traditional method of mooring vessels and a number of different patterns of ships’ anchor for temporary usage. The standard method multiplies the mass of the anchor by an efficiency factor which is dependent upon the seabed soil. Some of these factors for different patterns have been given. For 12 legged anchored laybarges, the typical arrangement and handling considerations is given along with dimensions of wire and anchor. Modern anchors are more efficient. Permanent moorings demand no slippage and a more analytical formula takes into account the area of the anchor and the actual properties of the soil. Clump anchors are another permanent means of mooring subsea buoys at flexible risers. A typical arrangement is shown. 465 Anchors and piles EXERCISE EXERCISE Estimate holding capacity of system in sand Assume a groundleg drag factor of 1 Estimate unit mass of studlink chain 2 2 3 3 Use M 21.9 tonnes/m Dlink 1367.2 lb/ft Dlink Check chain can hold anchor Use a proof stress for chain of 150 N/mm² (21.8 ksi) Check strength of wire 100 m (328 ft) groundleg 12 tonne (13.2 US ton) Danforth anchor 64 mm (2½ in) wire IPS Bridon Bluestrand 6x41 MBL 238 tonnef (524.7 kip) 90 mm (3½ in) S1 studlink chain proof load 1910 kN (430 kip) Sandy seabed Often laybarges will omit the groundleg chain to ease handling of the anchors. However, when in deeper water, the groundleg is essential to avoid uplift of the anchor. The compliant response of the vessel is determined from the combination of straightening of the catenary in the wire and lifting of the groundleg chain from the seabed. This example can use the lowest grade of steel for the chain because, in this instance, the weight of chain is more important. Where ■ M = unit weight of studlink anchor chain ■ Dlink = diameter of links 466 Installation calculations for subsea pipelines PILES TYPES OF PILE 1 2 3 4 5 6 7 Many types of pile exist. Some typical examples used in the offshore industry are shown above. [1] A closed pile may be driven using a drop hammer from within onto a dry-mix concrete pad set at the closed bottom of the pile. It can also be driven using conventional hammers set at the top. Closed piles rely on skin friction plus end bearing. Some piles are reamed out and concrete filled. [2] If the soil is suitable, under-reaming of bell-ended piles can provide extra end bearing. [3] Sleeved and grouted piles can be driven providing additional stiffness at the seabed. The inner pile is usually at least 150 mm smaller diameter than the sleeve. [4] Open piles fill with material and allowance during driving must be made for the additional skin friction of the internal soil. [5] With very long piles, this internal friction builds up and effectively forms a plug at the bottom of the pile. The pile then acts as an end bearing pile. 467 Anchors and piles [6] Open piles may encounter different soil types as they are driven. It is common to encounter lenses of sand and gravel that are more difficult to drive through. Installation testing of the pile is done by checking the rate of progress for each hammer impact. It is often quoted as the number of blows per metre. Care should be taken that the pile does not punch through the bottom of the lens during this testing - or even shortly following testing when the pore water pressure has had time to disperse. [7] It is common to jet or drill into rock and grout a pile into place. Care must be taken to avoid damaging the surrounding rock with jetting or creating a slick of silt on the sides of the drill. Both may reduce the efficiency of the pile holding capacity. CAN (SUCTION) ANCHORS ROV-operated pump removes water Differential pressure forces can into seabed Cohesive soils and fine sands ROV Can anchor Suction piles on deck A recently introduced method of installation makes use of the wide cross section of offshore piles to force a drum or can into the seabed. These were used by Saipem for the Christine development. An ROV is used to create a partial vacuum within the pile and the pressure differential at depth over the full area provides the insertion force. Suction anchors are good for clay soils and fine sands where a hydraulic seal can be made. In granular soils such as coarse sands and gravel, a water path may be created through the base of the anchor back up to the seabed. This prevents the formation of the partial vacuum. They tend to be 3 m to 5 m (10ft to 16ft) in diameter and up to 20 m (65 ft) long. It is common to locate the lug at a point ¼ to 1/3 up from the base. This ensures that the full resistance of the soil is developed. Any rotation of the cylinder will be about the seabed rather the base so the anchor will dig in rather than be pulled out. Guidance on the installation and design of these anchors in sands and clays is given in two recent (www.geotechnique-ice.com) papers by Houlsby, Kelly, Huxtable and Byrne : ■ Field trials of suction anchors in sand for offshore wind turbine foundations, February 2006 Géotechnique 56, N° 1, 3-10 468 ■ Installation calculations for subsea pipelines Field trials of suction anchors in clay for offshore wind turbine foundations, February 2006 Géotechnique 55, N° 4, 287-296 The new code DNV-RP-E303, ‘Geotechnical design and installation of suction anchors in clay’ was published in 2005. TUBULAR PILES Closed tubular or hollow Driven into seabed Percussion or vibration driver Hollow – drop hammer inside Vertical upward pull Skin friction resists pullout Check for wall tension Lateral pull Surface area of pile Check for bending of pile Tubular piles are widely used to fix structures to the seabed. They are normally held within a tubular positioning guide (not shown) prior to driving. They can be closed tubular or hollow. The latter permits the soil to enter the pile. This reduces the area of steel penetrating the soil, but for long piles, significantly increases the friction force. Percussive or vibrating piling hammers can be used. These are attached to the top of the pile and transmit loads down the pile wall. Sometimes in fine sands, it is quicker to use a smaller hammer. This is because bigger impacts cause higher pore water pressure between the grains. These effectively resist the driving action. A lesser force may cause the pile to enter the ground more quickly. The simplest form of driving and which can be used in shallow water, involves dropping a cylindrical steel weight down the inside of the tubular pile onto a pad of dry-mix concrete resting on the capped end at the bottom of the pile. Lateral forces on the pile are resisted by the area of the side of the pile in contact with the soil. Vertical forces are resisted by skin friction. Downward forces can also count on the end bearing of the pile. A combination of these effects can be used for diagonal forces. It is important to check for bending of the pile in case the upper portion moves laterally within the soil. Similar checks should be made for stresses in the steel wall itself. 469 Anchors and piles Where the attachment point is below the seabed and any groundleg anchor chain cannot be replaced during the life of the facility then it is common to increase the diameter of the permanently-buried section of chain by 25% to allow for corrosion. In other instances such as when vertical forces are being resisted, the lug can be at the surface. FAILURE MECHANISMS 2 3 4 ⅓D D ⅔D + 1½B 1 B Pressure bulb [1] Scour can occur rapidly in rapidly moving shallow tidal waters. With large diameter piles, the scour depths can be surprising. They are often a proportion of the pile diameter. For example, whilst fixing 4 m diameter piles for an offshore windfarm in Dublin Bay, within four tides (48 hours), a scour hole 6 m deep was formed in the seabed. Piles need to move slightly against the soil in order to develop lateral resistance. [2] Short piles may fail due to rotation. [3] Longer ones may fail due to bending overstress in the steel of the pile wall. [4] Groups of piles tend to act as one larger diameter pile and may not always provide as much total resistance as first thought. This is because the skin friction may drag a block of soil down as one. 470 Installation calculations for subsea pipelines DESIGN CODES API RP2A - WSD Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms - Working Stress Design Section 6.0 Foundation design DNV Classification notes N° 30.4 Foundations The recommended codes for pile design for offshore structures are API RP2A or the DNV classification notes N° 30.4 Foundations. They provide simple equations for design of piles and identifies where problems may demand further investigation. They provide a number of references to provide additional information. The following methods are take from the API code. PILING HAMMERS Driven piles Open ended piles Steam, diesel or hydraulic powered hammers Wave equation analysis for cushion and capblock Pile refusal before design penetration Review hammer performance Re-evaluate design penetration Modify piling procedure Plug removal Soil removed below pile tip Sleeved pile Most seabed piles are open ended cylinders. 471 Anchors and piles These may be driven using a variety of hammers. The subcontractor will use an in-house wave equation analysis to ensure driving forces do not exceed the strength of the pile. The minimum wall thickness, t is given by the empirical equation: t = 6.35 + D/100, where t is the diameter of the pile. It is normal practice to allow a cutoff length of between 0.5 m (1.6 ft) and 1.5 m (5 ft) at the top of the pile following driving. If driving becomes difficult, there are a number of methods listed above in order to achieve the design penetration. The last items must only be used as a last resort. Jetting of material below the pile tip is to be used with caution. PILE DRIVING Easy driving conditions Low stress wave Light, fast ram Stiff cushion Difficult driving conditions High stress wave Heavier, slow ram Soft cushion Hammer Single acting steam or air Efficiency 75% to 85% Hammer Cushion Drive cap Follower Joint (compression only) Pile Double acting steam or air 70% to 80% Diesel 85% to 100% Hydraulic 85% to 95% The above arrangement shows a typical arrangement for pile driving. The follower is used when driving a pile from above the water when the final position of the pile top is required to be below the water line. The joint between the follower and the pile itself can only take compression forces. The best way of studying the hammer/pile/soil system is using wave compression theory. When the hammer strikes the pile head, a stress wave travels (at the speed of sound in steel) from the pile head down towards the bottom of the pile. (Another stress wave travels up to the hammer.) The stress wave travelling down the pile is split, with one part reflected and one part continuing whenever a discontinuity occurs - such as at a change in wall thickness. On reaching the end of the pile, the remaining stress wave is partly reflected. Stress waves continue to bounce up and down the pile until their energy is dissipated by: ■ friction between the pile and the soil ■ plastic soil deformation ■ radiation of stress waves through the soil and away from the pile ■ internal natural damping in the pile 472 Installation calculations for subsea pipelines It has been found that both the energy transferred from the hammer and the shape of the stress wave affects the efficiency of the piling. The energy is determined primarily by the mass of the ram and its impact velocity: E =½ M V² (but only 60% to 70% is typically transferred to the drive cap from the ram). See the table above for different hammer types. Clearly the greater the energy the greater the penetration per blow and the greater the risk of damaging the pile. The maximum stress in the stress wave is largely determined by the velocity of the ram. For the same energy input to a given pile a lighter, faster ram and a stiff cushion will result in a higher stress of shorter duration than a heavier, slower ram and a soft cushion. For easy driving conditions, the long duration, low stress wave is best. For heavier driving conditions, a short duration, high stress wave is better. DYNAMIC ANALYSIS MODEL FOR DRIVING Mr M Mass Mc K Linear spring P z Mf Compression only spring P z Mf Damper P z• P z Kc Kf Kf Kf Mf Ks Kf/p Linear spring with maximum force limited by friction Mp Kp Mp Kp Mp Approximate dynamic soil model The mathematical model for the analysis includes the above elements, where the subscripts are as follows: ■ r = ram ■ c = cushion and cap ■ f = follower ■ p = pile Each element’s mass and spring stiffness is assessed. For the analysis, the pile and follower are generally divided into shorter sections 2 m (6.6 ft) to 3 m (9.8 ft) long. Hard cushion materials include hardwood, steel, aluminium and coils of wire ropes. Some hammers incorporate a gas cushion in the hammer body. This soft cushion sustains a soft push to be applied to the pile - an effective method of driving through soils with a low tip resistance such as North Sea clays. The soil resistance is also analysed in layers. Dynamic soil resistance, Rdy = Rst (1 + J v) and damping coefficient, Cs = Rst J Anchors and piles 473 Where: ■ Rst = static soil resistance ■ J = soil damping constant ■ v = velocity of pile element Wave equation analysis is subject to considerable uncertainty owing to the uncertain soil properties so approximations in the model are used. Nevertheless, it is a useful and widely used technique. For a fuller discussion refer to Dynamics of Fixed Marine Structures, by Barltop and Adams. DESIGN OF VERTICALLY LOADED PILES Typical factors of safety 1.5 to 2.0 Ultimate bearing capacity Qd Q f Q p f As q Ap Cohesive soils (clays) Skin friction – assess in intervals down to depth f cu 0.5 0.5 where 1.0 (lower layers) 0.5 0.25 where 1.0 (upper layers) ' cu /po End bearing q 9 cu Where: ■ Qd = ultimate bearing capacity of pile (kN) ■ Qf = skin friction resistance (kN) ■ Qp = total end bearing (kN) ■ f = unit skin friction capacity (kPa) ■ As = side surface area of pile (m²) ■ q = unit end bearing capacity (kPa) ■ Ap = gross area of pile (m²) = a dimensionless factor (1.0) ■ ■ cu = undrained shear strength of soil = a dimensionless factor ■ ■ p'0 = effective overburden pressure at the depth being considered (kPa) 474 Installation calculations for subsea pipelines DESIGN OF VERTICALLY LOADED PILES Cohesionless soils (sands and gravels) Shaft friction – assess in single layer down to depth f K po tan See table over for maximum limits on f and values for Open end piles, K = 0.8 in tension and compression Full displacement piles, K = 1.0 End bearing q po N q Where: ■ K = coefficient of lateral earth pressure ■ p0 = effective overburden pressure at the pile tip (kPa) = friction angle between soil and pile ■ ■ Nq = dimensionless bearing capacity factor TYPICAL PROPERTIES FOR SILT, SAND & GRAVEL (COHESIONLESS) Density Soil description Very loose Loose Medium Loose Medium Dense Medium Dense Dense Very dense Dense Very dense Sand Sand-silt Silt Sand Sand-silt Silt Sand Sand-silt Sand Sand-silt Gravel Sand Soil-pile friction angle, ° 15 Limiting skin friction values kPa (psi) 47.8 (6.93) Nq 20 67.0 (9.72) 12 2.9 (0.42) 25 81.3 (11.8) 95.7 (13.9) 114.8 (16.7) 20 4.8 (0.70) 9.6 (1.39) 12.0 (3.05) 30 35 8 40 50 Limiting unit end bearing values MPa (ksi) 1.9 (0.28) 475 Anchors and piles EXAMPLE VERTICAL PILE IN COHESIVE SOIL (CLAYS) Pile Diameter, = 305 mm (12 in) Length, L = 6 m (19.7 ft) Soft to medium CLAY Submerged density, 950 kg/m³ (53.9 lb/ft³) Undrained cohesion, cu = 45 kN/m² (6.5 psi) Use factor of safety of 2.0 Assess soil in three 2 m (78.7 in) steps More, thinner steps for more accurate pile capacity Assume a single full-depth, homogenous layer of soil Boreholes usually describe discrete horizons & properties We are using a simple example to demonstrate the method. Normally, a variety of soils each with its own properties will be encountered down the pile. The steps will normally be thinner and take into account these variations in density and cohesion with depth. When more steps are considered, then the crude determination of may either slightly increase or decrease the capacity. VERTICAL PILE IN COHESIVE SOIL Skin friction Use the middle level of each step to assess f For first 2 m step (assessment at 1 m down) Overburden at middle 1 m 950 kg m3 g 9.3 kPa cu ob 45 kN 9.3 kPa 4.8 0.5 Ψ 0.5 0.337 (upper layer with >1) f cu 0.337 45 kN m2 15.2 kPa Skin friction 2 m f 29.1 kN Second 2 m step, assess at 3 m, skin friction = 38.3 kN Bottom 2 m step, assess at 5 m, skin friction = 43.9 kN Total friction = 29.1 + 38.3 + 43.9 = 111.2 kN (25.0 kip) End bearing 9 cu 4 2 29.6 kN 6.7 kip SWL 111.2 29.6 FoS 70.4 kN 15.8 kip 476 Installation calculations for subsea pipelines VERTICAL PILE IN COHESIONLESS SOIL (SANDS AND GRAVELS) Pile Diameter, = 305 mm (12 in) Length, L = 6 m (19.7 ft) Closed end Medium dense SAND Submerged density, 1050 kg/m³ (65.5 lb/ft³) Coefficient of friction, = 25° Limiting skin friction, 81.3 kPa (11.8 psi) Nq = 20 Limiting end bearing, 4.8 MPa (696 psi) Use factor of safety of 2.0 Again we are using a simple example to demonstrate the method. The same pile is used but now it is in sand instead of clay. VERTICAL PILE IN COHESIONLESS SOIL Circumfl shaft area, As L 5.7 106 mm2 8911 in2 Effective overburden pressure at tip, p0 L g 6 m 1050 kg m3 g 61.8 kPa 9.0 psi K = 1.0 for closed end piles Skin friction, f K p0 tan 1 61.8 tan 25 28.8 kPa4.2 psi Less than the limiting skin friction, 81.3 kPa (11.8 psi) 6 Shaft friction, Q f f As 28.8 5.7 10 467 kN 105 kip ' End bearing area, Ap 4 2 73062 mm2 113 in2 End bearing pressure, q p0 N q 61.8 20 1.2 MPa 179 psi Less than the limiting end bearing, 4.8 MPa (696 psi) End capacity, Q p q Ap 1.2 73062 351 kN 79 kip Net, Qd Q f Qp 467 351 818 kN 184 kip Pile capacity or SWL Qd FoS 409 kN 92 kip Sands tend to have a substantially higher pile holding capacity than clays. In this case and for these soils, the pile capacity increase is some six times. Note that this capacity may be vertically up (pullout) or downward. Anchors and piles 477 PILES – SUMMARY Tubular and can (suction) anchors Failure mechanisms Design codes Piling hammers and driving analysis Design parameters for sands Comparison of design for tubular piles Combination of shaft friction plus end bearing Holding capacity of sand is better than clay Any questions? The main issues with tubular and suction piles have been described. Piling analysis using impact hammers has been described. Typical designs for identical piles in granular and cohesive soils have been compared for their safe working loads. 478 Installation calculations for subsea pipelines SHEET PILE ANCHORAGE SHEET PILING Used as dead man anchor for winches Coulomb method Corus/Arcelor’s Sheet-Piling Handbook Often used for landfalls’ beach cofferdam design Beware! The method is designed for retaining walls Has in-built safety factor of ~2 against active failure Holding the piles against the active soil We are using it in reverse We are forcing the piles against the passive load of soil Hidden safety factors overestimate the tendency of the soil to push the sheet-pile wall into a cofferdam or trench Sheet piling is typically used for the cofferdam and the anchorage at a landfall. Arcelor now produce the Piling Handbook (having bought the piling division of Corus Steel). However, Corus do provide information on their website (www.corusconstruction.com) in the form of an on-line technical manual. However, since the design of sheet piling is more commonly used to restrain soil at harbours or retaining walls, the user should be aware that there are hidden factors of safety that must be allowed for. The book recommends using a factor of 2 for any anchorages made from sheet piles. See www.arcelormittal.com/sheetpiling for available section sizes and design software. There are at least three competing theories to represent the peak strength of soil. These are linear Mohr-Coulomb; a power law; and stress dilatancy. We use the first as promoted by Terzaghi. However, each method has its champions and detractors. For a discussion, see the Editorial of Géotechnique June 2006 Vol LVI Number 5 p 289 and 479 Anchors and piles letter pp 357-358. See also A N Schofield and C P Wroth, Critical state soil mechanics, 1968 – available to download via http://www2.eng.cam.ac.uk/~ans/. SHEET PILE ‘DEADMAN’ ANCHORAGE Compacted sand berm on existing ground surface Need to assess capacity of soil when pushing both ways from piles The figure shows sheet piles driven into the sand of the beach. A wedge of sand resists the pull from the buried wires attached to the beam welded to the back of the piles. It is normal to have some stiffening where the wire hawsers are attached. This may be a purpose made clamp or perhaps just a half section of pipe with web stiffeners on the beam. The optimum location of the beam is 1/3 up from the bottom of the piles. All the sand wedge is then in compression. In truth, the shape of the volume of sand is a little greater than a simple wedge. Not only does it extend slightly deeper (along a spiral curve), but there are ‘wings’ either side which add to the resistance. In normal practice, the contribution of the wings is ignored. If the piles extend more than about 5 m (16.4 ft), it will generally be difficult to excavate down to the correct depth for the beam. This is especially true where the phreatic surface is close to ground level. In this instance, we need to mobilise the cantilever effect at the base of the piles. See corner sketch. The force at the beam and at the base of the pile balances the resistance of the soil. A check is needed to ensure that the pile will not bend when subject to these large forces. In this case, use has been made of a berm of well-compacted sand above the ground level. This can be an efficient way of using the excellent properties of dry sand. But the berm must extend out for a distance equal to the depth of pile divided by tan(). Use of a berm limits the amount of excavation for the wires (and consequential ground disturbance in front of the sheet piles). 480 Installation calculations for subsea pipelines TYPICAL SOIL PROPERTIES Internal friction Subm density, kg/m³ angle, ° Loose or compacted Loose Compacted Soil Bulk density, kg/m³ Loose Compacted Fine sand 1750 1900 1050 30 35 0 Coarse sand 1700 1850 1050 35 40 0 Gravel 1600 1750 1050 35 40 0 Peat - 1300 300 - 5 5 River mud 1450 1750 1000 - 5 5 Loamy soil 10 Cohesion cu kN/m² 1600 2000 1000 - 10 Silt - - 800 - 10 10 Sandy clay - - 900 - 0 15 to 40 Very soft clay - - 900 - 0 <20 Soft clay - - 900 - 0 20 to 40 Firm clay - - 1000 - 0 50 to 75 Stiff clay - - 1100 - 0 100 to 150 Very stiff clay - - 1200 - 0 >150 The above table abstracted from Corus’ Piling Handbook provides typical values for soils. This is a useful when no numerical data is available and we only have a description of the soil from the borehole log. Where better information exists from the ground investigation testing laboratory, then this should be used in preference. ACTIVE AND PASSIVE PRESSURES Active Passive Grains tumble down Grains lock together Pa K a Ob Pp K p Ob 481 Anchors and piles Where: ■ Ka = active coefficient ■ Kp = passive coefficient ■ Ob = overburden pressure ■ Pa = active pressure ■ Pp = passive pressure If the pile is pushed horizontally to the right of the figure, making a slight gap to the lefthand side of it, then the soil on that side would tend to slide down along a steep slope. This is termed the active pressure. On the right-hand side of the diagram, a large resistance would be built up as a shallow wedge of soil has to be raised. This is termed the passive pressure. In actual fact, the shape of the wedge is a logarithmic spiral (shown in amber) but for most soil analysis, a straight line can be assumed. For a typical pile then, the net resistance at any depth would be the passive minus the active pressure. For granular soils, both the active and passive pressures increase linearly with depth. They are proportional to the active and passive coefficients multiplied by the overburden pressure. This is shown on the two triangular pressure distributions either side of the pile. In the next slide, we show how these simple equations are modified to deal with mixed and cohesive soils. EARTH PRESSURE COEFFICIENTS Active pressure at depth h Pa γ h g tan 2 45 2 c tan 45 2 2 Pa γ h g K a 2 c K a 2 Active coefficient, K a tan 45 2 Normally ignore wall friction for Ka Wall friction, 0° 10° 20° 30° Soil friction, 25° 0.41 0.37 0.34 - 30° 0.33 0.31 0.28 0.26 35° 0.27 0.25 0.23 0.21 40° 0.22 0.20 0.19 0.17 45° 0.17 0.16 0.15 0.15 Where: ■ c = soil cohesion. Active pressures act upon the pile if it is moving away slightly from the face of soil. 482 Installation calculations for subsea pipelines They increase with depth using the above relationship. It is common to simplify the equation by substituting values for Ka as shown. Corus provides the above tabular relationship for Ka for different values of soil and wall friction. However, they recommend that wall friction be ignored for active pressures ( = 0). Note that some hidden factors have been allowed for in the table. EARTH PRESSURE COEFFICIENTS Passive pressure at depth h Pp γ h g tan 2 45 2 c tan 45 2 2 Pp γ h g K p 2 c K p 2 Passive coefficient, K p tan 45 2 Kp is modified for pile friction, Usually use ratio: = ⅔ Wall friction 0 1 /2 2 /3 Soil friction, 15° 20° 22° 24° 26° 28° 30° 32° 34° 36° 38° 40° 45° 1.7 2.0 2.0 2.2 2.7 3.0 2.4 3.0 3.3 2.5 3.4 3.8 2.8 3.8 4.3 3.0 4.4 5.0 3.3 5.0 5.8 3.6 5.8 6.8 3.9 6.6 7.8 4.2 7.8 9.0 4.6 9.0 - 5.8 - 2.1 2.5 2.7 Passive pressures act upon the pile if it is pushed towards the face of soil. They also increase with depth, using the similar above relationship. It is common to simplify the equation by substituting values for Kp as shown. Corus provides the above tabular relationship for Kp for different values of soil and wall friction. Here the relationship is tan = tan (2/3 ), but the simpler form shown above is sufficiently accurate for most calculations and can be read directly off the table. Note again that some hidden factors have been allowed for in the table. Note that the net result of passive minus active pressures are what provides restraint for the anchorage. Anchors and piles 483 SHEET PILE ANCHORAGE – SUMMARY Use of sheet piles for landfalls Arcelor’s Piling Handbook Pressure balance Triangular (or tapezoidal) soil pressure profile Typical soil properties Dry and submerged density, friction and cohesion Active and passive coefficients (Ka and Kp) Dependent upon friction, and cohesion, c Modification for pile friction and phreatic level Any questions? Sheet piling is commonly used as deadman anchors at landfalls. The recommended reference is the Piling handbook published by Corus Steel (formerly British Steel). Although this is originally designed for cofferdam construction and has hidden factors of safety, with care it can be used to design holdback anchors. Soil pressures either side of the piles increase with depth in a triangular profile. Typical values were given for a range of soils where no accurate survey information is provided. These include granular and cohesive soils as well as organic deposits. From the and c values, it is possible to determine the active and passive coefficients (Ka and Kp), which are used to multiply by the overburden at different depth horizons. The coefficients may be modified by the friction of the pile. Soil beneath the water level has a reduced density so is able to resist less pressure. 484 Installation calculations for subsea pipelines WORKED EXAMPLE IDEALISED SHEET PILE CROSS SECTION Wire to winch Triangular soil pressure distribution Ideally, beam located ⅓ up from the base Difficult in wet ground Trench for beam and wires disturbs ground Loss of strength Since for granular soils (such as beach sand) the soil pressure distribution is a triangle, then the ideal location for the back beam would be at the centroid. This is 1/3 up from the base, the same as for suction anchors. This would provide the maximum resistance from the soil and ensure the shortest length of sheet piling to resist the pull of the winches. However, in order to dig the trench to fit the back beam, and more importantly to position the wires to the winch, CDM and H&S regulations demand shoring to these trenches if they are more than about 1 m deep. This means that it is common to position the beam somewhat higher and use a reverse loading at the bottom of the piles to stop them rotating. Where the phreatic water table is above the excavation level, (such as near to a beach) then the soil pressure distribution is modified slightly by the reduction in resistance with depth. In short, anything which can lift the level of the beam assists construction. 485 Anchors and piles In the following more realistic example, a 2 m high berm is built at the front of the piles enabling trenches to be kept shallower than 1 m. But first, let us find the idealised holding capacity of a sheet pile with beam at the 1/3 position. IDEALISED SHEET PILE RESTANCE Wire to winch 0 6m 4 2m 6 Water level Sand, = 1700 kg/m³, = 35°, c = 0 kN/m² Pile length 6 m, beam at 4 m below ground Net pressure at 6 m Pp Pa K p K a Ob 3 Pnet 6 7.3 0.27 1700 kg m g 6 m 703 kPa Beam resistance 12 Pnet 6 6 m 2110 kN m In this example, the holding power of the beam is 2110 kN/m or 215.1 tonnef/m. The length of the beam could be determined from this for any required holding capacity. We will see later just how much reduction in holding power there would be for a real case when the excavation for beam and wires has to be limited. By cutting out slots in front of the sheet piling for the wires, the strength of the soil is reduced. Also, excavations down to 4 m must necessarily be supported in order to gain access to lay the wires and connect them to the beam. Remember that for a pair of winches and perhaps a sheave block there would be six wires connecting at the front. It is usually more cost effective to limit the depth of beam and provide a slightly longer sheet pile wall. 486 Installation calculations for subsea pipelines SHEET PILE CROSS SECTION 0 Compacted berm 2m 6m 1m 2 6 Wire to winch = 35° Water level Sand, = 1700 kg/m³, = 35°, c = 0 kN/m² 2 m high berm Beam located at 1 m below ground Pile length 6 m Phreatic level 4 m below surface Note the three red force arrows must balance to avoid rotation of the piles. Assume the above parameters. The berm has been constructed from the beach sand and has been well compacted to provide the same properties as the undisturbed natural layer beneath. We first need to assess the overburden on either side of the sheet piles. Use the top of the piles as reference point. Ignore the trench behind the piles when calculating overburden since loads are transmitted from beyond and the trench may become filled in. 487 Anchors and piles OVERBURDEN AND SOIL FORCES Depth Soil properties Overburden (kPa) from Rear Front Cu top of kg/m³ piles 0m 0 Berm 1700 Pra - Front Prp - Pfa Pfp 0x0.27 0x7.3 =0 =0 0x7.3 33.3x0.27 33.3x7.3 =0 = 9.0 = 243.4 33.3x7.3 = 33.3 35° 0 6m Rear Kp 2x1700xg 0 Beach 1700 Ka Soil pressures (kPa) 0.27 7.3 0x0.27 =0 35° 0 2m Coeffs 0.27 7.3 0x0.27 0x7.3 33.3x0.27 =0 =0 = 9.0 = 243.4 4x1700xg 6x1700xg 66.7x0.27 66.7x7.3 100x0.27 100x7.3 = 66.7 = 100.0 = 18.0 = 486.8 = 27.0 = 730.2 Depth from top of piles Net soil pressures = passive - active (kPa) Rear Pr 0 - 9.0 = -9.0 0 - 9.0 = -9.0 486.8 - 27.0 = 459.8 0m 2 m in berm 2 m in beach 6m Front Pf 0 243.4 - 0 = 243.4 243.4 - 0 = 243.4 730.2 – 18.0 = 712.2 It is necessary to evaluate pressures above and below soil interfaces. This is particularly important where cohesive and non-cohesive soils meet. It may be that pressures drop below zero with firm clays. EARTH PRESSURE BALANCE 0 m (top of pile) 2 m (beach) 3 m (beam) Net Net Active Passive Passive Active x 6 m (bottom of pile) Resistance at rear of pile (Push to left) Resistance in front of pile (Push to right) Net force balance Because the bottom section of the pile is resisting the tendency of the pile to rotate, we need to assess the resistance of the pile when moved to the right and to the left respectively. In either instance, the net resistance is the difference between the active and passive soil pressure. These increase with depth. Remember however, that the 488 Installation calculations for subsea pipelines surface is at a different level on both sides of the pile. We will ignore the trench cut to insert the beam. The left hand graph shows the active and passive pressures acting on the pile when considering the soil capacity at the rear of the pile. There is less overburden behind the pile than in front of it. The blue line gives the net soil pressure (passive in green minus active in red). The middle graph shows the equivalent resistance at the front of the pile. This net soil resistance is what will be utilised by the anchor beam and wire to the winches. Again, the blue line gives the net pressure. The right hand graph shows how the two interact. The red arrows show how the large force applied at the beam is resisted by the net force at the front of the pile with a counteracting toe force at the rear. We need to balance this force/moment system by finding the unknown level x from the bottom of the pile. EARTH PRESSURE BALANCE Solve for x using moments about the beam (beam contribution = 0) 0 m (top of pile) Pf0 Clockwise rotation is positive Area A x lever arm a + Area B x lever arm b + Area C x lever arm c + Area D x lever arm d + Area E x lever arm e =0 2m A + Therefore x = 0.745 m With x now known, we can algebraically sum the forces and find the force in the beam Beam resistance = 137.3 tonnes/m 1/ Pf2 laa 2 m (beach) B 3 m (beam) 1m lab 2/ · 3 (4 m-x ) C Pfx D 1/ 3 x E 2/ Pr6 6 m (bottom of pile) 3 x 1/ · 3 (4 m-x) lac lae lad Prx 3 ·2 m 3m x Moment and force balance Once the net forces are defined, the above procedure may be followed to balance the moments about the beam. The following slide shows how these values may be tabulated in a spreadsheet. The beam resistance now is just 137.3 tonnes/m compared with the 215.1 tonne/m of the simple case. 489 Anchors and piles TABLE Estimate a value for x = 0.745 m It is now convenient to use the following table: TriArea angle equals (kPa m) Lever arm equals Moment (m) = A x La A ½ Pf2 · 2 m 243.4 -1 m – ⅓ · 2 m -1.667 B ½ Pf2 · (4 m – x) 396.2 -⅓ · (4 m – x) 0.085 33.7 C ½ Pfx · (4 m – x) 1017.2 ⅔ · (4 m – x) 1.170 1190.3 D E -½ Prx · x -½ Pr6 · x -138.7 -171.2 3 m – 2/3 · x 3m–⅓·x 2.504 2.752 -347.2 -471.1 Sum -405.7 1346.8 0.0 Since the net moment is zero, we can read off the force, 1348.9 kPa m (137.3 tonnef/m) This table helps us to quickly establish the resistance of the beam. However, we do not know the value of x. This is done either by trial and error or using a solve facility of either Excel or MathCad. At each stage, the net soil pressures at level x can be established by proportionality, knowing the values at 2 m and 6 m below the top of the pile. Once we have established a zero net moment at bottom right, it is easy to total the force in the beam. BEAM LENGTH AND SIZE OF BERM Holding a pair of 300 tonne linear winches Factor of safety of 2 Resistance = 2 x 300 x 2 = 1200 tonnes Beam length = 1200 / 137.3 = 8.737 m For 600 mm wide piles, beam length = 9.0 m 9m 8.6 m 10. 6m Width of berm at top = 6 m/tan() = 8.6 m Add 2 m to bottom (for 45° batter) = 10.6 m Volume of berm = width x length of beam + allowance on 3 sides for batter = 217 m³ 490 Installation calculations for subsea pipelines Knowing what can be held by the soil at the level of the beam, we can work out the length of beam (and length of sheet pile wall) needed for a pair of 300 tonnef linear winches. We also need to establish how wide the soil berm needs to be in front of the piles. Using standard mensuration techniques, the volume of the berm and batter (the triangular area times the length at its centroid) can be established. 491 Anchors and piles EXERCISE EXERCISE 0 Compacted berm 2 Water level 6m 1m 2m Wire to winch = 25° 6 Silty sand, = 1800 kg/m³, ' = 800 kg/m³ = 25°, c = 15 kN/m² Phreatic level now at surface Berm, beam & pile heights remain the same Hint – net soil pressure >0 at top of pile Try x = 0.547 m For your exercise, we have slightly reduced the friction resistance in the soil but provided some cohesion, appropriate for silty sand. However, the main difference is that the phreatic level is now at the natural surface of the beach. The submerged density, rather than dry density, needs to be taken for soil beneath this level. Assume the above parameters. The berm has again been constructed from the beach sand and has been well compacted to provide the same properties as the undisturbed natural layer beneath. However, the calculation must use dry density for the berm only. Remember that because we have a c- soil, the net pressure will not be zero at the top of the pile. In fact, we may end up with negative active pressure (suction) at the very top. 492 Installation calculations for subsea pipelines ANCHORS AND PILES – SUMMARY Temporary or permanent points of fixity Anchors Ships’ anchors and handling of laybarge anchors Anchor efficiency multiplied by mass Modern vertical pull plate anchors and clump anchors Piles Can (suction) and tubular piles Design considerations for horizontal and vertical pulls Pile driving dynamic analysis Sheet pile anchorage Soil properties Landfall back-anchorage design Any questions? Both anchors and piles provide a point of fixity. This may either be permanent or temporary. Some movement may be acceptable with temporary anchoring of laybarges. We looked at different patterns of ships’ anchors and how laybarge anchors are moved. The traditional method of assessing holding power was to multiply the mass of the anchor by an efficiency factor which depended on the type of seabed soil. The exercise included the contribution from the groundleg studlink chain. More sophisticated analysis can be carried out for permanent anchors such as vertical pull plate anchors or even clump anchors. With these it is worth undertaking geotechnical survey testing at location. Suction anchors and tubular piles were described. Either can be designed to resist horizontal forces. The latter can also resist vertical pull out or downward forces. Design equations were given to estimate pile capacity. We described how to size the hammer to drive hollow cylindrical piles and some options when refusal is reached early. Some typical soil properties were given during the sheet pile anchorage design method which are also applicable to other piles (tubular or suction). The exercise showed the triangular net soil pressure diagram. 493 Anchors and piles BACKGROUND INFORMATION Survey methods GEOTECHNICAL SURVEY METHODS Permanent works Careful selection of borehole location Undisturbed sampling for clays Triaxial test to determine cohesion – c and cu Disturbed samples for sands Shear box test to determine angle of friction – Cores for rocks Triaxial shear strength test and assessment of bedding Temporary works Uses general data specified and collected for Client CPT, vibrocore or borehole sampling For permanent structures such as a platform, careful geotechnical assessment and testing is undertaken. The samples are collected from carefully selected locations and tested in the laboratory. Different tests are specified for the different types of soil and rocks. These determine the loads able to be withstood at the foundations. For temporary works, such as anchors and deadman piles, it is common to have to use general data specified and collected by the Client for other purposes. We may not have the samples taken from exactly the location that will be stressed. We do not know how carefully the samples were collected and tested in the laboratory meaning that tolerances on data may be higher than we would like. Typically, we have results from CPT or vibrocores on the seabed and boreholes at or near the shore (shallow water to 30 m or so). The samples taken but not tested have 494 Installation calculations for subsea pipelines long since been disposed of and in any case, there is no time or budget to undertake additional testing. We have to design the temporary structures with a certain amount of general appreciation both of the seabed variability and how the samples were obtained and tested. CONE PENETROMETER TESTING CPT test seabed soils Confirmation of seismic survey Correlated with grab samples Seismic surveys will first have established a descriptive nature of the surface of the seabed: whether it is sandy, silty or rocky. Grab samples may be used to extend these description. Geotechnical surveys establish numerical data for the soil along the pipeline route and extend this to the immediate subsurface layers. They typically use a cone penetrometer tester (CPT), as shown in the picture above. This is a small device dropped onto the seabed with a coiled probe which is forced into the seabed. On its way in, it measures the pressure at its tip and friction on its side. By cross-referring to calibration data, these features can be used to determine whether the soil is sand or clay and what strength or friction angle it has. The results of the CPTs are then used to confirm and strengthen the sub-bottom profile survey. Anchors and piles 495 CPT Types of soil or rock at or below seabed Thickness of layers Engineering characteristics Density Porosity Strength CPT looks at: cone resistance, sleeve friction, friction ratio and pore pressure to determine density, porosity and undrained shear strength of clay above sand at two points From the measurements of friction and tip resistance, the above mentioned data can be interpreted. The upper trace shows a layer of clay some 1.7 m (5.6 ft) thick overlying sand. The lower trace shows a similar arrangement but with a thin clay lens near the upper level of sand, just below the interface. Often these plots are originally in colour to differentiate the traces. The Client supplies the contractor with a monochrome photocopy to decipher. VIBROCORE Samples a column of soil Site or laboratory analysis 496 Installation calculations for subsea pipelines Vibrocoring is the state-of-the-art sediment sampling methodology for retrieving continuous, undisturbed cores. Vibrocorers can work in up to 5000 m of water and can retrieve core samples up to 12 m (40 ft) in length. The principle behind a vibrocore is the development of high-frequency, low amplitude vibration that is transferred from the vibrocore head, down through the attached barrel or core tube. This vibrational energy liquifies sediments, enabling the core barrel attached to the vibrocore unit to penetrate into the liquified sediments. A core-catcher is attached to the end of the barrel, which holds the sediment inside the barrel when withdrawn. A variety of vibrocore units are available. Some are small, lightweight and portable; others are large, heavy units that can only be deployed from large vessels. SHELL AND AUGER BOREHOLES Onshore or in shallow water 150 mm (6 in) diameter lining in loose ground 250 mm (10 in) diameter in shallow waters Fixed drop drives auger Requires careful interpretation of results Vane shear test and CPT Soil samples Disturbed and undisturbed Water level It is possible to use coring techniques to recover soil from boreholes on land or in shallow water. The latter can be done from a small flat-bottomed barge. A cheaper alternative shown above is the use of shell and auger equipment. Extreme care must be taken when making use of such data since a lot depends upon the skill of the operator in undertaking the work. Similar information is available as to the strength of the soil layers as that obtained using CPT or vibrocore. The borehole deepening is stopped and tools inserted. These include vane test to determine sand shear friction, a cone penetrometer to determine strength etc. Soil samples may also be obtained both disturbed (bag samples) and undisturbed (a sealed cylinder used for clays). These are then selected for testing in the laboratory. Clays are normally assessed for undrained cohesion using a triaxial testing machine. To assess the drained cohesion takes time and costs more. An important record is that of the water level. A high phreatic surface reduces the ability of soil to hold load. Anchors and piles 497 SOILS DATA ANALOGY Dart used to assess shed timber Strength of pine - needs 2 inch nail Doghouse may be softer (balsa) Buy a 3 inch nail to resist pull out forces Size hammer for 3 inch nail Assess for harder doghouse (mahogany) An analogy may be drawn by sizing a nail to hold the dog chain to the kennel. The strength of timber that holds the nail has been assessed by a third party. This was done by throwing a dart into the adjacent shed and determining its density and holding power. If done well, this could be within 10% of the true value. But we don’t know because all we have is a report. The pull of the dog can be determined exactly. If the wood is pine - as reported - then we need a 2 inch nail. We need to extrapolate to the kennel. Here the wood may be softer so we will have to buy a 3 inch nail in order to resist the pull forces. (Piles are not cheap and cannot be changed easily once on location at sea). Now we have a larger nail (with a good factor of safety against pullout), we need to consider the possibility that the doghouse wood is much harder than expected. Perhaps it is mahogany. So we need a larger hammer to ensure that we can get it to penetrate. The adage “You pay for soils investigations - whether you do them or not.” is true. Not only do we have a larger nail, but we have a bigger hammer too. In piling terms, the pile needs to be larger and the piling equipment must be much larger. 498 Installation calculations for subsea pipelines TYPICAL INFORMATION PROVIDED Not located exactly at anchorage May be along pipeline route Seismic, vibrocore or borehole Undertaken early in job for various purposes Non-homogeneous nature of soil Missing surface layer Not recovered Often a soupy nature with no strength Sensitivity analysis Typically check on ½ or 2 times soil strength This is typical soils information received by the contractor. The information is not exactly where needed. It has been gathered by the client for other purposes. Soil is not homogeneous (like other materials commonly used by mechanical engineers) and can vary from one area to another. Being more interested in the surface layer for anchorages, it is often just this layer that is missing from the geotechnical study. For this reason, it is often wise to undertake a sensitivity analysis to consider stronger or weaker soils than reported in the geotechnical report. Anchors and piles 499 TYPICAL SOIL LOG This is a typical soil log provided by a reputable company, and used by Jee to design a clump anchor which penetrated less than a metre into the seabed. Four layers of soil are indicated in the upper sections of this log. However, they were unable to recover any soil in the top 0.4 m (1.3 ft). To the left, we have depths of penetration with graphical indication of the materials are given. Location of the samples and tests is shown along with along with strata thicknesses. The description includes an indication of particle size and colour. It is normal to capitalise the principal constant of the soil for ease of understanding. Typical examples are: ■ fine to medium, grey, shelly SAND with clay lenses ■ green silty, sandy CLAY ■ light grey, decomposed GRANITE The two right hand columns graphically indicate the results of tests both in situ and laboratory results. The frequency of sampling and testing will be set by the geotechnical specification. Some samples will then be selected by the Client’s geologist to be fully tested in the laboratory. The tests represented in the left column are: Plastic limit, Water content %, Liquid limit, Carbonate content %, Relative density and Submerged unit weight kN/m³. In the rightmost column we have: Pocket penetrometer, Torvane, Miniature vane, Unconsolidated undrained triaxial, Consolidated undrained triaxial, In-situ vane, Estimated N values from CPT (with open symbols referring to remoulded tests) and Unconfined shear strength graph kPa. The right margin gives the percent passing a 75 m sieve. 500 Installation calculations for subsea pipelines Soil types and properties SOILS Granular material (sand or gravel) Classified by its density and angle of friction () Clays and silts Cohesive soil (c) – no friction Mixed soil Has both friction and cohesion (c, ) Harder material (rock and marl) Cored samples – triaxial shear strength and joints Carbonate soil Needs specialist geotechnical knowledge Sands and gravels are classified as granular material (or non-cohesive). The soil properties needed are its grain size distribution, its density, its angle of internal friction (ability to withstand loading) and its pore water pressure. Finer grains may mean that pore water rises quickly when loaded. For example, it may be difficult to cut through fine sandy soil with a plough or to easily hammer in a pile. This is because the interstitial water cannot flow through the pores quickly enough and pressure builds up, resisting the force. Sometimes a lighter hammer can insert a pile quicker than a heavier one. Clays have cohesion but no friction angle. They are made up of stacked microscopic platelets which deform slowly under pressure. For this reason, it is necessary to undertake tests which are relevant to the forces being applied. Long term drained triaxial tests are more appropriate to permanent structures. Undrained triaxial tests should be used for forces applied for a short fixed period, such as those of laybarge anchors. The results can be used to plot normal and shear stresses on Mohr’s circle for clays. Most soil is mixed - certainly on land - and has both friction and cohesion. With harder material, it may be better to assess it using cored samples. Triaxial strength testing can be carried out in the laboratory on recovered samples. Other parameters to consider are porosity, bedding plains, fissures and joints. Some rocks decompose or break down to form soils. Carbonate soils require specialist geotechnical knowledge to determine their engineering properties. These soils are found in tropical areas (between 30° latitude North and South of the equator) and cover some 35% of the ocean floor, including the Gulf of Mexico. They are composed of biogenic remains and may be well or only partly cemented. The optimum testing regime for these needs to be determined once their exact nature has been assessed. 501 Anchors and piles COHESIVE SOILS (CLAY AND SILT) Field identification Class Very soft Soft Firm Stiff Very stiff Description Exudes between fingers when squeezed in a fist Can be readily excavated with a spade and can be easily moulded with the fingers Can be excavated with a spade and can be moulded with substantial pressure in the fingers Requires a pick or pneumatic spade for its removal and cannot be moulded with the fingers Requires a pick or pneumatic spade for its removal and will be hard or brittle or very tough Very stiff clay has joints or fissure network Underwater samples Only partial recovery may be made of extremes The descriptive classification for clays comes from BS 5930 : 1999 Code of practice for site investigation. It may be that all we have for design are grab samples or description of the type of soil. Note that very stiff clays have fissures similar to a rock. Where the samples are not recovered in underwater cores, this may indicate one of the extremes (either very soft or very hard). Clays do not generally distribute pore water pressures during the time that they are stressed by applied anchors loads. 502 Installation calculations for subsea pipelines COHESIVE SOILS Shear strength Description PI % Cohesion kN/m² Undrained, cu Very soft Soft Firm Stiff Very stiff Over 80 80 50 30 15 Below 20 20 to 40 50 to 75 100 to 150 Over 150 Drained, c 0 0 0 0 0 ° 15 15 20 25 30 Typical tests Density Atterberg Limits: LL, PL and PI Typical shear strength properties for clays are given above. They follow the same classification as the previous table. Where testing has been carried out, then these values should take precedence. An approximate relationship that may be used to interpret the preliminary undrained cohesive strength from the cone resistance (qs) of a CPT is cu = qs / Nk, where Nk is 17 to 18 for normally consolidated clays or 20 for overconsolidated clays (such as London clay). Note that different countries have different classifications for soils. For example, ASTM D-2488 uses values for cohesion that are some 2/3 that of the UK values given in the above. 503 Anchors and piles COHESIONLESS SOIL (SAND AND GRAVEL) Relationship between in situ tests and RD Relative density SPT ‘N’ value CPT qs MN/m² Very loose Loose Medium dense Dense Very dense 0 to 4 4 to 10 10 to 30 30 to 50 Over 50 2.5 2.5 to 7.5 7.5 to 15.0 15.0 to 25.0 Over 25.0 Specific gravity Shear value Grain size distribution Compaction Clay 10 Clayey silt Sandy silt & silty clay & silt Silty sand Sand 8 (qc/pa)/N60 Typical laboratory tests ° 25 28 30 36 41 6 4 2 0 0.001 0.01 0.1 Mean particle size D50 (mm) 1 Sand and gravels are commonly assessed for density using the standard penetration test (SPT) and cone penetrometer test (CPT). These (SPT and CPT) are essentially standard empirical means to assess the properties of soil. Again, data obtained using laboratory tests should take precedence over field descriptions. A number of studies have been presented over the years relating SPT N-value to CPT. The graph shows one such relationship. Where: ■ qc = cone resistance in kPa ■ pa= atmospheric pressure (100 kPa) ■ N60 = SPT N-value (energy ratio of about 60%) ■ D50 = mean particle size in mm For non-cohesive soils, this gives the approximation: shear strength = 6 x N. Abandonment and recovery Abandonment and recovery 507 EXPECTATION EXPECTATION When is A&R used Case study for reel lay A&R Determined maximum seastate for recovery Software used for analysis Examine pipe stresses during recovery Pull head at a range of heights above seabed In this module, you will be able to identify why we need to determine pipeline and wire stresses for abandonment and recovery operations and how the first may need to take place during worsening sea states. We will provide typical analysis details of a reel lay barge lifting a pipe from 395 m (1300 ft) of water. The stresses were determined at critical steps such as when the pipe is lifted off the seabed (spanning as a beam) and also when the pulling head is being recovered at the water surface onto the barge. From these studies, the maximum operating sea state was determined. A range of analytical methods are described from simple hand calculations through to full dynamic analysis which takes account of the vessel and pipe-span motion in waves and currents. When the head is close to the seabed, the wire catenary becomes significant. As the head lifts more from the seabed, the weight of the wire becomes insignificant when compared with that of the pipeline, and the wire is assumed to be a straight line. In the exercise, you will compare the pipeline stresses using a simple procedure with the head at various heights above the seabed. 508 Installation calculations for subsea pipelines A&R OVERVIEW ABANDONMENT AND RECOVERY Abandonment – laying pipeline end down on seabed Recovery – picking pipeline end up from seabed Variations Initiation from pre-installed string (eg directionally drilled landfall) Initiation from landfall pulled off-shore Termination – end of pipelay The pipe is ‘abandoned’ when pipe laying operations stop and the end of the pipeline is set down on the seabed. The recovery operation is the reverse of this operation. Some variations on this may involve initiation or startup when an existing pipeline is recovered from shore. This may have been preinstalled either by directional drilling or using a pull barge and pipe-pull offshore. At the end of pipelay operations, the pipe is terminated often with a pigging skid or valve assembly attached. In all cases, similar analysis is needed. The only difference being the water depths and seastate likely to be encountered. 509 Abandonment and recovery BASIC PROCESS Terminate or cut end of pipeline Attach a cap to the end of the pipeline Shackle on a large diameter wire Lowered by winch on the barge Pipeline laid on seabed with recovery buoy Laydown head Wire Crimped ferrule Thimble Weld Pipeline D shackle The diagram shows the typical arrangement for a small diameter line in shallow water. The head may be either welded or bolted to the pipeline. The former demands a full weld time to complete. Where the loads are higher, a wire socket termination and purpose-made two-pin link would replace the thimble and D shackle. Once the head is fixed to the end of the pipeline, the wire from an A&R winch is connected to maintain tension in the pipe wall through the touchdown sag bend. The pipeline tensioners are released and the pipeline may be lowered to the seabed as the vessel moves forward. This process is reversed to recover the pipeline. 510 Installation calculations for subsea pipelines A&R OPERATION Barge direction Abandonment Recovery head Recovery Recovery buoy A&R winch Lowering cable Catenary profile Beam profile The figure illustrates the process of abandonment of the pipeline. The pipeline is being laid by the J-lay technique. The recovery head is lowered towards the seabed on a cable . The lowering is controlled by the A&R winch to maintain the correct tension in the pipeline to prevent sagbend buckling. Lowering involves moving the barge forwards whilst paying-out the winch cable from the rear. Calculations of the required tensions and resulting pipe stresses for abandonment should consider that as the length of winch cable paid-out increases it to will adopt a catenary profile. The bending profile of the pipe is initially assumed to be a catenary curve during pipe lay. At the start of abandonment the pipe maintains the catenary profile. However, as the pipe end approaches the seabed, the bending profile tends towards that of a deflected beam. With the smaller pipe deflections, the stiffness of the pipe becomes increasingly significant. The entire process is reversed for the later recovery of the pipeline. Abandonment and recovery 511 NEED FOR A&R OPERATION Emergency Breakdown on laybarge Problems with tensioners or dynamic positioning system Deteriorating weather Prevention of fatigue at sagbend touchdown Recovery from pipeline buckle Planned Normal pipelay during startup and termination Limit on reel barge length At end of each spool of pipeline Contingency A&R plans are needed for the exceptional emergency condition should the barge suffer a breakdown. In deteriorating sea conditions, when it is unsuitable to continue the laying process, the pipeline can be lowered to the seabed. This prevents fatigue and overstress due to the dynamic response of the pipeline span and the vessel in severe weather conditions. We may be unfortunate enough to buckle the pipe during laying. Then a contingency measure is needed to recover the pipe from the seabed and continue laying. All pipelines need to carry out the operation at the start and end of lay operations. There is a limit on the length of spooled pipe that can be carried on a reel barge. Therefore, at the end of each spool the pipe is laid on the seabed and recovered to connect the next section. At landfalls, we may need to recover the end of a pipeline pre-installed using the pulloffshore method, or lift the tail of a directionally drilled line. 512 Installation calculations for subsea pipelines SEA STATES AT DESIGN STAGE Beaufort wind speeds 13 levels related to speed in knots 0 is calm – 12 is hurricane Extended to sea states – but no wave heights Weather forecasting 3 or 5 days ahead Provides detailed wind speeds and wave heights Often errs on side of safety Improving weather Worsening conditions At all times ensure safe operation The classic description of wind speeds use the Beaufort scale. This translates sailors’ terms such as calm, light airs, light to strong breeze, moderate to whole gale up to hurricane force winds into a 13 level scale. From this, each description can be given a numerical range of wind speeds in knots. Although we are interested in wind speed - perhaps for safe operation at the top of a J-lay tower, vessels are designed to operate in certain wave heights and periods. It is now possible to obtain good 3 or 5 day ahead weather forecasting. We need to check for wind speeds and significant wave heights. A maximum operating level will have been specified prior to work commencing. If we are likely to reach the operating levels, we also need to look ahead to see if the conditions are improving or worsening. If work needs to stop but the weather is improving, then we may choose to ride the conditions out on station. However, if there is a likelihood of the weather worsening, it may be advisable to cut and run. Safety is the overriding parameter. Abandonment and recovery 513 WEATHER-RELATED ABANDONMENT Wind speeds Measured on board Waves – assess Hs and Hmax Modern lay vessel Stops laying at Hs= 5 m (16.4 ft) Abandons at Hs= 8.5 m (27.9 ft) Older barge half of these Limit of anchor-handling tugs Fatigue At touchdown and overbend Release short lengths Head released over stinger Normally, vessels receive regular weather forecasts giving wind speeds and wave heights. Currents can be predicted as they vary between spring and neap tides. Although used in the risk assessment, they are not part of the lay parameter specifications. Wind speeds can be measured using an anemometer. Whilst information will be available from the 3 or 5 day forecast, it is often the case that the report errs on the side of safety. The argument that then ensues in the bridge is, ‘Just what height are those waves that we are facing?’ Significant wave heights are those that an experienced sailor would assess them as. Consistency of assessment over the centuries has been found. However, DNV RP2 Sea Transportation gives a relationship between the visual and significant wave heights: Hs = 1.68 (Hv)0.75, where both heights are in metres. This reduces larger visual heights and increases smaller heights (<10 m). Hs is numerically equal to the mean of the highest one third of the waves and is sometimes written as H1/3. The maximum wave height is normally set at 1.8 or 1.85 times as high as Hs. As the weather worsens, pipelaying operations initially stop and the vessel holds station. Care needs to be taken to avoid fatigue in the pipe at touchdown or, for an S-lay barge, over the stinger. Sometimes, small lengths of pipe can be released from time to time to ensure that the fatigue is spread over different sections. If the weather forecast remains poor and worsening, at some stage the decision will be taken to abandon the line. This will have previously been set down in the lay procedures. The actual weather conditions are dependent upon the vessel and its capacity to operate in different sea conditions or wind speeds (especially for a lay tower). The first generation vessels could only withstand a few metres wave heights. However, modern vessels are designed for rougher seas. As an example, the reel lay vessel Deep Blue only abandons pipe when the significant wave height is approaching 8.5 m. However, older vessels – particularly anchored 514 Installation calculations for subsea pipelines laybarges – may need to stop operating at much lower wave heights. It may not be the vessel itself that is the limit, however, the anchor-handling tugs may be unable to move them to their new location in significant wave heights as low as 2 m (7 ft). The head is attached at a workstation and lowered into the water. The photograph shows it going down the firing line over an S-laybarge stinger. IT’S TOO LATE NOW! Courtesy of Barth Crane Inspections (http://www.craneoperator.com) Decisions to move off station should be taken early enough to safely disconnect and avoid emergency action. Perhaps the weather forecast was insufficient in this instance. Abandonment and recovery 515 A&R OVERVIEW – SUMMARY Meaning of A&R Needed at startup and termination of lay Needed in bad weather Specified levels for stopping lay operations Specified levels for cutting pipe and run for shelter Weather forecasts of wind speed and wave heights Any questions? We have looked at all the different operations which come under the category of A&R. All pipelines will require A&R at startup and termination of lay with long reeled pipelines needing intermediate operations. However, contingency plans are needed prior to laying operations for bad weather conditions. 516 Installation calculations for subsea pipelines CASE STUDY LAYDOWN HEAD Welded or bolted to pipe end on barge Seals pipeline end to prevent flooding Provides lug or hook for attachment of A&R cable Morgrip laydown head The laydown head is attached to the pipeline end prior to abandonment to seal the pipeline end to prevent flooding of the line. The head also provides a lug or hook to allow for attachment of the abandonment and recovery cable. Abandonment and recovery 517 BIG INCH LAYDOWN HEAD Bolted on to pipe end Buoy and pennant not needed ROV hooks on to recovery wire This connector makes use of a bolted connection between the pipeline and the laydown head. The strop and lug at the top (picture on the right) is used only for handling aboard the vessel. Note the use of a hook rather than a lug. This requires tension to be applied at all times but permits the use of an ROV to take down a wire and make or release the unit. It may not always be desirable to leave a wire and buoy attached – for example, in a shipping lane. ADDITIONAL EQUIPMENT Laydown heads can also be fitted with: Launcher for pigs Separates water from air during purge Handling lugs Flood and vent valves Protection framework Swivel Location devices 518 Installation calculations for subsea pipelines As mentioned, it is sometimes necessary to design laydown heads which incorporate other items such as listed above. If the line is to be hydrotested subsea using the head and a ROV operated flood skid, then it is useful to have a number of pigs set into a length of the pipe. Behind each is a tee and valve which permits the pigs to be launched individually. The slide photograph above also shows protection framework to prevent accidental damage during launch of the initiation head from a J-laybarge. It is important that the torque in the wire is not transmitted into the pipeline. This would mean that the pipe would tend to roll to one side during lay. This may result in the valves being unreachable by the ROV. A swivel between the wire and the head provides a means of removing any tendency of the pipe to rotate. If the laydown head will have the wire detached then a trisponder location device may be fitted. WET BUCKLE RECOVERY Cut damaged pipe subsea Install recovery head by ROV or diver Attach A&R cable In the event of a pipe buckle, it may be necessary to cut the pipeline and install a head subsea. The following slides illustrate some of the equipment necessary for these operations. 519 Abandonment and recovery BUCKLE REPAIR CUTTER Sonsub-Saipem Bluestream project Wet buckle repair system on ROV Diamond cutter unit This cutter can be used to repair a wet buckle at depths of up to 2200 m. It was developed for use on the Bluestream Project in the Black Sea, but fortunately was not needed. The intention was to cut the pipe below the buckle and insert the recovery head. The evacuated pipe would then be brought back to the surface and J-lay continued. BLUESTREAM HEAD Bluestream depth 2150 m (7054 ft) of water ROV-operated equipment Use of cutter to produce ‘square end’ Pressure to force pipe onto anvil Pipeline plastically deformed to form seal Single use unit Pipeline Anvil Deformed pipe wall Pressurised volume The second tool designed to be used by the ROV sealed the end of the pipe enabling water to be removed and allowing the pipe to be lifted. 520 Installation calculations for subsea pipelines A thin walled can was inserted inside the pipe and expanded. This plastically deformed the end of the pipe onto an anvil. The latter deformed elastically. When the pressure was released, the anvil recovered (elastically) and held the permanently deformed pipe wall using friction. Valves (not shown) permitted the pipe to be purged. This reduced the weight enabling the pipeline to be recovered onto the laybarge, where the unit could be cut off. Fortunately, it was a contingency item only. Although proven technology, it was not used in anger. FIRST SUBSEA (BSW) BALL GRAB RECOVERY TOOL Ball and taper mechanical connection This proprietary end can be used to recover pipes which have suffered damage (such as a buckle) and have been cut off by an ROV-operated saw. First Subsea was formerly known as BSW. The tool is still referred to as the Ball Grab. The small balls are retracted into tapered slots and the unit fed into the cut end of pipe. Once the unit is fitted, the balls are released and they exert a force which grips the inside of the pipe. Each ball individually adjusts and redistributes the load in response to vibration or cyclic forces. The benefits are that the unit is strong: once connected, it cannot release until the load is removed. There is no possibility of overloading since the benign compressive stress in the pipe wall remains well within the code allowances for normal handling. The unit is operated by pure mechanical lift: there are no hydraulics involved. It is self acting and is vibration and fatigue-resistant. Extensive use has been made of these units in the Gulf of Mexico for fixing anchor wires (and Dyneema ropes) for barges and semi-permanent structures. The anchor remains on the seabed and the ball grab can be fitted into a swivelling cradle using an ROV. Abandonment and recovery 521 A&R WINCH Alongside firing line on S-lay vessel Sheave arrangement to bring A&R wire down the firing line Accommodates large diameter wire Reeving device A typical A&R winch mounted adjacent to the firing line aboard an S-lay vessel. Note the reeving device, which ensures that the wraps of wire are evenly positioned. The wire passes through the arm suspended from the helically grooved steel bar. As the drum turns, the arm is moved from side to side laying an even wrap at each layer. BUOYS Size of buoy determined by weight of wire Wire length typically three times water depth Weight plus reserve buoyancy Needs to withstand hydrostatic pressure if likely to submerge Ensure that current forces on pipe do not move pipeline May need to attach a pennant Provides groundleg weight Can be smaller diameter If a buoy is to be used, then the submerged weight of the whole length of the cable is used to calculate its size. This ensures that it will not sink below the surface and be crushed. 522 Installation calculations for subsea pipelines Typically, the length of wire to a buoy is set at three times the water depth. This permits it to move and take up any current or wind forces acting on the system. The buoy is designed to carry 1½ the weight of the wire. Some codes quote the reciprocal of this, giving a value of carrying capacity of 65%, and a reserve buoyancy of 35%. The self weight of the buoy is included in the calculations. Alternatively, the buoy may be designed to submerge. These are more costly but can be used with shorter length of wire. They need to be at the surface in calm conditions for recovery. A check also needs to be made - especially with short wires and submerged buoys - that the force is not sufficient to move the pipeline end itself. If this is the case, then a length of additional pennant wire may be added to the pull wire in order to provide some additional groundleg weight. The pennant wire may be smaller diameter to lessen current forces. Buoys tend not to be used where the laydown is in busy shipping channels. A ROVoperable wire/head connection is required in such cases. 523 Abandonment and recovery CASE STUDY FIELD LAYOUT Grid North FPSO Anchors FPSO FPSO Anchors FPSO Anchors 4 32 m m 2 (1 ) in W I FPSO Anchors The FPSO anchors are arranged in four sets of four. Their exclusion area on the seabed is shown. Between them the Water Injection line approaches from the SW quadrant as close as possible to the FPSO. The area is congested with existing production lines, gas lift lines, water injection lines and control umbilical cables. From the laydown location of the FTA, a flexible riser connects to the FPSO vessel. 524 Installation calculations for subsea pipelines ANALYSIS Original study used Offpipe static analysis Final installation study used Orcaflex 3D, non-linear, finite-element software 15 minute (900 s) random sea simulation Full dynamic analysis Direction of sea considered Dominant wave direction is SW Vessel will face NE during installation of FPSO FTA Worst conditions under stern seas – Critical end for analysis is at FPSO TJA was involved in this particular recovery study located in the West of Shetland fields in some 395 m (1300 ft) of water. Two finite element packages were used: Offpipe and Orcaflex. The first was limited to static analysis to determine which load cases were critical. The second was used to undertake full dynamic analysis of the critical cases using random sea simulation. At this stage, it was necessary to examine which head (the laydown head at the FPSO or the initiation head at the field) caused most problems for the vessel rolling. The dominant wave direction is from the South West. The bow of the reel lay vessel had to face NE during installation of the flowline termination assembly (FTA) at the floating production storage and offloading (FPSO) vessel. At the field it would be facing the other direction. However, the worst conditions for laybarge stability is under stern seas. Therefore, we need to complete the analysis for the FPSO end. Abandonment and recovery 525 DESIGN INPUT DATA 323.9 mm (12 in) water injection line Water depth 395 m (1300 ft) at FPSO down to 450 m (1476 ft) at field Seven heights above seabed examined Two assessed in full using software package Just below surface and 6 m (19.7 ft) of wire – highest tension in FTA so highest stress in adjacent pipe wall Near seabed with maximum stress in sagbend Wave height, Hs = 3 m (9.8 ft) Range of four wave periods examined Tz = 5.7 s, 6.4 s, 7.1 s and 8.0 s Tp = 8.0 s, 9.0 s, 10.0 s and 11.3 s Four lines were to be laid by the same method: a 219.1 mm (8 in) gas line, a 273.1 mm (10 in) WI, a 273.1 mm (10 in) production and the 323.9 mm (12 in) WI line. The latter produced the critical stresses being the largest and heaviest and creating the longest spans. Water depths varied up to 395 m (1300 ft). The initial analysis assessed seven heights for the complete recovery operation: the head was at a water level +12 m (39 ft) for step 1 and at -84 m (276 ft), -176 m (577 ft), -266 m (873 ft), -322 m (1056 ft), -380 m (1247 ft), -395 m (1296 ft), for the subsequent steps up to 7 (the last is with the head on the bottom). The analysis was carried out for significant wave height of 3 m (9.8 ft). Since we do not know what the period might be (random seas), a check is made for four zero crossing periods (Tz) and their associated Tp. 526 Installation calculations for subsea pipelines LAYDOWN STEPS The seven steps of laydown are shown. Note the proximity of the laybarge to the FPSO during laybarge operations. With the laydown head near the surface, the wire is bar tight. Only with the pipeline head near the seabed is there a significant catenary in the wire. At each step, the wire angle near the barge is calculated. For much of the operation the angle remains fairly constant. There is a slight decrease as the pipeline approaches touch-down on the right. But then when the catenary in the wire becomes significant and the pipeline approximates a beam, the wire angle increases again immediately before touchdown. Abandonment and recovery 527 PIPESTRESS OUTPUT DATA Output data from static and dynamic analysis similar Near surface Offpipe was just 45 MPa (6.5 ksi) more than Orcaflex Due to the improved modelling of the FTA in latter Near seabed Almost identical results at sagbend with FTA Peak stress is 89% of SMYS DNV 1981 permits 72% using static analysis Results were factored for Hs = 2.5 m (8.2 ft) Operations can be undertaken in good weather The results from the two analyses were similar. Stresses for the static were slightly higher than for the dynamic for the step with the flowline termination assembly (FTA) head near the surface. This was due to the better detailed modelling of the head attachment points. With the head near the seabed, the results at the sagbend were almost identical. The peak equivalent stress for the full dynamic analysis is 89% of SMYS. However, DNV 1981 permits 72% using static analysis. For this reason, it was decided to factor the loads down for a smaller wave height. Since the timing for the operation could be selected and would only take 12 hours, recovery work could be carried out in good weather. 528 Installation calculations for subsea pipelines OPERATIONS OUTPUT DATA Ramp for reel pipe lay operations was 70° Wire departure angles varied from 65° at step 5 to 77° at step 7 Ramp angle had to be adjusted to suit Wire tensions varied from 420 kN (94 kip) at top (step 1) to 96 kN (22 kip) at bottom (step 7) Wire 800 m (2625 ft) long maximum: scope=2 Vessel moves axially 426 m (1398 ft) in steps At the vessel, the operational data is shown above. The ramp angles varied slightly throughout the operation. As expected, loads increased as more of the pipeline was raised from the seabed. The maximum length of wire on the drum was 800 m (2625 ft). This is approximately twice the water depth. That is, it has a scope of 2. (You are about to size the wire for a FoS of 5.) The vessel moves in discreet stages as the operation progresses. approximately the same as the water depth. The distance is Abandonment and recovery 529 CASE STUDY – SUMMARY Controlled recovery of pipe end to fit FTA Analysis carried out in 7 steps Initial study Offpipe static analysis Simplistic static design of forces 72% SMYS for pipe Full random sea analysis using Orcaflex Similar results for the calm sea conditions selected Determined maximum operational sea state Sized wire to be used FoS = 5 Any questions? The case study was to recover a pipeline end in order to fit the FTA. The pipe had been prelaid close to a FPSO. Seven steps or heights of the head were examined and two were deemed to be critical. One was with the head near the seabed and the other with it at the barge above water level. A simple static analysis was first carried out using Offpipe. This permitted a stress of 72% of yield in the pipeline. Then a full random sea state analysis was undertaken using Orcaflex. Both gave similar results because the sea was relatively calm with a maximum 3 m (9.7 ft) wave height. From this it was possible to use the permitted stress level in the pipe wall to scale down to a maximum wave height for operations. Wire sizing (diameter and length) were determined. 530 Installation calculations for subsea pipelines A&R ANALYSIS A&R ANALYSIS Determines pipe stresses as laid down or picked up Calculates loads on cable First pass analysis Static Catenary calculation Detailed analysis Dynamic Specialist software The objectives of the A&R analysis are: ■ determine the stresses in the pipe as it is laid-down or picked-up ■ determine the tensions necessary to maintain acceptable stresses in the pipe ■ determine the strength/size of cable required Simple first pass calculations can be performed using catenary calculations for the pipe and assuming a straight cable (ie ignoring the cable catenary as the weight is low compared to the pipe and the tension is high) A detailed analysis, incorporating dynamic effects of pipe and vessel, environmental loadings and non-linear reactions and loads is performed using specialist software. Abandonment and recovery 531 SOFTWARE Offpipe OrcaFlex Anflex MCS Pipelay and Flexcom FE packages Aqwa – good for ships’ motion Many different packages can be used to analyse the dynamic forces at A&R operations. Different companies also have their own in-house or bespoke software suited to the particular vessel being used and the particular RAOs in the waters being operated. The above is just a sample of what is available. SIMPLE J-LAY AND REEL LAY ANALYSIS Assumptions Ignore current forces on wire during operations Wire is almost straight - negligible catenary Compared to pipeline insignificant weight and current loads Angle of wire at barge varies only a little Different approach when head near seabed Pipe bending analysis Check for beam bending near seabed (small ) Check for catenary bending near surface (large ) Check stresses Immediately behind pull head In sagbend at touchdown point For Reel-lay and J-lay the wire is almost straight and at a steep angle (typically 70°) from the seabed to the vessel. We can assume that the wire catenary is almost negligible when the pipeline adopts a catenary shape. This is due to the effect of the relatively much 532 Installation calculations for subsea pipelines stiffer and heavier pipeline. To simplify the analysis, it is often the practice to ignore the effects of current on the wire and concentrate on the forces acting on the suspended length of pipeline. The only exception to this is when the pullhead is close to the seabed and the pipeline can be modelled as a beam. In this case, software must take into account the wire catenary. The assumption is made that at small deflections (height of head above the seabed), the pipeline acts as a beam. That is, it has some stiffness. When the deflection gets larger (as the head rises through the water column), the analysis changes to catenary bending. We need to check for stresses throughout the length of the pipeline. However, normally two areas need to be examined: these are in the pipe just behind the flange of the recovery head and at touchdown in the sagbend area. ADAPTION OF STANDARD CATENARY xab y Wire catenary equations b H w y V V cosh 1 w ab cosh sinh 1 a sinh 1 a ww H H H ww xab 1 Va Vb H sinh sinh H H H sab tanαb tanαa ww xab sab b Vb H yab A&R wire Heavier pipe origin H a a Va x When combining equations for the catenary in the pipeline and the lighter wire, it is necessary to account for the angle at the pulling head. This is particularly so for the condition when the head is close to the seabed and there is only a short length of pipeline suspended. The vertical lift on the end of the pipeline must equal the downwards force acting on the wire. The horizontal forces must also be equal. We need to use some of the equations appended to the catenary module to determine the geometry of the A&R wire, which does not pass through the origin. The necessary ones are shown above. In the above equations: ■ H = the horizontal force component, N (lbf) ■ Va = the vertical component of tension at the pullhead, point a, N (lbf) ■ Vb = the vertical component of tension at the barge, point b, N (lbf) ■ ww = the submerged weight per unit length of wire, N/m (lbf/ft) ■ yab = the vertical depth from barge to the pullhead, m (ft) Abandonment and recovery ■ ■ ■ 533 a = the angle at the pullhead, point a b = the angle of wire departure at the barge, point b sab = the length of wire span from barge to pullhead, m (ft) TYPICAL S-LAY A&R ANALYSIS S-lay requires higher tension Longer and stronger wire Now not vertical Subjected to current forces Barge may move laterally and axially Lateral current forces on pipe and wire Use of smaller pennant There are some changes with S-lay A&R. Because the pipe is under higher residual tension, the span is longer. The wire needs to be longer to prevent sagbend buckling. The diameter of the wire is larger to provide the additional tension. We now consider the effect of lateral currents on both the pipe and wire. This may mean that the barge will need to move laterally as well as axially during laydown and recovery to prevent sideways movement of the pipeline on the seabed. Again, with a larger diameter and heavier recovery wire, it is normal practice to connect a smaller pennant wire onto the end of the main wire up to the buoy. 534 Installation calculations for subsea pipelines A&R ANALYSIS – SUMMARY Simple static design and dynamic analysis Check pipe stress at pull head and touchdown Software packages Sized wire to be used - diameter and length Different approach needed with S-lay Any questions? We have looked at the two approaches needed for A&R analysis. It is common to undertake a simple static design first. Once this is complete, a more computer hungry full dynamic analysis is undertaken to fully assess the seastate in which A&R can be undertaken. Critical pipestresses are checked at the pull head and touchdown locations. A number of computer packages have been identified. A further requirement of A&R is to ensure that the wire is strong enough to withstand the operation and that sufficient length of that diameter can be held on the recovery winch on the vessel. The case study was for a relatively steep recovery angle associated with reel lay. J-lay has a similar configuration. We identified the differences needed with S-lay where longer wire is needed to ensure there is sufficient pipeline tension at touchdown to prevent overstressing. Abandonment and recovery 535 EXERCISE WIRE DIAMETER SIZING AND FINAL BARGE POSITION Maximum wire tension with head at surface Ignore Vessel movements Current and wave effects Assess using the catenary equations Adapt Catenary module exercise Limit pipe stress to 72% of SMYS Adjust tension at pulling head What diameter wire do you want? Use FoS = 5 for wire Assess barge position relative to target box For this simplified analysis, we can use the static catenary equations you used in the catenary module exercise. First, assume that the head is at the water level (zero length of wire) and there are no effects from vessel movement, current or wave. Limit the equivalent stress in the pipe steel to 72% SMYS. Adjust the pipe tension at the pulling head until stress is just below permitted. Select a wire diameter from the table given. Calculate where the barge needs to be in plan relative to the final target location on the seabed for the end of the pipeline pull head. 536 Installation calculations for subsea pipelines EXERCISE – WIRE PROPERTIES Diameter mm inch 40 137/64 Unit mass kg/m lb/ft 6.37 4.28 MBL tonnef US short ton 103 113.5 MBF kN kip 1008 227.1 44 149/64 7.71 5.18 124 136.7 1220 273.4 48 57 1 /64 9.17 6.16 148 163.1 1452 326.3 52 3 2 /64 10.76 7.23 174 191.8 1704 383.6 54 21/8 12.48 8.39 187 206.1 1837 412.3 56 213/64 14.33 9.63 201 221.6 1976 443.1 11.16 231 254.6 2268 509.3 60 23 2 /64 16.61 Steel core Blue strand wire rope 1770 tensile grade 6 x 49 construction from Bridon Rope catalogue. BARGE MOVEMENT DURING LOWERING Calculate unit weight of wire Now assume the head is a percentage up from the seabed - say 5%, 25%, 50% or 75% Adjust the tension at the pull head until the pipe stress is 72% of SMYS Use the angle of wire at pullhead to calculate where the barge must be, relative to target Estimate the tension needed in wire at barge In the worked example, we have actually calculated the catenary in the wire. In general there is not much curvature, so a straight wire set at the pipe angle for the pull head is a good approximation. Note that only for the height of 5% of water depth is there a significant difference between the true wire catenary length and the bar tight one. Abandonment and recovery 537 As guesses for the pull head tensions, use the following: ■ 5% -74.4 kN (16.7 kip) ■ 25% - 128.6 kN (28.9 kip) ■ 50% - 196.3 kN (44.1 kip) ■ 75% - 264.1 kN (59.4 kip) ■ 100% - 331.9 kN (74.6 kip) EXERCISE – INPUT DATA Pipe diameter = 273.1 mm (10.75 in) Wall thickness = 20.6 mm (0.82 in) Young’s modulus, E = 207 GPa (30 000 ksi) SMYS = 414 MPa (60 ksi) Allowable stress = 72% SMYS Water depth = 400 m (1313.3 ft) Initial wire length = 0 m (0 ft) Density of steel = 7850 kg/m² (490 kg/ft³) Density of seawater = 1025 kg/m² (64 kg/ft³) The original was in slightly less water depth and used marginally less lay tension (318 kN). EXERCISE – DERIVED DATA AND METHOD Area of pipe steel = 16.341 10³ mm² (25.3 in³) Subm. pipe weight = 669.2 N/m (45.9 lbf/ft) Selected wire diameter = 52 mm (2.0 in) MBL of wire = 174 tonnef (383.6 kip) Method Follow pipe stress method from catenary exercise Find vertical force at pull head using equations given earlier Calculate height and distance from pull head to barge Find angle at pull head and sea surface Determine tension in wire at barge Find length of wire 538 Installation calculations for subsea pipelines Note that only for the final height (5% of water depth) is there a significant difference between the true wire catenary length and the bar tight one. For information, the bar tight wire would be 664 m (2179 ft). ABANDONMENT AND RECOVERY – SUMMARY A&R overview Case study Analysis Any questions? We have compared why we need A&R analysis for all pipeline laying operations. A reel-laybarge case study showed the steps and two analysis packages used. Other techniques and packages have been identified along with the different approach needed with S-lay. Profiles Profiles 541 MIKE HAWKINS TECHNICAL DIRECTOR BTech (Hons) CEng MIMechE Education: Academic Qualifications: Professional Qualifications: Loughborough University Degree in Mechanical Engineering Member of the Institution of Mechanical Engineers Current Position at Jee Limited Mike is Jee’s Technical Director and a Principal Engineer with 25 years’ experience of pipeline engineering project work and development. Mike has been instrumental in writing the pipeline engineering courses at Jee and has travelled worldwide delivering them. Specific Expertise and Experience at Jee Limited In his time with Jee, Mike has been responsible for many studies and activities. Particular fields of expertise include: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Conceptual and detailed design of pipeline and rigid riser systems 3rd party verification of design Computer analysis of fluid, mechanical, structural, soils and thermal problems Trawl gear interaction with pipelines and the prediction of fishing gear loads Upheaval and lateral buckling Modelling of impact and denting Heat transfer and modelling of transient behaviours Creep of foam insulation systems Analysis of pipeline freespans, vortex induced vibrations and fatigue assessment Risk and reliability analysis ABAQUS finite element analysis Integrity management of pipelines 542 Installation calculations for subsea pipelines MARTIN EAST HEAD OF ANALYSIS BSc (Hons) MSc CEng MIMechE Education: Academic Qualifications: Professional Qualifications: University of Sheffield Loughborough University of Technology Honours degree in Mathematics MSc in Computer Integrated Engineering Chartered Engineer Member of the Institution of Mechanical Engineers NAFEMS Registered Analyst (Advanced) Current Position at Jee Limited Martin is Jee’s Head of Analysis and a Principal Engineer with nearly 20 years’ experience of pipeline engineering projects. Specific Expertise and Experience at Jee Limited Martin is responsible for supervising and carrying out most of the finite element analysis (FEA) and computational fluid dynamics (CFD) work at Jee limited. Specific experience includes: ■ ■ ■ ■ ■ ■ ■ ■ Limit-state design of HP/HT pipelines Lateral buckling assessments of surface-laid pipelines Upheaval buckling analysis of trenched and buried pipelines Thermal analysis of surface-laid and trenched umbilicals and pipelines Stability assessments of pipelines Pipeline spanning assessments Fracture assessment Overtrawling assessments and trawl gear interaction with subsea structures Martin was recently involved in the following projects and brings his experience straight to the classroom: ■ ■ ■ ■ Development of guidance to reduce the effects of flow-induced pulsations in gas risers Scale model tests of overtrawlable protection structures for use offshore West Africa Analysis of subsea Y piece connectors Assessment of trenching and backfill options for a major contractor in UKCS Profiles 543 ALAN KNOWLES CIVIL AND CONSTRUCTION SPECIALIST Eur Ing BSc(Hons) CEng MICE Education: Academic Qualifications: Professional Qualifications: Liverpool Polytechnic College Honours Degree in Civil Engineering Chartered Engineer Member of the Institution of Civil Engineers FEANI European Engineer Current Position at Jee Limited Alan is Jee’s Civil and Construction Specialist – a civil engineer with over 35 years’ experience in the design of oil, gas and water pipelines. He has also worked in the nuclear industry, substantiating structures for the safety issues associated with seismic events. Alan joined Jee Limited as a Senior Engineer in 2002. Specific Expertise and Experience at Jee Limited Alan has particular expertise in the following: ■ ■ ■ ■ ■ ■ ■ ■ ■ Installation methods for subsea lines for hydrocarbon developments Design and specification of onshore and offshore pipelines Soil assessment for pipeline trenching, burial and pile design Subsea pipe bundles Flowlines, landfalls, directional drilling, river crossings, marine structures and sea defences Both conventional and single-point moorings for tankers Finite element analysis of subsea equipment and finite difference analysis in soils Calculations for coated pipelines with regard to both stability and thermal insulation Design and construction of outfalls including investigation of primary and secondary effluent dispersion patterns 544 Installation calculations for subsea pipelines PHIL MEDLICOTT PRINCIPAL ENGINEER BSc PhD CEng MIMechE Education: Academic Qualifications: Professional Qualifications: Nottingham University BSc in Mechanical Engineering PhD in Acoustics – Mechanical Engineering Chartered Engineer Member of the Institution of Mechanical Engineers Current Position at Jee Limited Phil is one of Jee’s Principal Engineers with over 25 years’ experience in the oil industry and 10 years’ specialising in pipeline engineering studies. He joined Jee in July 2000. Specific Expertise and Experience at Jee Limited Phil has particular expertise in the following: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The use of polymer/composite materials in offshore applications FEED studies including use of Pipesim Pipeline piggability studies Pipeline stability analysis using PRCI software Fishing field trials to assess trawl gear interaction with pipelines Tank testing to assess trawl gear interaction behaviour with pipelines Verification, design and cost studies of alternative subsea pipeline schemes Preparation of ISO 21329:2004 Standard for testing of mechanical connectors for use in pipelines Determining the suitability of mechanical connectors for S and J-lay Presentation of training courses covering pipeline design, pipeline integrity management and use of composite materials in offshore applications Pipeline validation requirements Phil has recently been involved in the following projects and brings his experience straight to the classroom: ■ ■ ■ ■ Pipeline risk assessment ILI of subsea pipelines Pipeline validation requirements Corrosion management audit Profiles 545 JONATHAN FRANKLIN ENGINEERING MANAGER AND PRINCIPAL ENGINEER BEng (Hons) CEng MIMechE DipM CMgr MCMI Education: Academic Qualifications: Professional Qualifications: Brunel University Degree in Mechanical Engineering Diploma in Managemenr Member of the Institution of Mechanical Engineers Member of Chartered Management Institute Chartered Mechanical Engineer Chartered Manager Registered Offshore Survival Certificate Current Position at Jee Limited Jonathan is Jee’s Engineering Manager and a Principal Engineer with over 15 years’ experience of pipeline engineering project work and development. Jonathan has developed many of Jee’s courses and teaches most of the courses. Specific Expertise and Experience at Jee Limited Jonathan is involved in a wide range of pipeline engineering projects, specialities include: ■ ■ ■ ■ ■ ■ ■ ■ Pipeline span assessment and remediation Global buckling assessment Remnant life assessment Onshore pipeline design assessments Failure investigation Pipeline integrity management Defect and fracture assessment Use of mechanical connectors for pipeline construction 546 Installation calculations for subsea pipelines Acronyms and abbreviations Acronyms and abbreviations +ve -ve °C °F 30D 3D 3rd A&R AC AGA AGI Al AIS AISC ALARP ALS ANSI API APOS approx. ASB ASCII ASCE ASD ASM ASME ASTM AUT AUV AVTUR AWS Bar a Bar g BAT BBL BE BHP BLEVE BM BMP BoD BOP BP BPD BPEO BS BSI BSR C2H4 C2H6 C3H6 C3H8 positive negative degree Celsius degree Fahrenheit bend radius of 30 times the pipe diameter three-dimensional third Abandonment and recovery Alternating current American Gas Association Above-ground installation Aluminium Automatic identification system American Institute of Steel Construction As low as reasonably practical Accidental limit state American National Standards Institute American Petroleum Institute Acoustic positioning operating station Approximate Above seabed American Standard Code for Information Interchange (computer text) American Society of Civil Engineers Allowable stress (or strength) design American Society of Materials American Society of Mechanical Engineers American Society for Testing and Materials Automated ultrasonic test Autonomous underwater vehicle Aviation turbine (fuel) American Welding Society Bar absolute (1 bar = 100 kN/m²) Bar gauge (0 bar g = 1 bar a) Best available technology US oil barrel (1 bbl ≈ 0.159 m³) Bitumen-enamel Brake horse power (1 BHP ≈ 745.7 W) Boiling liquid, expanding vapour, explosion Bending moment Best management practice Basis of design Blow-out preventer British Petroleum Ltd Barrels per day (1 BPD ≈ 0.159 m³/day) Best practical environmental option British Standard British Standards Institute Bend strain reliever Ethene (ethylene) Ethane Propene (propylene) Propane 549 550 C4H10 C5H12 C6H5CH3 C6H6 C6H14 C10H8 CA CAA CAD CAE CALM CAPEX CAPS CATS CBM CBR CCTV CD CDM CDR CDST CDT CDTM CDUs CFD CGB CH4 CHP CHS CIPS CITHP Cl2 CLHR CNS CNOOC CO COOEC CO2 CoB CoG COMAH COR CP cP CPF CPT CPTU Installation calculations for subsea pipelines Butane or isobutane Pentane or isopentane Toluene Benzene n-Hexane Naphthalene Corrosion allowance Civil Aeronautical Administration Computer-aided design Computer-aided engineering Catenary anchor leg mooring Capital expenditure (construction and installation costs) Cranfield automated pipe-welding system Central Area Transmission System Conventional buoy mooring Catenary bundle riser Closed circuit television Chart datum (often defined at LAT) Construction design and management regulations (UK implementation of European Directive) Composite drilling riser Controlled-depth surface tow Controlled-depth tow Controlled-depth tow method Crude oil distillation units Computational fluid dynamics Concrete gravity based Methane Combined heating and power Circular hollow section (structural steel tubing) Close interval potential survey Closed-in tubing head pressure Chlorine Concentric leg hybrid riser Central nervous system Central North Sea China National Offshore Oil Corporation Carbon monoxide China Offshore Oil Engineering Corporation Carbon dioxide Centre of buoyancy Centre of gravity Control of major accident hazards Concentric offset riser (see CLHR) Cathodic protection Code of practice Centipoise (1 cP = 1 x 10-3 Pa·s) Central processing facility Cone penetrometer test CPT with undrained pore pressure measurement (also known as PCPT) Acronyms and abbreviations CRA CRT CSA CSO CSOL CSt CT CTE CTL CTOD CTR CVAR CVI C/WO D/t DA DAF DC DCVG DD DDCV DGPS DHL DHSS dia, diam. DIN DMaC DnV DoF DOI MMS DP DPI DPT DRA DSAW DSV DTI DTL DVL DWP DWT E ECA ECL ECDIS ED 50 EDP EEIPS e.g. EGNOS Corrosion resistant alloy Cathode ray tube Cross-sectional area Coflexip Stena Offshore Coflexip Stena Offshore Limited Centistokes (1 cSt = 1 x 10-6 m²/s) Computed tomography Coal tar enamel Cut to length Crack tip opening displacement Cost time resource Compliant vertical access riser Close visual inspection Completion/work-over (well development) Diameter to wall thickness (ratio) Double armour (cable) Dynamic amplification Factor Direct current Direct current voltage gradient Directional drilling Deep draught caisson vessel Differential global positioning system (see GPS) Dynamic hook load (lifting) Dual head scanning sonar Diameter Deutsches Institut für Normung e.V. (German standards) Diverless maintained cluster (connection system) Det Norske Veritas Degree of freedom Department of Industries Mineral Management Services (USA) Dynamic positioning (for vessels) Dye penetrant inspection Dye penetrant testing Drag reduction agent Double submerged arc welding Diver support vessel Department of Trade and Industry Dynamic tensioner limit Doppler velocity log Design working pressure Dead weight tonnage Young’s modulus of elasticity East Easting Engineering criticality assessment ELF communication link Electronic chart display information system European datum 1950 Emergency disconnect package Extra extra improved plow steel (for wire ropes) exempli gratia (= for example) European Geostationary Navigation Overlay Service 551 552 EI EIA EIPS EIS ELF EMIT EN EoFL EOR EP EPE EPDM EPIC ERD ERP ERW ESD ESDV ESIA ESV ETAP ETLP etc FAA FAD FBE FDS FDEMS FE FEA FEED FEHM FHM Fi Fi FJ FLAGS FLIP FLS FoS FP FPS FPSI FPSO FPU FRP FRSU FSM FSO FSU ft FTA GAEL Installation calculations for subsea pipelines Bending stiffness Environmental impact assessment Extra improved plow steel (for wire ropes) Environmental impact statement Extreme low frequency Examination, monitoring, inspection and testing Euronorm End of field life Enhanced oil recovery Evacuation plan Exploration and production (Europe) Ethylene propylene diene monomer Engineering, procurement, installation and commissioning Extended reach drilling Emergency recovery plan Electrical resistance welding Emergency shut-down Emergency shut-down valve Environmental and social impact assessment Emergency shut-down valve Eastern Trough area project Extended tension leg platform Et cetera (and other similar items) Federal Aviation Administration Failure assessment diagram Fish-attractant device Fusion bonded epoxy Field development ship Frequency-dependent electromagnetic sensing Finite element Finite element analysis Front end engineering design Fire and explosion hazard management Fire hazard management Fire fighting Flexible joint (FlexJoint) Far north associated gas system (North Sea) Flowline induced pulsation Fatigue limit state Factor of safety Foam pourers Fluoroprotein Forties pipeline system Forties pipeline system and infrastructure Floating production storage and offloading (facility) Floating production unit (or unloading) Fibre reinforced plastic Floating regasification and storage (unit for LNG) Field signature measurement (or method) Floating storage and offloading (facility) Floating storage unit foot (1 ft = 0.3048 m) Flowline termination assembly Graben area export line Acronyms and abbreviations gal US GBS GCHPL GEBCO GFRP GIS GLU GMAW GoM GOR GPR GPS GPSS GRP GSPU GTAW GVI H2 H2O H2S H&S HAL HAT HAZ HAZAN HAZID HAZOP HCl HCR HCV HDD HDPE He HEPC HFI HFW HIC HICC HiPAP HIPPS HM HMPE HMWPE HP HP/HT HSDG HSE HT HV HW HWM I US gallon (1 gal US ≈ 3.785 litre) Gravity based structure Grangemouth combined heat and power limited General bathymetric chart of the oceans Glass-fibre reinforced plastic Geographic information system Gas-lift umbilical Gas metal arc welding Gulf of Mexico Gas oil ratio Ground penetrating radar Global positioning system Government pipeline and storage system (UK) Glass-reinforced plastic Glass syntactic polyurethane Gas tungsten arc welding General visual inspection Hydrogen Water Hydrogen sulphide Health and safety Hiden Analytical Limited Highest astronomic tide Heat-affected zone Hazard analysis Hazard identification Hazard and operating assessment Hydrogen chloride High collapse resistance Hydrant control valve Horizontal directional drilling High density polyethylene (PE-HD) Helium Hose end pressure coupling High frequency induction High frequency welding Hydrogen-induced cracking Hydrogen-induced corrosion cracking High precision acoustic positioning High integrity pressure protection system High modulus High modulus polyethylene (man-made fibre ropes) High molecular weight polyethylene High pressure High pressure/high temperature High sulphur dyed gasoil Health and Safety Executive (UK) Health, safety and environment High tensile Vickers hardness High water High water mark Second moment of area (or moment of inertia) 553 554 ICS ID ID i.e. IFC IGEM in INS IP IPB IPS ISGOTT ISO IWRC JIP JIS JONSWAP kip KP KPI LAFB LAT LBL LC LCP LDPE LF LFS LFSS LHD LL LLDPE LMRP LNG LO LP LPG lpm LRFD LRJ LRP LSD LUSBL LW LWM LWP LWSCR Installation calculations for subsea pipelines The International Chamber of Shipping Internal diameter Density index (granular soil compaction) id est (= that is) International Finance Corporation Institution of Gas Engineers and Managers Inch (1 in = 25.4 mm) Inertial navigation system Institute of Petroleum Inspection plan Intersection point (between two straights of pipe route – with a horizontal radius between TPs) Intelligent pig (or pigging) Integrated production bundle Improved plow strength (for wire ropes) International safety guide for oil tankers and terminals International Standards Organisation Independent wire rope core Joint industry project Japanese Institute of Standards Joint North Sea wave project Kilopound (= 1000 lbf) Kilometre point (chainage in km along pipe route) Key performance indicator Local authority fire brigade Lowest astronomical tide Long base line (survey) Liquid crystal Lack of cross penetration Low density polyethylene Low frequency Lack of fusion surface Lack of fusion subsurface Linear heat detection Liquid limit (clay soils) Linear low density polyethylene Lower marine riser package Liquid natural gas Lift-off point (or touch down point for pipe catenary) Low pressure Liquefied petroleum gas Litres per minute Load and resistance factor design Lower riser joint Lead replacement petrol Level sensor device (survey) Limit state design Long and ultra-short base line Light weight Low water Low water mark Light weight protected (cable) Lazy wave steel catenary riser Acronyms and abbreviations M MA MAG MAOH MAOP MATIS max MBD MBF MBL MBP MBR MDPE MEG MFL MHR MHWN MHWS MIG mil mile min MK MLWN MLWS MMA MMBD mmboe MMS MMscfpd Mn MODU MOL MP MPI MPRE MPT MRS MRU MSL MSV N N° N2 NA NACE NAD27 NAM Monitors Mechanical advantage (pulley systems) Metal active gas Maximum allowable operating head Maximum allowable operating pressure Modular advanced tie-in system Maximum Thousand barrels per day (see mmbd) Minimum breaking force Minimum breaking load Minimum burst pressure Minimum bend radius Medium density polyethylene Monoethylene glycol Magnetic flux leakage Multibore hybrid riser Mean high water neap (tide) Mean high water spring (tide) Metal inert gas (welding) (see GMAW) Thousandth of an inch (1 mil = 25.4 m) 1 mile ≈ 1.609 km Minimum Minutes Mark Mean low water neap (tide) Mean low water spring (tide) Manual metal arc (welding) Million barrels per day (see MBD) Million barrels of oil equivalent US Minerals Management Service Million standard cubic feet per day (gas flow) 1 MMscfpd ≈ 28 317 m³/day Manganese Mobile offshore drilling unit Main oil line Medium pressure Marriage point Magnetic particle inspection Military pipeline repair equipment Multifunction positioning transponder Main riser section Motion reference unit Mean sea level Multi-support vessel North Northing Number Nitrogen Neutral axis National association of corrosion engineers North American Datum 1927 Nederlandsche Aardolie Maatschappij (Dutch Petroleum Company) 555 556 NAP Nd NDE NDT NE NFPA NGL NGO NH3 NNF NORSOK NPSH NUI NWECS (UK) NW O&M OCIMF OD ODN OHTC OPEC OOS op. OPA OPEX ORQ OS OSGB 36 OTDR PA 11 PCPT PCR PD PDAM PDF PDQ PE PFD PFP PGA PGD pGMAW PI PIG PIMS PIT PL PLEM PLET Installation calculations for subsea pipelines Nieuw Amsterdams Peil (Dutch land survey height datum) Neodymium Non-destructive examination (see NDT) Non-destructive testing (AUT or radiography) North east National fire protection association Natural gas liquid Non-governmental organisation Ammonia Normally no flow Norsk Sokels Konkurranseposisjon (Norwegian Contracting Guidance and Standards) Net positive suction head Normally unattended installation Northwest European continental shelf (United Kingdom) North west Operations and maintenance Oil companies international marine forum Outer diameter Ordnance datum Ordnance datum Newlyn (UK land survey height datum) Overall heat transfer coefficient Organization of Petroleum Exporting Countries Out of straightness (pipeline survey) Operating Oil and Pipelines Agency (UK) Operating expenditure (through-life costs) Oil rig quality (wire rope) Offshore standard Ordnance Survey of Great Britain 1936 triangulation Optical time-domain reflectometry Polyamide 11 (Nylon) Piezometer cone penetration test (see CPTU) Pipeline cost reduction Positive displacement (flow meters) Published document (BSI) Pipeline defect assessment manual Probability density function Production, drilling and quarters (platform areas) Polyethylene Penetrant flaw detection Passive fire prevention Peak ground acceleration Permanent ground deformation Pulsed gas metal arc welding (see STT) Plasticity index (clay soils) Pipeline inspection gauge Pipeline integrity management system Pull-in tool Pipeline Plastic limit (clay soils) Pipe line end manifold Pipe line end termination Acronyms and abbreviations PP PPA PPE PPF PPM PRC PRISM PS psi PSV PT PU PUF PVC PVDF PWHT QA QC QC/DC RA RAO RAS RC RD RHS ROT ROTV ROV ROVNAV ROW RP RSJ RST RT RTK RTU RxV S SA SAC SAF SAGE SAL SALM SAW SAWH SAWL SBM SBL SCADA SCC Polypropylene Pressure point analysis Personal protection equipment Polypropylene foam parts per million Pipelines Research Council Pipeline reporting inspection system multimedia Plow steel (for wire ropes) pounds per square inch (1 psi ≈ 0.069 bar) Pressure safety valve Point of tangency Polyurethane Polyurethane foam Polyvinyl chloride Polyvinylidene fluoride Post-weld heat treatment Quality assurance Quality control Quick connect/disconnect (coupling) Rock armour (cable) Response amplitude operator Remote activation system Reinforced concrete Relative density Rectangular hollow section (structural steel tubing) Remotely operated tool Remotely operated towed vehicle Remotely operated vehicle ROV navigation (position fixing) Right of way Recommended practice Reference publication Recovery plan Rolled steel joist (structural section) Releasable swivel tensioner Radiographic testing Real time kinematic Remote terminal unit Receiver verify (DGBS) South Single armour (cable) Special area of conservation Stress amplification factor Scottish area gas evacuation Single armour light (cable) Single anchor leg mooring Submerged arc welding Submerged arc welding (helical seam) Submerged arc welding (longitudinal seam) Single buoy mooring (see SPM) Short base line (survey) Supervisory control and data acquisition Stress corrosion cracking 557 558 SCF SCR SCSSV SDGPS SE sec SFPS SFR SG SIL SIWP SKL S.L. SLHR SLOR SLS SM SMAW SME SMYS SOW SQEP S-N SPL SPM SPARNAV SPT SPU SRB SS SSBL SSC STT SSIV SSSI STP SW SWL T&C T&I TAPS TARA TD TDP TDR TDZ TEG TFHE Installation calculations for subsea pipelines Stress concentration factor Single column floaters Steel catenary riser Surface-controlled sub-surface safety valve Satellite differential global positioning system South east second Semi-floating production system Semi-submersible floating production systems Strategic fuel reserve Specific gravity – density of material compared with water or air (SG for soil compaction in USA, see ID) Safety-integrity level Shut-in wellhead pressure Skew load factor (lifting) Sensu lato (= in the broad sense), Satellite link Single leg hybrid riser Single leg offset riser (see SLHR) Serviceability limit state Standard modulus Submerged metal arc welding Subject matter expert Specified minimum yield strength Scope of works Suitably qualified and experienced Stress – number of cycles (fatigue) Special load (tugger and guide lines for lifting) Single point mooring – can be buoy (see SBM) or tower system Spar navigation Standard penetration test Syntactic polyurethane Sulphate-reducing bacteria Stainless steel Super-short base line Sulphide stress cracking Surface tension transfer welding (pulsed GMAW) Subsea isolation valve Subsea intervention valve Site of special scientific interest Standard temperature and pressure South west Safe working load Threaded and coupled (joints) Transport and installation (project) Trans-Alaska pipeline system Tartan riser access Transmission and distribution Touch down point Time reflectometer Touch down zone Tri-ethylene glycol Tactical fuel handling equipment Acronyms and abbreviations TFL Tg TGP Ti TIG TLP TLWP TM TMAW TMS T-N TOFD TOM TP TRB TRF TS TSA TSBR TSJ TTR UB UC U/C UD UDL UDW UI UK UKCS UKOOA ULS ULSD UO UOE UPC URJ US USA USBL UT UTA UTH UTM UTS UV UVCE V Through flowline systems Glass transition temperature Temporary guide plate Titanium Tungsten inert gas (welding) (see GTAW) Tethered leg platform Tensioned leg platform Tension leg wellhead platform (unmanned) Transverse mercator non-standard zone (see UTM) Tungsten metal arc welding Tethered management system (‘tophat’ system for launching ROVs) Tension – number of cycles (fatigue – US usage) Time of flight diffraction (AUT) Total oil marine Tangent point (between a straight and horizontal curve along pipe route) Through (or three) roller bending Thermal radiation flux Threaded riser and flowline Tensile strength Thermally-sprayed aluminium Top sliding bundle riser Tapered stress joint Top-tensioned riser Universal beam (structural steel section) Universal column (structural steel section) Undercut Uni-directional Uniformly distributed load Ultra-deep water Ultrasonic inspection United Kingdom United Kingdom continental shelf United Kingdom offshore operators association Ultimate limit state Ultra-low sulphur diesel U-ing, O-ing (SAW method of pipe manufacture) U-ing, O-ing and expanding (SAW method of pipe manufacture) Ultimate pull-in capacity Upper riser joint United States United States of America Ultra-short base line Ultrasonic testing or thickness (measurement) Umbilical termination assembly Umbilical termination housing Universal transverse mercator (world projection) Ultimate tensile strength Ultra violet Unconfined vapour cloud explosion Vanadium 559 560 VI VIV viz. VLA VLS VOC VP VPR W WAAS WD WF WGS 84 WHSIP WI WL WoS WSA WSD X52, X65, X80 XLPE YAG YS P eq h l Installation calculations for subsea pipelines Volt Visual inspection Vortex-induced vibration Videlicit (= namely) Vertical load anchor Vertical lay system Volatile organic content Vapour pressure Vertical production riser Tungsten (formerly Wolfram) Watt West Wide area augmentation system Water depth Wave frequency World Geodetic System 1984 Wellhead shut-in pressure Water injection Water line Water level West of Shetland Wye sled assembly Working stress design API pipe steel grades Cross-linked polyethylene Yttrium aluminium garnet (laser) Yield stress (see SMYS) Change in pressure Equivalent stress Hoop stress Longitudinal stress Acknowledgements and references Acknowledgements and references 563 The following companies have kindly provided images, videos or help with this course. Their help is gratefully acknowledged. ABANDONRITE See Nabors Industries ltd AQUADEVICE www.aquadevice.com ABAQUS FINITE ELEMENT SOFTWARE See Simulia ARABIAN OIL AND GAS www.arabianoilandgas.com ACERGY MS LTD (Formerly Stolt Offshore) (MATIS Modular advanced tie-in system and Talon trencher) www.acergy-group.com ADAS ENVIRONMENTAL MANAGEMENT SERVICES www.adas.co.uk ADVANTICA TECHNOLOGIES LTD www.advanticatech.com AEI CABLES LIMITED www.aeicables.co.uk AKER SOLUTIONS (Formerly Aker Kvaerner) www.akersolutions.com ALLEN WATSON LTD www.allenwatson.com ALLSEAS GROUP SA www.allseas.com ALYESKA PIPELINE SERVICE COMPANY (Trans-Alaska pipeline system (TAPS)) www.alyeska-pipe.com ANKER ADVIES BUREAU BV (Anchors) www.flipperdelta.com APPLIED INSPECTION LTD (NDT) www.appliedinspection.co.uk APPLUS RTD LTD www.applusrtd.com ARC MACHINES, INC (Automatic pipe welding equipment) www.arcmachines.com ARCELORMITTAL Sheet piling www.arcelormittal.com ARTUS www.psartus.com ASHTON GATE ENGINEERING LTD ASPLUNDH TREE EXPERT CO www.asplundh.com ASTEC UNDERGROUND (Landline trenching machines) www.astecunderground.com BALDT ANCHOR & CHAIN www.baldt.com BEXCO NV (Polypropylene, polyester, polyamide, Dyneema and Aramid ropes) (Associated with Vryhof Anchors bv) www.bexco.be BIG INCH MARINE SYSTEMS INC See Oil States Industries BJ PROCESS AND PIPELINE SERVICES www.bjservices.com BODEWES WINCHES www.bodewes.com BOSKALIS OFFSHORE BV www.boskalis.nl 564 or WESTMINSTER DREDGING COMPANY www.boskalis.co.uk BP PLC (Study for Shah Deniz in Azerbaij – with Advantica and Transco) www.bp.com BREDERO PRICE COATERS LTD (BPCL) www.bredero-shaw.com BRIDON INTERNATIONAL LIMITED www.bridon.com BRITISH GAS (Walney Channel crossing case study) See National Grid Plc Installation calculations for subsea pipelines CONOCO FLOW IMPROVER SOLUTIONS (LiquidPower™ DRA, Texaco Basin Case Study and Heidrun drilling riser) www.conocophillips.com CORROCEAN Now known as Roxar ASA CORROSION CLUB www.corrosion-club.com CORROSION CONTROL PRODUCTS COMPANY See CCP CORROSION COST www.corrosioncost.com CORTEC® CORPORATION (Corrosion inhibitor) www.cortecvci.com BROWN AND ROOT See KBR CORUS (Steel & Hydrotherm) www.corusgroup.com CATHODIC TECHNOLOGY LTD www.cath-tech.com CRANFIELD UNIVERSITY www.cranfield.ac.uk CCP (CORROSION CONTROL PRODUCTS COMPANY) AND PACTIV CORPORATION (Rockguard foam pipe coating) www.farwst.com/ccp CRC-EVANS PIPELINE INTERNATIONAL INC (Automatic welding, pipe installation equipment and PIH) www.crc-evans.com CEBO HOLLAND (Rubber hoses) www.ceboholland.nl or AUTOMATIC WELDING www.crc-evans.com CLOCK SPRING COMPANY, LP (Pipeline repair) www.clockspring.com CREST See Sapura Crest CLYDE PUMPS LTD www.clydepumps.com CSO, CSOL See Technip COBHAM www.cobham.com CRP GROUP LIMITED (Now part of the Trelleborg Group) www.crpgroup.com COFLEXIP SA See Technip CTC MARINE PROJECTS LTD (Trenching equipment) (Now part of DeepOcean Subsea Services) www.ctcmarine.com Acknowledgements and references DEEPGULF INC www.deep-gulf.com DEEPOCEAN ASA (See also CTC) www.deepocean.no DIGGING DONALD AND SUPPORT VESSEL, TRENCHSETTER (Mechanical subsea trencher) See Allseas DIXON MARINE CONSULTING LTD www.dmcltd.com DORIS ENGINEERING www.doris-engineering.com DSM DYNEEMA (Man-made fibre for ropes) www.dsm.com DUCO See Technip EMC MARINE CONTROL www.emc-offshore.com ESSO PETROLEUM (Chad-Camaroon pipeline and UK multi-product lines) www.esso.com 565 FMC MEASUREMENT SOLUTIONS (Oil and gas flowmeters) www.fmctechnologies.com/Measureme ntSolutions FMC TECHNOLOGIES (UTIS - Universal tie-in system) (An FMC Corporation subsidiary) www.fmctechnologies.com/subsea FORCE TECHNOLOGY www.force.dk FOSTER WHEELER PETROLEUM DEVELOPMENT (Kadanwari field case study) www.fwc.com FOUNDOCEAN (Formerly SeaMark Systems Ltd) www.foundocean.com www.seamarksystems.com FUEL SUBSEA ENGINEERING (DMaC umbilical connector tool) (Now part of Intec Engineering/Heerema) www.intecengineering.com FUGRO NV (Marine survey) www.fugro.nl EUROPIPE www.europipe.com GARDLINE MARINE SCIENCES (Marine survey) www.gardline.com FINE TUBES LTD www.finetubes.com GE OIL AND GAS www.geoilandgas.com FL SMIDTH RAHCO (Onshore pipeline construction vehicle) www.rahco.com GEO-GRAF, INC (GPR gas pipeline leak detection) www.geo-graf.com FLEXCOM AND FREECOM 3D OFFSHORE SOFTWARE See MCS: Advanced Engineering Solutions GEOLINE APS Sage Profile (Subsea pipeline analysis) www.geoline.dk GETMAPPING PLC (Aerial photography) www.getmapping.com 566 Installation calculations for subsea pipelines GLOBAL INDUSTRIES (Pipeline and derrick operations) www.globalind.com INSTITUTE FOR TECHNOLOGY www.ife.no GRENLAND GROUP ASA (Offshore fabricators) www.grenlandgroup.com INTERLIANCE LLC. Associates for the California Energy Commission (Gulf Coast to California pipeline case study) www.interliance.com GROUNDFORCE SHORCO (Trenching equipment systems) www.groundforce.co.uk/GroundforceS horco GUSTO MSC INC and IHC GUSTO BV (Now part of SBM Offshore group) See SBM www.gusto.nl HALMA PLC www.halma.com HDI HORIZONTAL DRILLING INTERNATIONAL (Colville River HDD case study) www.hdi.fr HEAMAN PIPE BENDING INC www.heaman.com HEAT TRACE LTD (Pipeline heat tracing) www.heat-trace.ltd.uk HEEREMA MARINE CONTRACTORS NEDERLAND BV (Balder laybarge) www.heerema.com HELIX ENERGY SOLUTIONS GROUP, INC (Well operations, production and Caldive) www.helixesg.com HIBBITT, KARLSSON & SORENSEN INC (Abaqus finite element software) See Simulia HYDRATITE (Morgrip subsea connectors) (Formally Hydratite Sweeney) www.hydratight.com ENERGY ITAS (Pigging and isolation plugs) www.itas.biz ITP INDUSTRIAL THERMO POLYMERS LTD (Pipeline insulation) www.tundrafoam.com J RAY MCDERMOTT See McDermott International www.jraymcdermott.com JME LTD (NDT equipment) www.jme.co.uk KBR (Formally Kellogg, Brown and Root) www.kbr.com KMC ATLANTA INC www.kmcatlanta.com KONGSBERG (UTIS - Universal tie-in system) See FMC technologies LAND AND MARINE PROJECT ENGINEERING LTD (Directional drilling, landfalls and bundles) (Formerly part of Costain / Smit Groups) www.landandmarine.com LANKELMA LTD (Soils investigation) www.lankelma.com LEIGHS PAINTS www.leighspaints.co.uk Acknowledgements and references 567 LIFTEX CORPORATION (Pipeline lifting slings) www.liftex.com MORGRIP (Underwater connector) See Hydratight LINCO EQUIPMENT INC (Mobile soil sampling) www.linco.com NABORS INDUSTRIES LTD (Workovers) www.nabors.com LMR DRILLING UK LTD (Horizontal directional drilling) www.lmrdrilling.co.uk NATIONAL GRID PLC (Gas transmission pipelines for British Gas) (Study for Shah Deniz in Azerbaij, – with BP and Advantica) www.nationalgrid.com LØGSTØR RØR A/S (Pre-insulated pipelines, pipe-in-pipe) www.logstor.com MACCAFERRI LTD (Gabions and geotextiles, Severn river bank case study) www.maccaferri.co.uk MANNESMANN www.mannesmann.com MAT AND TIMBER SERVICES Division of Sarum Hardwood Structures Ltd www.grootlemmer.com MATIS MODULAR ADVANCED TIE-IN SYSTEM See Acergy MCCONNELL DOWELL (Natural gas line Australia) www.macdow.com.au MCDERMOTT INTERNATIONAL www.mcdermott.com MCS: ADVANCED ENGINEERING SOLUTIONS (Flexcom & Freecom 3D offshore software) www.mcs.com MERLIN CONNECTORS See Oil States Industries MILLER ELECTRIC MANUFACTURING CO (Welding equipment) www.millerwelds.com NEXANS (Spider trenching excavator and cable manufacturers) www.nexans.com NEXEN INC (Energy company) www.nexeninc.com NKT FLEXIBLES I/S (Flexible subsea pipelines) www.nktflexibles.com NORFRA A/S (Dunkirk landfall) www.norfra.no NSW (Umbilical cables) www.nsw.com OCEAN ENGINEERING SYSTEMS www.oes.net.au OCEANEERING INTERNATIONAL, INC. (Umbilical cables) www.oceaneering.com OCEANTEAM POWER & UMBILICAL ASA www.oceanteam.nl OFFPIPE www.offpipe.com OIL AND GAS UK www.oilandgas.org.uk 568 Installation calculations for subsea pipelines OIL STATES INDUSTRIES LTD (Merlin pipe connectors) www.oilstates.com PSL ENERGY SERVICES LTD (Jet prop and clay cutter trenchers) www.psles.com OLYMPIC PIPELINE COMPANY (Whatcom Creek / Bellingham gas pipeline case study) www.olympicpipeline.com PRYSMIAN CABLES SYSTEMS www.nl.prysmian.com OMS (OPTICAL METROLOGY SERVICES) (Pipe Checker ™) www.optical-metrology-services.com AND R J BROWN See Technip REDFERN AMIFLEX HOSE www.redfern.co.uk ROCKWATER (CDT) See Subsea 7 ORBIT ENGINEERING CO www.orbitengineering.com ORCINA LTD (Orcaflex software) www.orcina.com ROTECH www.rotech.co.uk PACTIV CORPORATION See CCP ROXAR ASA (Reservoir management) www.roxar.com PETROBRAS www.petrobras.com.br ROYAL DUTCH SHELL GROUP See Shell PHOENIX BEATTIE (Rubber hoses) www.phoenixbeattie.co.uk RSK ENVIRONMENT LTD www.rsk.co.uk PII PIPELINE SOLUTIONS (Pipeline inspection) Now part of GE Oil and Gas RUPTURE PIN TECHNOLOGY (Pressure safety systems ESDVs) www.rupturepin.com PIPE INDUCTION HEAT LTD (PIH) See CRC-Evans SAAB SEAEYE LTD Sister company to Hydrovision (Panther ROV) www.seaeye.com PIPESHIELD INTERNATIONAL LTD www.pipeshield.co.uk PIRELLI SUBMARINE CABLES See Prysmain Cables and Systems PIGGING PRODUCTS SERVICES ASSOCIATION www.ppsa-online.com AND PSI PLUGGING SPECIALISTS INTERNATIONAL AS Now TDW Offshore Services AS SAGE PROFILE (Subsea pipeline and plough analysis) See Fugro and GeoLine www.sage-profile.com SAIPEM www.saipem.eni.it SAPURA CREST PETROLEUM BERHAD (Incorporating Teknik Lengkap, TL Geosciences and TL Offshore) www.crest.com.my Acknowledgements and references 569 SASOL GAS LTD (Mozambique river crossing case study) www.sasol.com SRD SONAR RESEARCH & DEVELOPMENT LTD (Underwater video) SRD are part of Tritech and Halma Group www.srduk.com SBM OFFSHORE NV (Single buoy moorings, FSOs and FPSOs) www.singlebuoy.com STARTRAK PIGGING TECHNOLOGIES (Pigging and river crossing inspections) www.starpig.com SEABED SCOUR CONTROL SYSTEMS LIMITED www.scourcontrol.co.uk STATOILHYDRO www.statoilhydro.com SAS GOUDA BV www.sasgouda.nl SEAMARK SYSTEMS LTD (Concrete mattresses) See Foundocean www.seamarksystems.com STOLT COMEX SEAWAY MS LTD See Acergy SEAWAY FALCON (Reel barge) See Acergy SUBSEA 7 (Formed from Halliburton Subsea (Rockwater) and the subsea activities of DSND) www.subsea7.com SERIMAX (Automated pipe welding) www.serimax.com SUBSEA PROTECTION SYSTEMS LIMITED www.sps.gb.com SHELL EXPLORATION & PRODUCTION (Nigerian Pipeline sabotage) www.shell.com SUPERPESA www.superpesa.com.br SIERRA PACIFIC CORP (Infrared thermography) www.x20.org SIMULIA (Abaqus finite element software) www.simulia.com SMD HYDROVISION www.smd.co.uk SMIT INTERNATIONALE N.V. (CDT – see also Land and Marine) www.smit.com SPM INSTRUMENT AB (Condition monitoring systems) www.spminstrument.se TALON SUBSEA TRENCHER See Acergy TAPS TRANS-ALASKA PIPELINE SYSTEM See Alyeska TD WILLIAMSON INC (Shortstopp® connection, pipeline inspection and commissioning) www.tdwilliamson.com TDW OFFSHORE SERVICES www.tdwoffshore.com TECHNICAL TOOLBOXES INC (Software products for the energy industry) www.ttoolboxes.com 570 TECHNIP (Apache, Pliant wave and S risers) (Formerly Technip-Coflexip) www.technip.com TEKNIK LENGKAP See Sapura Crest TESMEC (Landline trenching and stringing machine manufacturers/suppliers) www.tesmec.it TESMEC USA INC (Landline trenching and stringing machine manufacturers/suppliers) www.tesmec.com Installation calculations for subsea pipelines TRIAD WESTERN CONSTRUCTORS INC (Auger boring, pipe ramming and HDD) www.triadwestern.com TRITECH See SRD www.tritech.co.uk TWI LTD (The Welding Institute) www.twi.co.uk TYCO TELECOMMUNICATIONS www.tycotelecom.com THRUST SHORE See Groundforce Shorco UNITED OFFSHORE SERVICES (Cable-laid slings and grommets) www.uos-nl.com TIG TITANIUM INFORMATION GROUP www.titaniuminfogroup.co.uk UNITED STATES DEPARTMENT OF AGRICULTURE www.usda.gov TL (TEKNIK LENGKAP) OFFSHORE See Sapura Crest VERMEER MANUFACTURING COMPANY (Rock trenchers and HDD) www.vermeer.com TOTAL DUNBAR (Insulated pipe connector) See Total TOTAL EXPLORATION UK PLC (Formally TotalFinaElf) www.uk.total.com TTI See Technical Toolboxes Inc TRANSCANADA www.transcanada.com VIA+ VISITLESS INTEGRITY ASSESSMENT LTD (Satellite earth condition monitoring) www.via-plus.net VIA VALVOLE www.viavalvole.com VOSTA-LMG (Dredging technology) www.vostalmg.com TRELLEBORG CRP AB www.trelleborg.com VRYHOF ANCHORS (Anchors and manmade fibre ropes) www.vryhof.com TRENCH SHORE LTD See Groundforce Shorco WELLSTREAM INTERNATIONAL www.wellstream.com TRENCOR INC See Astec underground X100 STUDIES See Shell Global Solutions, TransCanada, Advantica, Serimer Dasa, Cranfield University and BP Acknowledgements and references 571 Additional Help Additional help was provided by individuals: Cyril Bishop (Pipe freezing and hot tapping) Herman Duff (Malaysian pipeline) Mike Mosedale (Cartoonist) Frank Gibbons (Marsh and wetlands) References “Corrosion Costs and Preventive Strategies in the United States”, G.H. Koch, M.P.H. Brongers, N.G. Thompson, Y.P. Virmani, and J.H. Payer, Study by CC Technologies, Report FHWA-RD-01-156 (September 2001). “Oman India Pipeline: Development of Design Methods for Hydrostatic Collapse in Deep Water”, C Tam, P Raven, R Robinson, T Stensgaard, A M Al-Sharif & R Preston, Offshore Pipeline Technology Conference (OPT96) Amsterdam (15-16 February). “Liquefaction hazards and their effects on buried pipelines”, T D O’Rourke and P A Lane (1989), Tech Rep NCEER-89-0007, National Center for Earthquake Engineering Research, Buffalo, NY (1 February). “Guide to purchasing, manufacturing, and testing of loading and discharge hoses for offshore moorings”, Oil Companies International Marine Forum (1991) “Ultimate Pipe Strength under Bending, Collapse and Fatigue”, C E Murphey & CG Langner, Proceedings of the 4th OMAE Symposium, Volume 1 (1985). “Reeled Pipe-in-Pipe for Ultra Deepwater”, David Kaye and Vincent Ledoux of Coflexip, Presented at Deepwater Offshore Technology Conference, Rio de Janeiro, (October 2001). “Editorial of Géotechnique”, Vol LVI, Number 5 p 289 and letter pp 357-358 (June 2006). “Critical state soil mechanics”, A N Schofield and C P Wroth (1968) – available to download via http://www2.eng.cam.ac.uk/~ans/ “Reeling of pipelines with thick insulation coating, finite element analysis of local buckling”, Tim Crome; OTC, Houston (1999). “‘Factors Affecting Pipe Collapse”, S Kyriakides, and M.K. Yeh, Engineering Mechanics Research Laboratory, EMRL Report No 85/1, A.G.A Catalogue No. L51479 Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin (1985) “Calculating the service life of running steel wire ropes”, Dipl Ing Roland Verreet, Casar Drahtseilwerk Saar GMBH (www.casar.de) (Aug 1998) 572 Installation calculations for subsea pipelines Web Sites The following web contact addresses may also be of use: API American Petroleum Institute www.api.org ASME American Society of Mechanical Engineers www.asme.org ANSI American National Standards Institute www.ansi.org BS British Standards Institute www.bsi-global.com BERR Department for Business Enterprise and regulatory reform www.berr.gov.uk DEAL DATA REGISTRY FOR UK OFFSHORE OIL AND GAS Data and information about offshore oil and gas exploration and production for the UK www.ukdeal.co.uk DEEP ROPE MANUAL 2004 www.offshoreengineering.org/moorings/Downloads/deepropemanual.pdf DNV Det Norske Veritas www.dnv.com HSE UK Health and Safety Executive (Offshore Safety Reports and Contact Research Reports) www.hse.gov.uk ENERGY INSTITUTE Previously the Institute of Petroleum www.energyinst.org.uk ISO International Organisation for Standardization www.iso.org MINERALS MANAGEMENT SERVICE (MMS) USA Authority for Pipelines – Offshore incidents in Pacific and Gulf of Mexico www.mms.gov and www.mms.gov/offshore/index.htm NACE - THE CORROSION SOCIETY National Association of Corrosion Engineers www.nace.org PIGGING PRODUCTS AND SERVICES ASSOCIATION Information on pigging manufacturers and suppliers www.ppsa-online.com Acknowledgements and references OS Ordnance Survey (of Great Britain) www.ordanancesurvey.co.uk SAFEBUCK JIP Design guideline for on-bottom lateral buckling www.safebuck.com SHEET PILING SPECIFICATIONS Search engine for sheet piling specifications www.pilespecs.com US DEPARTMENT OF ENERGY www.energy.gov WORKSAFE VICTORIA Australian State of Victoria Health and Safety Accident Prevention Arm (Good international contacts worldwide) www.workcover.vic.gov.au 573
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