Strength of Self-Elevating Unit - Part 1 18th January 2010 Strength of Self-Elevating Unit - Part 1 1. Definitions Definitions ► Configuration in Elevated Position: Leg length reserve Jackhouse Pontoon Air gap Water level Leg Seabed Leg length penetration Spudcan Strength of self elevating units Part 1 _ January 2011 3 Definitions Chord Spudhouse Bracings Strength of self elevating units Part 1 _ January 2011 4 Definition ► Configuration in Transit Position Legs are fully retracted (or one bay lowered) Strength of self elevating units Part 1 _ January 2011 5 Modes of Operation ►Transit: The unit moves from one location to another. Field / Ocean tow: short / long distance. Wet / Dry tow: self-floating / supported by barge. ►Installation: The unit is lowering legs and elevating hull. The legs are preloaded. Wet tow ►Operating: The unit is supported on the seabed. The combined environmental and operating loadings are within the appropriate design limits established for operations (e.g. drilling). Strength of self elevating units Part 1 _ January 2011 Dry tow 6 Modes of Operation ►Survival (extreme storm): The unit is supported on the seabed. Condition during which the unit may be subject to the most severe environmental loadings for which the unit is designed. Operation may be interrupted. Operating / Survival ►Retrieval: The unit is lowering hull and elevating legs. Wind ►Accidental (if relevant): May be overturning, broken bracing, leg deformation, punch through, boat impact, etc. Wave + current Break surface Punch through Strength of self elevating units Part 1 _ January 2011 7 Mode of operations Transit mode Retrieval mode JACK-UP Is designed to resist the loads that may occur during all stages of the life-cycle of the unit Survival mode Installation mode Operating mode Strength of self elevating units Part 1 _ January 2011 8 Overturning moment and base shear Wind Environmental loads OVERTURNING MOMENT Wave Current BASE SHEAR Strength of self elevating units Part 1 _ January 2011 9 Strength of Self-Elevating Unit - Part 1 2. Design principles Design Principles - Hull ► Hull form: triangular, rectangular, etc. ► General requirements of Ship Rules Pt B, Ch 4 to be applied. ► For small jack-ups, the hull may be made of containers (e.g. Flexifloat) Triangular hull Rectangular hull Strength of self elevating units Part 1 _ January 2011 11 Design Principles - Hull ► The framing system of the pontoon is to consider the global stress flow. ► Requirements for double bottom of oil fuel tank as requested by MARPOL 12 A are not applicable to SEU. Strength of self elevating units Part 1 _ January 2011 12 Design Principle - Legs ► Legs are critical components for determining the performances of a jack- up. They are sensitive for weight, stiffness, drag/inertia coefficient, etc. ► Shell type: rectangular, cylindrical, … Cylindrical shell Rectangular shell Strength of self elevating units Part 1 _ January 2011 13 Design Principle - Legs ► Truss type: Square or triangular cross section; K bracing, X bracing, Inverse-K bracing. Bracing Bay spacing Chord K bracing Same Bracing Weight X bracing Inverse-K Same Wave Load Same Strength Wave Load Strength Weight Strength Weight Wave Load K 127% 38% 100% 25% 238% 142% X 100% 100% 100% 100% 100% 100% inv-K 100% 48% 100% 48% 187% 112% Strength of self elevating units Part 1 _ January 2011 14 Design Principle - Legs ► Examples of jack-up legs: Strength of self elevating units Part 1 _ January 2011 15 Design Principle - Legs ► Examples of jack-up legs: Strength of self elevating units Part 1 _ January 2011 16 Design Principle – Mat footing ► Mat footing connects all Jack-up unit’s legs to one common footing. Mat footings typically are rectangular structures, flat on the top and bottom. ► Due to their larger size, mat footings exert a lower bearing pressure on the soil than Units with spudcans. ► Mats cannot be used on uneven seabeds or those with large slopes. Strength of self elevating units Part 1 _ January 2011 17 Design Principle – Spudcan ► Spudcan is used to transfer the leg loads into the seabed below. ► Size and shapes depends on the properties of the soils. ► Connection with the leg is critical. Strength of self elevating units Part 1 _ January 2011 18 Steel grades ► Structural Categories: Second category elements are structural elements of minor importance, the failure of which might induce only localized effects, First category elements are main load carrying elements essential to the overall structural integrity of the unit, Special category elements are parts of the first category elements located in way or at the vicinity of critical load transmission areas and of stress concentration loads. ► The category of each structural elements is defined in the NI534. ► Structural categories define the steel grade, welding and NDT to be performed for the selected structural element. ► For higher strength steels (Re > 460 N/mm2), IACS Rec.11 (Rev 1. 1996) “Material Selection Guideline for Mobile Offshore Drilling” is to be considered. Strength of self elevating units Part 1 _ January 2011 19 Steel grades ► According to Pt B, Ch 2, Sec 2, [6], the design temperature is, the mean air temperature of the coldest day (24 h) during the whole anticipated life of the vessel. ► By default, the design air temperature is 0˚C for units not intended to operate in cold areas, -10˚C for units intended to operate in cold areas. ► For steel grade determination, one specific diagram for each category. Strength of self elevating units Part 1 _ January 2011 20 Steel grades Strength of self elevating units Part 1 _ January 2011 21 Strength of Self-Elevating Unit - Part 1 3. Jackhouses & Elevating Systems Jacking System: General ► Jackhouse: structure in way of the jacking system / elevating arrangement to properly transmit the load between legs and pontoon. ► To be (type)-approved. ► To be arranged with redundancy to avoid any uncontrolled descent of the unit and impair the possibility to jack the unit to a safe position. ► Types of jacking systems: Pin & hole jacking system, Rack & pinion jacking system, Others. Strength of self elevating units Part 1 _ January 2011 23 Jacking Systems: Pin & Hole Jacking System ► Pins (in jack house) move and reposition from one hole (on legs) to the next. ► Made up of : a travelling piece connecting to hydraulic jacks, and a fixed piece connecting to the hull. ► Pins are generally drived by hydraulic cylinders. Pins drived by hydraulic cylinders Pinholes Strength of self elevating units Part 1 _ January 2011 24 Jacking Systems: Pin & Hole Jacking System ► Relative slow jacking speed, ► High jacking/holding capacity, (Max. holding capacity = 9000t/leg for MSC latest jacking system.) ► Usually applied for smooth, large hollow shape of legs, ► Normally applied for units working in shallow water. ► In normal operation, Depending on the design, vertical axial loads in the legs can be permanently taken either by the fixed or travelling parts. Bending moment and shear force are taken by guides. Strength of self elevating units Part 1 _ January 2011 25 Jacking Systems: Rack & Pinions ► Rack & Pinion Jacking System Jacking by relative movement between rack (leg) and pinion (jackhouse). Top of jackhouse Upper guide Pinion ► Typical leg/jackhouse configuration Rack & opposed pinion jacking system (4, 6 or 8 pinions per chord), Fixation system may be fitted, Lower and upper guides are fitted. Main deck Fixation system Lower guide Bottom Strength of self elevating units Part 1 _ January 2011 26 Jacking Systems: Rack & Pinions ► Rack & Pinion Jacking System (continued) Racks are welded onto chord section of legs, Usually applied for truss type legs but not only. Chord Rack Chord-rack cross section Leg/Jackhouse arrangement Strength of self elevating units Part 1 _ January 2011 27 Jacking Systems: Rack & Pinions ► Rack & Pinion Jacking System (continued) Racks drived by the rotation of pinions (jacks), Higher jacking speed than pin and hole system, Jacking/holding capacity depends on pinion capacity and number of pinions. e.g. F&G MOD II: jacking/holding capacity = 200/454 tons per pinion. Rack (a part of leg) Rack Pinion Jacking System (lowering leg) Pinion with electric motor Strength of self elevating units Part 1 _ January 2011 28 Jacking Systems: Rack & Pinions ► Rack & Pinion Jacking System (continued) Unopposed pinion (single sided) elevating unit, Opposed pinion (double sided) elevating unit, - Local deflection into chord Additional loads on chord & bracing (Not for opposed pinion) - Better load-sharing between stacked pinions Strength of self elevating units Part 1 _ January 2011 29 Jacking Systems: Rack & Pinions ► Rack & Pinion Jacking System (continued) Fixed jacking system: welded connection between jackhouse and hull, Floating jacking system: rubber pads are utilized for connection. Upper shock pad Floating jack case tends to rotate Lower shock pad Weld attachment Fixed jacking system Floating jacking system Strength of self elevating units Part 1 _ January 2011 30 Jacking Systems: Rack & Pinions ► Fixation/Locking System (may be fitted or not) Each fixation system (rack chock) has a counter rack which is moved up/down and in and out the rack of the leg chord. Disengaged during hull or leg elevation, Engaged with rack and holding it in position during extreme conditions, - If NOT fitted, pinions will always sustain vertical loads, - Normally one pair of fixation systems for each chord. Rack chock Strength of self elevating units Part 1 _ January 2011 31 Jacking Systems: Rack & Pinions ► Upper / Lower Guides Guides fitted in order to protect pinions and hull, To maintain the rack at a constant distance away from the pinions, Doubling plates may be used to reinforce locally, Strength of self elevating units Part 1 _ January 2011 32 Jacking Systems: Rack & Pinions ► Rack & Pinion Jacking System Jacking / fixation systems sustain vertical loads Lower / upper guides sustain horizontal loads Absorb one part of bending moments of legs due to environmental loads. Absorb the rest. Strength of self elevating units Part 1 _ January 2011 33 Jacking Systems: Rack & Pinions Strength of self elevating units Part 1 _ January 2011 34 Other Jacking systems: strand jack ► Another type of jacking system: strand jack system Jack Strand Jack Strength of self elevating units Part 1 _ January 2011 35 Strength of Self-Elevating Unit - Part 1 4. Loads and non linear load effects in elevated position Loads and non linear load effect in elevated position ► Global and local structural analysis shall be performed taking adequately into account Fixed and operational loads Static inclination of the legs Non linear wave loading effects Morison loads (effect of drag) • Wave theory • Variations of submerged portions of legs Wind Weight • Non linear amplification of large displacements of the unit • P-D effect • Euler effect Wave Dynamic amplification loads Non-linear interaction • Leg/hull interaction • Leg/sea bottom interaction Current Strength of self elevating units Part 1 _ January 2011 37 Load – Fixed and operational loads ► Fixed loads: loads which are not expected to vary during service life, e.g. self-weight. An accurate weight distribution is necessary for evaluating the mass matrix used in both hydrodynamic and global analysis. ► Operational loads: loads that could vary in magnitude, position and direction during service life of the unit and are related to the operation condition, e.g. deck loads, weight of consumables, ballast, riser tensioning loads… - For check of overturning stability: • 50% of operational loads - For check of leg strength: • 100% of operational loads Strength of self elevating units Part 1 _ January 2011 38 Environmental conditions – Necessary data for calculation ► Wave: Maximum wave height Hmax Associated period Tass (a wider range ±15% is recommended for analysis) ► Current: Current velocity (at least: V at sea surface & V at sea bottom) ► Wind: Wind profile: MODU code wind profile is to be considered as a minimum standard Wind velocity. Where no particular data are specified, wind speed at 10 m above the mean water level is: Condition of operation Transit Wind speed (m/s) 51.3 Working 36.0 Severe storm 51.5 ► Marine growth (if applicable): Increase of thickness on the outside boundary of each structural members under water. Strength of self elevating units Part 1 _ January 2011 39 Hydrodynamic loads ► Morison loads Wave length > 5 D F = Fdrag + Finertia = 1 ρDC D vn vn + ρCM Aun 2 D: Reference dimension A: Cross sectional area of member u�, n vn Fluid particle acceleration and velocity normal to member ρ CD CM Vn Density of water (1025 kg/m3) Drag coefficient Inertia coefficient Relative fluid particle velocity resolved normal to the member axis taking into account current and member velocity Strength of self elevating units Part 1 _ January 2011 40 Hydrodynamic loads ► Non-linear loading effect: for deterministic analyses appropriate wave theory is to be used Strength of self elevating units Part 1 _ January 2011 41 Hydrodynamic loads ► The resulting hydrodynamic loads depends on the heading d Wave Current COG 0° d: distance between legs α O x α 90°- α Strength of self elevating units Part 1 _ January 2011 90° 42 Static leg inclination ► e0 = e1 + e2 + e3 (Reaction force at spudcan) with P e0 : Total horizontal offset of leg base with respect to hull, e1: Offset due to leg hull clearances, Leg / hull clearance e0 e2 : Offset due to maximum hull inclination permitted by the operating manual, e3 : Offset due to leg fabrication tolerances. e0 ► Additional bending moment inside the leg in way of the pontoon equal to P*e0. Strength of self elevating units Part 1 _ January 2011 43 P-delta effect: ► A jack-up is a flexible structure. ► This implies that lateral motions of the hull are induced by the environmental loads. ► Lever arm (∆) is created between the vertical reaction on the spudcan and the center of the leg at pontoon level. ∆ Weight Wind Additional bending equals to R1*∆ Wave Current R2 Strength of self elevating units Part 1 _ January 2011 R1 44 Euler effect (large displacements) ► Due to high axial loads, the lateral/bending stiffness of the legs is reduced. ► The deflection is then larger than standard beam theory. ► The increase of deflection (Δ’) is a function of the ratio of the applied axial load to the critical buckling Euler load. ∆’ ∆' ≈ Weight Wind ∆ P 1− PE With ∆: The linear-elastic hull pontoon displacement, P: The average axial load in the leg at the pontoon, Wave PE: Euler buckling load of an individual leg. Current R2 R1 Strength of self elevating units Part 1 _ January 2011 45 Resonance ► Dynamic amplification loads (Fdyn) Typical jack-up natural periods fall within the range of wave period. The wave loadings will amplify the quasi-static responses. Dynamic amplification loads are to be considered. ► Natural period of a jack-up is (see NI 534): Tn = 2π M P K 1 − PE Strength of self elevating units Part 1 _ January 2011 46 Resonance ► Dynamic amplification forces depend on Viscous damping, Ratio between Tn and wave periods. ► SDOF (Single Degree of Freedom) methodology is generally applied to determine dynamic amplification forces at COG of the platform Fdyn (NI 534, Sec 5, [4.5]): FDyn = ( DAF − 1) BS BS = BS max − BS min 2 DAF = 1 2 Tn2 T 1 − 2 + 2ξ n Tass Tass Strength of self elevating units Part 1 _ January 2011 2 47 Resonance ► Rarely: MDOF method and Determination of Most Probable Maximum Extreme Frequency domain simulation • Wave loads to be linearised and calculation of RAOs, • Uncoupled assessment of the dynamic and static parts of the response, • Evaluation of the MPME for both inertia and drag force, • Evaluation of the correlation of both inertia and drag force, • Assessment of the DAF. Time domain simulation • The response is evaluated at each time step for several sea states chosen at random, • MPME to be evaluated using Weibull (3 parameters) distribution, Gumbel distribution or Winterstein’s Hermite polynomial method, • More accurate method but difficult to implement. Strength of self elevating units Part 1 _ January 2011 48 Question?
