DEP SPECIFICATION
DESIGN OF MULTIPLE-PIPE SLUG CATCHERS
DEP 31.40.10.12-Gen.
February 2011
DESIGN AND ENGINEERING PRACTICE
DEM1
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written permission of the copyright owner or Shell Global Solutions International BV.
DEP 31.40.10.12-Gen.
February 2011
Page 2
PREFACE
DEP (Design and Engineering Practice) publications reflect the views, at the time of publication, of Shell Global
Solutions International B.V. (Shell GSI) and, in some cases, of other Shell Companies.
These views are based on the experience acquired during involvement with the design, construction, operation and
maintenance of processing units and facilities. Where deemed appropriate DEPs are based on, or reference
international, regional, national and industry standards.
The objective is to set the recommended standard for good design and engineering practice to be applied by Shell
companies in oil and gas production, oil refining, gas handling, gasification, chemical processing, or any other such
facility, and thereby to help achieve maximum technical and economic benefit from standardization.
The information set forth in these publications is provided to Shell companies for their consideration and decision to
implement. This is of particular importance where DEPs may not cover every requirement or diversity of condition at
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information set forth in DEPs to their own environment and requirements.
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requirements not specifically covered, the Principal will typically expect them to follow those design and engineering
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1)
Operating Units having a Service Agreement with Shell GSI or another Shell Company. The use of DEPs by
these Operating Units is subject in all respects to the terms and conditions of the relevant Service Agreement.
2)
Other parties who are authorised to use DEPs subject to appropriate contractual arrangements (whether as part
of a Service Agreement or otherwise).
3)
Contractors/subcontractors and Manufacturers/Suppliers under a contract with users referred to under 1) or 2)
which requires that tenders for projects, materials supplied or - generally - work performed on behalf of the said
users comply with the relevant standards.
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DEP 31.40.10.12-Gen.
February 2011
Page 3
TABLE OF CONTENTS
1.
1.1
1.2
1.3
1.4
1.5
1.6
1.7
INTRODUCTION ........................................................................................................4
SCOPE........................................................................................................................4
DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS .........4
DEFINITIONS .............................................................................................................4
SYMBOLS AND ABBREVIATIONS............................................................................5
CROSS-REFERENCES .............................................................................................7
SUMMARY OF MAIN CHANGES...............................................................................7
COMMENTS ON THIS DEP .......................................................................................7
2.
GENERAL DESIGN ASPECTS..................................................................................8
3.
3.1
3.2
3.3
HYDRAULIC DESIGN ................................................................................................9
GENERAL ...................................................................................................................9
SLUG CATCHER SIZE...............................................................................................9
SLUG CATCHER GEOMETRY AND COMPONENTS ..............................................9
4.
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
MECHANICAL DESIGN ...........................................................................................10
GENERAL .................................................................................................................10
MAXIMUM ALLOWABLE OPERATING PRESSURE ..............................................10
DESIGN FACTOR FOR HOOP STRESS.................................................................10
OTHER STRESS ......................................................................................................10
SUPPORTS AND ANCHORS...................................................................................11
MATERIAL SELECTION...........................................................................................11
INTERNAL/EXTERNAL CORROSION PROTECTION ............................................11
OVERPRESSURE PROTECTION ...........................................................................11
MAINTENANCE........................................................................................................12
EMERGENCY DEPRESSURISATION.....................................................................12
MISCELLANEOUS ...................................................................................................13
5.
5.1
5.2
5.3
HAZARD AND RISK MANAGEMENT .....................................................................14
GENERAL .................................................................................................................14
APPLICATION OF FORMAL HEMP BASED STUDIES...........................................14
DESIGN CONSIDERATIONS...................................................................................15
6.
REFERENCES .........................................................................................................18
APPENDICES
APPENDIX A
Figures .............................................................................................................19
DEP 31.40.10.12-Gen.
February 2011
Page 4
1.
INTRODUCTION
1.1
SCOPE
This DEP specifies requirements and gives recommendations for design of multiple pipe
slug catchers to be installed on land.
This DEP contains mandatory requirements to mitigate process safety risks in accordance
with Design Engineering Manual DEM 1 – Application of Technical Standards.
This is a revision of the DEP of the same number dated January 2010; see (1.5) regarding
the changes.
1.2
DISTRIBUTION, INTENDED USE AND REGULATORY CONSIDERATIONS
Unless otherwise authorised by Shell GSI, the distribution of this DEP is confined to Shell
companies and, where necessary, to Contractors and Manufacturers/Suppliers nominated
by them. Any authorised access to DEPs does not for that reason constitute an
authorization to any documents, data or information to which the DEPs may refer.
This DEP is intended for use in facilities related to oil and gas production and gas handling.
This DEP may also be applied in other similar facilities.
When DEPs are applied, a Management of Change (MOC) process should be
implemented; this is of particular importance when existing facilities are to be modified.
If national and/or local regulations exist in which some of the requirements could be more
stringent than in this DEP, the Contractor shall determine by careful scrutiny which of the
requirements are the more stringent and which combination of requirements will be
acceptable with regards to the safety, environmental, economic and legal aspects. In all
cases the Contractor shall inform the Principal of any deviation from the requirements of
this DEP which is considered to be necessary in order to comply with national and/or local
regulations. The Principal may then negotiate with the Authorities concerned, the objective
being to obtain agreement to follow this DEP as closely as possible.
1.3
DEFINITIONS
1.3.1
General definitions
The Contractor is the party that carries out all or part of the design, engineering,
procurement, construction, commissioning or management of a project or operation of a
facility. The Principal may undertake all or part of the duties of the Contractor.
The Manufacturer/Supplier is the party that manufactures or supplies equipment and
services to perform the duties specified by the Contractor.
The Principal is the party that initiates the project and ultimately pays for it. The Principal
may also include an agent or consultant authorised to act for, and on behalf of, the
Principal.
The word shall indicates a requirement.
The capitalised term SHALL [PS] indicates a process safety requirement.
The word should indicates a recommendation.
1.3.2
Specific definitions
Term
Definition
Bottle,
primary
Pipe-type part of a multiple-pipe slug catcher for separation of the fluid and
storage of the liquids, sloping down under small angle(s) from the lower end
of the downcomer to the liquid outlet header (see Figure 2).
Bottle,
secondary
Pipe-type part of a multiple-pipe slug catcher for storage of the liquids only,
sloping up from the liquid outlet header, and ending at the gas risers (see
Figure 2b).
DEP 31.40.10.12-Gen.
February 2011
Page 5
Term
Definition
Downcomer
Vertical or steeply sloping pipes of a multiple-pipe slug catcher between the
inlet header and the primary bottles (see Figure 2).
Entrance
length
The section of a primary bottle from the inlet to the gas risers.
Gas outlet
header
Part of a multiple-pipe slug catcher on top of and connecting all risers for
collection of the out going gas streams (see Figure 2).
Inlet header
Part of a multiple-pipe slug catcher in which the fluid is evenly distributed
before entering the downcomers and in which fluid flow conditions are
further improved for separation (see Figure 2). Also called distribution
header.
Liquid outlet
header
The lowest part of the multiple-pipe slug catcher, in which the liquid is
collected for export. Also called liquid header.
Gas riser
Vertical pipe on top of a bottle, through which gas exits (see Figure 2).
Separator
Equipment for separating gas and liquids or various liquids of a mixed fluid.
Slug catcher
Part of a multi-phase pipeline system for separation of the gas and liquid
phases and for temporary storage of the liquids.
Slug catcher,
multiple-pipe
type
Slug catcher, made of several parallel pipe bottles.
Slug catcher,
parking-loop
type
Slug catcher, where the separator is located apart from the storage part.
The separator can be a vessel type and the storage part is made of pipeline
material, which can be located away from the separator.
Slug catcher,
vessel type
Slug catcher of limited dimensions and capacity, where separator and
storage parts are combined within one or a combination of vessels.
Splitter
Inlet of a multiple-pipe slug catcher in which the fluid stream is split up for
even distribution over the inlet header (see Figure 2).
1.4
SYMBOLS AND ABBREVIATIONS
1.4.1
Symbols
A
Cross-sectional area
Cw
Drag coefficient
d
Diameter of droplet (m)
D
Diameter (m)
H
Hold-up fraction
g
Gravity constant (m/s2)
L
Length (m)
m
Number of risers per bottle
n
Number of bottles
V
Velocity (m/s)
Vol
Volume (m3)
Q
Gas flow rate (m3/s)
DEP 31.40.10.12-Gen.
February 2011
Page 6
Re
Reynolds number
Greek symbols
1.4.2
1.4.3
η
Dynamic viscosity (N.s/m2)
λ
Load factor (m/s) or Volumetric fraction of liquid in two-phase flow
θ
Angle between bottle and horizontal plane
ρ
Density (kg/m3)
Subscripts
b
Bottle
buffer
Buffer
C
Constrictor
G
Gas
IH
Inlet header
int
Intercept
L
Liquid
goh
Gas outlet header
loh
Liquid outlet header
outlet
Outlet
pb
Primary bottle
pipeline
Pipeline
R1
First riser on a bottle (from inlet)
R2
Second riser on a bottle (from inlet)
riser
Riser
s
Settle
sb
Secondary bottle
SC
Slug catcher
SG
Superficial gas
SL
Superficial liquid
Abbreviations
BLEVE
Boiling Liquid Expanding Vapour Explosion
CFD
Computational Fluid Dynamics
DEP
Design and engineering practice
Dbottle
Internal diameter of the multiple-pipe slug catcher bottles
Ddowncorner
Internal diameter of the downcomer
Dgoh
Internal diameter of the gas outlet header
Doutlet
Internal diameter of the gas outlet
Driser
Internal diameter of the gas risers
dP
Differential pressure
DEP 31.40.10.12-Gen.
February 2011
Page 7
1.5
GtL
Gas to Liquids
HEMP
Hazards Effect Management Process
KHI
Kinetic Hydrate Inhibitor
LNG
Liquefied Natural Gas
MAOP
Maximum Allowable Operating Pressure
MEG
Mono Ethylene Glycol
PSV
Pressure Safety Valve
SMYS
Specified Minimum Yield Strength
SGV
Sphere-generated Volume
CROSS-REFERENCES
Where cross-references to other parts of this DEP are made, the referenced section
number is shown in brackets. Other documents referenced by this DEP are listed in (6).
1.6
SUMMARY OF MAIN CHANGES
This DEP is a revision of the DEP of the same number dated January 2010.
The only main change made is that the previous DEP has been split into this Specification
and an adjoining Informative DEP
1.7
COMMENTS ON THIS DEP
Comments on this DEP may be sent to the Administrator at standards@shell.com, using
the DEP Feedback Form. The DEP Feedback Form can be found on the main page of
“DEPs on the Web”, available through the Global Technical Standards web portal
http://sww.shell.com/standards and on the main page of the DEPs DVD-ROM.
DEP 31.40.10.12-Gen.
February 2011
Page 8
2.
GENERAL DESIGN ASPECTS
A multi-phase flow pipeline is intended for transporting the gas and liquid phases
simultaneously. The slug catcher situated at the end of the pipeline is intended to separate
the phases and to provide temporary storage for the liquid received.
DEP 31.40.10.12-Gen.
February 2011
Page 9
3.
HYDRAULIC DESIGN
3.1
GENERAL
The hydraulic design of slug catchers is based on theoretical and experimental work and
field experience with existing slug catchers, and shall be performed by competent
specialists and approved by Principal.
3.2
SLUG CATCHER SIZE
The size of a slug catcher (Volsc) is directly related to the maximum liquid volume it has to
hold. It SHALL [PS] be able to intercept the maximum possible slug or surge (Volint)
emerging from the multi-phase pipeline at any part of its agreed operating envelope. It
should also contain a buffer volume (Volbuffer) in order to guarantee liquid supply to treating
facilities downstream of the slug catcher. Hence:
Volsc = Volint + Volbuffer
Steady state and dynamic simulation tools for multiphase flow in pipelines shall be used for
this purpose.
3.3
SLUG CATCHER GEOMETRY AND COMPONENTS
The inlet pipe shall also be configured to avoid flow maldistribution in the inlet header. The
inlet pipe shall have a straight length of 10 pipe diameters with no bends in the horizontal
plane before it connects to the inlet header. Bends in the vertical plane are acceptable as
they do not induce lateral flow maldistribution. Similarly, it is not necessary for the inlet pipe
to connect horizontally to the inlet header.
As the number of downcomers (and hence of primary bottles) per inlet manifold increases,
it becomes more difficult to evenly distribute the fluid over them. If there are more than eight
downcomers, the splitter arrangement shall be confirmed with Computational Fluid
Dynamics (CFD). Expert advice is needed to define the CFD scope of work and to interpret
the results. CFD may also be usefully used to confirm the effect of the inlet pipe on flow
distribution and also if there are less than eight bottles.
The approval of the Principal shall be sought for any design which uses secondary bottles.
It shall be ensured that the liquid outlet header is accessible for cleaning.
The designs of slug catchers for water-wet pipeline operation are still in development and
all designs shall be approved by the Principal.
Since one of the main HSE risks is loss of containment, the number of flanges, fittings
should be kept to a minimum and instrument bridles should not be used for slug catchers.
Special fluid aspects, such as corrosiveness, toxicity and presence of solids or
contaminants, shall be considered. Special risks include foaming, fouling, plugging and
deposits.
Pressure tappings in the liquid outlet header shall be positioned to avoid the effect of solid /
sludge build up.
Slug catchers are maintenance items that need on occasion be taken out of service for
internal inspection, cleaning out solids, repairs etc. Even if such outages cannot readily be
aligned with the outages of the downstream plant, then provision shall still be made for
internal maintenance by the use of a splitable slug catcher or other equivalent means.
DEP 31.40.10.12-Gen.
February 2011
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4.
MECHANICAL DESIGN
4.1
GENERAL
Slug catchers are an integral part of a pipeline system and SHALL [PS] be designed in
accordance with DEP 31.40.00.10-Gen, which is an amendment to ISO 13623. In applying
DEP 31.40.00.10-Gen, the following should be taken into consideration:
•
Under ISO 13623, a mixture of hydrocarbon gas and hydrocarbon liquid will be fluid
category E.
•
Depending on the applicable location class, as described in Annex B of ISO 13623,
the design factor on hoop stress for category D and E fluids ranges from 0.45 to
0.67.
The demarcation of the slug catcher from downstream facilities shall be indicated on the
process engineering flow schemes by indicating the design code breaks.
In some jurisdictions, it may be necessary to design the slug catcher to the ASME B31.8
Pipeline code, and not to the ISO 13623 code. In this case note the following:
•
4.2
The ASME B31.8 code is limited to temperatures down to –29°C (–20°F). The
lower design temperature resulting from depressurisation (see section 4.10) may
be lower, in which case either an alternative design code should be used or the
blowdown should be changed to avoid the low temperature.
MAXIMUM ALLOWABLE OPERATING PRESSURE
If line packing is required for the operation of the pipeline system, the MAOP of the slug
catcher SHALL [PS] not be less than the MAOP of the feeding pipeline system. If line
packing is not required, then maintaining the MAOP of the slug catcher at the MAOP of the
feeding pipeline system is preferred as this will eliminate the requirement for separate
overpressure protection. If line packing is not required the MAOP of the slug catcher may
be lower than for the feeding pipeline system if a significant net lifecycle cost benefit can be
demonstrated from lowering the MAOP and installing dedicated overpressure protection for
the slug catcher.
4.3
DESIGN FACTOR FOR HOOP STRESS
The design factor to limit the hoop stress arising from pressure containment shall be
selected according to DEP 31.40.00.10-Gen.
NOTE:
4.4
The design factor applies to the minimum wall thickness.
OTHER STRESS
All applicable loads SHALL [PS] be considered when determining combined stresses
during pressure testing and operations. These loads will include:
a)
pressure;
b)
thermal expansion;
c)
passage of slugs or surges;
d)
foundation and support reaction;
e)
settlement;
f)
environmental loads.
Settlement includes deformations due to the possible different settlements of the slug
catcher and the connecting structures and for buried slug catchers, possible deformations
predicted at the transitions between exposed and buried parts, due to different settlements
of the slug catcher and the connecting structures.
The equivalent stresses shall not exceed the limits specified in DEP 31.40.00.10-Gen.
DEP 31.40.10.12-Gen.
February 2011
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4.5
SUPPORTS AND ANCHORS
The foundation of locally supported slug catchers shall be in accordance with
DEP 34.19.20.31-Gen.
The bottles of a buried slug catcher should be situated entirely below original ground level
in areas with a natural slope similar to the downward slope necessary for the gravity fill of
the bottles.
On a level plot, the bottles may be buried in a stable sand mound above the original ground
level, with the liquid outlet header above the original ground level, or to reduce the height of
the mound, in a shallow pit.See Figure 3 for examples of burial design for slug catchers.
Due consideration shall be given to the fact that inspection of a buried slug catcher is
difficult.
The visual impact of the slug catcher on the environment should also be considered when
determining its level and requirements for burial.
4.6
MATERIAL SELECTION
4.6.1
General
The materials of the slug catcher shall be selected as part of the project’s overall material
selection study and in accordance with DEP 39.01.10.11-Gen. and DEP 39.01.10.12-Gen.
The following should be used as guidance:
4.6.2
•
Solids can build up in the bottles, so under-deposit corrosion can occur.
•
If the ambient temperature is colder than the fluid arrival temperature, then the gas
will cool and water will condense out of it. This can especially be a problem in parts
of the slug catcher where the gas is normally not flowing, such as the bottles. Then
the condensing water can cause top of line corrosion.
Prevention of brittle fracture
The lowest temperature occurring in the slug catcher during depressurisation shall be
determined in accordance with DEP 80.45.10.12-Gen, the resulting low temperature
SHALL [PS] be used as the slug catcher low design temperature.
For guidance on the selection of the lower design temperature for slug catcher materials,
reference is made to DEP 30.10.02.31-Gen. Although this DEP specifically applies to
pressure vessels and piping systems (and associated codes PD 5500, ASME VIII and
ASME B31.3), its content may be useful to determine the applicable minimum design
temperature in consultation with the appropriate Technical Authorities.
4.7
INTERNAL/EXTERNAL CORROSION PROTECTION
The potential for internal corrosion in the slug catcher shall be evaluated to determine the
need for a corrosion allowance or other measures to mitigate internal corrosion.
Cathodic protection of
DEP 30.10.73.31-Gen.
a
buried
slug
catcher
shall
be
in
accordance
with
Anti-corrosion coating of both buried and non-buried slug catchers shall be in accordance
with DEP 31.40.00.10-Gen.
4.8
OVERPRESSURE PROTECTION
Separate dedicated overpressure protection is not required if the MAOP of the slug catcher
is not less than the MAOP of the feeding pipeline system. (NB it may still require PSVs for
fire relief.)
If the MAOP of the slug catcher is less than the MAOP of the feeding pipeline system then
it
SHALL
[PS]
be
safeguarded
against
overpressure
accordance
with
DEP 31.40.10.14-Gen.
DEP 31.40.10.12-Gen.
February 2011
Page 12
Note that high capacity PSVs on a pipeline are prone to chattering and that this can cause
severe damage and even loss of containment. Modulating PSVs can avoid chatter.
4.9
MAINTENANCE
The plant’s maintenance philosophy shall consider the need to maintain the slug catcher
and its ancillary equipment (such as PSVs, blowdown valves and liquid level shut down
valves). It may for instance be necessary to provide installed spare PSVs (with isolation) so
that they can be maintained without taking the slug catcher out of service. The slug catcher
may need to be a splitable type so that the gas plant can continue production during slug
catcher maintenance. The means of maintaining a slug catcher shall be assessed and
incorporated into its design. For instance:
4.10
•
If certain tasks require entry into the slug catcher, then the design shall make this
access feasible. If the Job Hazard Analysis concludes that this shall be done
wearing breathing apparatus, then the pipe size and manhole size shall be sized
accordingly.
•
Removal of solids from the liquid header is most conveniently done if there are end
flanges on both ends of the header, however safety analyses (5.3) has the aim of
reducing potential leak paths by eliminating flanges. An option assessment shall be
made to determine the optimal solution based on the balance of loss of
containment safety risk and maintenance safety risk.
•
Requirements for the condition monitoring of buried slug catchers shall be
established during the design stage and be designed to minimise the requirement
for future excavation.
EMERGENCY DEPRESSURISATION
The evaluation of the slug catcher depressurisation shall be in accordance with
DEP 80.45.10.12-Gen. The following shall be considered in this evaluation:
a)
Due to their very large inventory, it is usually impracticable to depressure a slug
catcher at the same rate as the rest of the plant.
b)
Lower depressurisation rates have to be justified by analysing the loss of pressure
retaining ability (due to the affects of pool fires etc.) against the depressurisation
profile in time.
c)
It may be necessary to increase the wall thickness of small diameter components
to bring their period of fire resistance up the same value as the rest of the slug
catcher.
d)
Because slug catchers are depressurised quicker than their pipelines, the slug
catcher’s lower design temperature is usually lower than that of the pipeline and
hence the material specification may be different. For instance, the pipeline may be
specified for the minimum design ambient temperature (e.g. 0°C), but the slug
catcher may be specified for the blowdown temperature (e.g. –29°C). It is thus
common that materials that were specified for the pipeline cannot be used for the
construction of the slug catcher.
e)
Care has to be taken when calculating the slug catcher minimum blowdown
temperature, particularly if the slug catcher has a very large and complicated
geometry as the metal wall temperature will not be uniform during the blowdown.
The two parts that may be particularly subjected to low temperatures are:
•
The pipe wall in the vicinity of the blowdown nozzle. The heat transfer
coefficient between the high velocity cold gas and the nozzle will be
relatively high, bringing the metal temperature close to that of the cold gas.
It is common to use an insert (e.g. of stainless steel) to shield the nozzle
from the high velocity cold gas.
•
The slug catcher low points, particularly the bottom of the bottles and the
liquid outlet header. In the early part of depressurisation, the slug catcher
DEP 31.40.10.12-Gen.
February 2011
Page 13
inventory may enter the retrograde region, forming light hydrocarbon
condensate which will run to the bottom of the slug catcher. Later in the
depressurisation, when the pressure is below the retrograde region, this
liquid will boil, generating low temperatures.
It is possible that most of the rest of the slug catcher walls have only dropped a few
degrees below the ambient temperature.
4.11
MISCELLANEOUS
The constrictor at the start of each downcomer can be achieved by an unequal tee or by a
forging. There is no minimum pipe length at the constricting diameter, so the swage to the
larger diameter should be welded fitting to fitting with the tee.
The transition in bottle slope for dual slope slug catchers is best achieved with a bend. The
bend can be several metres long to avoid difficulty in fabrication.
The gas risers should be approximately vertical. It is acceptable to use a tee with a 90°
branch as the resulting riser will only be approximately 1.5° from the vertical.
The spacing between bottles shall be sufficient to allow construction and maintenance (e.g.
painting, inspection). In the concept identification and concept selection project phases, the
minimum spacing allowed is 1.5 bottle diameters. In project specification and detailed
design project phases, the construction of the slug catcher will be determined and this will
affect the bottle spacing. For instance, if the liquid outlet manifold is to be constructed from
tees, then the closest mechanically feasible bottle spacing will be when the tees are welded
fitting to fitting, without intervening spools.
Consideration should be given to not constructing the manifolds from tees, but as a single
item with extruded branches. This may be cheaper, require less welding on site and allow
closer bottle spacing. This would also overcome an additional disadvantage of using tees they are usually internally profiled like a barrel and these form pockets that cannot drain.
DEP 31.40.10.12-Gen.
February 2011
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5.
HAZARD AND RISK MANAGEMENT
5.1
GENERAL
Loss of containment from a slug catcher presents a major potential contribution to the level
of facility safety risk both for personnel within, and members of the public outside, the
fence. Slug catchers typically contain a large inventory of non-stabilised flammable
hydrocarbons that cannot be rapidly disposed of to flare in the event of an emergency. In
addition, if the reservoir fluids are sour, the associated toxic gas risk in the event of a
release can be far reaching.
The application of the Hazards Effect Management Process (HEMP) is a mandatory
requirement of EP 2005-0300 for any major facility component, including slug catchers.
This process begins at the concept identification phase and continues through to the
finalisation of detailed design.
Selection of the slug catcher concept and its location effectively determine the residual HSE
risk level to be managed during the commissioning and operating phases. During
subsequent design work the focus is on optimising the design details that ensure
constructability, technical integrity, operability and maintainability. Relevant formal HEMP
based studies and ALARP demonstration requirements are summarised in (5.2), and
design considerations in (5.3).
5.2
APPLICATION OF FORMAL HEMP BASED STUDIES
Table 5.1 summarises the formal HEMP (EP2005-0300) based studies relevant to the
achievement and demonstration of ALARP HSE risk levels. This list is derived from the
Opportunity and Project Management Guide, EP 2006-5500, Chapter 4 and its companion
document, GS.06.50034. Specialist HSE advice should be sought to identify the optimum
type and timing of studies for inclusion in the project HSE Plan.
DEP 31.40.10.12-Gen.
February 2011
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Table 5.1
HSE in slug catcher design
Activity
Key Reference
Concept Identification & Concept Selection
Concept Phase HAZID:
Identify major hazards and coarsely assess associated
risks.
HAZID, EP95-0312
Concept Layout Study:
Assess slug catcher location options to optimise process,
safety and operability benefits.
None specific.
Major Hazards Assessment:
For each option being considered; carry out plausible
scenario based Physical Effects Modelling, PEM, and
preliminary QRA to produce fire/explosion and toxic risk
contours; also cover events that may impact the slug
catcher.
(Note that additional, or complementary, risk assessment
may be necessary to meet regulatory requirements).
Physical Effects Modelling,
EP 95-0314
Quantitative Risk Assessment,
EP 95-0352
Coarse HAZOP and Initial Instrumented Protective
Function, IPF, and Safety Integrity Levels, SIL studies.
HAZOP EP 95-0313
Classification and implementation
of instrumented protective
functions DEP 32.80.10.10-Gen.
Include HSE assessment and ALARP justification for the
slug catcher in the concept selection report.
HSE Cases EP2005-0310
Front End Engineering Design, FEED
Detailed Layout Study.
None specific.
Preliminary Bow-tie Study, identification of HSE (Safety)
Critical Elements, SCEs, and Performance Criteria.
Bow-Ties EP 2005-0300-SP-02
Detailed PEM and Fire and Explosion Assessment (may
also involve a FIREPRAN study) leading to a Fire Safety
Report.
FIREPRAN EP 95-0350
DEP 80.47.10.30-Gen
Assessment of Fire Safety of
onshore Installations
Detailed QRA.
As above
Main HAZOP and IPF studies.
As above
Human Factors Engineering, HFE, Analysis
(covering maintainability and operability including Job
Hazard Analyses)
HFE in New Facilities Projects,
Yellow Guide
Preliminary Construction Health Risk Assessment, HRA
HRA EP 2005-0300-PR-10
Issue End FEED Design HSE Case
HSE Cases EP 2005-0310
Detailed Design
Update and finalise all FEED studies
Finalise Design HSE Case
5.3
HSE Cases EP 2005-0310
DESIGN CONSIDERATIONS
Concept identification will consider different sizes and types of slug catcher, and possibly
options that do not require a slug catcher, for example:
•
A single large two phase pipeline will need a larger slug catcher than two smaller
pipelines.
•
Dehydrating the gas and condensate before they enter the two-phase pipeline,
whilst adding upstream facilities, may eliminate the need for frequent corrosion
inhibition and associated sphering. The slug catcher would then only need to be
DEP 31.40.10.12-Gen.
February 2011
Page 16
sized for the slug resulting from flowrate changes, not the sphere generated slug.
Furthermore, frequent solids removal is also eliminated.
These are important issues that impact ongoing operational exposure and risk, which shall
be taken into account in concept selection. After concept selection has been made, scope
for risk reduction is limited to refinement of the chosen design and related operation.
Once the concept is selected the factors that influence safety in design are addressed, with
Bow-tie Analysis also contributing to identification of risk reduction opportunities. Some key
considerations that may influence loss of containment risk levels and contribute to
achievement of ALARP are:
a)
Location of slug catcher within a facility in terms of jet fire related threats that the
slug catcher may pose to the adjacent facilities and vice versa. Jet fire
impingement on adjacent facilities should either not be able to occur or not occur
for long enough to lead to rupture.
b)
Overpressure protection (including consideration of process unit interfaces and
incoming pipeline packing expectations), isolation, and blowdown rate.
c)
Corrosion allowances, corrosion management.
d)
Reduction in potential leak paths (e.g. number of flanges, fittings and nozzle
connections). This includes minimising the number of small bore connections, as
they are more easily broken by mechanical impact, vibration etc. Reduction in
potential leak paths is a very effective way of reducing risk, however note the
requirement in (4.9) for maintainability.
e)
Reduction in opportunities for escalation due to flame impingement and radiation
(e.g. orientation of flanges, routing of connecting pipework, proximity of other
process units). A specific example here is the addition of solid plate decking on
platforms installed to access flanged manways; this decking will deflect any
potential jet fires that may otherwise impinge directly onto the slug catcher.
f)
Small bore connections to the slug catcher should be DN 50 (NPS 2) minimum.
Temperature Indicators should be located in thermowells directly in the DN 50
connections. Pressure Indicators should be via small bore instrument tubing
(< 10 mm) connected to the DN 50 connection to the slug catcher thereby limiting
the release should these be damaged or fail. Note that this same approach should
be adopted for adjacent facilities where there is a potential for jet fire impingement.
g)
Connections (of any size) to the slug catcher can be broken and as the slug
catcher inventory is typically very large, it is important to provide the means of
stopping the release of the inventory. Isolation valves SHALL [PS] therefore be
installed directly on the slug catcher nozzles for all connecting pipework. This
includes connections to appurtenances such as level bridles. As it is necessary to
close these valves in an emergency, they shall be either manual (in which case
they need to be accessible to be able to operate them) or automatic (in which case
they need to be accessible to carry out their maintenance).
h)
Active and Passive fire protection: Consideration could be given to remotely
operated, manually activated, water monitors. Passive fire protection could be
considered, but only where jet fire impingement could credibly occur.
i)
Segregated catchment and fire safe drainage systems graded to prevent escaping
liquids being able to form pool fires under the slug catcher (assuming that released
fluids are capable of forming flammable pools – this should be determined as part
of the hazard assessment).
Considerations that influence other slug catcher related safety and health risks are:
•
Identification of operating, maintenance and emergency response procedures and
any required implementation in the design. These barriers or controls and those
responsible for making the controls work shall be included in the bowtie analysis.
DEP 31.40.10.12-Gen.
February 2011
Page 17
•
Safe entry to the slug catcher (including when one piece of the slug catcher is in
operation and the other is under maintenance) shall undergo a design phase Job
Hazard Analysis as part of the design development to determine all requirements,
including:
o
Isolation requirements (locations of blinds etc)
o
How to purge and ventilate
o
Safe removal and disposal of pyrophoric solids
Other considerations are:
•
Loss of containment risk assessments for slug catchers and other facility
components with large inventories include an assessment of the likelihood and
consequences of a potential catastrophic failure. The likelihood of a well designed,
operated and maintained slug catcher rupturing catastrophically is extremely low.
However the use of historical data (often correlated with other data for facility
components judged similar) does generate a frequency. The modelling of potential
consequences (amount released, fire and explosion effects, toxic effects) as a
result of such catastrophic failures often falls outside the modelling range. The
tendency to use of the BLEVE (Boiling Liquid Expanding Vapour Explosion) model
that assumes that the entire inventory of the vessel failing undergoes vapour
expansion, something that is not credible for a multiple pipe slug catcher simply
because of its size, can serve to exaggerate assessments. However unlikely, the
consequences of a catastrophic failure of a slug catcher will be dramatic and an
aversion to such consequences, regardless of likelihood, tends to dominate
societal risk discussions with local and regulating authorities. Specialist expert
advice shall be sought to support the assessment and communication of societal
risk with respect to such catastrophic events.
•
Although burial appears an attractive way to reduce risk of leaks, fire and
escalation, burial makes inspection of the slug catcher difficult and hence they shall
not be buried unless approved by the Principal.
NOTE:
Although pipelines are buried, they are inspected by intelligent pigging, whereas it is currently
not possible to inspect a slug catcher that way.
DEP 31.40.10.12-Gen.
February 2011
Page 18
6.
REFERENCES
In this DEP, reference is made to the following publications:
NOTES:
1. Unless specifically designated by date, the latest edition of each publication shall be used,
together with any amendments/supplements/revisions thereto.
2. The DEPs and most referenced external standards are available to Shell staff on the SWW (Shell
Wide Web) at http://sww.shell.com/standards/.
SHELL STANDARDS
Prevention of brittle fracture in New Equipment
DEP 30.10.02.31-Gen.
Design of cathodic protection systems for onshore buried pipelines
DEP 30.10.73.31-Gen.
Pipeline engineering (amendments/supplements to ISO 13623)
DEP 31.40.00.10-Gen.
Pipeline overpressure protection
DEP 31.40.10.14-Gen.
Classification and implementation of instrumented protective
functions
DEP 32.80.10.10-Gen.
Reinforced concrete structures
DEP 34.19.20.31-Gen.
Selection of materials for life cycle performance (EP) – Materials
selection process
DEP 39.01.10.11-Gen.
Selection of materials for life cycle performance (EP) – Upstream
equipment
DEP 39.01.10.12-Gen.
Emergency depressuring and sectionalizing
DEP 80.45.10.12-Gen.
Assessment of the fire safety of onshore installations
DEP 80.47.10.30-Gen.
AMERICAN STANDARDS
Process piping
ASME B31.3
Gas Transmission and Distribution Piping Systems
ASME B31.8
ASME Boiler and Pressure Vessel Code – Section VIII: Rules for
construction of pressure vessels
ASME VIII
Issued by:
American Society of Mechanical Engineers
ASME International
Three Park Avenue, M/S 10E
New York, NY 10016-5990
USA
BRITISH STANDARDS
Unfired fusion welded pressure vessels
PD 5500
Issued by:
British Standards Institution
389 Chiswick High Road
London W4 4AL
UK
INTERNATIONAL STANDARDS
Petroleum and natural gas industries - Pipeline transportation
systems
Issued by:
ISO Central Secretariat
1, ch. de la Voie-Creuse
Case postale 56
CH-1211 Genève 20
Switzerland
Copies can also be obtained from national standards organizations.
ISO 13623
DEP 31.40.10.12-Gen.
February 2011
Page 19
APPENDIX A
Figures
1
Typical multiple pipe slug catcher
2
Multiple pipe slug catcher details
3
Examples of burial design
DEP 31.40.10.12-Gen.
February 2011
Page 20
me
t re
s
liquid outlet section
20
0
to
40
0
bottle section
≈2
5m
e tr
es
gas outlet
section
inlet section
Figure 1
Typical multiple pipe slug catcher
DEP 31.40.10.12-Gen.
February 2011
Page 21
inlet
splitter
inlet header
gas outlet
gas outlet
header
downcomer
primary bottle
1st gas riser
2nd gas riser
Figure 2a
Multiple pipe slug catcher details
primary bottle
secondary bottles are
not fed from the inlet
manifold
secondary bottle
Figure 2b
Multiple pipe slug catcher details - Primary and secondary bottles
DEP 31.40.10.12-Gen.
February 2011
Page 22
Figure 3
Examples of burial design