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Offshore Petroleum Technology Conference 2007

Advantages of Pipeline Bundles for HP/HT Systems

Dr T Sriskandarajah (Global Pipelines Engineering Manager), J Brydon (Engineering Manager), W Watt (Pipeline Construction Manager) and R Wilkins (Principal Engineer), Subsea 7

Abstract

Finding cost-effective solutions to HP / HT fields in shallow and deep water is a challenge faced by the offshore oil and gas industry. Pipeline bundles have been around for a number of years, and due to the construction methods the pipeline operational (axial) stresses are typically lower than those in conventionally laid pipelines due to the relieving of the pipeline thermal stresses.

The use of advanced non-linear Finite Element modelling of the bundle system demonstrates clearly that pipelines typically do not reach full axial constraint and enable an optimised, cost-effective design to be achieved. This analysis also shows that a stress-based approach can be maintained which has potential benefits in terms design, fabrication and material specification compared to strain based designs.

This paper describes how pipeline bundles are modelled in ABAQUS with practical examples, and addresses the implication of design codes applied to bundle pipeline design.

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1. Introduction Pipeline bundles offer an alternative for installing short-to-medium tie-backs of wells to process / production facilities. They offer the benefits of a single installation when lines, i.e. production flowlines, test lines, gas / water injection or gas lift together with control / injection umbilicals need to be installed together.

Bundles are fabricated onshore and installed by towing offshore, suspended between two tugs. The various pipelines/umbilical systems are contained within a carrier pipe which is designed to provide buoyancy to the system in order to facilitate the tow. The primary function of the carrier pipe is to provide this buoyancy but following bundle installation and flooding of the carrier pipe annulus, the carrier pipe provides secondary benefits eg, On Bottom Stability, Trawl Board Protection, Controlled Environment for the internal pipelines and Control of Upheaval Buckling.

The construction of a pipe in pipe system is similar in principle whether installed by reeling, S-lay, J-lay or tow methods the insulated pipe is supported within the sleeve pipe by means of polymer centralisers clamped to the production pipe. The insulated production pipe is inserted into the sleeve pipe supported by the centralisers. These centralisers preferably have a low friction co-efficient.

The construction of the bundle requires a further spacer with polymer rollers to be clamped around the sleeve pipe for insertion into the carrier pipe. These spacers remain functional during operation to hold the pipes in their configuration, but these spacers also allow some axial freedom within the constraints of the end bulkheads.

The bundle design is usually performed along the same lines as for pipe-in-pipe systems in respect to thermal expansion. Although the mechanics of the thermal expansion design are the same for single and multiple pipe systems, there are some important consequences in considering the behaviour of bundle systems. For short-to-medium length tie-backs with high pressure lines with temperatures up to 130C there are cost benefits to be gained from analysing the specific mechanics of the bundle system as demonstrated.

2. Bundle and Pipe-in-Pipe Configurations Typical bundle and pipe-in-pipe system configurations used in the analyses for the parametric studies are shown below, in Figure 1. Typically, in shallow water locations (~100m water depth) a 24 diameter bundle would be left on the seabed surface, whereas a 12 diameter pipe-in-pipe would need to be buried for protection against third party activities.

Figure 1: Typical Bundle and Pipe-in-Pipe Configurations

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3. Thermal Expansion Analysis Pipeline systems, either laying on the seabed or trenched and buried, will tend to expand when subjected to internal pressure and elevated temperature. The friction that develops between the pipe system and the seabed soil provides resistance to the expansion, which in turn develops compressive force within the system. This compressive or effective axial force is a function of the internal and external pressures, the design temperature, the pipeline submerged weight, pipeline/soil friction, and friction between the internal system components.

Mechanics of Thermal Expansion The pipeline lying on the seabed will develop effective axial compressive forces within the system when subjected to operating temperature and pressure. As it expands under operating conditions, soil friction forces between the outer pipe and the seabed oppose the free thermal expansion of the assembly and results in an overall effective axial compressive force developing within the system. The magnitude of the maximum overall effective axial force depends on whether or not the pipeline develops full axial constraint. If the pipeline is operating in the end expansion zone, then the overall effective axial force, PEFF (compression positive), is a function of the soil friction and submerged weight and distance from the end, given as:

( ) += TAPEff R.WP where Wp represents the pipeline submerged weight; A is the pipeline/soil axial friction coefficient and RT is the resistance provided by external forces. The overall effective axial force increases from the pipeline end location until it develops full axial constraint, given by:

eiTWEff PPPP += where PTW is the true wall force, Pi and Pe are the forces due to internal and external pressure (peAe). Note that the effect of the external pressure needs careful consideration with respect to the contribution to the effective tension, but for shallow waters (~100m) this effect is negligible.

For multi-pipe systems, the true wall force comprises contributions from both the inner and outer pipes, i.e.

21TW PPP += where P1 and P2 are the true wall forces in the inner and outer pipes, respectively. At full constraint, the effective axial force for the inner and outer pipe are given by

1i1h11S1Eff PTAEP += and

2i2h22S2Eff PTAEP += where E represents the modulus of elasticity, AS is cross sectional area of the pipe wall, is the coefficient of thermal expansion, T is the temperature of the pipe wall, represents Poissons ratio and subscripts 1 and 2 refer to inner and outer pipes, respectively. The Poisson effect of the hoop stress needs to be computed based on either thin or thick wall theory, as appropriate.

For composite and lined / clad production / service pipes the thermal expansion effects of the liner may be included by means of an effective coefficient of expansion applied as:

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Lp

L

p

Lpeq A

AEE +=

For bundle systems in general, the effective axial force develops due to the internal pressure and temperature applied to the production pipe(s) and service (gas / water injection, gas lift etc). The other service pipes within the system i.e. the sleeve and carrier pipes provide restraint against the thermal expansion, which tends to put the pressurised / heated pipes into compression and the other pipes into tension.

System Compliance Multi-pipe systems may be classified as being either compliant or non-compliant, depending on the method of load transfer between the inner and outer pipes. A compliant system is defined as one in which continuous force transfer (axial and bending) occurs between the inner and outer pipes along the length of a pipe system. A typical example of a compliant system is the use of polyurethane foam, in which continuous force transfer occurs between the inner and outer pipes. The use of discrete nylon or similar centralisers to hold the inner and outer pipes apart represents an example of a non-compliant system.

For bending compliance the inner and outer pipes must remain concentric during flexure, which implies equivalence of curvature. If bending compliance is achieved, then multi-pipe systems act as an equivalent single pipe in bending, with a flexural rigidity equal to:

( ) ( ) ( ) ( )n21EQ EIEIEIEI +++= L In cases where discrete spacers are used bending compliance may not achieved, and the curvature of the inner and does not match that of the outer pipe, then the analysis becomes more complex. There is no simple analytical method of determining the equivalent pipe section properties, and FE analysis represents the only way to realistically model the behaviour.

Force Transfer The overall expansion of the production and sleeve pipes for both compliant and non-compliant systems is constrained to be equal by end bulkheads. The compliant or non-compliant behaviour only comes into effect in the way in which the frictional forces are transferred between the various component pipes.

The effective axial force builds up from the soil friction effects as the system expands. In the absence of pipe-soil friction the strain in the inner pipes would be constant along the length of the system, which would represent the best possible case with regard to design of production pipes. As soil-friction effects are included, the effective axial force builds up from the ends until a condition of full axial constraint is reached in the sleeve, or carrier pipe in contact with the soil. This change in the effective axial force drives the change in inner production pipes via the pipe-in-pipe friction. The relative displacement and force transfer can occur for a considerable distance past the carrier / sleeve pipe anchor point.

Compliant behaviour alters the axial force distribution on the inner and outer pipes for systems that do not reach full axial constraint. The compliant system achieves the optimum force distribution in the outer pipe, by reducing the maximum tensile force that occurs at the pipeline end locations. This is achieved, however, at the expense of an increase in the maximum compressive force that occurs at the mid-point of the production pipe. Note, however, that for any particular system, the overall effective axial force is unaffected by the compliant or non-compliant behaviour.

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Pipe-in-Pipe System Expansion

Under the effects of temperature and pressure, the production pipe will tend to expand at the ends. The sleeve pipe provides constraint as the two pipes are connected by bulkheads at the end locations. The production pipe therefore goes into compression and the sleeve pipe into tension.

The tension in the sleeve reduces from the end bulkhead location due to the action of pipe-soil friction, until the sleeve pipe reaches a condition of full axial constraint. The compressive axial force in the production pipe builds up from the end due to the friction force induced between the production pipe and the sleeve pipe. Depending on the length of the pipeline, the production pipeline will reach full axial constraint at some distance past the location past that at of the sleeve pipe. Figure 2, shows a typical force distribution for a pipe-in-pipe system.

Figure 2: Force Distribution in Pipe-in-Pipe System

Bundle System Expansion

Similar to the pipe-in-pipe system, the production pipe will tend to expand under the effects of temperature and pressure. The carrier, sleeve and product pipes are constrained to move together at the ends of the bundle by bulkheads. The production pipe therefore goes into compression and the carrier pipe goes into tension, similar to the sleeve pipe of the pipe-in-pipe system. Figure 3 shows the force distribution of the pipes within the bundle system.

The behaviour of the bundle differs from that of the pipe-in-pipe system in that there is an additional interface between the carrier pipe and the production pipe. The two interfaces in the bundle system are:

- The sleeve to production pipe interface, the friction force of which develops from sliding friction of the polymer centralises. This is a Coulomb type friction, with typical values of friction coefficient of the order of 0.43;

- The carrier to sleeve pipe interface, the friction force of which develops as the main spacers translate within the carrier pipe. This is a roller type friction, for which the equivalent value of Coulomb friction is somewhat less than that of the sleeve to production pipe.

As the thermal expansion takes place, the driving force is that of the production pipe expanding, and the resistance is provided by the carrier pipe contact with the seabed. The actual friction force that develops within the production pipe is dictated by the lesser of the forces that can develop between the two frictional interfaces, in this case the frictional force

Production Pipe

Sleeve Pipe

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less than that of the carrier and sleeve pipe. This is the important difference in the behaviour of the bundle compared to that of a pipe-in-pipe system.

Figure 3: Force Distribution in Bundle System

4. Pipeline Bundle / Pipe-in-Pipe Design Strategy Traditionally, pipeline design codes have used allowable stress design (ASD) methods based on a straightforward application the Barlow / Lame equations for circumferential stress and von Mises criterion for equivalent stress. More recently, limit-state design codes have been introduced in which the load and resistance factor design method (LRFD) is used with partial safety factors on loads and resistances.

Each of these methods has its own advantages when applied to pipeline bundle / pipe-in-pipe design, and these are assessed below.

Stress Based Design The allowable stress design methods of PD8010 (Ref. 1) adopt yielding as the criteria for the design checks. The stresses are limited to some fraction of the SMYS, dependant on the design case under consideration. Two checks are performed, based on hoop and equivalent stress criteria, and the stress within the pipeline should meet the inequality:

A d yf < where fd is the usage factor, Y is the specified minimum yield. The formulation for calculating the hoop stress, based on thick wall theory, is based on the Lame formulae:

( ) ( )( )2 2o i

h i o 2 2o i

D DP P

D D

+ = where Pi and Po are the internal and external pressures, Do and Di are the outside and inside diameter, respectively.

The combined, or equivalent stress criterion limits is based on von Mises yield criteria, the 3-dimensional form of which is:

( ) ( ) ( )2 2 2e h l l R h R12 = + The criterion given in PD8010 is intended for thin walled pipes, as it neglects the radial stress effect. This is not considered to be consistent with the approach to hoop stress calculation, in

Production Pipe

Carrier Pipe

Sleeve Pipe

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which Lame theory is used for thick wall pipe. In HP pipelines, the radial stress at shut-in can reach values of 60MPa and above, which cannot be considered to be negligible.

As far as the design is concerned, limiting the hoop and equivalent stresses to some fraction of the pipeline material specified minimum yield stress (SMYS) presents several distinct advantages, namely:

material behaviour is assumed to be elastic, hence simplified, linear analyses may be used;

keeping the stress below yield means that the corresponding strains (elastic + plastic) are kept below 0.2%. Hence fracture and accumulated plastic strains are not a problem;

years of experience have shown that designs based on the ASD method are inherently reliable.

It should be noted, however, that the ASD method has certain limitations with regards to temperatures to which it may be applied. Attempting to satisfy the equivalent stress criterion for pipelines operating at temperatures in excess of 140C may severely limit the hoop stress capacity due to internal pressure and the design exercise becomes one of chasing the wall thickness. Figure 4 shows how the wall thickness requirements vary with temperature based on the equivalent stress criterion.

0.0

10.0

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Wal

l Thi

ckne

ss t

(mm

)

Figure 4: Wall Thickness Requirements vs Temperature based on Equivalent Stress Check

This does not mean that the pipeline will fail at this pressure/temperature: rather it illustrates a limitation of using the equivalent stress criteria. This is in area where the limit-state / strain based design may be usefully employed.

Limit State / Strain Based Design The limitations imposed by the equivalent stress criterion may be overcome by taking advantage of the fact that linepipe steels to API 5L possess significant ductility, and can undergo a certain amount of plasticity.

The DNV OS-F101 Offshore Standard (Ref 2) adopts a limit-state approach, with criteria for pressure containment (bursting and yielding), local buckling and limits on allowable strain. Other limit-states are defined, but these are not considered here. OS-F101 replaces the equivalent stress check with local buckling checks on the pipe section. Two criteria are

Key:

Hoop Stress based on 0.72 SMYS

Equivalent Stress based on 0.96 SMYS

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included: one for a section under load control, based on pressure, bending moment and axial force, and one for a section under displacement control, based on limiting strain.

In addition, limits on accumulated plastic strain for which additional requirements in terms of materials and welding are required. The limits are as follows:

When the equivalent plastic strain, p 0.3% then the requirements for materials and welding etc defined in OS-F101 are considered adequate, and no special requirements on materials are needed;

When the equivalent plastic strain, p > 0.3% a fracture assessment is required for the girth welds;

When the equivalent plastic strain, p > 2.0% then in addition to the fracture assessment, the material shall meet the supplementary requirements P.

The local buckling criteria require a combined loading check for interaction between pressure, bending moment and axial force which are presented in two forms: a load controlled and a displacement controlled condition.

The displacement controlled situation is defined when deformation of a pipe section is limited by either the restraints (such as a trenched and buried pipeline) or possibly in the case of secondary loads (i.e. temperature). In cases where the displacements are not bounded, then the situation is considered to be load controlled. Note that the load controlled equations in OS-F101 are reckoned to be unduly conservative.

To adopt a strain-based design, it is necessary to demonstrate that the production pipeline is under displacement control. In addition, the requirements for hoop strain ratcheting place a limit on the accumulated plastic strain. Under maximum design conditions of shut-in pressure and design temperature, the maximum permitted accumulated plastic strain shall not exceed 0.1%. A further requirement is that the pipeline shakes down to elastic behaviour after the first loading.

It must be noted that a strain based design adds significant costs to a project in terms of design manhours, material specifications, welding and NDE requirements.

5. Analysis of Bundle and Pipe-in-Pipe Systems In order to compare the performance of bundle and pipe-in-pipe systems a parametric study was performed for a situation typical of that where a bundle or pipe-in-pipe system would be used, i.e. a shallow-water (~100m depth) North Sea tie-back from a well to a production / process facility, located in an area with a sandy seabed.

The comparison of a high temperature pipe in pipe system installed by reeling and a similar pipe in pipe installed inside a pipeline bundle can be demonstrated by Finite Element Analysis. Analyses were performed in order to assess the benefits of the two systems, with sensitivity studies on the following:

Pipeline system length: lengths of 2.5, 5.0 and 7.5km were considered for the bundle and pipe-in-pipe systems;

Internal pressure: design pressures ranging from 300 500barg were considered; Temperature: design temperatures ranging from 90 140C were considered.

In order to conduct a meaningful assessment, the properties of the common pipe components were kept the same for both the bundle and the pipe-in-pipe system, as given below:

Product Pipe:

Outside Diameter : 168.3mm Wall Thickness : varies, depending on design pressure Material : API 5L X65 (SMYS 448 MPa)

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Sleeve Pipe:

Outside Diameter : 273.1mm Wall Thickness : 7.1mm Material : API 5L X52 (SMYS 359 MPa)

For the bundle system, the outer, carrier pipe properties are given below:

Outside Diameter : 622.3mm Wall Thickness : 9.6mm Material : API 5L X52 (SMYS 359 MPa)

Analyses were run for both the pipe-in-pipe and bundle cases with similar pipe properties, so as to enable a direct comparison of two systems to be made. The properties remained constant for all sensitivities, apart from the wall thickness of the production pipeline, which varied as the internal pressure increased. For the sensitivity on the pipeline length, the design pressure was maintained at 400barg and the temperature was maintained at 120C. The seabed soil in both cases was assumed to be medium sand, with friction factors appropriate to the surface laid / burial condition. For the pipe-in-pipe system, burial depths of 0.5, 1.0 and 1.5m were assessed in order to determine an appropriate value. An initial FE analysis indicated that burial to a depth of 0.5m left the pipe-in-pipe system susceptible to the effects of upheaval buckling, hence, a burial depth of 1.0m was adopted for the analyses.

Finite Element Analyses Finite element analysis of the pipe-in-pipe and bundle systems was performed using the ABAQUS general-purpose finite element package version 6.6 (Ref 3). The static, non-linear formulation was used, which accounts for both material and geometric non-linearity effects. The formulation also models the effects of friction, which have a significant effect on the expansion behaviour of the pipeline. The non-linear behaviour of the pipe material is introduced by explicitly defining the stress-strain curve for the steel, and the large displacements and rotations that can occur as the pipeline breaks-out are handled by the geometric non-linear facility.

The complete pipe-in-pipe / bundle lengths were modelled from end-to-end, which allows for the correct force transfer between the component pipes / spacers due to friction. This has important consequences in the development of the effective axial force within the production pipe of the two systems. Typically, models comprised upwards of 17,000 elements for the bundle systems.

Pipe-in-Pipe Model The model for the pipe-in-pipe system was constructed using ABAQUS 2-noded 3-D stress-resultant pipe elements, PIPE31. These elements, based on Timoshenko theory, account for the effects of transverse shear strain and allow for axial strains of arbitrary magnitude. The Poissons coupling effect between hoop and axial strains are included within the formulation which correctly models any hoop stresses developed by flexure. The constitutive equations are evaluated at several points around the pipe section circumference and model the gradual spread of plasticity that may occur across a pipe section.

For large displacement analysis, the non-linear contribution to the load stiffness due to internal overpressure is included, as is the end cap force that develops at pipeline ends. Internal pressure is specified as acting on the inside diameter of the product pipe, and external pressure is specified as acting on the outer diameter of the sleeve pipe, so as to give the correct effective axial force. The thermal effects due to the expansion of the product pipe are also included in the effective axial driving force.

The pipe-to-seabed interface was modelled via the ABAQUS finite-sliding rigid contact capability. This finite-sliding rigid contact capability is implemented by means of contact

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elements based on measures of the overclosure (or penetration) into a rigid surface) and relative shear sliding, which are then used, together with appropriate Lagrange multiplier techniques, to introduce surface interaction theories (contact and friction). The softened contact facility was used, which gives the seabed some flexibility, by allowing the contact between the carrier pipe and the seabed to build up gradually. The classical Coulomb friction law was used for the carrier pipe-seabed contact, with different friction factors in the both the axial and lateral direction.

The out-of-straightness of the trench bottom was modelled by adjusting the seabed profile to introduce a typical seabed imperfection of 0.2m height with a wavelength of 1.5 times that of a prop imperfection. The effect of burial for the pipe-in-pipe model was modelled by imposing the additional weight of the soil on top of the pipe and the increased friction on the sleeve pipe surface.

The pipe-in-pipe system was modelled with the ABAQUS ITT31 tube-to-tube contact facility which constrains the production within the sleeve pipe. These are slide line contact elements, in the sense that they assume that the relative motion of the two tubes or pipes is predominantly along the line defined by the axis of one of the tubes (the relative rotations of the tube or pipe axis are assumed to be small).

Bundle Model The bundle model was constructed using 2-noded 3-D stress-resultant pipe elements, PIPE31, described above. The sleeve pipe of the pipe-in-pipe system were attached to the spacers by beam elements, which in turn used the ITT31 contact slide-line facility to allow the spacers to slide within the carrier pipe. The three component pipes within the bundle were constrained at the end bulkhead locations with multipoint constraints.

The effects of the towheads were modelled as simple beam elements which can take a distributed weight and react on the seabed friction surface, thus providing a reaction force at the end bulkhead locations.

6. Stress Comparison of Bundle and Pipe-in-pipe Systems A series of analyses were performed to investigate the performance of pipe-in-pipe and bundle systems, assess the relative merits and to determine the limits (if any) of the two systems. Parametric studies were performed in order to determine the boundaries in terms of length, pressure and temperature. Analyses were performed for lengths of 2.5km, 5.0km and 7.5km, pressures ranging from 300 500barg and temperatures ranging from 90 - 140C. Friction plays an important part in the behaviour of both the pipe-in-pipe and bundle systems. There are, however, two important differences that need to be considered in behaviour of the systems. Firstly, the pipe-in-pipe system needs to be buried which will significantly increase the axial friction factor due to the increased contact area and the additional overburden. Secondly, for the pipe-in-pipe system the axial force build-up in the product pipe comes from the soil-pipe friction onto the sleeve pipe and via the pipe-in-pipe spacers onto the product pipe. For the bundle system, on the other hand, there is the additional interface of the bundle main spacers between the carrier pipe and the outer sleeve of the pipe-in-pipe system.

The effect of soil cover depth on the effective axial force in the production pipe of the pipe-in-pipe system is shown in Figure 5 for the 2.5km long system. For 1m soil cover the inner pipe is just below full axial constraint.

The spacers within the pipe-in-pipe system are assumed to be similar for the pipe-in-pipe within the bundle, i.e. these are based on a frictional coefficient of 0.43 for the type of spacers used.

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Effect of Variation in Pipeline Length The displacements for the pipe-in-pipe system are shown in Figure 7 for the inner and outer pipes, from which it may be seen that the sleeve pipe reaches full axial constraint due to pipe-soil friction approximately 450m from the end of the pipe system. The inner production pipe reaches full axial constraint approximately 1.5km from the end of the pipe system, which is evident from the plots for the 5km and 7.5km cases. These analyses were carried out for a pressure of 400barg with a temperature of 120C. The displacements of the component parts within the bundle system are shown in Figure 6 for the three lengths considered. As may be seen, the outer carrier pipe reaches full axial constraint approximately 1.2km from the towhead at the end of the bundle. The sleeve and production pipes do not however reach full axial constraint, even for the longer 7.5km case.

The effective axial forces within the pipe-in-pipe and bundle systems are shown in Figure 8. It may be seen for the pipe-in-pipe system that the 2.5km length is only just below full axial constraint and that both the 5km and 7.5km sections both reach full axial constraint. For the bundle system, however, the effective axial force for the 7.5km section is still below the limit of full axial constraint.

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2.5km Product Pipe Longitudinal Displacement

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Figure 5: Effect of Variation in Burial Depth for Pipe-in-Pipe System

Key:

Soil Cover 0.5m

Soil Cover 1.0m

Soil Cover 1.5m

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2.5km BundleLongitudinal Displacement

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Figure 6: Effect of Variation in Length on Displacements within Bundle System

Key:

Longitudinal displacement in Carrier Pipe

Longitudinal displacement in Product Pipe

Longitudinal displacement in Sleeve Pipe

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2.5km Pipe-in-PipeLongitudinal Displacement

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Figure 7: Effect of Variation in Length on Displacements within Pipe-in-Pipe System

Key:

Longitudinal displacement in Sleeve Pipe

Longitudinal displacement in Product Pipe

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Effective Axial Force in Product Pipewithin Bundle System

-2.60E+03

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Key:

2.5km Long System

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Full Axial Constraint

Figure 8: Effect of Variation in Length on Effective Axial Force in Product Pipe

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Effect of Variation in Pressure Pressure containment requirements dictate larger wall thicknesses, which in turn result in larger effective axial driving forces due to both the internal pressure and temperature effects. A series of analyses were performed to determine the utilisation ratios for pipe-in-pipe and bundle systems for pressures ranging from 300 500barg, the results of which are shown in Figure 9. It may be seen that the utilisation ratios, based on equivalent stress checks, are significantly less for the bundle than the pipe-in-pipe system.

Effect of Variation in Temperature A similar parametric study to that of pressure containment was performed for temperature ranges from 90 - 140C, the results of which are shown in Figure 10. It may be seen that the utilisation ratios, based on equivalent stress checks, are significantly less for the bundle than the pipe-in-pipe system.

It should be noted, however, that the equivalent stress check used in computing the utilisation ratios rapidly approaches a limit at around 150C. As may be seen from Figure 4, at 180C there is no wall thickness which satisfies the equivalent stress criterion. The use of limit-state design with a strain-based criterion (Ref. 4) would need to be adopted for assessing design temperatures above 140 - 150C. 7. Conclusions The additional frictional interface within a pipeline bundle system reduces the axial constraint for production pipes compared to a comparable pipe-in-pipe system. Taking advantage of the low friction of the main spacer assemblies results in reduced axial forces / stresses with comparable savings in material costs for the production pipe.

The reduced stresses with the production pipes in a bundle system permit stress-based design criteria to be used whereas for a pipe-in-pipe system, a strain-based design may be typically be required.

The use of a stress-based design avoids the need to procure linepipe to the DNV OS-F101 specifications.

A stress-based design avoids the more stringent welding and NDE requirements associated with a strain-based design.

8. References 1. PD8010 Code of Practice for Pipelines Part 2: Subsea pipelines, BSI 2004

2. OS-F101 Offshore Standard - Submarine Pipeline Systems, DNV 2000

3. ABAQUS Standard, version 6.6, 2006, ABAQUS Inc.

4 Klever FJ et al Limit-State Design of High Temperature Pipelines, OMAE 1994

17

Usage Factors for 2.5km Pressure Containment

0.7

0.74

0.78

0.82

0.86

0.9

250 300 350 400 450 500 550

Pressure (barg)

U

s

a

g

e

F

a

c

t

o

r

s

Pipe-in-Pipe Bundle

Usage Factors for 2.5km Temperature

0.7

0.74

0.78

0.82

0.86

0.9

80 90 100 110 120 130 140

Temperature (C)

U

s

a

g

e

F

a

c

t

o

r

s

Pipe-in-Pipe Bundle

Figure 10: Usage Factors for Temperature Variation Figure 9: Usage Factors for Pressure Containment