OTC 21671

Overview of the SAFEBUCK JIP D. A. S. Bruton - Atkins; M. Carr - Votadini

Copyright 2011, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 2–5 May 2011. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract

This paper summarizes key areas of research and development undertaken by the SAFEBUCK Joint Industry Project (JIP), including the development of engineering models and the testing of pipe-soil interaction, fatigue and local buckling. The SAFEBUCK Joint Industry Project set out to address one of the key design challenges for pipelines - buckling due to the compressive forces created by internal pressure and temperature - by supporting and guiding a design approach that deliberately encourages pipelines to buckle, but in a controlled way. By controlled initiation of lateral buckles at regular intervals, the loads are shared and reduced at each buckle site. Early application of this idea was hounded by problems due to a lack of know-how, which led to a number of failures, including three full-bore ruptures and one abandonment, mostly due to the issue not being addressed correctly in design. A related challenge addressed by the JIP, is pipeline walking, which has been observed on a number of pipelines, leading to one failure and a number subsea interventions to prevent future failure.

With the JIP now entering its third and final phase, the design guideline and research findings have been applied on a number of projects. This is therefore a good opportunity to present the SAFEBUCK design methodologies and research to a wider audience, and outline work underway now, in the final phase. A series of papers follow in the same session at OTC 2011, to provide more detail on the SAFEBUCK Design Guidance, research findings and the practical application of this technology to the lateral buckling and walking of pipelines.

 

Figure 1 – Side scan sonar image of lateral buckle Introduction

Background Subsea pipelines are increasingly being required to operate at higher temperatures and pressures. The natural tendency of such a pipeline is to relieve the resulting high axial stress in the pipe-wall by buckling. Uncontrolled buckling can have serious consequences for the integrity of a pipeline. An elegant and cost-effective design solution to this problem is to work with rather than against the pipeline by controlling the formation of lateral buckles (Figure 1) along the pipeline. Controlled lateral

2 OTC 21671

buckling is an efficient solution to the relief of axial compression. Indeed, as temperature and pressures increase further, lateral buckling may be the only economic solution. With the increase in deepwater developments, where pipeline trenching is uncommon, even low temperature pipelines such as water injection flowlines become susceptible to lateral buckling. This challenge has led to a radical advance in pipeline engineering with a greater need for a robust lateral buckling design solutions. Prior to the launch of SAFEBUCK, the industry’s understanding of lateral buckling was immature; many projects had undertaken research work and considerable engineering with the aim of developing a robust solution. However, project timescales frequently necessitated alternative fallback solutions and this approach led to some high profile project issues, in some cases risks were not identified early enough, leading to late design changes that incurred considerable project costs. This immaturity in knowledge was sadly demonstrated by the occurrence of unexpected lateral buckles in a number of pipelines, leading in some cases to pipeline failure. These failures included three catastrophic full-bore pipeline ruptures in the North Sea, West Africa and Brazil. In the first two cases, lateral buckling was not addressed correctly in design; in the last case, buckling was caused by the unexpected exposure and global instability of a pipeline buried in very soft clay. A similar story emerged with the issue of pipeline walking, which can cause global axial movement of a whole pipeline. Again, understanding in the industry was immature, although pipeline walking had been observed on a number of pipelines, leading to subsea intervention to eliminate walking on some pipelines and at least one full-bore connection failure. The SAFEBUCK JIP (joint industry project) was initiated in 2002, with the support of key operators in the industry to address these uncertainties and deliver a demonstrably safe and effective lateral buckling design approach, based on targeted experimental research and the development of Design Guidance. Groundbreaking research was carried out by a world-class team, of leaders in their field, to contribute to the design guidance and research programs including fatigue, local buckling, buckle initiation and pipe-soil interaction. The sharing of knowledge and data by participants within the JIP has also been exceptional. Many novel test-methods developed for the JIP are now used routinely for oil and gas projects.

The level of understanding, research and technology, developed by the JIP over the last nine years, is well beyond what can be achieved in any project timescale, showing how a JIP can take the long-term view to help improve the way we do projects. JIP Scope of Work, Technology and Data Sharing The scope of work for Phase I of the SAFEBUCK JIP is summarized in Figure 2 and discussed in this paper, which highlights key findings, design methodologies and test results.

Figure 2 Scope of Work for the SAFEBUCK JIP The starting point of the JIP was to assemble a knowledge base of participant’s and contractor’s experience, including test data and lessons learned from lateral buckling design and operating experience. This process of gathering of data has continued throughout the JIP. The JIP has therefore been incredibly successful at forming a global knowledge base, with relevant data

OTC 21671 3

donated by JIP contractors and JIP participants, on operating pipeline behavior and relevant project-specific test programs. The spirit of sharing knowledge and data within the JIP is exceptional, with participants donating data with estimated research costs far exceeding the total value of the JIP. This has provided a valuable, generic industry view of the data and lessons learned - a significant improvement over short-term, operator-specific or project-specific reviews. Design Methodology and Guidance

Design Guideline The SAFEBUCK Design Guideline, by Carr et al[10] is the most important updated deliverable from the JIP, which has now been employed on a large number of international projects. The Design Guideline outlines a clear, robust methodology, which addresses the inherent uncertainty in the lateral buckling problem and the complexity of the analysis required. The Guideline provides a framework that allows all stakeholders in the design process to focus on the key challenges, at all stages in the project lifecycle. Use of the Guideline is independent of any specific design code but is intended to work alongside the leading pipeline design codes DNV-OS-F101[15] and API 1111[14]. While the guideline is available to participants and their immediate subcontractors, a key aim of the current Phase III of SAFEBUCK is to formalize the Guideline into an Industry Recommended Practice. This will ensure that the work embodied in the Guideline is ultimately available to as large a number of projects as possible, for the benefit of the whole industry. The Guideline captures, at a high level, all the work that has been carried out to support the JIP, summarized in this paper, including the development of design and analysis methods, supported by extensive research into some of the most challenging uncertainties that designers face.

Design Methodology SAFEBUCK has developed a design methodology for lateral buckling and walking of pipelines, covering single pipe and pipe-in-pipe systems. This methodology is based on encouraging the deliberate formation of lateral buckles by pre-installing triggers at intervals along a pipeline that is susceptible to buckling. The assessment of susceptibility to pipeline walking and control of walking is also addressed. These methodologies are summarized in Figure 3 and the terminology is explained in the following sections.

Is pipeline susceptible to

buckling?

Select Initiation Method

Determine peak loads and cyclic stresses for Characteristic VAS

Are Key Limit States Exceeded?

Check Buckle Formation ReliabilityCheck ‘Characteristic’ spacing (VAS) Modify Initiation Method

or Select New Initiation Method

Design Parameters

Lateral Buckling Design Solution

No

Yes

No

Yes

Simplest firstMore complex with increasing P&T

• Local Buckling • Fatigue & ECA• Peak Strain Limit

Tolerable VAS > Characteristic VAS

Review Design

Parameters

Yes

Is pipeline susceptible to

walking?

Define rate of walking for each driving mechanism & potential contribution of each

For multiple buckles – assess localised & global walking response & buckle stability*

Are Displacement Limits Exceeded?

Define end and mid-line connection displacement limits Select Anchoring method to

control walking & define anchor location(s)

Design Parameters

Pipeline Walking Design Solution

No

Yes

No

Yes

• Slopes• Liquid hold up• Thermal Transients• SCR loads

Review Design

Parameters

Yes* May require reassessment

of lateral buckling solution

Figure 3 Design Roadmap for (a) Lateral Buckling and (b) Pipeline Walking Virtual Anchor Spacing A key component to the design approach is the adoption of a technique that simplifies design by analyzing an isolated lateral buckle between virtual anchors, in what is now commonly called “VAS Analysis”, where VAS refers to the virtual anchor spacing. As the flowline is encouraged to form a number of discrete lateral buckles along its length to share the load between them, the force in the pipe reduces at each lateral buckle. Approximately half-way between the buckles the pipe forms a virtual anchor point, from where feed-in occurs towards each buckle. The level of feed-in (flowline axial expansion) defines the extent of growth and load in the lateral buckles. The design methodology is then based around the ‘Characteristic VAS’ (previously defined as the ‘Probable VAS’) which defines the maximum spacing expected at any point along a pipeline, from probabilistic analysis. This can then be compared to the ‘Tolerable VAS’ which is the spacing at which one of the design limit states is exceeded.

4 OTC 21671

buck

le n

o. 2

buck

le n

o. 1

buck

le n

o. 3

buck

le n

o. 4

VA VA VA VA VA VA VA

fully developed force profile

distance along pipeline

effe

ctiv

e ax

ial f

orce

Virtual Anchors

Figure 4 Force profile along pipeline showing Virtual Anchor Spacing (VAS) and reduction in force at each lateral buckle

Effective Force and Influences As the pressure and temperature in a pipeline increases, the pipe attempts to expand but this expansion is resisted by the axial friction between the pipe and the seabed. The resulting build-up of axial force in the pipeline is defined by the ‘effective force’, which drives the structural response and is made up of the (true) force in the pipe wall and the pressure induced axial force. The natural tendency of such a pipeline is to relieve the resulting high axial stress in the pipe-wall by buckling. The assessment of effective force is therefore fundamental to the pipeline response. While thermal loading generally dominates the effective force in production systems, pressure dominates the effective force in low temperature water injection systems. However the effective force in the pipeline can also be moderated by the residual tension from installation, or by deliberately flooding the pipeline during installation, as described by Sinclair et al[19]. The advantages of reducing the axial force has led to flooded installation being adopted on some recent projects. Residual tensile load can also be generated when installing an insulated pipeline that has insufficient time to cool to ambient temperature before reaching the seabed, for which an analytical model was developed by Anderson et al[1]. Any significant locked in tension will influence the buckling behavior of the pipeline by inhibiting buckle formation and reducing pipeline expansion.

Models for the Assessment of Lateral Buckling and Pipeline Walking To support the design methodology, analytical solutions were developed for conceptual design assessments of lateral buckling and walking and software was created to tackle some of the more challenging detailed design requirements, such as buckle formation and pipe-soil interaction; these are described below. Finite Element Analysis (FEA) methods were also reviewed to provide guidance on their application.

Analytical Models To ensure that the risk associated with lateral buckling and pipeline walking is managed within a project, the technical

challenges must be thoroughly explored at the conceptual design stage. The use of non-linear FEA (finite element analysis) at the conceptual stage of a project is generally not realistic, nor is it a practical use of resources. SAFEBUCK therefore developed analytical models to evaluate both lateral buckling and pipeline walking: 1. For lateral buckling assessments, analytic models were developed (for single pipe and pipe-in-pipe systems) that

incorporate first load plasticity and cyclic fatigue in operation. These models were implemented in Mathcad and successfully validated against FE analysis. They are a significant improvement over published analytical models that are based purely on elastic stress limits, when in reality lateral buckling is generally accompanied by moderate plastic deformation on the first load cycle. Consequently, the existing elastic models are excessively conservative and do not address the true design limit states.

2. Pipeline walking assessments - after evaluating and developing a sound understanding of the underlying driving mechanisms, analytic models were developed to evaluate the rate of walking. Due to its fundamental importance to pipeline integrity, this important work was published by Carr et al 2006 [11].

FEA Analysis In the detailed design phase, analysis will be based upon FE analysis techniques. With the advent of modern FE packages,

the analysis technique has become more and more like a black box. However, FE analysis, like any other analysis tool, is simply a mathematical model of the physical world. If the model omits important physical parameters then it is possible to produce results that are misleading at best and catastrophic at worst. The work performed for the JIP investigated the effect of modeling assumptions in four broad categories:

• Mathematical modeling issues;

OTC 21671 5

• Strain localization phenomena; • Material modeling; • Pipe-soil interaction modeling.

This investigation assessed the importance of the various physical effects, to produce guidance on the parameters that must be incorporated within the design analysis. To ensure a robust assessment of the loads within a buckle, the JIP made a number of recommendations for FE modeling of lateral buckling, previously summarized by Bruton et al, 2005[3].

Buckle formation Reliability Assessment Buckle formation is an imperfection sensitive process that is intimately linked to the initial condition of the pipeline – the

out-of-straightness (OOS) and the breakout lateral friction, which is a function of the initial pipeline embedment and the soil properties. The buckling response of a pipeline will always be inherently uncertain because the project specific OOS and level of embedment along the pipeline are not known prior to pipe lay and there will be uncertainty in the soil properties along the route. The designer must therefore understand this uncertainty and reduce it to levels whereby the project can proceed with confidence.

Lateral buckle spacing, defined by the VAS is fundamental to this design approach; a smaller distance between intended buckle sites implies less severe buckles, but the likelihood of buckles forming is also reduced. This means that buckling may not occur at some of the intended sites, so that the robustness of the solution decreases. Within the SAFEBUCK design methodology; this challenge is addressed by defining an acceptable level of reliability, which by necessity requires a probabilistic design approach.

SAFEBUCK developed a structural reliability model of the pipeline expansion process to calculate the probability of buckling and the likely spacing between buckles, using probabilistic methods (Figure 5). An overview of the model is given by Cosham et al 2009[13], who demonstrate a significant advance on current methods for estimating buckle formation reliability bringing some welcome simplicity to the design process. This model has been coded into Fortran software called BUCKFAST, which is available to participants of the JIP. The approach is ideally suited to assessing the benefits of engineered buckle initiators.

0

1000

2000

3000

4000

5000

6000

7000

8000

0 5 10 15 20KP

Pro

babl

e V

AS

(m)

No Triggers 5 Triggers (Rogues)

5 Triggers (Planned)

Figure 5 Prediction of ‘Characteristic’ VAS - Example

Engineered Buckle Initiators Lateral buckling has steadily matured as a design solution, with a focus on reliable buckle formation reducing peak loads, and controlling fatigue. As design conditions become more challenging, primarily due to increasing operating temperatures, more complex buckle initiation methods are required to achieve reliable formation and satisfy design limit states. Lateral buckles are initiated deliberately by introducing planned initiation sites (buckle triggers) along the pipeline route that significantly reduce the buckle initiation force to increase the likelihood that buckles will form as planned. This has a profound influence on the design methodology. The planned initiation methods, illustrated in Figure 6, are described more fully with operational experience by Sinclair et al. 2009[19]. The engineered buckle initiation methods that have been used on many deepwater projects to promote the reliable formation of lateral buckles and control the buckle spacing and operating loads, include:

• Snake-lay • Vertical Upset • Local weight reduction

Snake Lay Snake lay relies on regular, as-laid, lateral curvature imperfections installed along the pipeline route, to trigger lateral

buckles. The reliability of buckle formation can be increased by reducing the radius or increasing the length of curvature in

6 OTC 21671

each snake. The post-buckle shape is also modified by the snake-shape, generally reducing the load in comparison with an unplanned buckle.

Vertical Upset Vertical upsets, the most common being a large diameter pipe section (sleeper), are preinstalled perpendicular to the pipe

route at the desired buckle spacing. The vertical out-of-straightness and reduction in lateral soil restraint, reduces the buckle initiation force and lifts the high curvature section of the buckle off the seabed, reducing lateral restraint on the pipe, reducing peak and cyclic loads. This technique can only be used where there is little chance of snagging due to fishing and span lengths are acceptable.

Localized Weight Reduction Reducing pipe weight over a short section (typically up to 100m) of the pipeline is most commonly achieved using

buoyancy at the desired buckle initiation sites. The localized light section with an increase in outside diameter creates a vertical out-of-straightness and a reduction in lateral soil restraint that reduces the buckle initiation force. The reduced operational submerged weight within the buoyant section also reduces the lateral frictional restraint on the pipe, reducing peak and cyclic loads. This technique can be used with any pipeline, although fishing interaction and on-bottom stability must also be addressed in design. A variation of this technique for concrete coated pipelines is to reduce or eliminate the weight coating over a short section.

Figure 6 Engineered buckle initiators Lateral Buckling - Failure Modes and Limit States The limit states that usually dominate lateral buckle loading are local buckling or fatigue, although weld fracture and defect tolerance must also be assessed. Local buckling failure is of most concern when a buckle is formed, and generally defines the maximum plastic deformation on first load. Subsequent cyclic loading is generally in the elastic range and defines the allowable fatigue life and fracture requirements. In addition to these limit states; the pipeline material specification must be consistent with the high levels of imposed load. A limit is placed on the first load strain, to ensure that the uniform strain capacity of the material is not approached. Sometimes, in conditions of extreme sour service, an additional strain limit is applied to minimize or avoid plasticity in the presence of high levels of H2S. A limit is also placed on the maximum axial stress range, to prevent cyclic plasticity; this limit considers the biaxial nature of the stresses and incorporates an allowance for the Bauschinger effect. Key limit state design checks are performed for peak loads (local buckling of the pipeline wall and fracture) and also cyclic loads (Girth weld SN fatigue and ECA). Design criteria for local buckling and SN fatigue are given in DNV OS-F101[15] and API1111[14]. The SAFEBUCK Design Guideline[10] encapsulates the above code requirements, and recommends a further

OTC 21671 7

limit state check on peak strains, such that the strain capacity of the flowline is not exceeded. However SAFEBUCK also recommends modifications to the safety factors in these codes, based on testing specific to the lateral buckling limit state. Local Buckling One key limit state for designing to accommodate lateral buckling is local buckling of the pipe cross-section. Local buckling is driven by the high imposed bending in combination with internal and external pressure. Relatively little work has addressed the effect of these combined loads for the D/t (diameter/thickness) range of interest to submarine pipelines (between 10 and 45). Further, little attention has been paid to the importance of Lüder banding. This type of behavior is normal in seamless linepipe, which is used for most in-field flowlines, and can be extremely detrimental to the local buckling capacity of a pipe. Carr et al 2011[9] addressed this issue by performing a combination of full-scale testing and numerical modeling to investigate the local buckling behavior of seamless linepipe. The work showed that the local buckling response is fundamentally influenced by the Lüder plateau. Pipes with a low D/t ratio buckle at strains far above the Lüder strain and have a high buckling capacity. However, pipes with a high D/t ratio may buckle below the Lüder strain, in which case there is essentially no beneficial effect of strain hardening and the pipe has a very low buckling capacity. This work looked at buckling capacity across a wide D/t range, including the D/t transition zone where the behavior changes from one response to the other. Current design equations do not capture the influence of the Lüder plateau, and the design margin implied by the equation varies considerably over the range of parameters considered; it was concluded that more robust design equations should be developed.

Fatigue Another key limit state for designing to accommodate lateral buckling is low-frequency high-stress fatigue. This design limit is driven by shutdown-restart cycles and is often the overriding design concern, particularly if the pipeline is exposed to a corrosive or sour environment. In pipelines designed to buckle laterally, shutdown–restart cycles result in significant axial cyclic stress ranges experienced at the buckle crown. The fatigue performance of girth welds in this high-stress low-cycle regime is therefore often a critical aspect of overall pipeline design. Although thermal cycling is usually intended to be entirely elastic, stress ranges may approach or even marginally exceed the uniaxial yield stress, particularly early in life. Under these conditions, material response may differ from that conventionally seen under low-stress high-cycle fatigue loading, such as that resulting from wave or VIV loading. In particular, the possibility of cyclic softening needs to be considered, and specific boundaries set to ensure such behavior is avoided. The specific nature of the fatigue loading associated with lateral buckling, also presents a significant challenge when considering the likely corrosion fatigue performance of girth welds exposed to either seawater or produced fluids. Corrosion fatigue performance is known to depend on the frequency of cyclic loading, with lower frequency loading incurring greater fatigue damage in each cycle. Unfortunately, the cyclic loading frequency associated with lateral buckling is very low (at least several hours per cycle) and this is beyond the range of conventional laboratory testing. Special techniques and methods of analysis are therefore needed to determine an appropriate ‘fatigue life reduction factor’ for use in design. Baxter et al 2011[2] describe the SAFEBUCK fatigue testing program, which evaluated cyclic softening at high stress ranges, as well as fatigue performance under the low frequency loading associated with lateral buckles in corrosive environment. This included a series of crack-growth and fatigue endurance tests in seawater with CP, representing conditions on the outside of the pipe; and in sour production fluid environment, representing conditions on the inside of the pipe. This assessment and testing of fatigue performance in a lateral buckle has highlighted a key concern because the fatigue loading frequency is incredibly low (a few hundred cycles over a twenty-year design life). This combination of low frequency, corrosive environment and high stress range means that fatigue lives can be reduced by a factor of ten or more.

Pipeline Walking Assessment and Control Pipeline walking is a stepwise ratcheting mechanism that occurs during changes in operating conditions, particularly during shutdown and restart operations. Over a number of cycles, this movement can lead to very large global axial displacements, with associated overload of any connections. Where the predicted pipe walking displacements threaten system integrity, walking may be controlled by the use of pipeline anchors, typically installed at the end of the pipeline from which it is walking. However, pipeline anchors result in very high levels of tension at shutdown, which can lead to large radius route-curves becoming unstable. This is usually overcome by increasing the route-curve radius, or removing route curves altogether by changing the field layout. For these reasons, the pipeline buckling and walking responses are key drivers in the layout of a field development. A number of deepwater projects have identified pipeline-walking issues in design that required mitigation, typically by installation of pipeline anchors, typically suction piles with a capacity of 50 to 350 tons. Several of these projects are now in operation and feedback on the walking response is gradually emerging. This includes the observation and measurement of an

8 OTC 21671

unexpectedly rapid pipeline walking response, which led to the identification of the multiphase flow mechanism as the root cause.

A number of contributory mechanisms, illustrated schematically in Figure 7, are now known to cause pipeline walking:

• Sustained tension - applied to the end of a pipeline by a steel catenary riser (SCR); • Seabed slopes - along the pipe length, defined by route bathymetry; • Thermal transients - changes in fluid temperature and thermal loading during shutdown and restart operations; • Multiphase flow behavior - during shutdown and restart operations, driven by segregation of gas and liquids.

The first three walking mechanism are explained by Carr et al, 2006[11], the last mechanism is explained by Bruton et al 2010[6]; in each case the authors developed analytical models verified against FEA, to quantify the rate of walking.

Figure 7 Pipeline Walking Mechanisms

Pipe-Soil Interaction Longitudinal expansion of a pipeline is resisted by the axial resistance between the pipe and the seabed. This restraint causes an axial compressive force to develop in the pipeline, which can lead to buckling; if such a pipeline has sufficient out of straightness to start a buckle forming, there is only lateral soil resistance to prevent it. Once a buckle starts to form, pipe feeds into the buckle longitudinally, resisted by axial friction along the pipe and lateral resistance at the buckle, where the level of lateral resistance controls the shape of the buckle and the loads that the pipe will experience. Once the pipeline shuts down, the resistance provided by the soil is reversed; the axial resistance limits contraction of the pipe and lateral resistance resists the straightening of the buckle. It is clear that the resistance generated by the soil around the pipe is critical to establishing the response of the system and the loads it experiences. The pipe-soil response therefore affects the design limit states associated with lateral buckling, pipeline walking, route-curve pullout and flowline anchoring. These behaviors are all extremely sensitive to pipe-soil interaction forces and there is significant uncertainty associated with the characterization of these forces in design. The complexity that lies behind the lateral and axial pipe-soil response in soft clays has led to a radical over-haul of established geotechnical practice in pipe-soil interaction. SAFEBUCK and many recent projects have invested significant research effort to evaluate, quantify and understand these pipe-soil interaction mechanisms. A rigorous theoretical understanding of the basic phenomena involved has only recently emerged, principally through research activity associated with the SAFEBUCK JIP. Lateral Response SAFEBUCK started research into the large displacement cyclic lateral response of pipelines on very soft deep-water marine clays at Cambridge University in 2002 (Figure 8a). This early work strongly influenced lateral buckling design in soft clays by demonstrating the large displacement response and the important influence of soil berms on cyclic lateral response. The data was reviewed, alongside data donated to the JIP, by Bruton et al 2006[4]. Further large-scale tests (Figure 8b) at the Norwegian Geotechnical Institute (NGI) were reported by Jayson et al [17]. These targeted relatively light wet-insulated pipelines, although a few tests with heavier pipe identified a radically different response, with higher levels of friction that increased with displacement.

OTC 21671 9

Axial test set up

Lateral test set up

Figure 8 Large-Scale Pipe-soil Interaction Testing at (a) Cambridge University (b) Norwegian Geotechnical Institute SAFEBUCK pioneered the idea of small-scale lateral pipe-soil tests in a centrifuge, firstly at Cambridge University in the UK and then at the University of Western Australia (UWA). Such tests are now established as a reliable evaluation method for new projects. The repeatability of such tests and their validation against large-scale tests has allowed more precise light pipe models to be developed for JIP Participants.

Figure 9 Small-Scale Centrifuge Testing of Lateral Pipe-soil Interaction at (a) Cambridge Uni. (b) Uni. of Western Australia Centrifuge tests were also ideally suited to evaluate this ‘heavy pipe response with a series of small-scale, deep bin tests carried out at UWA for the JIP (Bruton et al 2008[7]) and then for a project (Bruton et al 2009[8]). These tests, and the work that followed, provided a good understanding of the complex lateral cyclic response of pipe that is heavy in relation to the soil shear strength. Such pipes tend to dive under cyclic displacement, causing the contact pressure to reduce, as the lobes of the buckle displace laterally and penetrate vertically; the rate of reduction in contact pressure with increasing penetration of the buckle lobes is approximately linear with depth and is defined by the structural response of the pipe in the lateral buckle. Clearly, this reduction in contact pressure means that the rate of penetration reduces and the lateral resistance eventually plateaus to a steady-state cyclic resistance (Bruton et al, 2009[8]). The unfortunate complexity of such models is being addressed in the next phase of the JIP. Axial Response For some years, interface friction tests for pipelines have been carried out at the University of Texas using a Tilt Table device to measure the fully-drained interface friction (Najjar et al 2003[18]). SAFEBUCK initiated research at Cambridge University to assess the influence of pipe roughness and pipe velocity on the interface resistance of pipelines on soft clays, using the Camshear device; with the specific intent of evaluating partially drained interface resistance, which was expected to be less in many cases than the drained friction response. This was a concern for lateral buckling and particularly pipeline walking, as the lower bound friction is often the most critical in pipeline expansion and walking rate assessments.

10 OTC 21671

This work confirmed that lower bound partially-drained frictional resistance was a concern; this led to the development and improvement of test methods, within the JIP and on current projects. A series of short and long pipe tests were proposed to capture the excess pore-water drainage paths around the pipe circumference. These tests, summarized by White et al 2011[22], also confirmed that axial resistance is strongly link to the pipe velocity.

Meanwhile a number of projects also carried out some large-scale axial tests at NGI (Figure 8b), summarized by Jayson et al. 2008 [17] and Bruton et al 2009[8]. This development and improvement to testing methods has also contributed significantly to the development of a large-scale test rig called SMARTPIPE® described by Hill & Jacob 2008[16], using the lessons learned from large-scale laboratory tests to carry out in-situ tests in deep water offshore West Africa. Integrity Monitoring in Operation and Feedback on Performance

While integrity monitoring is vitally important to the safe long-term performance of the system, it is clear to all those involved in the design of such systems that the feedback from good integrity monitoring of operational systems is also of vital benefit to future projects. To demonstrate that a system, susceptible to laterally buckling or pipeline walking, is fit for service, it is essential to obtain flowline positional data during operation to be able to assess the true lateral buckle response, end expansions and rates of walking. Performing detailed out-of-straightness (OOS) and visual surveys on flowlines that are susceptible to lateral buckling or pipeline walking is essential to assess flowline integrity and to advance the understanding of this complex and uncertain flowline system behavior. Surveys of sufficient accuracy must be performed to verify the location, buckling mode, curvature and amplitude of the lateral buckles along the whole pipeline, to be able to assess the loads in the system and the probable load history based on operational pressure and temperature data. This integrity monitoring approach is critical to system integrity and the ongoing fatigue performance of any pipeline, indeed it is often a requirement of the design approach that fatigue damage is monitored (specifically this affects the selection of an appropriate Fatigue Usage factor). Detailed pipeline surveys conducted in the North Sea, West Africa and the Gulf of Mexico have confirmed the value of integrity monitoring to long-term system integrity. In particular, recent surveys have identified unexpected behavior, providing important lessons for future projects that are now being addressed early in design, and in some cases resulting in subsea mitigation works to ensure future system integrity (Watson et al. 2011[21]). It is important to remember that many historically significant pipeline design issues were first recognized from integrity monitoring surveys, including early observations of lateral buckling and pipeline walking. Recent examples of unexpected behavior described by Watson et al[20] include the link between internal flow regimes and the global pipeline response. Surveys have also been vital in confirming expected behavior, such as the clear interaction between lateral buckling and pipeline walking, with pipeline expansion passing-through lateral buckles and migrating to the free-end of a pipeline. The Current Phases of SAFEBUCK The latest phases of the JIP, SAFEBUCK III and SAFEBUCK GEO, were launched in early 2010 to continue and complete the development of methodologies to address the significant design challenge of controlling pipeline lateral buckling and walking, particularly for deepwater, and high-temperature, high-pressure developments.

The key areas of work to be undertaken in SAFEBUCK Phase III, are:- • Verification of the reliability of the design methodology using structural reliability analysis, with the aim of reducing

unnecessary conservatism in design; • Collection and sharing of data and lessons learned from operating pipelines on recent projects without sharing potentially

sensitive project specific data; • Formalization of the SAFEBUCK Design Guideline into an Industry Recommended Practice, described further by

Collberg et al 2011[12]. The key areas of work to be undertaken in SAFEBUCK GEO (the Geotechnical scope) are:- • The development of a new ‘force-resultant plasticity model’ to run inside standard software packages, which will capture

the considerable experience from modeling and testing lateral pipe-soil interaction. This approach will fundamentally improve the way that lateral pipe-soil response is addressed in design and help resolve the design challenges described by Bruton et al 2010[5].

• A detailed review of axial friction response, based on recent project-specific tests, supplemented by additional JIP tests, to improve our understanding and quantify key uncertainties that influence cyclic expansion and pipe-walking, described further by White et al 2011[22].

These latest phases have advanced well in the last year. In particular, SAFEBUCK III has commenced workshops with DNV to progress the formalisation of the SAFEBUCK Design Guidelines into an Industry Recommended Practice by merging with

OTC 21671 11

and updating DNV RP-F110. The generosity of the participants has allowed data and lessons learned from operating pipelines and recent projects to be collated and shared, enabling our design methodologies to be calibrated and refined. Meanwhile, SAFEBUCK GEO is conducting further research into pipe-soil interaction, with focused small-scale centrifuge testing of lateral pipe motion at the University of Western Australia and a detailed review of axial pipe motion in preparation for axial testing at the Norwegian Geotechnical Institute. This work will fundamentally improve the way that pipe-soil response is addressed in design One key aim in Phase III is to collate performance data from operating pipelines that were designed to buckle laterally, to improve our understanding of how they behave and better address the design challenges encountered on recent projects. The data being collected includes information on pipeline embedment, OOS, buckle formation reliability and mode shape, operational loads and responses

Conclusions - Advances in Technology The lateral buckling and walking of pipelines creates design challenges that can affect the layout and architecture of new developments, requiring careful definition of pipeline routes, bathymetric profiles and the location of production facilities, as well as the design of pipeline crossings and tie-ins to risers and subsea structures. The SAFEBUCK JIP has improved confidence in the lateral buckling and walking design methods and cost has been saved in a variety of different ways, including shorter design times, avoiding over-conservative design solutions, reduced installation costs and mitigating against operational issues and potential failures, with the obvious impact on production losses and the environment. The intellectual property generated by the JIP includes the Design Guideline, the research results and design software written for the JIP, which is licensed to each Participant for use on their own field developments. A key contributor to the increasing design certainty is the research and development undertaken by the JIP, including the development of engineering models and the testing of pipe-soil interaction, fatigue and local buckling. The software and the evolution of the design methodology was a significant investment for the JIP and are fundamental to the acceleration of the project design process and schedule. The design cycle for such developments has reduced markedly over the duration of the JIP, reducing the key aspects of the engineering design cycle from years to months. Indeed, recent projects would not have been engineered as quickly or with as much design certainty or future operational reliability without the SAFEBUCK design methodology.

The early adopters of the JIP technology were the companies that participated in the first phases of the JIP for their most challenging deepwater developments. Many of these developments are now in production and the integrity of their systems is being assessed against the same SAFEBUCK methodology. Acknowledgements The authors would like to acknowledge the support and contribution of the SAFEBUCK Joint Industry Projects participants, (ABS, Acergy (now Subsea 7), BOEM/MMS, BP, Bureau Veritas, Chevron, ConocoPhillips, DNV, ExxonMobil, Fugro, JFE, Petrobras, Saipem, Shell, Statoil, Technip, Tenaris, Total and Woodside) and the JIP Partners (found at www.safebuck.com) . References

1. Anderson, M., Bruton, D., Carr, M. 'The Influence of Pipeline Insulation On Installation Temperature, Effective Force and Pipeline Buckling'. 26th International Conference on Offshore Mechanics and Arctic Engineering. OMAE-29317 Jun 2007.

2. Baxter, D., Tubby, P. SAFEBUCK JIP: Critical Aspects of the Fatigue Limit State for Pipelines Designed to Laterally Buckle. Offshore Technology Conference. OTC 21510 May 2011.

3. Bruton, D., Carr, M. Crawford, M. Poiate, E. 'The Safe Design of Hot On-Bottom Pipelines with Lateral Buckling using the Design Guideline Developed by the SAFEBUCK Joint Industry Project.'. Deep Offshore Technology Conference. Nov 2005.

4. Bruton, D., White, D., Bolton, M. Cheuk C., Carr, M. 'Pipe/Soil Interaction Behavior during Lateral Buckling'. SPE Projects, Facilities & Construction. SPEPFC106847 Sep 2006.

5. Bruton, D.A.S., Sinclair, F., Carr, M. Geotechnical challenges for deepwater pipeline design - SAFEBUCK JIP. Frontiers in Offshore Geotechnics II. P779 Nov 2010.

6. Bruton, D.A.S., Sinclair, F., Carr, M. Lessons Learned From Observing Walking of Pipelines with Lateral Buckles, Including New Driving Mechanisms and Updated Analysis Models. Offshore Technology Conference. OTC 20750 May 2010.

7. Bruton, D.A.S., White, D. Carr, M. Cheuk C. 'Pipe-Soil Interaction During Lateral Buckling and Pipeline Walking - The SAFEBUCK JIP'. Offshore Technology Conference. OTC19589 May 2008.

8. Bruton, D.A.S., White, D. Langford, T.Hill, A. J. 'Techniques for the assessment of pipe-soil interaction forces for future deepwater developments'. Offshore Technology Conference. OTC 20096 May 2009.

12 OTC 21671

9. Carr, M. MacRae, I., Bruton, D.A.S., SAFEBUCK: Local Buckling Limit State. Offshore Technology Conference. OTC 21668 May 2011.

10. Carr, M., Bruton, D., and Cosham, A. Design Guideline. Safebuck JIP. December 2008. (Confidential to JIP Participants). 11. Carr, M., Sinclair, F. Bruton, D. 'Pipeline Walking – Understanding the Field Layout Challenges, and Analytical Solutions

developed for the SAFEBUCK JIP'. Offshore Technology Conference. OTC-17945 May 2006. 12. Collberg, L., Carr, M. Bruton, D.A.S., Levold, E., Safebuck Design Guideline. Offshore Technology Conference. OTC 21575

May 2011. 13. Cosham, A., Carr, M. Bruton, D.A.S. Leslie, D. 'The reliable initiation of lateral buckles – SAFEBUCK JIP'. SUT Annual

Conference. Feb 2009. 14. Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design). API Recommended

Practice 1111. Third Edition, July 1999. American Petroleum Institute 15. Det Norske Veritas, “Submarine Pipeline Systems”, DNV-OS-F101, October 2007. 16. Hill, A.J. and Jacob, H “In-Situ Measurement of Pipe-Soil Interaction in Deep Water” Proc. Offshore Technology Conference,

Houston, USA. Paper OTC19528 (2008). 17. Jayson, D., Delaporte, P. Albert, JP., Provost, ME. Bruton, D., Sinclair, F. 'Greater Plutonio Project – Subsea Flowline Design and

Performance'. Offshore Pipeline Technology Conference. Feb 2008. 18. Najjar, S.N., Gilbert, R.B., Liedtke, E.A., McCarron, W. “Tilt Table Test for Interface Shear Resistance between Flowlines and

Soils”. OMAE (2003). 19. Sinclair, F., Carr, M. Bruton, D., Farrant, T. 'Design Challenges and Experience with Controlled Lateral Buckle Initiation

Methods'. International Conference on Ocean, Offshore and Arctic Engineering. OMAE-79434 Jun 2009. 20. Watson, R., Bruton, D.A.S., Sinclair, F. Influence of Multiphase Flow on Global Displacement of Deepwater Pipelines – In Design

and Operation. 5th International Offshore Pipeline Forum. IOPF-6002 Oct 2010. 21. Watson, R., Sinclair, F., Bruton, D.A.S., SAFEBUCK: Operational Integrity of Deepwater Flowlines. Offshore Technology

Conference. OTC 21724 May 2011. 22. White, D. Ganesan, S.A., Bolton, M.D., Bruton, D.A.S., Ballard, J-C., Langford, T. SAFEBUCK JIP: Observations of axial pipe-

soil interaction from testing on a soft natural clays. Offshore Technology Conference. OTC 21249 May 2011.