Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design

Westgate, Z.J.; Haneberg, William C.; White, David

doi:10.1139/cgj-2016-0091

Cited by 8 publications

(6 citation statements)

References 21 publications

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“…In summary, Westgate et al (2016) and Dr. McCarron's discussion of this paper show that spatial variability -from both geotechnical and structural effects, both natural and installation-induced -is an important topic to focus on when assessing the response of subsea pipelines. Pipeline lateral buckles are a relatively local phenomenon, whereas the overall expansion and force build-up are global, and these behaviours interact.…”

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confidence: 89%

“…McCarron for his insightful discussion of the Westgate et al (2016) paper on spatial embedment variability of subsea pipelines, and for sharing our enthusiasm for this topic. We are pleased that our paper attracted this discussion.…”

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confidence: 99%

“…(6). However, this is the standard approach to assess buckle susceptibility (based on the SAFEBUCK JIP Guideline, Atkins 2015), so provides a valid comparison with conventional practice when we introduce the relevant geotechnical variability in Westgate et al (2016). The tendency for a buckle to initiate due to reduced lateral resistance over a length less than the critical buckle length has been demonstrated by finite element analysis using field data of as-laid embedment and calculated lateral resistance (White et al 2014).…”

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Reply to the discussion by McCarron on “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”

2017

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We would like to thank W.O. McCarron for his insightful discussion of the Westgate et al. (2016) paper on spatial embedment variability of subsea pipelines, and for sharing our enthusiasm for this topic. We are pleased that our paper attracted this discussion. Below are specific responses to each of the discusser's key points.In response to the first and second points on buckle length and averaging length, we appreciate the approximate nature of eq. (6). However, this is the standard approach to assess buckle susceptibility (based on the SAFEBUCK JIP Guideline, Atkins 2015), so provides a valid comparison with conventional practice when we introduce the relevant geotechnical variability in Westgate et al. (2016). The tendency for a buckle to initiate due to reduced lateral resistance over a length less than the critical buckle length has been demonstrated by finite element analysis using field data of as-laid embedment and calculated lateral resistance (White et al. 2014). In this analysis, for a similar pipeline to pipe A, lateral buckles formed at locations where the embedment (and resulting lateral resistance) was reduced over a pipeline length of approximately 10-20 m -a much shorter length than the resulting buckle that formed. We do note, however, that such examples are specific to the pipeline properties and out-of-straightness features involved, but the analyses serve their purpose of highlighting the influence that geotechnical variability can have and therefore the potential value of including it realistically in design.The third point relates to the horizontal correlation length of lateral resistance, due to the variable embedment and soil properties. The discusser queries the value adopted in the example analysis, which could represent a soil heterogeneity present in situ or a heterogeneity created through the process of installing a pipeline on the seabed and locally remoulding the soil to different degrees. Ultimately our overall choice for length scales of the various processes that affect embedment was driven by the field observations, which match our example case. We agree that soft clay deposits in the deep ocean, such as the Gulf of Mexico, may be more homogenous and have a longer correlation length scale than used in our example. However, we have previously published a collection of undrained strength profiles derived from offshore T-bar tests in soft clay from deep water off the coast of West Africa that provides a useful example (White et al. 2014). Within the upper 0.5 m, the coefficient of variation (COV) for intact undrained strength, from this set of 12 profiles, varied in the range 0.10-0.42, with an average value of 0.23. Including variability due to the level of remoulding of the soil can increase the applicable COV. This supports our selected level of heterogeneity, although we recognise that the measurement error itself adds uncertainty onto the natural variability. This study gave no information on the length scale of the variability due to the wide spacing of the tests. Reviews f...

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Reply to the discussion by McCarron on “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”

2017

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“…It is evident from works by [59][60][61][62][63][64][65][66][67][68] that variations in soil conditions can impact pipeline structural response significantly. Additionally, pipe geometry, including diameter and wall thickness, can also influence the pipeline's structural response, as established by [63,65,66].…”

Section: Uncertainty Quantification Using Multi-fidelity Surrogate Mo...mentioning

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Probabilistic Seismic Risk Analysis of Buried Pipelines Due to Permanent Ground Deformation for Victoria, BC

Dey

Tesfamariam

2022

Geotechnics

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Buried continuous pipelines are prone to failure due to permanent ground deformation as a result of fault rupture. Since the failure mode is dependent on a number of factors, a probabilistic approach is necessary to correctly compute the seismic risk. In this study, a novel method to estimate regional seismic risk to buried continuous pipelines is presented. The seismic risk assessment method is thereafter illustrated for buried gas pipelines in the City of Victoria, British Columbia. The illustrated example considers seismic hazard from the Leech River Valley Fault Zone (LRVFZ). The risk assessment approach considers uncertainties of earthquake rupture, soil properties at the site concerned, geometric properties of pipes and operating conditions. Major improvements in this method over existing comparable studies include the use of stochastic earthquake source modeling and analytical Okada solutions to generate regional ground deformation, probabilistically. Previous studies used regression equations to define probabilistic ground deformations along a fault. Secondly, in the current study, experimentally evaluated 3D shell and continuum pipe–soil finite element models were used to compute pipeline responses. Earlier investigations used simple soil spring–beam element pipe models to evaluate the pipeline response. Finally, the current approach uses the multi-fidelity Gaussian process surrogate model to ensure efficiency and limit required computational resources. The developed multi-fidelity Gaussian process surrogate model was successfully cross-validated with high coefficients of determination of 0.92 and 0.96. A fragility curve was generated based on failure criteria from ALA strain limits. The seismic risks of pipeline failure due to compressive buckling and tensile rupture at the given site considered were computed to be 1.5 percent and 0.6 percent in 50 years, respectively.

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The authors are thanked for their contribution regarding the spatial embedment variability of subsea pipelines (Westgate et al 2016). The design of such pipelines against temperature-induced buckles is directly influenced by their embedment in the seafloor.

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Discussion of “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”

McCarron¹

2017

Can. Geotech. J.

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The authors are thanked for their contribution regarding the spatial embedment variability of subsea pipelines (Westgate et al. 2016). The design of such pipelines against temperature-induced buckles is directly influenced by their embedment in the seafloor. The industry benefits from the presentation of observed installed embedments and the discussion of how such data can be used in design environments. The writer would like to draw the authors' attention to several points.Firstly, the expression shown in eq. (D1) (eq. (6) in the paper under discussion) is a simplification of the Hobbs solution for the infinite mode buckle capacity. The simplification obscures the dependency of the axial capacity on a characteristic, or effective, length of the pipeline buckle. The expression presented in the paper and the original Hobbs infinite mode expression follow in eqs. (D1) and (D2), respectively.In eq. (D2), E is the elastic modulus, I is the pipe moment of inertia, A is the pipe cross-sectional area, L is the lateral friction coefficient, w is the pipe weight, and L is the buckle length, which for this mode is found by a minimization procedure (Hobbs 1984) to have a critical value, L , ofEquation (D1) can be derived from eqs. (D2) and (D3) in two steps. Substituting (D3) in to (D2) yields, after some manipulation,Then using the thin-wall approximations for A and I in L , eq. (D1) is obtained, while noting in the authors' terminology L w = H brk . Equations (D1) and (D4) begin to diverge as the thin-wall pipe approximations lose accuracy.The physical reality is that there is a strict relation between the lateral soil resistance, the length of the structural buckle obtained in the solution (eq. (D4)), and the buckle capacity. Similar requirements develop if the lateral resistance is elastic, as for the beamon-elastic foundation problem described by Timoshenko and Gere (1961), or if elastic-plastic lateral resistance is simulated as is often the case for finite element solutions. The Hobbs infinite mode capacity assumes that a uniform lateral soil resistance to the buckle is fully mobilized over the entire length of the buckle. The illustrative conditions used in the paper under discussion render the use of eq. (D1) inappropriate unless H brk has a scale length longer than L . For the pipe A conditions in the paper, this dimension is approximately 35 m, depending on the choice of H brk . The Hobbs paper also includes buckle capacity solutions for several other modes involving isolated buckles in a long pipeline as opposed to the continuous sinusoidal shape for the infinite mode. Those other solutions, originally developed by Kerr (1978), all result in lower buckle capacities compared to the infinite mode, and all have longer buckle lengths (perhaps 60-100 m) than the infinite mode.The second point is then that, regardless of the buckle mode investigated, the averaging length for determining H brk should be greater than the associated critical length L , and therefore the reduction in buckle capacity described in the pape...

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Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design

Cited by 8 publications

References 21 publications

Reply to the discussion by McCarron on “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”

Reply to the discussion by McCarron on “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”

Probabilistic Seismic Risk Analysis of Buried Pipelines Due to Permanent Ground Deformation for Victoria, BC

Discussion of “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”

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