Elsevier

Ocean Engineering

Volume 217, 1 December 2020, 107963
Ocean Engineering

Finite element modelling for as-laid embedment of pipeline in clayey sediments

https://doi.org/10.1016/j.oceaneng.2020.107963Get rights and content

Highlights

  • A numerical modelling technique for estimating as-laid pipeline embedment in clayey sediments.

  • Pipe-soil interaction force related to soil buoyancy is quantified in the estimation of as-laid pipeline embedment.

  • The as-laid pipeline embedment may be overestimated significantly if the soil buoyancy is ignored in the analyses.

Abstract

A numerical modelling technique for the estimation of as-laid pipeline embedment in clayey sediments is introduced in the paper. The process of pipe-laying is modelled numerically with the pipe-soil interaction defined using a nonlinear elasto-plastic model, therefore the soil resistance contributed by soil buoyancy can be well considered in the evaluation of as-laid pipeline embedment. The impact of the soil buoyancy in the estimate of as-laid embedment is then quantitatively assessed using the proposed numerical modelling technique, and the results show that the as-laid pipeline embedment can be significantly overestimated if the soil buoyancy is ignored in the analysis.

Introduction

Offshore pipelines are commonly laid on the seabed by a laying barge in deep waters without prior trenching or burial (Fig. 1), where the typical deposit is normally consolidated fine-grained soil. The embedment of the pipeline after installation, so called as-laid pipeline embedment, is a function of the pipeline self-weight relative to the seabed strength, but is complicated by the laying process. Firstly, the contact force between the pipe and soil in the touchdown zone (TDZ) is larger than the self-weight of the pipeline, due to bending of the pipeline caused by the catenary laying shape (shown in Fig. 1). Secondly, the pipeline dynamic motion during the laying process would lead to remoulding of soil within the TDZ, water entrainment and possible trenching to further increase the penetration (Westgate et al., 2012).

Accurate assessment of the as-laid embedment is essential for pipeline stability and integrity designs as the embedment substantially affects pipe-soil interaction and thus the pipeline behaviour of lateral buckling, on-bottom stability, axial friction resistance and heat transition (White et al., 2017). There is no ‘conservative’ estimation of the as-laid pipeline embedment. For example, underestimation of the embedment may eventually result in more heat losses in the design and therefore underestimation of the thermal expansion which increase the risk of ‘snap-through’ buckling, while a higher hydrodynamic load would be used in stability analysis with possible unnecessary costly stabilisation solutions to be taken. Therefore, the accurate estimation of the as-laid embedment is concerned in this paper.

The major efforts have been devoted to the modelling of soil responses experimentally and numerically, such as the softening of soil stiffness (Shiri and Randolph, 2010; Clukey et al., 2011; Aubeny et al., 2015; Zhao et al., 2020) and the effect of consolidation (Hodder et al., 2009; Yuan et al., 2017; Zargar et al., 2019). The effects of remoulding for short-term loadings are majorly considered in the analysis of the pipeline as-laid embedment, and the typical studies are summarised in Table 1, including the analytical solutions and the finite element (FE) analysis by commercial software Orcaflex (Orcina, 2012). The pipe penetration resistance was approximately assumed to increase linearly with depth in the theoretical analyses (Lenci and Callegari, 2005; Yuan et al., 2012), due to the difficulties in solving the differential equations. Although a hysteretic seabed model is adopted for pipe-soil interaction in Orcaflex, the resistance related to soil buoyancy is ignored. The contribution of soil buoyancy to the total soil resistance may become significant as the undrained strength of clay is reduced due to soil remoulding during the laying process (Randolph and Gourvenec, 2010; Chatterjee et al., 2012; Clukey et al., 2017).

The aim of this note is to develop a FE modelling technique to evaluate the as-laid pipeline embedment, in which the normally consolidated clayey seabed with shear strength increasing proportionally with depth is considered and the entire pipe-laying process is reproduced to obtain the enhanced contact force in the TDZ. The anisotropy, structure and time-dependency of soil strength (Randolph, 2004; Yin et al., 2011; Grimstad et al., 2012) are not considered here. Nonlinear elasto-plastic pipe-soil interaction can be implemented easily in FE modelling, and therefore the soil buoyancy is taken into account in estimation of as-laid embedment. The dynamic lay effects, such as soil remoulding due to cyclic loadings and water entrainment, are implicitly accounted for by using a remoulded soil shear strength profile.

Section snippets

Finite element modelling

A numerical modelling technique for the pipe-laying process is developed within commercial FE package Abaqus. The pipeline is discretised using beam elements (B21 in Abaqus) and the continuity of bending moment and shear along the pipeline can be well simulated in the pipe-laying process. The seabed is represented by a series of macro ‘springs’ defined at each node of the beam elements with the spacing of 1 m (shown in Fig. 2). Based on the plasticity theory, the loading responses of pipe-soil

Verification of the modelling technique

The modelling technique proposed is verified by comparison with the existing solutions, with concerns on (a) the pipeline laying shape which dominates the contact force in the TDZ; and (b) the as-laid pipeline embedment in the TDZ. All comparisons are against the properties of pipeline and soil and laying conditions provided in Table 2.

As-laid pipeline embedment with consideration of soil buoyancy

In this section, the influence of soil buoyancy on the as-laid embedment is investigated using the modelling technique proposed, in which the nonlinear elasto-plastic pipe-soil interaction shown in Fig. 3c is adopted. The intact shear strength gradient k is assumed as 12 kPa/m; a variety of soil effective unit weight is studied as γ′/γw = 0.3, 0.5, 0.7 and 1.2, where γw = 9.8 kN/m3 is the unit weight of water; and the reduction of shear strength caused by soil remoulding due to pipe dynamic

Conclusions

A modelling technique incorporating finite element simulation and the pipe-soil interaction model is developed to evaluate the as-laid pipeline embedment. This technique can estimate the contact force in the touchdown zone reasonably, given that a sophisticated pipe-soil interaction model, such as the nonlinear elasto-plastic model used, is incorporated.

The modelling technique is validated by comparison with the existing analytical solutions. The penetration resistance contributed by soil

CRediT authorship contribution statement

Liang Zhao: Conceptualization, Methodology, Software, Validation, Data curation, Formal analysis, Writing - original draft, Visualization. Dong Wang: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Yinghui Tian: Conceptualization, Methodology, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The second author would like to acknowledge funding from the National Natural Science Foundation of China (through the grants of No. U1806230 and 41772294).

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