Numerical study on lateral buckling of pipelines with imperfection and sleeper

doi:10.1016/j.apor.2017.08.010

Applied Ocean Research

Volume 68, October 2017, Pages 103-113

https://doi.org/10.1016/j.apor.2017.08.010 Get rights and content

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Abstract

Lateral buckling is an important issue in unburied high-temperature and high-pressure (HT/HP) subsea pipelines systems. The imperfection–sleeper method is one of the most well-known methods used to control lateral buckling of HT/HP pipelines. Pipelines–sleeper–seabed numerical models are established and verified to analyze the buckling behavior of pipelines using the imperfection–sleeper method. The influence of six main factors on lateral buckling behavior is investigated in details based on the numerical results. Equations of buckling displacement (buckling displacement is defined by the final displacement of the middle point of the pipelines), critical buckling force, and buckling stress (Mises stress) are proposed using the gene expression programming technique. These equations show good accuracy and can be used to assist in the design of sleepers and assess the compressive and stress levels of pipelines.

Introduction

Subsea pipelines operated under high temperature and high inner pressure will develop compressive force because of seabed soil restraining its axial expansion. If the compressive force is sufficiently high, then global buckling will occur, which is similar to the buckling of a steel bar. For unburied subsea pipelines, which are common in deep-sea conditions, global buckling usually occurs in the lateral direction [1], [2]. Lateral buckling may lead to fracture, collapse, or buckling propagation [3]. With the increase in operation temperature and pressure, controlling lateral buckling becomes an essential problem in the design of subsea HT/HP pipelines.

A number of methods are used, such as snaked lay (imperfection) method [4], sleeper method [5], and distributed buoyancy method [6], to control lateral buckling responses. All of these methods are aimed at reducing the compressive force levels and buckling responses. This study focuses on the method that combines imperfections and sleepers.

For pipelines with imperfections, a number of studies on lateral buckling of imperfect pipelines have been conducted in recent years. Miles and Calladine [7] provided a design formula to calculate the maximum buckling strain on the basis of the experimental and numerical results. Karampour et al. [3] derived an analytical solution for pipelines with half-wavelength sinusoidal imperfection. Hong et al. [8] used the energy method to calculate the analytical solution for pipelines lateral buckling with a single-arch initial imperfection and indicated that a small single-arch initial imperfection is associated with snap buckling. Most of the lateral buckling studies have a reasonable two-dimensional assumption. However, the buckling problem becomes a three-dimensional (3-D) problem when a sleeper is considered. For pipelines with sleepers, Sinclair et al. [5] indicated that buckling mode 2 may occur in pipelines with sleepers, which should be ignored. One effective way to avoid mode 2 is to combine the sleeper method with the imperfection method. The initial imperfection not only reduces the critical buckling force but also guarantees that only mode 1 occurs, which will be discussed in Section 3. The proposed critical buckling force and maximum strain formulas cannot be applied directly to the pipelines with imperfections and sleepers. Therefore, formulas of critical buckling force, buckling displacement, and buckling stress are necessary to improve the design of pipelines with imperfections and sleepers.

The lateral buckling behavior of subsea pipelines with imperfection and sleeper has been investigated numerically. First, a 3-D finite element model of a 2000 m-long pipelines–sleeper–seabed system is developed in Abaqus. Then, six factors which influence the buckling response are investigated to analyze the lateral buckling behavior. Finally, on the basis of the numerical results, three formulas (critical buckling force, buckling displacement, and buckling stress) related to pipelines with imperfections and sleepers are proposed by the gene expression programming (GEP) method. These equations can be used to design pipelines with both imperfection and sleeper.

Section snippets

Finite element model

The finite element model provides a convenient method to calculate the lateral buckling of pipelines. The finite element model of the pipelines with imperfection and sleeper is described on the basis of the following four aspects:

First, the element types are selected. The beam element is suitable for simulating pipelines structures because the subsea pipelines is an ultra-slender structure, which indicates that the length of one direction (pipelines axial direction) is more than the length of

Lateral buckling behavior

Factors that influence the buckling behavior of the pipelines are studied in order to derive the formulas that control the buckling behavior of pipelines with imperfections and sleepers. The influences of different parameters on buckling responses are investigated in detail. Six parameters are selected, namely, bending stiffness (EI), imperfections (h₀/l₀), submerged weight (q), friction coefficient of the seabed (μ₁), friction coefficient of the sleeper (μ₂), and sleeper height (h₂). The

Genetic programming analysis

In the design of lateral buckling of pipelines with imperfections and sleepers, buckling displacement, critical buckling force, and buckling stress are the key parameters. Buckling displacement indicates the length of the sleeper and critical buckling force indicates the axial force level along the pipelines, whereas buckling stress is used to assess the integrity of the pipelines. As a result, in this section, we generate three GEP [16] models to derive the formulas of the aforementioned key

Conclusion

A numerical model of pipelines with imperfections and sleepers is generated to analyze the lateral buckling behavior of the pipelines. The numerical model shows good consistency with the existing experiment. Based on the simulated results, the GEP technique is used to establish three equations to calculate buckling displacement, critical buckling force, and buckling stress. The following main conclusions are drawn to summarize this study:

Bending stiffness, submerged weight, seabed friction, and

Acknowledgment

The authors are grateful for the support provided by the National Basic Research Program of China (NO. 2014CB046801).

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Full length articleNumerical study on lateral buckling of pipelines with imperfection and sleeper

Highlights

Abstract

Introduction

Section snippets

Finite element model

Lateral buckling behavior

Genetic programming analysis

Conclusion

Acknowledgment

Eng. Struct.

Ocean Eng.

Eng. Struct.

Ocean Eng.

Ocean Eng.

Adv. Eng. Softw.

Eng. Struct.

Ocean Eng.

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J. Transport Eng.

Overview of global buckling of deep-sea pipelines

J. Tianjin Univ. Sci. Technol.

Postbuckling analysis of snaked-Lay pipelines based on a new deformation shape

J. Offshore Mech. Arct.

Full length article
Numerical study on lateral buckling of pipelines with imperfection and sleeper