Reliability-based design of subsea light weight pipeline against lateral stability
Introduction
With increased corrosion failures and the degradation of rigid steel pipelines in subsea system for sour crude oil, anti-corrosion non-metallic light weight pipelines (LWPs) are considered to replace the rigid steel pipe due to their flexibility and ductility. However, one of the greatest challenges for the application of LWP to subsea systems is the on-bottom stability problem of the pipe because of its light weight [1]. The selection of additional weight for subsea pipeline on-bottom stability analysis is critical to decrease the cost of the pipeline system.
A comprehensive understanding of the fluids-pipe-soil interaction for subsea pipelines is very important to determine the additional weight required for the pipe on-bottom stability. Factors included are the hydrodynamic forces due to waves and currents (including lift, drag and inertia forces) and passive soil resistance [2]. The most common approach used in industry to assess the on-bottom stability of submerged pipelines is making conservative assumptions based on hydrodynamic loadings and soil resistance. However, the evaluation of hydrodynamic forces introduces large uncertainty into the analysis. For example, the universal force coefficients do not consider the effects of the body of pipeline-itself [3], [4], [5]. The evaluation of the interactive effects between the seabed and the pipeline is far more complex. The development of pipe-soil interaction models can be divided into three stages. Stage I: DNV RP-E305 [6] recommends conservative soil friction coefficients, usually, 0.7 for sand soil and 0.2 for clay soil to model the soil resistance. However, tests show that even for the same type of soil, friction coefficients show a considerable scatter [7], [8]. The recommended friction coefficients are selected based on the lower bound values and therefore may under represent actual values. Stage involves two-component empirical models that are proposed and developed based on the theoretical and experimental work [9], [10], [11], [12], [13]. The models consist of a basic coulomb friction component and a passive component. Yet for different models, the passive component depends on soil strength, unit weight of soil, embedment, loading history, etc. Stage III is the plasticity pipe-soil interaction model first introduced by Zhang et al. [14], [15], [16] and developed further by Tian and Cassidy [17], [18], Tian et al. [19] and Youssef et al. [23]. Compared to the previous models, the plasticity model provides a much more comprehensive understanding of the mechanics involved in the pipe-soil interaction. However, it is known that there are large uncertainties in governing soil parameters, load effects etc. in the model. Thus, it is still not a valid method regarding pipeline on-bottom stability.
The probabilistic analysis shall be used in the model when large uncertainties exist in the relevant parameters, where the uncertainties in the range of the basic variables are described statistically. The statistical values used to describe a random variable are the mean value and the coefficient of variation (CoV). These values may be obtained from recognized data sources in normal cases. It is then possible to use reliability analysis, taking account for all relevant variables and the uncertainties related to them. Once the probability functions of all parameters are obtained, the probability of pipeline failure due to on-bottom stability can be evaluated. The application of response surface model for the probabilistic analysis of pipeline on-bottom stability was first conducted by Youssef et al. [24].
Excessive lateral displacement of pipeline under wave and current loads may be considered to be a serviceability limit state (SLS) with a recommended target failure probability of 10−2 per pipeline per relevant period [25]. The target reliability levels have to be met in design to ensure certain level of safety. The aim of this paper is to provide an approach for the reliability-based determination of subsea pipeline additional weight allowance against on-bottom lateral instability. The paper first introduces a non-linear finite element model (FEM) that takes account of the fluids-pipe-soil interaction model. Then it performs a probabilistic analysis that takes into account all relevant parameters and the uncertainties in the FEM model. A response surface model in combination with the Monte Carlo simulation is used to calculate the safety indices and the required submerged weight can be determined to meet the target levels of failure probability.
Section snippets
Structural model
A non-linear FEM is constructed to model hydrodynamic loads on subsea pipelines resulting from irregular waves and currents. Fig. 1 shows a flow chart of the non-linear FEM for pipeline on-bottom stability analysis. The general non-linear FE software ABAQUS is used to model the fluid-pipe-soil system. The beam element PIPE31H is used to simulate the pipeline and the beam section of the pipe is determined by pipe diameter and wall thickness. The subsea pipeline is subjected to hydrodynamic
Case study
A flexible non-metallic light weight pipeline is selected for the pipe on-bottom stability analysis. Table 1 lists the input parameters used in the case study. The wave and current data are selected from a typical area in the South China Sea [27]. The soil type is calcareous sand. A finite element model representing a 500 m pipeline was configured using ABAQUS/Standard PIPE31H structural elements. A 500 m long pipeline is divided into 50PIPE31H elements. The pipe-soil interaction model is
Reliability assessment
Failure caused by excessive lateral displacement of the pipeline because of the loss of pipe on-bottom stability due to hydrodynamic loads may be considered to be a serviceability limit state (SLS). The limit state can be specified by the operator, guided by a tolerable displacement limit:
Where g(Z) is the failure function, Z is a vector of all uncertainty variables, R(Z) is pipeline design resistance, S is the serviceability limit state. According to structural reliability theory,
Pipeline submerged weight study
According to the case study in the last section, the pipeline lateral displacement will definitely exceed the design criterion. To determine what pipe submerged weight should have been required to meet the target probability level, pipeline cases with different submerged weight values using FE analysis were studied. Stabilization method such as trenching, anchoring, rock dumping and mattress have been used in the past to ensure the stability of the pipeline. It is assumed that in this paper the
Conclusion
Reliability-based analysis of subsea light weight pipeline against lateral stability based on a non-linear finite element model is introduced. In the FEM model, the hydrodynamic loads generated by irregular waves and soil resistance by pipe-soil interaction are included through Abaqus subroutines. An example of on-bottom stability analysis for flexible non-metallic light weight pipeline is selected as a case study. The analysis results show that pipeline displacement behavior varies widely
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