Elsevier

Marine Structures

Volume 76, March 2021, 102891
Marine Structures

Efficient methods to mitigate SCR-induced walking of short subsea flowlines

https://doi.org/10.1016/j.marstruc.2020.102891Get rights and content

Highlights

  • Cost-effective walking and buckling mitigation techniques are investigated.

  • Innovative pre-deformed pipeline strategies are proposed to substantially reduce effective axial force along pipelines.

  • Novel deformed pipe pieces cause plenty of small deformations that gives benefit to structural response of the pipeline.

  • As-laid configurations effect on thermomechanical response of the pipeline is explored.

Abstract

Steel catenary riser is a long-established option for subsea projects in deep-water regions. Sustained pulling force of steel catenary risers on subsea flowlines in combination with cyclic thermal load throughout the system lifetime may lead to progressive global axial displacement of subsea pipelines which has been termed as ‘walking’. One of the challenges in the deep-water industry is long-term walking of subsea flowlines in a cumulative manner. Common practice methods for walking mitigation are quite expensive operations. State-of-the-art mitigation strategies are proposed in the paper by means of modifying pipe pieces before the installation operation. Bowed pipe pieces and miter joints are two recommended approaches for walking mitigation. The presented mitigation strategies are relatively cost-effective solutions for the pipe-walking challenge and they are able to considerably cease the potential cyclic walking. Comprehensive FE analyses in ABAQUS software are performed to evaluate the proposed deformed pipelines response subject to two loading conditions. Through-life integrity of the suggested pre-deformed pipeline is assessed in terms of effective axial force, local buckles and excessive axial strains. A comparison of the presented method with conventional techniques shows the effectiveness of the proposed configuration. The proposed methods can significantly reduce effective axial force throughout the subsea pipeline by means of artificially introduced deformations. The cumulative walking of the presented method is practically zero. In addition, the influence of combined triggering mechanisms to the walking phenomenon is assessed when the pipeline is located on a sloping seabed and it is subject to non-uniform thermal loads. A parametric study is performed to improve confidence in design and provide a reasonably practical technique with an optimal shape.

Introduction

Subsea pipelines are designed to convey high-temperature well-stream products from offshore oil and gas fields. Subsea pipelines must tolerate the elevated temperature in conjunction with further harsh operational and environmental loads over their lifetime. High temperature and pressure loads normally build up high axial compression in pipe's wall-thickness; accordingly, the pipeline naturally tends to react by peripheral expansion. In addition, periodic shutdowns and startups throughout the system lifetime may promote longitudinal movement of the entire short flowlines which is known as the ‘walking’ phenomenon.

The cumulative lifetime movement of the pipeline may result in catastrophic failures. Pipeline walking with serious consequences has initially been reported with documents in the North Sea [1]. The walking occurrence is not an immediate incident and the failure may occur many years after installation. Nowadays, the phenomenon is fairly understood in the offshore industry. Nevertheless, careful design and accurate prediction of walking action are crucial to ensure satisfactory long time performance.

SCR, Steel Catenary Riser, is a common option in deep-water projects [2]. Permanent pulling force at one end of the heated pipeline may lead to asymmetric contraction and expansion along the pipeline length and walking per each cycle of shutdown and startup. Other governing factors such as seabed slope and uneven thermal loads may similarly drive the walking.

Pipeline shift towards the riser-side end that is imposed by the presence of the SCR may result in severe consequences. Walking is not considered as a limit state criterion in the design process but it must be predicted accurately. One major consequence can be overstressing end facilities and tie-in structures such as PLETs, spools and jumpers. Changes in shape and loss of tension in SCR are potential significant concerns caused by accumulative axial movement [2]. In addition, there are some other possible consequences comprising straightening the bends and losing its original route, failure at connections and risers, uncontrolled lateral buckling, and changes in prediction of end expansion in design stage [3].

It is important to mitigate axial instability of subsea pipelines in order to avoid its unwanted effects. Walking mitigation is the major challenge in deep-water short flowlines that are connected to a SCR. The required cost for prevailing methodologies designed for walking mitigation might soar to about 10% of the global budget of a subsea pipeline project [4], as some common mitigation measures in subsea industry might be unfeasible for some cases in deep-water fields.

Holding pipelines opposite to the anticipated walking direction was used widely in the deep offshore industry [5]. Anchoring is the most common solution for pausing the walking phenomenon [5]. Typical anchors are normally connected to one section of the pipeline and the resistance force is mainly concentrated in one section along the pipe. It causes an abrupt change in EAF distribution throughout the pipeline. Industry experience has shown that post installed anchoring facilities are generally very large and expensive. Anchors and retrofit clamps are used in several projects in the West Africa [4]. The required holding capacity of walking anchors might be about 100 mT [6]. Some larger anchors such as a 350 ton anchor might be utilized for controlling the walking [3]. These intervention systems may become a million-dollar project [7] and it was observed that some walking cycles occurred after anchor installation [8].

Restraining the pipeline in some way may cease the axial walking. A common alternative for preventing flowlines movement is rock dump [9]. The cost of rock dumping operation may increase with water depth [10]. Clump weights and concrete coats can also be employed for stopping pipeline walking [5]. Moreover, flexible concrete mattresses installation reduces end expansion and walking rate [11].

Another option to the axial restraint system is buckle initiation concept. Pipeline configuration on seabed with a couple of initial out-of-straightness (OOS) can trigger buckles at regular intervals. Despite the restraining methods that may lead to high axial force in pipelines, the initial deformations encourage the pipeline to release its high axial force. The initial deformation strategy has been used in several common mitigation techniques such as snake lay [12], sleeper [13], distributed buoyancy [14], zero bending radius [15], and residual curvature [16]. Carr et al. outlined the issues associated with interaction of lateral buckling and walking [17]. It is investigated that gradient temperature profiles cause a greater length of buckle in the hotter segment of the pipeline. Perinet et al. investigated the risks of subsea pipeline failures as a result of walking and buckling interaction [4]. It was shown that soft clay soils usually aggravate the walking and buckling interaction situation.

The snake-lay method is a conventional option to create predefined OSS with about 2–5 km VAS. It has been utilized widely in projects such as Penguins Field in the United Kingdom [18]. Large radius bends can be formed with no trouble by vessel route, but buckle initiation is not reliable at each bend.

An alternative way to make OOS is using reel laying vessels to create residual curvatures during the laying operation. Residual curvature method is successfully utilized in Statoil's Skuld project with the intention of lateral buckling mitigation [19]. This method is applicable just for installation of limited size pipeline by reel laying vessels and it cannot be used during s-laying or j-laying installations.

Conventional methods for buckling mitigation usually cause relatively large buckles. Besides, there is uncertainty with buckle formation in these methods because of long VAS. In case of any long VAS case, seabed undulations may trigger unexpected buckles between to planed OSS [[20], [21], [22]]. Large deformations put the pipeline at risk with generation of high strain in the pipeline and pipeline damages. As an illustration, Erksine pipeline in Britain was damaged by excessive strain induced by buckling [10].

Some aspects such as pipe-soil interaction may play an important role in buckle formation and restrain the pipeline against expansion. Uncertainty in soil behavior, variation of axial and lateral fiction and OOS reduce the reliability of triggering buckles in the established SL method [4]. Serious consequences such as pipeline rupture because of uncontrolled buckling are reported in the Guanabara Bay of Brazil [23]. Full-bore ruptures caused by lateral buckling were observed in the North Sea, West Africa [14].

Several research projects such as SAFEBUCK JIP are performed dealing with pipe-soil interaction and its influence on buckle development. The tendency to walk toward the riser primarily relies on axial soil friction. Hong et al. evaluated the accumulated pipeline walking with considering changes in axial soil friction [2].

It has been observed that relatively short VAS can benefit the buckling response of subsea pipelines. Lateral buckling assessment of a specific pipeline with sinusoidal out-of-straightness was carried out by Chee et al. [24]. It was shown that numerous controlled buckle formations with short virtual anchor spacing can reduce uncertainties in subsea pipeline design and lateral buckling analysis. It was also observed that buckle initiation temperature rises by implementing continuously pre-deformed pipeline [25].

The aim of this paper is to propose safe and cost-effective methods for walking mitigation. It focuses on the SCR force induced walking of a typical short flowline. Walking of the straight pipeline (SP) is also assessed to capture the mechanism in comparison with proposed mitigation techniques. In addition, the influence of combined triggering mechanisms to the walking phenomenon is assessed when the pipeline is located on a sloping seabed and it is subject to non-uniform thermal loads. FEAs are carried out to investigate pipeline structural response with different as-laid configurations of the pipeline on seabed as to highlight the difference of the newly proposed design solution with a conventional mitigation approach. In the end, a parametric study is implemented to investigate structural response and the impact of design dimensions in the initially deformed pipeline.

Section snippets

Model description

Deep-water pipelines usually are installed unburied over the seabed. A 14-inch exposed short flowline which is laid on the seabed is studied in the present paper. The most common pipe sizes for deep-water subsea flowlines are in the range of 12in to 16in and usually do not have concrete coats especially in projects that are installed by reel lay vessels. Therefore, deep-water flow lines have a comparatively lower submerged weight than the shallow water ones.

The walking phenomenon can be

Straight pipeline (SP)

In order to highlight the necessity for proper mitigation, this section focuses on the modeling of an ideally straight pipeline (SP) with no OOS which imposes the highest rate of walking. The characteristics of model for SP case are in conformity with data described earlier in Table 1. Pipeline as-laid configuration on seabed is assumed perfectly straight with no initial deformation. The SP is evaluated to provide a comprehensive understanding of the phenomenon to demonstrate the walking

Finite element analysis (FEA)

Several detailed FE analyses are undertaken in the current research to anticipate the walking phenomenon accurately and at the same time, present novel mitigation measures. Three-dimensional models in ABAQUS software are utilized to assess 14” subsea flowline example.

Use is made of PIPE31H elements with 0.5 m mesh size to model 10 km deformable pipeline. The PIPE31H is a Timoshenko beam with a hybrid formulation that takes into account transverse shear deformation and it is well-suited for

Walking of the straight pipeline

Walking estimation of the SP which is induced by SCR is reviewed according to the available information in the literature. The theoretical prediction of the walking is provided to validate the FE model results. After verifying the FEA, further comparative analyses of SL, BPP, and MJ are performed by utilizing similar FE models.

The highest potential axial load can be emerged in the pipeline when it is axially locked-in with fixed ends without any expansion. In this case, the significant load

Parametric study on as-laid configuration

With the aim of achieving a mitigated walking response, two novel pipeline configurations are proposed in the paper (Sections 3.3 Bowed pipe pieces (BPP), 3.4 Miter joints (MJ)) by means of slight modifications in pipe pieces. Welding of the modified pipe pieces can result in predefined initial imperfections at regular intervals throughout the pipeline. The quantity and magnitude of intended OSS have influences on the pipeline response. They may affect walking rate, susceptibility to buckle,

SP results

Cumulative axial displacement of the SP is presented in Fig. 7. Cumulative riser-side movement of the mid-line section (KP5) shown in this graph during ten cycles of pipeline start-ups and shut-downs indicates pipeline walking.

A verification analysis is carried out to validate the finite element model of the SP. Subsequently, the configuration of the pipeline on seabed is modified and the BPP and MJ models are developed based on the verified SP model. So, all the analyses in this paper are

Conclusions

The walking mechanism as a major design challenge for deep-water short flowlines is discussed in the paper. The prediction of the phenomenon is carried out considering two different loading conditions. Primary analyses are carried out on the assumption that the walking is induced solely by the pulling force of steel catenary riser, and then further analyses are performed for a situation where it is subject to coupled triggering mechanisms. This paper has introduced innovative walking mitigation

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.

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