ABSTRACT

This paper describes the use of the pipe clamping mattress (PCM) option as an alternative and effective solution to mitigate the pipeline walking. The description provided in this paper is for a generic case study performed on a specific flowline. The flowline is designed to have several buckle triggers at predetermined locations and PCMs acting as walking restraint are then installed at specific locations prior to startup on top of the flowline, to reduce the walking rate by adding extra weight to the flowline. The PCM system comprises two components: the PCM mechanism itself clamping around the flowline and additional log mattresses placed above achieving an effective translation of the overall PCM system weight to axial frictional resistance on the seabed. The case study of the walking assessment for the flowline was performed using Finite Element Method (FEM) and had concluded that the flowline is susceptible to axial ratcheting, mainly governed by the temperature gradient during operational startup and shutdown cycles. The total amount of walking at the structures, over the design life was calculated and has exceeded the design limit, hence requiring deployment of walking restraint measures. By installing PCMs at specific locations, the total walking of the flowline was reduced to acceptable levels. PCMs were utilised as the walking restraint and were grouped together at 4 different locations (at the proximity of end structures and in-line structure). The effectiveness of PCM as a versatile alternative to conventional methods of mitigating pipeline walking with positive impact on different areas (design, cost, transportation, and installation) has been demonstrated.

INTRODUCTION

Pipeline walking is a phenomenon exhibited by relatively short pipelines or pipelines with lateral buckles where cyclic axial displacement occurs towards one end or towards the buckles. Pipeline walking occurs due to the following influencing factors:- gradients of transient temperature for operational startup and shutdowns, steep seabed slope, tension from dynamic riser and lateral buckles along the pipeline which split a pipeline into ‘short’ pipeline segments (short pipeline is described as pipeline that is not long enough to develop a fully restrained section) (Solano et al. 2014). If walking is not controlled by appropriate mitigation measures, the cumulative axial displacement throughout the design life could impact the integrity of the pipeline systems specifically at tie-in spools and structures (Perinet and Simon, 2011). Different mitigation types to anchor pipeline have been widely used in the industry such as suction piles with steel chains (Verdeil and Mourad, 2018), rock dumping and conventional concrete mattress. Depending on requirements of the anchoring system, the selection of walking mitigation scheme can significantly impact the cost of the projects. Hence, more reliable and cost-effective solutions are often preferred.

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