Lateral buckling and axial walking of surface laid subsea pipeline

Abstract

Subsea pipelines are increasingly being required to operate at high temperature and pressure HT/HP. The pipeline installed on the seabed and left exposed have a potential to buckle, walk and change configuration under high temperature and pressure (HT/HP). This could lead to failure of the Pipeline if buckling and walking is not properly controlled or mitigated. The objective of the thesis work is to study and understand the influence of pipeline-soil interaction on the design of surface laid subsea pipeline susceptible to lateral buckling and pipeline walking. The main focus of the thesis work is on the use of snake-lay configuration as a mitigating measure under controlled buckling design and rock dumping if needed to limit feed-in into buckle and end expansions; the effect of thermal gradient on axial walking and the use of direct electric heating (DEH) to reduce rate of walking. The snake-lay configuration is achieved by installing deliberate horizontal lay imperfection to trigger a sufficient number of thermal buckles at a pre-determined location along the pipeline. The desire is to limit pipeline expansion at the connecting ends by using snake-lay design with intermittent rock dumping. The work includes performing non-linear finite element analysis (FEA) and modeling the soilpipeline interaction of as-laid pipeline using general finite (FE) element software ANSYS. The results are discussed against the relevant design criteria based on design codes DNV-OS-F101, DNV-RP-F110 and Subsea 7 Lateral Buckling Analysis Design Guideline. FE analyses were performed to study the lateral buckling of a 2km VAS model with an initial out-of-straightness (OOS) under operating temperature and pressure. Consequently, the effect of thermal gradient of an asymmetric heating process in pipeline walking phenomenon is investigated. Based on FE analyses findings, the use of direct electric heating (DEH) system to reduce the rate of axial walking is proposed and explained. Keywords: Initial imperfection, Lay radius, walking, lateral buckling, Out-of-straightness (OOS), pipe-soil interaction, end expansion, effective axial force, Feed-in, Virtual anchor, snake-lay, thermal gradient, pipeline, direct electric heating (DEH).