Lateral buckling induced by trawl gears pull-over loads on high temperature/high pressure subsea pipeline

Abstract

This thesis work is performed to study and understand the pipeline global response as a result of trawl gears pull-over loads triggering lateral buckling on high temperature/high pressure subsea pipeline. The external interference from trawl gears pull-over loads may create substantial imperfection or out-of-straightness on the pipeline to trigger lateral buckling. The pull-over loads also may induce excessive bending moments and strains in the buckle region. To be able to understand the global response of the pipeline under pull-over loading condition, a finite element analysis is carried out based on DNV OS F101 [1], DNV RP F110 [2] and DNV RP F111 [1] . The analysis is carried out using general finite element analysis software ANSYS v13. Nonlinear-transient analysis is used to incorporate the non-linear effects, such as the nonlinearities of pipeline material, and the response of a structure under the action of pull-over time-dependent loads. The finite element analysis covers two period of duration i.e. during pull-over loads duration and after pull-over loads duration. The analysis during pull-over loads duration deals with the pipeline global response as a result of trawl gears pull-over loads. The pipeline is subjected to substantial horizontal and vertical pull-over forces from the trawl gears. The pull-over forces are applied based on force-time history as recommended in DNV RP F111 [3]. This thesis work also study the pipeline post buckling condition. After pull-over loads duration, the pipeline global response may be different. The analysis is carried out for additional 5 seconds from the end of pull-over loads duration. In this period, the pull-over loads are no longer applied. The pipeline may expand further more caused by temperature and pressure loads on pipeline. There are four (4) types of trawl gear considered in the analysis i.e. polyvalent and rectangular trawl gear, industrial v-board, beam trawl and clump weight. All the trawl gears data are taken from DNV RP F111 [3]. The thesis work calculates the pull-over loads and durations from all type of trawl gears. In the finite element analysis, only the pull-over loads from polyvalent and rectangular trawl gear are used as this is considered to adequately represent the cases. The finite element analysis of trawl gear pull-over shows the trawl gears pull-over loads induces lateral buckling on pipeline. The pipeline deform laterally at the pull-over loads location. High bending moments and strain occur at the apex of the buckle. The DNV displacement condition code check shows that the integrity of the pipeline is not satisfy the safety level hence the pipeline cannot withstand the trawl gears pull-over loads. The analysis with variation of operating temperature and pressure should be carried out as further works. The further works shall also take into account a full comparison of lateral and axial soil friction combination. This way, a more accurate result can be obtained and better conclusion can be verified.