The majority of the deepwater pipelines operating in the Gulf of Mexico (GoM) carry hydrocarbons at a high pressure and temperature resulting in the system's thermal expansion. In order to relieve the high stresses due to thermal expansion, these pipelines are designed to use buckle triggers in order to buckle laterally at pre-determined locations. Since these assets are expected to last a few decades, it is imperative that a robust long-term integrity management program is in place to effectively operate these pipelines, monitor their long term performance, verify design assumptions and thus, reduce any potential risks.
As a part of BP's comprehensive In Water Integrity Management (IWIM) program, three pipelines were surveyed during a 2012 Multi-Beam Echosounder (MBES) survey campaign. The surveys checked for buckle initiation, buckle locations and obtained accurate buckle shapes to allow subsequent integrity assessments to be performed. Ten out of ten planned buckles formed across the three operational pipelines; a mixture of sleepers and buoyancy modules initiated these buckles at pre-determined locations. Smaller buckles were also observed at unplanned locations in the pipeline system. These pipelines terminate at subsea structures situated at the ends of the pipelines. In order to track the pipeline's long-term global movement; the end displacements at these structures were recorded as well.
During its operation, a pipeline system will be shut-down and re-started multiple times, thus creating a series of thermal cyclic loadings. In order to ascertain the long-term performance of the pipelines under such sustained thermal loading cycles, a Finite Element (FE) model is built using the pipeline data, seabed bathymetry and other environmental conditions. The FE model is calibrated using the recorded 2012 MBES survey in-service pipeline lateral buckling profiles. The calibrated FE model generates axial stress ranges at critical locations (buckle crown); these stress ranges are subsequently used to perform fatigue analyses. The estimated fatigue damage is used to forecast the pipelines' long-term fatigue performance, which will determine the future inspection and monitoring plans.
This paper presents an overview of the 2012 MBES Survey conducted on three of BP's operational pipelines as well as the subsequent FE and fatigue analyses performed to assess the long-term integrity within lateral buckles.