A 12" rigid production flowline of approximately 27 km in length and a 6" rigid MEG supply flowline of approximately 38.5 km in length were installed in the Liu Hua 29-1 field in South China Sea on a fairly rough seabed in water depth varying from 520m to 1120m. The preliminary bottom roughness assessment in basic design gives an indication that a large number of the excessive spans may occur based on the seabed profile along the selected route. Following that, a pipeline route optimization was carried out using DTM tools in Fledermaus software firstly to avoid the pipeline being routed through rough & undulating areas as much as possible. During the EPC stage, a proper seabed characterization and a refined pipe soil interaction (PSI) assessment were carried out to predict the pipeline as-laid embedment using site specific soil parameters and project specific pipelay parameters. The bottom roughness assessment was then re-run using the pipeline as-laid embedment predicted by the refined PSI assessment. The predicted number of spans finally dropped to less than 10, all of which require no correction measures and was confirmed by the pipeline as-built survey results. The aim of this paper is to showcase the design optimization process used in the design of deepwater pipelines installed on a rough and undulating seabed terrain to minimize the pipeline post-lay span correction work and how the advanced PSI assessment can benefit the prediction of pipeline frees spans. As the results of this design optimization process, an optimized result in terms of most techno-economically feasible route was achieved in the end.
The subsea gas production system of Liu Hua 29-1 field in South China Sea is composed basically of a rigid 12" production flowline approximately 27 km long and a 6" rigid MEG supply flowline approximately 38.5km long installed on a fairly rough seabed in water depth varying from 520m to 1120m. During the basic design, the free span assessment was performed considering the maximum allowable free span length determined by the screening method recommended in DNVGL-RP-F105  and pipeline bottom roughness analysis was performed using the pipe vertical load penetration response model recommended in DNVGL-RP-F105 . This approach resulted in a large number of predicted free span requiring corrections, leading to a higher capital cost estimate for the project.