ESCARPMENT CROSSING DESIGN AND ANALYSES OF LARGE DIAMETER OFFSHORE PIPELINES

Subsea pipeline design for escarpment crossings proves complicated and time-consuming, primarily attributable to the excessive plastic strains encountered at the overbend and the extraordinary fatigue damage over long spans, exacerbated by the steep seabed conditions. Due to substantial uncertainties, the design approach for pipeline escarpment crossings leans towards conservatism by limiting pipeline bending moment/strain and span length. Consequently, the implementation of mitigation measures becomes a common necessity. In pursuit of a straightforward and cost-effective crossing design, this paper offers a comprehensive insight into the in-place behavior of large diameter pipeline crossing over escarpments. This study investigates a 24-in gas export pipeline in an African field from a water depth of 25m to 250m. The pipeline strength assessment employs the displacement-controlled condition (DCC) at the escarpment top and load-controlled condition (LCC) for the remaining pipeline, aligning with DNV offshore pipeline design standards. The numerical model, utilizing Abaqus elbow elements, simulates the S-lay, pre-commissioning, and operating processes. The mitigations of the strain and DCC utilization are then examined. This involves optimizing pipeline route, wall thickness, lay tension, and crossing protection.