Use of Stabilisation Chains to Optimise Lazy Wave Flexible Risers in Harsh Environments

Abstract

AbstractThe paper addresses the use of steel chains in a lazy wave configured flexible riser system to provide an alternative flexible riser configuration for use in challenging environments including large vessel offsets and motions, and large ranges of riser internal fluid properties. While the compliant nature of flexible pipe provides excellent fatigue and strength resistance, flexible risers typically experience larger deflections when compared with rigid risers, which results in greater challenges managing interference issues with adjacent structures. Different lengths and variable masses of chain are installed at locations along the hog bend of the flexible riser configuration. The arrangement of the chain masses, length and positioning along the line are developed to primarily prevent contact with the seabed and the hull of the FPSO when a range of heavy and light internal fluids are considered. A number of weighted steel chain configurations are evaluated and presented through an analytical case study in order to demonstrate the benefits of this approach for a typical generic shallow water application FPSO system. Installation and hardware design aspects are additional requirements that may need to be addressed in further assessments.Through the in-place case study, comparisons are made between the performance of the flexible riser system with and without the weighted steel chains. Global finite element models are developed to simulate the performance of the different flexible riser configurations when subject to a range of loading scenarios covering large FPSO offsets, harsh environmental conditions and a range of riser internal fluid densities. Performance criteria of the flexible riser such as tensile loading, curvature and motion envelopes are presented to show the improvements derived though optimisation of the chains. It is also demonstrated that the chain section that extends along the seabed helps to reduce the transverse displacement and “lateral walking” thus reducing the risk of clashing with adjacent structures and changes in line lay azimuth under strong transverse current loading. The cost effectiveness of the chain weighted flexible is also compared to other solutions considering new and retro-fit applications. This work demonstrates that an improved and cost effective solution is developed to provide an acceptable flexible riser dynamic response for the range of operational fluid densities that may be experienced in its operational lifetime.