Steel catenary risers (SCR) have been an attractive choice for deep-water field developments due to its simplicity. However, design of SCRs from large motion host platforms has been a great challenge. The key issues for the design of SCR in such conditions are the fatigue near hang-off and at touch down point.
Design of SCRs by distributing weight along the riser enhances the fatigue performance of SCRs thereby extends the feasibility of SCRs from large motion floaters, fulfilling both strength and fatigue requirement. The weight distribution along the SCR is obtained by fully qualified ballast modules and also fully qualified light weight add-ons at TDP which are designed for mud-line application. Further, by high quality welding and optimized weld geometry the fatigue performance of the SCR welds are improved and optimized. This paper also describes the mechanism that drives the weight distributed SCR design and also fatigue test data from optimized weld geometry.
The results clearly indicate that it is possible to have a robust design of SCRs from large motion host platforms using presently qualified material and technology. These qualified products extend the applicability of the weight distributed SCR concept to most dimensions and applications. This concept is also applicable in less demanding environments.
Fatigue test results will be presented from both carbon steel and clad/lined riser qualification programmes which will demonstrate that high fatigue performance can be sustained with established girth welding procedures with appropriate control of pipe fit up tolerances and weld soundness.
The weight distributed SCR concept combined with high performance weld, will make the applicability of SCRs a credible alternative option for use from host platforms with large motions. Since the ballast and light weight add-ons are fully qualified material they are cost effective and ready for project application.