Fatigue Enhancement of SCRs: Design Applying Weight Distribution and Optimized Fabrication
Authors: Legras Jean-Luc , Karunakaran Daniel N. , Jones Richard Lloyd 
Published: May 2013

Steel catenary risers (SCR) have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environment or from large motion host platforms has been a great challenge. The key issues for the design of SCRs in harsh environment are the fatigue near the hang-off and at the touch down point.

Design of SCRs by distributing weight along the riser enhances the fatigue performance of SCRs thereby extending the feasibility of SCRs for deep water harsh environment whilst fulfilling both strength and fatigue requirements. 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 in harsh environments using presently qualified materials 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. Based on the work, the applicability of this concept is mapped out for various floater motions.

Fatigue test results are presented from both carbon steel and clad/lined riser qualification programs 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 welds, will make the applicability of SCRs a credible alternative option for use in increasingly harsh environments or 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.

Introduction

Steel catenary risers have been an attractive choice for recent deep-water field developments. However, design of SCRs for harsh environment has been a great challenge, especially from large motion host platforms such as semi-submersible platforms. The application of SCR from such semi-submersible Floating Production Units (FPU) in harsh environments presents design challenges due to the large motions of the vessel from waves and large vessel offsets from wind, current and slow-drift wave motions. There are buckling issues at touch down point (TDP) due to large heave and surge motions, and fatigue problems due to vessel motions and soil-riser interaction. These design challenges can be successfully addressed by introducing a SCR concept with varying weight along the riser and with the lightest possible cross-sections in the Touch Down Zone (TDZ), Karunakaran et al. (2002), Karunakaran et al. (2005) and Foyt et al. (2007). This concept is schematically shown in Figure 1.

The effects of such weight distributed design of SCRs are demonstrated for North Sea condition and for West Africa conditions. These example studies are performed using nonlinear time domain analysis and the fatigue analyses are performed applying a comprehensive irregular wave time domain analysis procedure.

Further, by high quality welding and optimized weld geometry the fatigue performance of the SCR welds are improved. Fatigue test results will be presented from both carbon steel and clad/lined riser qualification programs 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.