Steel Catenary Risers for Deepwater Environments
Authors: Hatton Stephen A. , Willis Neil 
Published: October 2008

Steel catenary risers are a potential solution for future deep and ultra deepwater production applications. However, the complexity of design and installation is greater than for flexible riser systems. The challenge is even more significant when steel catenaries are considered for harsh environments such as West of Shetland, where extreme and long-term environmental conditions are amongst the most severe in the world, causing the risers to be highly dynamic and fatigue sensitive. The STRIDE JIP (STeel RIsers for Deepwater Environments) was initiated with the purpose of increasing confidence levels in the design methods and defining practical installation procedures suitable for harsh environments. In these locations weather windows are short, installation vessel motions high and fabrication quality is paramount. The paper presents the scope of the STRIDE JIP and key findings of Phase I. Remaining areas of uncertainty and the proposed approach for resolution of these during Phase II are discussed. These include improved VIV prediction, touch down point response confirmation, definition of weld quality requirements and effects of plastic deformation.

Introduction

2H Offshore Engineering initiated the STRIDE JIP in January 1997. The main objective of the JIP is to develop steel catenary technology to a level whereby it may be applied with confidence. The impetus for developing steel catenary technology is the continuing push into deep and ultra deep water, and the growing belief that riser systems based on steel pipe, rather than flexible, offer the best technical and commercial solution. However, current industry knowledge and capability is largely based on flexible dynamic risers and static steel pipelines. Consequently, successful application of steel catenary riser systems requires designers and installation contractors to develop new capabilities. An important feature of the STRIDE JIP is the participation and involvement of Operators, installation contractors and vessel contractors. This is seen as critical to the resolution of important interfaces and development of practical fabrication and installation solutions. The JIP has a total duration of 4 years with three distinct phases covering design, testing and a largelfull scale installation. 14 Operators and 6 Contractors supported phase I of the JIP, which was completed in January 1998.

Phase I Scope

The scope of Phase I covers riser system design and installation methods. The riser design scope, conducted 2H, includes pipe sizing, riser analysis (extreme storm, fatigue and parametric studies), material and welding requirements and hardware selection. The installation scope, conducted by the installation Contractors as payment 'in kind', covers a general review of steel catenary riser installation strategies, documentation of previous experience, current capability and an installation study covering detailed procedures, installation sequence drawings, schedule and cost information. A wide range of riser configurations are considered, Table 1, covering risers of 10-30 inches diameter suspended from TLP, Spar and FPSO production platforms. Water depths in the range of 400111 to 2000m are evaluated with the majority of work concerned with harsh environments typical of the West of Shetland and Voring Plateau. Benign environments such as the Gulf of Mexicio and West Africa are also considered but form a smaller proportion of the analytical effort.