Pragmatic Solutions to Touch-Down Zone Fatigue Challenges in Steel Catenary Risers
Authors: Bhat Shankar , Dutta Amit , Wu Jack , Sarkar Indranil 
Published: October 2008

The Steel Catenary Riser (SCR) is often the preferred riser concept in deep and ultra deepwaters when it is proven to be a feasible riser solution. One of the first tasks in any front end engineering study is to assess the feasibility of SCRs for the applicable design criteria and the floating system of interest. A number of SCRs have been installed worldwide over the years and SCR technology is gradually becoming mature. However, the expanding needs of the offshore industry tend to continuously push the limits of SCR technology. Engineers are often asked to assess the feasibility of SCRs outside the limits of what has been done to-date. SCR feasibility is controlled by fatigue in the touch-down zone. The purpose of this paper is to describe some recent studies on pragmatic ways to enhance fatigue performance of SCRs. These studies include a number of insightful sensitivity analyses and innovative solutions which can help SCR engineers throughout the industry.

1.0 Introduction

A recent survey [1] indicates that there are more than seventy Steel Catenary Risers (SCRs) in operation connecting both import and export lines to fixed and floating platforms, and at least eighty additional SCRs are in the planning and fabrication stages. This indicates the growing maturity of SCR technology. However, design challenges remain, and a combination of larger vessel motions, lower water depth, larger pipe diameter and/or degradation of the fatigue S-N curve due to sour service application can result in conventional SCRs being found to be infeasible for a project.

It is generally considered a greater challenge to suspend SCRs from vessels with relatively higher motions, such as semi-submersibles and FPSOs. In the present paper, attention is focused on SCRs attached to semi-submersible vessels, although much of the findings are general enough to be applicable to other floating platform types also.

The schematic of a typical SCR is shown in Figure 1. In general, fatigue performance, especially in the Touchdown Zone (TDZ), poses a greater challenge to SCR design than strength response. In instances where low fatigue life in the TDZ of the SCR poses seemingly insurmountable difficulties, instead of switching to an entirely different riser concept, it may be possible to add enhancements to the basic SCR configuration in order to achieve acceptable performance. This paper discusses the factors that control the fatigue response of SCRs suspended from floating production systems. Solutions to the TDZ fatigue problem using currently available techniques/technologies are presented, along with the results of sensitivity studies.

2.0 Factors Controlling SCR TDZ Response

Typically, the water depth, SCR diameter, pressure requirements, soil properties and environmental conditions are fixed by specific project requirement. All other factors are open to variation, especially in the early "system selection" phase of a project. References [2,3,4,5] provide interesting insights on the effects of soil nonlinearities, self trenching and incorporating K-C number dependent hydrodynamic coefficients, each of which can impact (and, for the most part, improve) the calculated fatigue life at the TDZ significantly. These are general topics on which research has been done, and continues to be performed. This paper will not address these issues. Instead, this paper will concentrate on fundamental aspects related to response and feasibility enhancement of SCRs attached to Semi-FPS vessels.