Development, Testing & Application of High Fatigue Performance Riser Joints
Authors: Baxter Carl , Boster Patrick , Caldwell Chris , Halley Jim 
Published: October 2008

Several fields have recently come on stream using TLP's or SPAR platforms that required production risers with high strength and fatigue resistance. This paper gives details of the critical features affecting fatigue, and issues in the design, manufacture and testing of riser joints for these applications.

Results are presented showing the fatigue quality that has been achieved.

Many aspects are also applicable to Steel Catenary Risers, and these are identified in the paper.

Finally, design criteria for fatigue critical riser joints are given.

Introduction

The growth of floating production systems has led a corresponding growth in dynamic risers, and a consequent concern for fatigue performance. Dynamic steel risers were initially developed for top tensioned riser systems and steel has subsequently been used for catenary risers.

This paper examines the development of fatigue resistant riser joints for top tensioned, dry completion risers typically found on TLP's and Spar's. It covers design, material and fabrication aspects pertinent to good fatigue performance, and gives an overview of how this technology can be applied to steel catenary risers.

Fatigue design criteria are developed for critical applications.

Riser Joint Assembly

A top tensioned riser system comprises a number of separate joints, see Figure 1, connected together when the riser is run offshore. The connection is mostly a threaded connector, either a threaded forging weld-on type for high fatigue applications or a thread integral to the pipe with a separate coupling sleeve (Thread and Coupled) used in low to moderate fatigue applications. These are shown in Figures 2 and 3.

Purpose designed flanges have also been used with success, notably on the Snorre TLP in Norway where they were used in high fatigue loaded regions of the riser string, and it is likely that other forms of connector could be used successfully. However, this paper will focus on the weld-on threaded connector, being the current form used in high fatigue applications.

The riser joint can be considered as three components: the pipe, the connector and the weld. Each of these must be addressed properly if high fatigue performance is required.

Riser Materials

Metallurgy

Pipe

Top tensioned risers typically use Grade 80 steel for the pipe, being the highest strength that can be successfully welded today.

Basic pipe specification for riser pipe is defined in API 5L1. This is for general line pipe but risers demand a higher performance and most companies have their own variations to the specification to achieve this.

A number of mills are equipped to supply riser pipe to API 5LG-X80 PSL2 criteria. Typical sizes range from 9 5/8" to 14". All projects have developed specific riser pipe specifications. These are generally required to define further and/or limit chemical composition, manufacturing process variables, physical dimensions and mechanical properties. In addition the defect type and size has to be specified if the pipe itself is to meet fatigue requirements. Variations in mill operations require each mill to develop and qualify materials suitable for subsequent weld fabrication assemblies.