Design, Qualification and Fabrication of Fatigue Critical Girth Welds for Free Standing Hybrid Riser Systems
Authors: Kristiansen Per , O&apos;Donnell Jeffrey , Hoopes Gary , Masson Craig , Fazackerley Bill , Novelli Paolo 
Published: October 2011

Metocean conditions and connections to dynamic floating facilities make riser systems for deepwater projects among the most challenging equipment to design and fabricate. The ends of the risers experience substantial complicated fatigue and tensile loads, and when production fluid includes hydrogen sulfide (H2S) the riser materials' resistance to fatigue and fracture can be diminished. The riser girth welds, in particular, must be specially designed and qualified on a project-to-project basis.

Forged high-strength steel tapered stress joints at the ends of free-standing risers address the high tensile loads and manage the fatigue demand. However, the girth welds connecting the tapered stress joint to seamless pipe must be specially qualified for fatigue. This paper will discuss the design, qualification and fabrication of the tapered stress joint girth welds for superior fatigue and fracture performance in mildly sour conditions for service in deepwater regions, like the Gulf of Mexico. The tapered stress joints were of 80ksi A182-F22 material, and the pipe was API 5L X70. Qualification included an elaborate testing program of the sensitivity of welding parameters, such as post-weld heat treatment, on weld hardness required for mildly sour service and pipe yield and tensile strength. Full-scale resonant fatigue testing demonstrated reliable C-class fatigue performance. In addition, a thorough engineering critical assessment (ECA) provided a basis for NDE system design and flaw acceptance criteria.

Introduction

Deepwater riser systems are susceptible to fatigue damage when subjected to the motions of the floating vessel to which they are attached and to currents that may produce vortex induced vibrations (VIV). Tapered stress joints (TSJs) at the top and bottom ends of riser systems are where fatigue demand is highest. These components are fracture-critical items requiring high reliability. For this reason, it is important to follow an established fatigue and fracture design philosophy, such as [Buitrago, Weir & Kan], for critical deepwater riser systems and tapered stress joints (TSJs). The overall strategy to ensure fatigue and fracture performance is illustrated in Figure 1. It is a structured process involving coordinated technology blocks such as materials, welding, fracture mechanics (FM) analyses, non-destructive examination, and full-scale test verification. The program presented here describes the pragmatic process followed and associated experience with implementing such a program for a free-standing hybrid riser (FSHR) system.

A typical FSHR system is illustrated in Figure 2. The FSHR can be used for either production to or export from a deepwater floating facility. Girth welded steel pipe makes up the lower portion of the hybrid riser system from the seafloor and flexible pipe makes up the upper portion to the floating facility. The rigid pipe is supported by buoyancy cans, and the bottom of the rigid pipe is attached to a pile on the seafloor. TSJs make up the uppermost and lowermost rigid pipe joints in the rigid pipe portion to manage bending loads and associated fatigue demand. The TSJs provide a constant inside diameter, while the outside diameter and wall thickness increase towards the ends, effectively spreading the bending moment and associated fatigue demand through the length of the TSJs.