Advances in Materials Selection and Welding for High-Strength Top-Tensioned Riser Systems
Authors: Warner Thomas D. , Garden Bill 
Published: April 2012

This paper describes the design and qualification approach for the materialsand weld engineering solution required to fabricate 80 ksi (552 MPa) yieldstrength (YS) welded riser joints suitable for sour service. Manyowner/operators prefer the more robust pin and box connector design overthreaded-and-coupled designs for top-tensioned production riser systems, especially when used in high fatigue applications. Such designs involvingwelded fabrication have been limited to 70 ksi (483 MPa) YS when compliancewith the NACE MR0175/ISO 15156 standard for materials used in H2S containingenvironments is a requirement. The new solution described in this paper enabledthe fabrication of 80 ksi YS standard pin and box riser joints which are nowbeing used to replace existing lower-strength riser systems for a tension legplatform (TLP) operating in the North Sea. Welded risers are significantlylower cost than the alternative solution where the connectors and the tubularsection are integrally forged in one piece.

The solution required a new materials and welding approach to comply with allproject requirements including base metal, weld metal, and weld heat-affectedzone (HAZ) hardness meeting the ISO 15156 standard using the Vickers hardnessmethod. The qualification welds were subjected to full scale resonance fatiguetesting that returned values exceeding the C1 (in air) fatigue curve defined inDNV-RP-C203 with 95% confidence limits. Test data is provided as well as thesteps used to achieve this solution.

The process used to develop the materials and weld engineering solution hasinterest for other subsea applications such as high pressure/high temperature(HPHT), where the base materials are to be used at strengths approaching theirengineering limits.

Introduction

A TLP owned and operated by a major global oil and gas company has beenproducing hydrocarbon liquids from a formation in the North Sea at a waterdepth of 1,015 ft (310 m) since the early 1990s. The original 9? in. (244.5 mm)outside diameter (OD) production risers were designed for a 20 year fatiguelife, but the projected life of the field has since been increased from 20 to50 years and some of the original production risers are approaching the end oftheir fatigue life. The owner elected to replace these risers with 9? in.(250.8 mm) OD systems that are lighter, necessitating the design YS to beincreased from 70 to 80 ksi. The nominal wall thickness is 0.600 in. (15.2 mm). The design pressure is 5,365 psi (37 MPa). The owner also specified that therisers had to be designed for a 300 year fatigue life (10:1 safety factor)using the C1 (in air) fatigue curve defined in DNV-RP-C203. In their designbasis, the owner specified that the fabrication standard for the project wouldbe DNV-OS-F101.

The riser design is based upon exposure to bore fluids containing H2S invokingcompliance with ISO 15156-2. The design basis requires the use of the Vickershardness methodology (250 Hv10 maximum at the weld root, 275 Hv10 maximumbeneath the weld cap) rather than the less stringent Rockwell methodology (HRC22 maximum in the weld metal and HAZ).