Advances in Materials Selection and Welding for High Strength Top-Tensioned Riser Systems

摘要

This paper describes the design and qualification approach for the materials and weld engineering solution required tornfabricate 80 ksi (552 MPa) yield strength (YS) welded riser joints suitable for sour service. Many owner/operators prefer thernmore robust pin and box connector design over threaded-and-coupled designs for top-tensioned production riser systems,rnespecially when used in high fatigue applications. Such designs involving welded fabrication have been limited to 70 ksirn(483 MPa) YS when compliance with the NACE MR0175/ISO 15156 standard for materials used in H2S-containingrnenvironments is a requirement. The new solution described in this paper enabled the fabrication of 80 ksi YS standard pinrnand box riser joints which are now being used to replace existing lower-strength riser systems for a tension leg platformrn(TLP) operating in the North Sea. Welded risers are significantly lower cost than the alternative solution where thernconnectors and the tubular section are integrally forged in one piece.rnThe solution required a new materials and welding approach to comply with all project requirements including base metal,rnweld metal, and weld heat-affected zone (HAZ) hardness meeting the ISO 15156 standard using the Vickers hardnessrnmethod. The qualification welds were subjected to full scale resonance fatigue testing that returned values exceeding the C1rn(in air) fatigue curve defined in DNV-RP-C203 with 95% confidence limits. Test data is provided as well as the steps used tornachieve this solution.rnThe process used to develop the materials and weld engineering solution has interest for other subsea applications such asrnhigh pressure/high temperature (HPHT), where the base materials are to be used at strengths approaching their engineeringrnlimits.