Weld Microstructural Control for Sour Service Drill Pipe Riser

Abstract

Drillpipe risers (DPR) constructed with high sulfide stress cracking (SSC) resistance are designed and fabricated to suit subsea completion and early production operations in sour environments (H2S). Early production operations typically include extended well testing (EWT), which is required to evaluate production characteristics. The riser string, among other components, is composed of drillpipe riser joints. The DPR joint manufacturing process includes welding a tool joint with a pipe body via rotary friction. The tool joint is a forged, quenched and tempered high strength low alloy steel piece with threaded connectors. The pipe body is a high strength low alloy seamless tube with upset ends, which are also fully quenched and tempered. After the friction weld process, the heat affected zone (HAZ) of the weld is heat treated to meet material properties and performance requirements.

Performance requirements in sour service wells include material low hardness which is difficult to obtain on high strength quenched and tempered steel, especially on a weld zone. With this challenge in mind, a DPR manufacturer engineered a superior combination of chemistry, friction welding conditions, and heat treatment parameters that are designed to meet both mechanical and SSC resistance requirements for sour service.

This paper presents the results of the testing qualification process, which was co-developed together by the involved companies, using joints with extremely accurate thermal cycles control and chemistry balance, to meet mechanical properties in the weld line region and microstructures that make the DPR suitable for operations in H2S enriched environments. The post welding heat treatment (PWHT) variables were critical, since tiny variations resulted in major differences among samples. Each sample was evaluated through optical microscopy (OM) and scanning electron microscopy (SEM), mechanical testing and SSC resistance test according NACE TM0177. The temperatures and time limits for the PWHT cycles associated with the tempering effects on the microstructure were determinant to maintain optimal performance in the H2S test.