ON THE FATIGUE CAPACITY OF A SUBSEA INTERVENTION SYSTEM TOOL

The offshore oil and gas industry is drilling into and producing from wells at pressures and temperatures greater than ambient conditions. Wells that are considered high pressure, high temperature (HPHT) experience even more challenging operating conditions. These HPHT wells have pressures greater than 15,000 psi (103.4 MPa) and/or temperatures of more than 350 degrees F (450 degrees K). To safely overcome the challenges in these operations, there is a greater demand for more advanced tools and new technology. Because of the sensitivity and potential impact on the environment, the industry is striving to homogenize the design and acceptance criteria. The Bureau of Safety and Environmental Enforcement (BSEE) requires equipment operating at high pressures and temperatures to pass a design verification analysis. Structural and fatigue capacity analyses are performed using finite element analysis (FEA) per ASME's Boiler and Pressure Vessel Code, Section VIII, Division 3, API Standard 17G, and API TR 17TR8. In service, threaded connections are often subjected to dynamic loads, and to maintain a secure connection, they are generally preloaded. The combination of preload and cyclic loads results in a multiaxial stress distribution over the connection, where the coupling's threads act to increase the stress, initiating fatigue cracks. The objective of this study is to evaluate fatigue service life per the methodologies outlined in API Standard 17G and DNV-RP-C203 for a subsea intervention system tool. The fatigue life evaluation of the subsea intervention system tool is performed with a finite element model using a 3D 180 degrees geometry. In this study, the fatigue characteristics (Delta M-N curves) of the tool are evaluated in accordance with API Standard 17G and DNV- RP-C203 using elastic-plastic material properties. The tool design fatigue capacity curves are generated per the S-N method, with no reduction factor (RF = 1) and use of a fatigue life reduction factor of RF = 4 per DNV-RP-C203. The S-N curve for subsea steel forgings in air subjected to a high mean tensile stress from DNV-RP-C203 for base material hot spots is used in this analysis. A sensitivity study is also performed to determine the effect of the external tension (riser tension) on the fatigue capacity.