Coiled tubing was utilized as a conduit-type riser to deliver high-pressure fluids from a rigless multi-service vessel (MSV) during a GOM multi-well treatment campaign. The campaign consisted of high pressure pumping into a number of subsea wells for extended durations. The coiled tubing (CT) downlines are deployed with a clump weight through open water and connected to the subsea safety module and well stimulation tool.
The depths of these operations were on the order of 1,372m (4,500 ft) and quite similar (±100 ft). Fluids (including acid) were pumped continually through the conduit at pressures up to 10,000 psi for extended periods of duration (on the order of several days) from the MSV to the fixed subsea safety module. Waves and currents impose pitch and roll on the MSV, along with the need for continual dynamic repositioning. This motion causes discrete wrapping and unwrapping at the point where the tubing just exits the sheave (the "hot spot"). This leads to elastic strain fluctuations that can lead to significant high cycle fatigue (HCF) damage.
The MSV must continually adjust its heading for optimal metocean response. A unique system was designed and implemented to coordinate this positioning with minimized CT HCF damage accumulation. The system uses a robust position transducer to provide continual input about the critical components of vessel motion. This information is used by a program to compute the near real-time dynamic stress response of the tubing at the hot spot and the corresponding HCF damage accumulation, along with the low cycle fatigue (LCF) damage accumulation that occurs along the entire length of the tubing as it is deployed during each job. The system facilitated the management of multiple jobs, assuring that hot spots were not over-exercised from job to job. This was important due to the similar depths of the wells. The hot spots were tagged with paint to confirm their location.
The mechanics underlying the fluctuating stresses at the hot spot are described along with how these are computed from the MSV motion. Post campaign fatigue testing was conducted to validate the residual life in the tubing, as predicted by the program.