Subsea wellhead solutions are increasing each year with over 2,600 new installations projected worldwide from 2001 to 2007. The number of subsea well intervention activities has been limited to date due to the lack of available safe and efficient intervention systems in the market. The increasing number of subsea completed wells and the drive to reduce operating expenditures as fields mature will require a higher level of subsea well intervention activities in the future. Therefore, improvements to intervention technology are needed to facilitate enhanced production performance of subsea fields in the North Sea and around the world.
Current practices for coiled tubing intervention on floaters normally involve a time consuming and complicated rig-up/rig-down processes that in most cases have to be performed on rig time. Work performed in the heave compensated tension frame, during rigging and deployment of tools, is restricted by weather limitations and represents risk to the operations personnel due to extensive handling of heavy equipment in a physical environment that accelerates and moves in all dimensions of space.
Process improvement, step changes in safety and work environment, increased efficiency and reduced cost, have been the result of the following projects recently performed in Norway:
Prior to the start up of the completion work on the Åsgard sub sea development, the semi-submersible rig Transocean Searcher was modified following a joint Statoil / Schlumberger initiative to improve the CT / rig interface. The modifications included installation of a new work-deck at the side of the derrick. This introduced a dedicated area for the coiled tubing equipment without conflicting with other activities on-board. Smaller modifications were performed to the derrick to allow the coil to enter, and to be operated from the new deck. In addition a set-back arrangement for the injector head was installed inside the derrick structure itself, this set back feature permits stabbing of pipe and servicing of the injector without removing it from the derrick. The new set up allowed for safer operations performed several days faster than the previously established standard.
During May 2001 the fist ever CT operation on a RAM rig ever, was performed on Bideford Dolphin for Norsk Hydro. Interfacing with the RAM rig (where the hoisting system is based on hydraulic rams rather than conventional drawworks)represented new operational challenges. A tension frame was custom-built to fit inside the limited space inside the derrick. In order to install the tension frame in the derrick, a purpose-designed chute was constructed, allowing the tension frame with pre-installed injector head to be efficiently installed in the derrick in a single lift operation.
A customized intervention vessel is currently under development by ProSafe and Schlumberger. The rig will allow the entire coiled tubing rig-up to be performed outside the derrick on a dedicated acess path towards the derrick, not interfering with other ongoing rig activities. This design also permits installation of the injector and well control equipment inside the upright tension frame prior to transfer into the derrick structure. Following complete rig-up (including stabbing of the coiled tubing pipe into the injectorhead, the tension frame will be skidded (still in the upright position) into the derrick, latched and be ready for deployment of the tool string.