This paper presents results from theoretical studies and a full scale test program carried out as a joint industry project with AGIP, FINA, CHEVRON, TOTAL, HISPANOIL and NORSK HYDRO on diverless installation and maintenance of subsea production systems.
Costed at US$ 3 million, the project falls into three parts covering X-mas tree installation, remote flowline connections and methods for diverless underwater intervention respectively.
Five underwater vehicles - JIM and WASP (ADS), TROV S7 (ROV), MANTIS (one-man sub), MERMAID IV (large manned sub) - have been through trials on a complete assistance/maintenance program for subsea production systems. These tests - carried out on a Xmas tree installed in 30 m water depth - gave extensive performance information and enabled a choice of systems for an extended 300 m deepwater trial program. The 300 m test represents some of the deepest installation work up to present and has four major tasks, including installation of the tree, layaway flowline connection, pull-in flowline connection and extension of the diverless intervention program. The installation consisted of operations on guidelines, lowering/connecting of the tree followed by functional and pressure testing. The first flowline connection took place with an off bottom pull-in of a welded flowline laid prior to installing the tree and with a 900 tie-in angle. The second flowline was made up of production tubing and installed by the lay-away method. To enable evaluation of results and comparison with computer simulation, accurate monitoring took place throughout the operation both with regard to geometry of flowlines and stress/strain.
The results of the project are contributing towards the needed knowledge for undertaking deepwater field developments using subsea production systems, limitations of existing technology are pinpointed and guidance is provided for operational planning and system design.
In particular, the feasibility of most tasks related to underwater maintenance as replacing subsea valves, making up electrical connections, stabbing hydraulic hot lines and so forth, has been proven. Proper planning, development of tools and minor modifications of the subsea equipment enable cost effective and reliable underwater intervention. The relationship between the vehicle concept and the performance of different tasks are identified and give input for improving the subsea equipment design.
Exploitation of deep water fields calls for extensive use of subsea production systems. This brings technology into rather undeveloped areas and the need for R&D work including studies, laboratory and full scale tests are obvious in order to promote future knowledge. For deep water it is mandatory to have first hand knowledge on operations and maintainability in order to make valid safety assessments and so forth. This was the principal reasons why VERITAS in 1979 initiated a large industry sponsored R&D program which led to the development of the project.
