In October 1973 SEAL PETROLEUM LTD. started a new series of tests on their subsea wellhead completion system SIS in the Ashtart oil field, Golfe de Gabes, Tunisia. The purpose of this programme is to evaluate the handling of the module string from a support vessel using a drill pipe riser and to install the production module which will then be overrated throughout the winter of1973 - 1974 from a nearby platform.
For this test series an existing vessel, the "Terebel", belonging to the Institute François du Petrole, has been modified.-A special derrick designed for guidance of the module string and for drill pipe techniques was added together with a set of riser tensionless and other handling devices. The ship is fitted with three ‘Schottel’ thrusters for automatic dynamic station keeping, for which a taut wire inclinometer system is employed.
The functions to be performed at the wellhead, as well as the requirements derived from the rigid-pipe lowering procedure, made the design of special tools and provisions in the module string necessary. Especially interesting is the production module handling tool which combines a number of unique features.
Although the tests are not yet completely finished at the date of writing this paper (December 1973), sufficient information has been obtained to conclude that the handling system as designed and built will allow the SIS system to be installed in deeper waters.
The SEAL SUBSEA WELLHEAD COMPLETION SYSTEM is designed to produce oil from wells in water depths up to 500 m. using remotely controlled and modularly installed wellhead equipment.
In short the complete installation sequence is as follows: (see also Figure 1).
A base support structure, with a guidance frame seated on top, is lowered onto the wellhead and hydraulically locked. The next step is to bring down a specially designed Manned Work Enclosure (MWE) containing a master valve assembly which can be aligned and attached onto the wellhead by personnel working inside the enclosure under atmospheric conditions. These technicians are transported in a Personnel Transfer Chamber (PTC),(ref.l). After retrieval of the MWR a remotely controlled, production control assembly is put down and latched to the master valve assembly without human intervention. Finally, after a flowline and electrical cables have been connected and run to a production platform, the well is completely controlled, from the platform mounted terminal.
Whereas the installation of the master valve assembly still needs human intervention, the production control assembly is lowered and operated completely remotely thereby being easily rechangeable.
A particular requirement of the SEAL approach is the fact that the modules are run from a drill pipe thus allowing wireline operations and well circulation to be performed from the support vessel. The design for such a support unit with its handling system will be discussed in this paper.