The rapidly expanding development of marginal fields using subsea production systems with long flow lines from the subsea facility to the surface installation has generated the need to consider routine pigging operations as an integral part of the pipeline maintenance programme.
This paper describes the design of subsea multiple pig launching systems based on optimising CAPEX/OPEX costs, and utilising conventional surface vessel deployment with diverless subsea operations, Provision of a reliable ROV or surface umbilical operated pig release mechanism is fundamental in the successful operation of such systems.
The supply of a multiple pig launching facility for installation on the MACHAR subsea production manifold, which forms part of the Eastern Trough Area Project (ETAP) field development is fully described. Although pig launchers have previously been deployed for subsea use, the specific requirement for continuous operation as part of the pipeline maintenance programme will be the first application of this type of equipment in the North Sea. The key design elements are fully discussed together with working alliance principles and risk/reward concepts adopted for the ETAP project.
A further development of this design utilises a pre-loaded lightweight pig cassette designed for diverless insertion into a permanently installed subsea launcher using ROV intervention.
The most significant aspect of the designs described is the provision of reliable, cost effective, life of field pigging facilities for subsea operations based on remote operation and minimum intervention times.
The provision of a subsea multiple pig launching system for BP ETAP represents an example of a temporarily installed launcher deployed subsea only when pigging operations are required.
The 1.6 billion GBP development of the Eastern Trough Area Project was granted in December 1995. It involves the development of seven distinct fields each of which would have been marginal, stand alone, developments but have been combined into an integrated project by alliance partnerships. Fundamental to the success of the project is the application of alliance working concepts embracing the principles of the industry's CRINE (Cost Reduction Initiative for the New Era) initiative. BP, with a 58% interest in the development, will operate the ETAP facilities on behalf of the partners: BP. Shell, Esso, Agip, Murphy, TOTAL and Mitsubishi Oil.
The fields involved in the development are Marnock, Mungo, Monan and Machar, Heron, Egret and Skua along with two future fields, Mirren and Scoter will be operated by BP. They have combined estimated reserves of some 400 million barrels of oil. 35 million barrels of natural gas liquids and 1.1 trillion cubic feet of sales gas. Peak export volume is anticipated to be some 210,000 barrels a day and the average gas export volume is expected to be around 360 million cubic feet a day. It is the largest North Sea development for a decade and also one of the most complex. A two-platform Central Processing Facility, located over the Marnock field, provides processing facilities, utilities and accommodation. A normally unmanned platform is located over the Mungo field and the Machar, Monan, Heron, Skua and Egret fields will produce through subsea manifolds tied back to the C.P.F. The 16-inch infield flowline between the Machar manifold and the C.P.F. is the longest step out production line in the North Sea.
The 22 mile length of this flowline carrying multiphase product and its resulting temperature drop from the 120 C well temperature (sea temperature of 5 C) means that heavy wax deposition could be expected in the pipeline. A series of process studies was carried out by BP and Brown & Root to determine the wax drop-out rate and identify the most cost effective solution to deal with the problem. P. 409
