Objective
A new system based on a dynamically positioned (DP) tanker vessel, progressing from previous work on the SWOPS concept is to be developed to be used for extended well testing and for production and delivery of oil from marginal fields.
The primary aims are to increase the efficiency and to reduce the capital and operating costs of oil extraction in the North Sea, and to extend the concept for worldwide application.
This project has resulted in the outline design for a low cost `sailaway' dynamically positioned (DP) floating production system. The DP production tanker is a feasible concept. The cost of the vessel was kept down in two main ways. Firstly, by converting an existing tanker rather than commissioning a newbuilt vessel. Process units would be installed on the deck, and designed for a minimum of structural modification and hook-up. Secondly, DP power would be concentrated at the ship's bow, with the stern being permitted to weathervane about a bow mounted flexible riser.
The feasibility of this bow-centred DP, as opposed to the midships-centred system used by conventional DP vessels, was tested by simulation studies and by ship model testing. Power savings of the order of 30 % were demonstrated.
The process design work covered several modules, on the basis that relevant modules could be selected and a detailed design completed once a field application were identified. These includedmodules for separation, metering, water injection, and gas injection/gas lift. Weight and cost estimates were prepared on the basis of a 20 MBD production rate with entrained gas up to 1000 SCF/BBL.
Three studies were undertaken on the riser system. The first concerned a retractible cantilevered riser handling unit, to support the flexible riser safely over the ship's bow. This incorporated a conventional multipath swivel. A parallel study concerned the development of BP's heliflex swivel concept. This device, based on coiled flexible pipes, could replace the toroidal swivel and lead to increased reliability. A third study concerned the configuration and maximum water depth potential of the risers themselves. After examining the configuration options this work showed that flexible riser systems could be engineered for the DP production tanker in water depths up to at least 600 metres.
The cost of purchasing a suitable trading tanker, and converting it into a DP production tanker,would be of the order of L50 m.
The system involves a converted tanker connected via flexible risers to one or more subsea wellheads, or to a riser base and flowline manifold system, handling systems for deployment of risers, a riser swivel unit, process equipment and a novel DPP thruster system concept for maintening the vessel on station.
The programme of work for the project includes development work on the new DP thruster system with model simulation, tank testing and control system design; development of the technology of multiple, flexible riser systems and methods of riser suspension; and design studies of devices and systems to accommodate rotation of the riser and swivel-less capabilities particular to this concept.
It will be necessary to verify the operational behaviour of the process units such as separators, water separation, injection and lift systems, as subjected to the motions of the vessel. Development work will also be required to ensure cost reduction through reconsideration of space and weight of process and related plant and economic utilisation of other types of production system. A thorough examination of the design will be made to reduce the costs of all the major system components.
The feasibility of the DP Tanker Scheme has been studied by BP using a purpose designed computer simulation. Using this tool, a novel arrangement of thruster units was conceived which promised lower power requirements than the conventional designs. Model ship tank testing and further simulation work is necessary to confirm and extend the concept.
Design work during 1985 developed the conventional DP tanker concept for extended well testing (EWT). More recently, it became apparent that early production could be achieved and hence extended production, using the DP Tanker. The advantages over single point mooring systems are essentially due to the mobility of the vessel from one oilfield to another, the system's independence in environmental survival conditions and its particular suitability for deep water oilfields.
The project was executed in three phases. Phase 1, which was completed in 1986, included DP simulation and ship model testing, modular process design studies, and the design of riser handling equipment. In 1987 phase 2A covered conceptual work on the heliflex swivel concept (which was patented), and preliminary riser analysis work. Phase 2B, in 1988, comprised completion of the riser analysis and heliflex swivel work, and also two new studies covering flare design and an offshore loading option.
Programme(s)
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
AB2 0PB Aberdeen
United Kingdom