Exmar has developed a new deep-draft production semi for deepwater fields in mild environments. Among the concept’s stronger selling points are hull stability and topsides versatility, in relation to risers and moorings. The platform would also be suited to relocation under a range of scenarios, including early production or short-term duty on marginal fields.
Exmar’s Semisubmersible Deepwater Development Solution (SDDS) is based on widely used and proven technology, and is designed to be economic both to build and install. Initially, Exmar has targeted operators in the Gulf of Mexico, particularly independents looking for solutions for stranded gas or small fields. These could be tied in to existing pipeline networks, according to Exmar’s offshore division managing director Carl Hansen. But the concept could also be applicable in more infrastructure-deficient deepwater regions such as Brazil, West Africa, Southeast Asia, and Western Australia, probably in tandem with an FSO.
As for construction, the emphasis would be on tightly managed integration procedures and minimized offshore commissioning. Materials procurement, assembly, and offshore installation would be handled by Samsung Korea, under an alliance that includes Hannover for process equipment, 2H Offshore for riser design and hull integration analysis, and Mustang Engineering for topsides engineering, process design, and integration management. Exmar, as the alliance leader, is responsible for naval architecture, deck and hull integration, and mooring system design.
Scalable solution
The SDDS hull is based on Exmar’s Deep Draft OPTI design, and has undergone extensive analysis and refinement over the past three years. The hull’s motion responses have been shown to provide a stable working platform, allowing relatively simple integration with steel catenary risers, the company says. Furthermore, it is a scalable solution that can accommodate larger payloads, in terms of topsides, additional risers, and moorings. Topsides payload can vary between 4,000 and 8,632 tons, with a draft of 30-36 m.
Schematic of Exmar�s DD-OPT semisubmersible.
Good motion response is accomplished through positioning part of the pontoons ahead of the columns. This reduces significantly wave-induced hydrodynamic forces. The main benefit, aside from improved platform operability and crew comfort, relates to dynamic loading of the riser system.
Another feature of the hull configuration is its large deck, spanning freely between the tops of the columns. A “top of the range” SDDS could handle fluid intake of 100,000 b/d and up to 150 MMcf/d of gas on a single deck configuration, thereby minimizing the risk of wind loads and over-turning moments.
Seakeeping model tests carried out at the Samsung Model Basin in Korea have verified the semisubmersible’s design and motion calculations, as well as the wave clearance (air gap) beneath the deck truss for a wide range of metocean conditions.
As implied above, production and export risers would typically be steel catenary, either single pipe or pipe-in-pipe. However, Exmar is examining alternative deepwater field layout arrangements in cases where SCRs are not applicable, i.e. in extremely harsh environments, or where the type of oil being produced is too demanding for SCRs.
One possibility is 2H Offshore’s single line offset riser that decouples vessel motions from vessel motions. This concept has been proven on Kizomba A and B offshore Angola.
The mooring system would be tailored to the offshore environment. At present, the SDDS is designed for station-keeping in water depths ranging from 300 to 7,000 ft. It can accommodate up to 12 mooring lines comprising 100% chain for shallow water applications, and a combination of taut wire/studless chain or studless chain, polyester rope and studless (bottom section) for deepwater, with vertically loaded anchors.
As part of the OPTI design development, using state of the art motions prediction programs such as Wamit and model tests to verify the results, Exmar undertook motion response comparisons between the OPTI design, a conventional semisubmersible and the Neptune-type spar platforms in both 10-year Gulf of Mexico storm and 100-year Gulf of Mexico hurricane conditions. In most cases, it claims, the OPTI performed favourably.
Exmar claims to have achieved economies in terms of construction, transportation and offshore installation. The lower hull pontoons and columns are designed to be built and erected at a single site (dry land or dry-dock), and would be floated out or skidded out with or without the topsides. The latter (deck plus facilities) could be installed at the shipyard, either on the fabrication site or at a quayside.
Samsung’s high capacity, floating ship yard cranes could be used to lift the single-train topsides onto the columns. The platform design also allows for the unit to be ballasted to suit loading and offloading to and from a transportation vessel.
Integration of the hull with the topsides would be performed onshore, the aim being to minimize offshore commissioning costs and to limit risks associated with integration offshore. Detailed case studies for the Gulf of Mexico suggest that the installation and hook-up costs are much lower than other competing deepwater floating production systems. This is due to the fact that offshore works are minimized, since risers and moorings are pre-deployed before arrival of the semi on location, in term limiting mooring/riser hook-up requirements. Also, the hull design and construction is designed to simplify inspection and maintenance: all internal tanks are accessible, and the platform can be de-ballasted to ease access to riser porches and fairleads.
Lump sum proposal
Exmar has undertaken studies for projectswith facilities ranging from 30,000-100,000 b/d. In December 2003, Exmar and Samsung provided an update to interested oil companies in Houston, including a proposed lump sum price based on an engineering, procurement, and installation program. What emerged, particularly in discussions with the smaller independents, was a preference for an EPCI contract, but with operation of the semi being entrusted to a third-party on a short or long-term, dayrate lease to lessen the risk to the oil company.•
