After a series of dialogs with major oil companies and independent opera-tors alike, GVA Consultants, a Halli-burton company based in Sweden, concluded that the market was calling for a new semisubmersible prod-uction platform design. The need, they decided, was for a production unit that could work in water depths of 3,000 to 10,000 ft, would be tailored for the requirements of steel catenary risers, and would be cheaper and faster to build.
The result of this research is the GVA 4000 ASU, a new design in semi production units.
As deepwater operations become more efficient and less costly and engineers familiarize themselves with available technology, major oil and gas companies are choosing to develop fields once considered marginal (i.e., production levels from 30,000 to 70,000 b/d) or uneconomic to produce. This trend, in combination with independent oil and gas companies moving their focus from shallow-water plays to deeper waters, has created an increased need for small production units that can operate in water depths from 3,000 to 10,000 ft and, in some cases, act as a central hub for subsea tiebacks of several fields. For example, 18 small production units have been sanctioned in the US Gulf of Mexico since 1996, and estimates suggest that another five to seven small production units will be sanctioned this year.
The GVA 4000 ASU is based on a concept meant to compete directly with mini TLPs and mini spars operating in the US Gulf of Mexico or offshore Australia. The idea for the ASU design can be attributed to studies of riser motion optimization done by GVA in 2003. With a clear understanding of how a semisubmersible's hull design impacts fatigue and extreme motions of SCRs, the company concepted a semisubmersible tailor-made to SCRs with an asymmetrical hull that moved the vessel's center of rotation aft of center for reduced motions at the SCR hang-off point. This design required the team to completely revamp the pontoon and column diameters and locations. Competitive semisubmersible designs are similar to the GVA 3000 and do not feature an asymmetrical hull or some of the ASU's motion and safety advantages, according to GVA.
The slanted column legs of the ASU connect underwater via rectangular pontoons and above water via a deck box. The aft pontoon is smaller than the fore pontoon, while the aft columns are larger than the fore columns. The ASU includes oil and gas processing equipment (that may be contained in a single module or integrated into the deck box), quarters with an integrated helideck for operational personnel, and safety systems in case of an emergency. Due to the nature of semisubmersibles, it is possible to expand the unit's topside facility with additional production, water injection, and/or gas injection equipment and re-deploy it upon project completion.
Wave tank tests verified the stability characteristics of the new design.
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The altered shape moves the unit's center of rotation aft of center, reducing the motions around the pontoons so that SCRs can be positioned off the side of the vessel. This configuration provides better vertical motions to the SCR flex joint and horizontal motions, an important benefit because once the vertical motions are reduced horizontal motions are largely responsible for SCR fatigue life. The slanted legs on the 4000 ASU also reduce its deck weight, scaling down the vessel's overall size to economically compete with the TLP and spar designs, according to the company.
According to GSA, the ASU's riser positioning also makes it easier to configure the SCRs and provides more flexibility during SCR installation. Safety considerations include the C-deck, which allows the topside to be set inside the deck box, as opposed to on top of it. This lowers the vessel's center of gravity for overall better motions than other small production semisubmersibles, the company says. Further, the topsides can be installed onshore, allowing easier and less expensive hook-up and commissioning.
The hull of the ASU is designed to allow a choice between integrated or modularized topsides. With an integrated topside, the unit can be constructed and pre-commissioned in a single shipyard, saving both time and money. It is also possible to seek lower steel and labor costs at international shipyards and still build the modularized topside in a US-based yard. Though a more expensive option compared to an integrated topside and hull, the fact that the topside can be installed in a single lift and commissioned onshore eliminates the need for offshore topside installation and unit commissioning. Additionally, the hull of the ASU is designed to require less steel and a less complicated mooring system.
Like the mini TLP and mini spar, the ASU can handle approximately six 12-in. production SCRs and two 18, 20, or 24-in. export SCRs. According to GVA, the advantage with the ASU design is the SCRs' position at the vessel's side, which it says makes the general riser configuration simpler, safer, and faster to install and repair. Because semisubmersibles can be repositioned by winching the mooring lines, it is possible to increase riser fatigue life by occasionally changing the SCRs touchdown point on the sea floor. It is also possible to de-mobilize a semisubmersible in one piece and mobilize it to a new production project.
By standardizing the hull of the ASU to a topside weight of less than 4,500 tons, the company estimates the unit's design and fabrication schedule will be about two months shorter than a purpose-built hull. Although it is possible to tailor the unit to meet project specifics, the hull's standardized design means its fabrication schedule will decrease incrementally over time as the construction process becomes more efficient, with the ultimate schedule driver being the unit's topsides.
