All semis great and small
  from: Offshore Engineer
  by: Darius Snieckus
  Sunday, August 01, 2004

Size matters it's true. But for GVA Consultants - the brains behind semisubmersibles ranging most recently from breakthrough heavyweight production units for the US Gulf Thunder Horse and Atlantis fields down to a new asymmetrical semi concept for marginal deepwater developments, the accent is more on fit. Darius Snieckus reports from Gothenburg.

Word from BP in 2001 that it had awarded GVA Consultants the contract to design and build the world's largest combined production and drilling semisubmersible for the oil company's giant Thunder Horse field in the US Gulf once again raised the bar for the Gothenburg-based semi specialist. A mainstay of the semi market since its pioneering GVA 5000 sailed out to the UK North Sea Balmoral field in 1985, GVA nonetheless had its work cut out for it on the 1 billion barrel development then still known as Crazy Horse.

Designed with a payload capacity of 40,000t (hence the design name), the GVA 40000 for Thunder Horse has a dry weight of 60,000t and a displacement of 130,000t. Built at Daewoo's Okpo yard in South Korea, the semi hull began its journey to Kiewit Offshore Services yard in Ingleside, Texas last month onboard Dockwise's jumboised Blue Marlin vessel (pictured) in the industry's heaviest dry tow to-date (OE April).

Once outfitted with its integrated topsides modules, engineered by Mustang Engineering and fabricated at J Ray McDermott's Morgan City, Louisiana facility, the unit will be towed out to Mississippi Canyon block 778 and moored at the field using a 16-point semi-taut chain-wire-chain system.

Peak production of 250,000b/d of oil and 200mmcf/d of gas will flow through steel catenary risers and be conveyed via a 24in export SCR linked to the planned 70 mile, 28in Proteus pipeline. Some 300,000b/d of water will be injected into field reservoirs using flexible risers.

Among the marketed strengths of the GVA 40000 design are 'flexibility, quayside completion and vertical wellhead access'. The unit has also been designed with an emphasis on 'a high level of safety with regard to fire hazards, stability and structural collapse', according to GVA vice president of business development Robert Ludwigson.

Since the contract award three years ago, GVA's work on the production and drilling semi for Thunder Horse has been far-reaching, he underlines, kicking off with a 12-month FEED stage that involved more than 120 project personnel and continuing through the construction phase providing support for BP at the Daewoo yard.

The company's involvement with development of the riser system for the unit was particularly intensive, Ludwigson notes.

'Though we did not design the riser system [contracted by BP to Intec and CSO], we did spend a lot of time on this aspect of the project, especially related to motion characteristics of the GVA 40000,' he explains. 'This has been a very large scope of work because SCRs are still a relatively new technology.

'We have learned a great deal during Thunder Horse and are still learning because the industry's knowledge of SCRs is still developing,' he adds.

As if the Thunder Horse job was not enough to keep its engineers busy, GVA also won the contract to provide a second, smaller semi for BP's US Gulf Atlantis project. For this the contractor put forward a GVA 27000 production and quarters semi, like the GVA 40000 a fourcolumn and ring pontoon unit with integrated deck structure for flexible topsides arrangement and enhanced safety. The 89,000t displacement unit, presently under construction at Daewoo Okpo, will have deck payload of 27,000t.

While also envisaged for the 'very large field sizes' that continue to populate the US Gulf region, the Atlantis semi will be able to handle a somewhat lower throughput than Thunder Horse, around 150,000b/d of oil and 180mmcf/d of gas through SCRs. Water injection will have a top flow rate of 75,000b/d. Accommodation will be limited to 60 personnel.

Station keeping in 7100ft of water at Atlantis will be ensured by a semi-taut catenary mooring system using 12 chainwire- chain lines.

Distinct from the drilling system-fitted Thunder Horse semi, the Atlantis unit will handle hydrocarbon flow from subsea tiebacks by 'moving about' the field within the scope of its moorings to adjust the touchdown point of the SCRs so as to further improve the risers' fatigue-life. 'We have demonstrated that if you move the semi twice during the life of the field - by about 1% of water depth, ie around 20-40m - you extend the life of the SCRs by a factor of 2.5,' explains Ludwigson.

Though it has more than half of the world's purpose-built production semis on its reference lift, GVA did not rest on its laurels when awarded these major contracts and has been able to derive 'significant' benefits from running Thunder Horse and Atlantis in overlap. 'There is no substitute for the experience gained from these jobs,' he underlines.

The wider resonance of the Thunder Horse and Atlantis semi-based projects to the offshore oil and gas business, particularly in the US Gulf of Mexico, is that - along with the Na Kika development - they are tilting the balance away from the present primacy of spar field development concepts in the region.

'These are the first semi projects in a region that has historically been quite conservative and has a very strong preference for dry trees,' states Ludwigson. 'But now we are beginning to see a shift related to the fact that subsea trees in many cases bring field development cost savings and this is very good for the argument in favour of the semi. The greater acceptance of subsea trees will lead to a greater acceptance of semis.'

Influenced by the learning process on Thunder Horse and Atlantis, GVA is taking aim at further altering the offshore landscape in the US Gulf with a new marginal deepwater field development semi, the GVA 4000 ASU.

Based on an 'optimised version' of its four-column frame pontoon design, the 30,000t displacement 4000 ASU (asymmetrical unit) is expected to compete with the mini-TLPs and spars targeting the 60% of deepwater US Gulf fields that hold reserves of between 30-50 million barrels. The unit can handle oil production of 30,000-60,000b/d with direct export through large diameter SCRs, its hull configuration having been fashioned to have the risers installed on the outside of the structure's pontoon to simplify installation and 'give minimum motion at the risers hang-off positions'.

Topsides arrangement can be either integrated or accommodate single or multiple production equipment modules using the GVA C-deck configuration, a novel deck structure allowing a traditional topside module to be installed on a semi without being effected by the wave-forces normally working on semi's deck modules. This translates into to a 'simplified and reduced interface' between the semi hull and topsides, making for a more streamlined construction. Deck load capacity for the GVA 4000 ASU is 4000t-plus and, notes Ludwigson, there is also an enlarged GVA 7500 ASU version available.

Envisaged for fields standing in water depths of 3000-10,000ft, the GVA 4000 ASU - which has already been given conceptual approval by ABS based on model tests at the Offshore Technology Research Centre in College Station, Texas - is designed to be moored using a 10-12 point semi-taut combined chain-wire or taut-leg combined chain-rope system.

'We have learned a lot about SCRs and semis through our work on Thunder Horse and Atlantis,' says Ludwigson. 'We have seen some of the challenges presented by these projects and it made us think harder about how to design a smaller semi for marginal fields - after all, the smaller the semi the more it moves around. From an extensive motion optimisation study, we came up with the ASU.'

The asymmetrical semi makes it possible to move the centre of rotation aft of centre, thereby reducing the pitchinfluence on the vertical motion of the SCR hang-offs. Moreover, the slanted legs of the GVA 4000 ASU curb deck weight, downsizing the unit in line with the smaller-scale TLPs and spars that are foreseen to be its competition.

'There are two aspects of SCRs being considered,' he continues. 'One is the extreme stresses in 100-year hurricane effecting the vertical motion of the risers; then you have fatigue due to the heave that is horizontally dominated. The ASU answers these concerns.'

As with any semi, the GVA 4000 ASU can be moved about a given field to reduce riser fatigue. The smaller unit can also be swiftly demobilised and - unlike a 'more site-specifically designed' TLP or spar - be sailed on to take up duties at a new development after production at a field winds down.

'When you look at the US Gulf, it is true you have Thunder Horse, Atlantis, Na Kika and the rest that are all big projects, but, after these, most projects are much smaller, marginal fields more or less. This is the market we are after with the ASU.'

GVA, whether with its large-scale production and drilling semis or its newbreed ASU concept, is betting on operators in the US Gulf - and in the future provinces including offshore West Africa - embracing the 'selection advantages' offered by the semi. Ludwigson reckons there is already a change in the wind. 'I think you will soon see a shift toward greater use of semis because there really are such cost gains to be had,' he concludes. OE

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