High subsea well intervention costs offshore Norway and a desire to lower them have sparked development of a new design for a triple-hulled intervention unit which offers the sea-keeping characteristics and deck area of a semisubmersible and an unprecedented five moonpools without splash-zones, plus the high transit speed of a monohull.

The SWATH design has been developed by naval architects at QinetiQ in association with Norway's Oilfield Technical Group (OTG), based on the belief that there will be much more demand for well intervention work on the Norwegian Continental Shelf in future to eke out production from mature wells.
Chris Ross, chief naval architect at QinetiQ, explained the thinking behind the SWATH concept.

"OTG came to use with a very simple question," Ross said. "Can you design a vessel which combined the operability of a semisubmersible with the sea-keeping and economic operating costs of a large offshore construction vessel? They wanted a combination of the two, with relatively cheap construction costs.
"We have come up with something which is a lot more like a ship than a rig. It looks like a very big, wide multi-service vessel."

Three submerged pontoon hulls are featured on the concept - hence the similarity with a semisubmersible design. It has two working decks - with 16.4 ft (5 m) of clear working height between - and the five splash zone-free moonpools. One is a small moonpool intended for deployment and recovery of an observation-class remotely operated vehicle (ROV); another for a work class ROV. A third is located directly below a derrick, intended as the main intervention moonpool. Two additional moonpools were incorporated with the design, at OTG's request, for running smaller equipment items - such as subsea tree caps, intervention strings and other equipment in parallel operations.

There is no free-lifting during operations with objects swinging about on deck either, because the deck is designed with skid rails allowed controlled movement of bulk items on board.

Designed with a high transit speed up to 15 knots, the vessel is intended to allow rapid improvement between well sites, helping to minimize the mobilization period for an intervention campaign, thereby lowering the overall cost.

Sea-keeping is provided by a class 3 dynamic positioning system, while the vessel is also intended to operate up to sea-state 7 - a full storm - allowing it to withstand 21.9-ft (6.7-m) significant wave heights over 10 seconds, whereas most intervention ships would have to disconnect from a well and seek shelter, while a semisubmersible intervention unit could ride it out, because of their inherently greater stability.
QinetiQ has been researching SWATH vessels for around 30 years - principally for naval applications - but discussion over an oil and gas industry concept has been underway for 2 years, in association with OTG. "We have been in an intense technical phase," Ross said. "This is not a fully worked up design, it is a concept." Nevertheless he said rules from certification authorities Det Norske Veritas and the American Bureau of Shipping have been followed, but the concept has not been taken to classification stage yet. "All of this has been developed with Norwegian Continental Shelf (NCS) standards in mind." This approach includes providing single cabin accommodation for marine and operational crews.

However, Ross said if a buyer wanted to write a check immediately, he could deliver a vessel within two years.

Phase 1 development funding has been applied from a potential commercial project sponsor with two principal objectives: First to clarify a SWATH vessels functional requirements, and second to determine the likely technical and commercial partners to take the design forward.

"There are around 450 subsea wells on the NCS," Ross said. "They produce significantly less than platform-controlled wells because it is more difficult to intervene on a subsea-completed well.
"With more and more subsea wells being drilled on the NCS, the operators are going to have to find a way of dealing with them in a satisfactory - not an ad hoc - manner."

This is no small undertaking though: This unit is intended to be about 330 ft (100 m) long, with a beam of 160 ft (49 m) and a displacement of 12,500 tonnes.

"It ought to be feasible to construct this unit for the same sort of money as a large multi-support or service vessel," Ross said. "You are looking at a very large vessel, but aft of the blast wall, it is relatively empty." He said it would be up to a vessel owner to decide the exact configuration of equipment on board, so there is plenty of free deck space "You keep the money [value] where it belongs - on the deck space."
This also retains versatility, allowing an owner to customize the design. "We have always tried to keep the ship as a modular unit - to give it plug and play flexibility."

Consequently, he said hard wiring of too many systems has been avoided. "The customers bring what they want. We will build a ship that will adapt to most requirements."

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