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With older fields in areas like the North Sea requiring more field maintenance, purpose-built light well intervention vessels are designed to provide an alternative to using drilling rigs for workovers.
The North Sea has a lot of subsea wells on fields that require maintenance and workover to maintain flows. Staoil has about 500 operated subsea wells on the Norwegian Continental Shelf (NCS) alone.
The company has been pursuing riserless well intervention in its subsea wells since 2000. The company's newest light well intervention (LWI) system is a purpose-built vessel that has been chartered by Statoil for eight years from Eide Well Intervention AS.
The Category "A" LWI vessel can perform logging, perforation, workovers, and well plugging. During intervention, the equipment downhole is remotely operated via wireline from the surface.
The new vessel is a hybrid design that sails like a vessel yet operates like a semisubmersible rig. The design improves seaworthiness, allowing more uptime compared to monohull LWI vessels.
The unit is 122-m (402.6-ft) long by 45-m (148-ft) wide with accommodations for 120 people. The vessel has 3,400 sq m (37,026 sq ft) of deck space.
Eide stated this will be the first purpose-built vessel of this design. The motion characteristics of the vessel allow mobilization and demobilization of crew and equipment even in rough weather conditions.
Companies providing services for the vessel include Halliburton, wireline and downhole services; Oceaneering, ROVs; Advantec, well control system; and Subsea Technologies, stack solution.
Statoil estimates it will be able to reduce well intervention costs by 60% with these vessels, which is important with mature fields.
"Performing these types of conventional jobs on subsea wells with low volumes of oil in place is expensive," explained Oystein Arvid Haland, Statoil's head of drilling and well servicing. "The LWI vessels ensure both cost-efficient and safe operations.
"Having more and new vessels of this category also helps increase recovery from fields onstream by opening new zones in the well and stopping water production downhole," he said.
Jon Arnt Jacobsen, Statoil's chief procurement officer, added, "We have great ambitions and a long-term perspective on the NCS. Using purpose-built rigs and vessels in our operations is an important part of Statoil's rig strategy.
"The high number of subsea wells will require maintenance, and we are securing capacity in order to meet this need," he explained.
Another LWI system being introduced in the North Sea is from the Expro Group. The AX-S system uses a vessel of convenience for well maintenance.
The company recently completed its final commissioning test on a subsea well in a fjord in Norway. All the subsea packages were fully deployed twice in a complete stack-up. Thirty-four tool runs were performed including calipers, production logging, casing collar logging, gamma ray, deep-set plug, crown plugs, and wireline tractor.