Navion expanding flexibility of the small production ship

Navion's Arctic Shuttle, an oil barge with STL system, propelled from the rear by an icebreaker unit. [21,623 bytes] The Symrig semisubmersible rig design is one of a number of new concepts developed by Navion. [36,092 bytes]

Submerged turret, suction anchor production ship add value

Nick Terdre
Contributing Editor
Norway
The new suction anchor loading system (SAL) developed by Navion comes into operation for the first time later this year on Statoil's Siri Field in Denmark. The SAL is one of a number of new concepts from Navion, the shipping and offshore company which started life as Statoil's shipping and marine technology division, and is now close to completing its first year as an independent concern.

Navion is already known for its submerged turret loading (STL) system and the multipurpose shuttle tanker (MST). With the addition of a high-pressure swivel and rotating connector, the submerged turret concept has been further developed into the submerged turret production (STP) system. In combination with the MST, the end result is a production ship for small and marginal fields. This concept has become a reality on Statoil's Lufeng Field in China in the shape of the Navion Munin, and on Enterprise's Pierce Field in the UK, where first oil through the Berge Hugin is expected in autumn.

The MST itself is proving a prolific concept, and has been further developed both as a deepwater drillship, of which two units are currently under construction, and for extended well testing.

Relationships

A number of core relationships are involved in Navion's pursuit of technology development. It is a one-third owner of Offshore Invest, a technology holding company in which the other two thirds are held by Statoil. Offshore Invest owns 60% of Advanced Production and Loading (APL) and 50% of Framo Engineering. These two companies, whose deliveries include components and systems for offshore loading and production, are key Navion partners.

The STL system is currently in use on a number of Norwegian and UK fields. Because the loading buoy is submerged, operations can continue even in extreme weather conditions. On Statoil's Heidrun Field, the use of a twin system means one tanker is always available for loading, which in turn means that storage facilities are not required. "There was 100% regularity at Heidrun over the last winter," says Kare Breivik, senior vice president for technical maritime services. "The operation is going very smoothly."

SAL or STL

For fields where storage is available and weather conditions are less demanding, the SAL provides a cheaper alternative to the STL. In this case, the suction anchor provides not only the mooring point for the tanker but a platform for loading. Installed on top of the anchor are a mooring turret, to which the mooring line is attached, and a loading swivel. Oil is transferred through a flexible hose to the tanker, which can weathervane around the anchor while in passive mode, thus saving fuel. Operations are possible in conditions of 6-7 meters significant wave height, compared with 5.5 meters for a single point mooring system such as is used on Statfjord, according to Breivik.

Compared with the STL, the SAL requires less conversion work to be carried out on the tanker. Interest has been shown by other potential buyers, and further sales have been secured to PGS for Conoco's Banff project in the UK and to Rockwater for Amerada Hess's South Arne development in Denmark.

The SAL system is now being further developed for production in the form of the single anchor production (SAP) system. A high-pressure STP swivel is placed on top of the suction anchor, from which four risers run up to the tanker, two on either side. The sides of the tanker are extended outwards to receive the risers. A verification program is being performed this year.

SAP is aimed mainly at well testing and early production, and possibly the sequential production of several marginal fields. It is thought capable of allowing a flow of up to 125,000 b/d, according to Breivik.

Other developments

Other developments have their starting point in Navion's hull experience.

  • Arctic Shuttle: The company recently launched an Arctic tanker concept, with one eye on future field developments off Russia's north coast. The Arctic Shuttle consists of two units - an oil barge propelled by an icebreaker unit which fits into a slot at the rear of the barge. When necessary, the icebreaker can decouple from the barge to break a path through the ice.
    The barge is equipped to load using the STL system specially reinforced for use in ice. While it is loading, the icebreaker can be used for other functions on the field, such as providing supply services.
  • Strengthened hull semi: Another new concept is the Symrig semisubmersible platform for drilling, well testing, and production. The Symrig has six or eight equal sides supported at each corner by a column, the base of which sits on a ring pontoon. The symmetrical design gives the structure great strength and stiffness, with optimal transfer of forces from topside to hull.
    Preliminary analyses indicate very good motion characteristics for the structure, Breivik says. Movement is largely independent of changes in wind and wave direction, which means fuel consumption when the rig is on the move or holding station in DP mode is low. The structure presents low construction and maintenance costs.

VOCs, gas-to-liquids

Navion is also participating in projects aimed at curbing the release of volatile organic compounds (VOCs) into the atmosphere. A full-scale prototype of a system for absorbing VOC gases into the crude is to be installed on one of its tankers this year. A second project, backed by the European Union, aims to liquefy VOC gases and burn them as fuel. Testing of the system will again take place on a Navion tanker - conversion of the first engine began this spring.

Navion is also involved, with responsibility for the vessel side, in Statoil's project to convert gas to syncrude at sea. It has also studied liquefying natural gas liquids from a gas/condensate stream about to be reinjected into the reservoir to aid liquids recovery. The gas/condensate flow would be routed from the production platform via an STL system to a Navion vessel where an expansion process would be used to liquefy the heavier components, and the flash gas would be returned to the platform for reinjection. The technology involved is fully feasible, Breivik says.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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