- Technical impression of the Varg monotower in its vertical position onboard the S-7000 crane vessel. [107,806 bytes]
Saga's Varg is the latest in a legion of marginal North Sea fields to be developed via a production vessel. But just as crucial to the economics of the project will be the associated wellhead platform and installation technique.
Varg (formerly called Fenris) was not a high priority development until Saga assumed operatorship from Statoil last year. The field, in 84 metres of water in Norway block 15/12, was considered complex, comprising numerous faulted structures.
Known accessible oil reserves are put at just 33 million bbl, although Saga and Statoil have identified potential for a further 20 million bbl through appraisal drilling. This scenario means that field life could be anything from five to ten years.
To cover these uncertainties, the central hardware - the production vessel and the 16-slot wellhead platform - will be able to handle tie-ins from other fields in the area, or should be easily disengaged at Varg's demise for use on other projects.
Assuming sanction for the PDO comes from the Norwegian authorities, production should start by the second quarter of 1998. FELS will build the vessel in Singapore under a NKr1.5 billion contract, with production capacity of 57,000 b/d and 1.5 MMcm/d of gas. The latter will be used to generate electricty, or to improve reservoir productivity through reinjection. Coflexip-Stena Offshore will perform pipeline installation and riser hook-up, and Neddrill looks set to drill the wells.
By anouncing that Varg was marginal, the field partners put the onus on contractors to produce especially cost-effective solutions. Saipem and Aker Verdal responded in their joint bid for the wellhead platform, leading to a NKr250 million EPCI contract. The finished platform, to be built at Verdal, should be installed in the field by the S7000 in summer next year.
The S7000 is the world's largest construction vessel, capable of enormous jacket and topsides placements. It can also serve the industry on smaller-scale projects. However, in order to be competitive, Saipem believes that it is necessary to break from tradition whereby design houses produce a base case platform suited to the capabilities of a smaller installation vessel, with installation tenders then being solicited from a wide range of vessels.
In Saipem's opinion, this method of contracting can lead to higher fabrication costs and in the case of the S7000, higher installation costs since the vessel's operation is not optimized.
In order to be competitive on smaller structures, Saipem has recognized the need to bid on an EPIC basis allowing savings in fabrication costs to be achieved by designing the structures around the S7000 and taking full advantage of the vessel's capability. This philosophy was successfully adopted for the Varg wellhead platform.
The Aker Verdal yard can accommodate construction of Saipem's 104-meter-tall monotower concept in a vertical position. Due to an innovative substructure arrangement, also involving transportation vertically, the entire 4,000 tonne platform can be taken onto the S7000 and then installed with minimal temporary appurtenances. This solution results in virtually no redundancy of structure or equipment after installation, in turn reducing construction and installation costs.
Otherwise, the basic platform design is straightforward, and likewise the installation, which should take 10 days. The jacket legs will terminate in four sleeves with 96-in. diameter piles, to be driven 50 meters into unproblematic seabed foundations.
In line with today's trend of designing new platforms with eventual decommissioning in mind, the Varg monotower should be another straightforward vertical removal exercise following cutting of the piles. Saipem compares the process to running the installation video backwards.
The Saipem/Aker Verdal solution proved to be the winner not only over rival wellhead platform designs, but also over proposals involving subsea wells tied to a production jack-up. Overall development costs at Varg are now estimated at NKr3 billion.
Saipem sees Varg as a test case for an alternative, low-cost fixed installation concept which would also justify S7000's continued presence in the North Sea's high season. The company is looking to work closely with other operators to maximise development of marginal fields. It beliieves that by making the S7000 more competitive (as with Varg), the number of potential developments would increase, and greater utilisation of the vessel would promote further reductions in installation costs.
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