Conoco Norway's pioneering plans for a concrete hull, tension leg platform for the deepwater Heidrun oil and gas field in the Haltenbanken area of the Norwegian Sea have finally made their way to the Ministry of Petroleum and Energy in Oslo for processing.
If Norway's Storting (parliament) approves the 19 billion kroner ($2.93 billion) project during the current spring session, Conoco will start work on preliminary engineering in the third quarter of this year with the aim of starting oil production in summer 1995.
NO POLITICAL CHOICE
Given Norway's expertise in building concrete structures for the offshore industry and the government's desire to ensure that operators turn to foreign construction yards only as a last resort, Conoco's choice of the concrete TLP might seem to have political connotations.
Conoco, however, says there are sound economic arguments for the concrete TLP. It estimates the concrete hull will cost 50% less than steel and construction time will be 15 1/2 months compared with 27 months for a steel hull.
Those figures emerged from Conoco's decision to develop conceptual designs for concrete and steel TLPS. Terry Wilson, the structural supervisor for the Heidrun project, outlined to the Deep Offshore Technology conference last year other major advantages of a concrete TLP over steel.
On the construction side, techniques are well established and relatively simple for concrete. The project would require a largely unskilled workforce, compared with substantial numbers of highly skilled welders and inspectors that would be required for the complex steel fabrication project.
Another key advantage, said Wilson, is the ability of a concrete TLP to support greater topside weight than a steel unit. A concrete hull also could accommodate changes in the topside weight that would present serious difficulties for a steel unit.
Finally, reinforced concrete is fatigue resistant and durable. When combined with the simple structural design of the hull, inspection and maintenance requirements are minimized.
The concrete hull TLP is not the only innovation for a project in Norwegian waters. Conoco intends to use offshore tanker loading to export the oil, a technique that has been refined into an extremely reliable way of handling crude by Mobil and later Statoil in Statfjord and Gullfaks fields.
Reliability of offshore loading systems in the Norwegian North Sea is high despite poor weather conditions during the winter. Statoil says offshore loading provides the same degree of reliability as a pipeline.
However, Statfjord and Gullfaks platforms have large storage capacity in their concrete substructures to ensure that production can continue unhindered on the few occasions when bad weather prevents tankers from loading.
NO STORAGE ADVANTAGE
The advantages of a concrete TLP do not run to storage facilities.
Instead, Conoco is considering a custom built storage tanker equipped with a sophisticated turret mooring system.
In the past, tanker storage has been frowned on by Norwegian authorities. The alternative would be a concrete storage tank installed on the seabed in more than 1,000 ft of water that would be connected to a loading buoy on the surface.
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