The loading buoys on Statfjord

Finn Harald Sandberg, Norwegian Petroleum Museum

The Norwegian Statfjord licensees had to find solutions for transporting the huge volumes of oil and gas involved. While the gas went by pipeline to Germany, the stabilised crude oil was exported by specially equipped shuttle tankers to refineries and terminals throughout northern Europe. Such offshore loading failed to comply with the “10 commandments” of Norwegian petroleum policy, which said that all the country’s oil and gas production should be landed in Norway. That made it difficult to approve the system.

— A tanker loads oil from a loading buoy on the Statfjord field. Photo: Mobil Exploration Norway Inc./Norwegian Petroleum Museum

installations at Statfjord with landings to Sotra - from Storting Report no. 90

Many strong advocates of piping the oil to Norway were to be found both in the political community and in state oil company Statoil, which was responsible for transporting all Norwegian oil and gas from Statfjord. Offshore loading was regarded as an unavoidable interim solution until a pipeline could be put in place. But shuttle tankers proved cheaper, even over the field’s total producing life.Offshore loading into tankers on the field was one of several options when Statoil launched a preliminary study on transporting crude oil in the North Sea during the autumn of 1974. Nor did it have a high priority. Nevertheless, the report presented in November 1975 concluded that this would be the cheapest option – although great uncertainty prevailed about its reliability. This was specially discussed in chapter 6.2 of White Paper no 90 (1975-76) to the Storting (parliament) on the development and landing of petroleum from the Statfjord field and a gas gathering pipeline.

Little hard data existed on climatic conditions in the northernmost North Sea, which could affect the profitability of the field. Loading regularity in winter – when demand of oil was at its highest – could be reduced by adverse wind and weather.

Lack of experience with the recommended type of loading buoy meant that the Ministry of Industry would not approve the use of such installations for the field’s whole producing life.

At the same time, the ministry appreciated the importance of gaining experience with these structures for future use, particularly in North Sea weather conditions. Such lessons could have great significance for developing small oil fields and possible discoveries north of the 62nd parallel (which marks the northern limit of the Norwegian North Sea).

According to the White Paper, two loading buoys were to be installed on Statfjord. They would have a conditional operating life of five years. The structures chosen were both of the same type, but with some design differences. They were initially intended to serve production from the A and B platforms respectively.

The first loading buoys on the Norwegian continental shelf (NCS) became operational in the summer of 1971 during test production on the Ekofisk field further south. These units left much to be desired in terms of operational reliability. They could only be used in good weather, and their availability – the time they could be used for loading – was as little as 40 per cent.

In the British sector of the North Sea, work was in full swing on adopting the second generation of such units (see illustration). These were popularly known as “Alps”, from the abbreviation of their full name: articulated loading platforms.

Statoil awarded a five-year contract to Norway’s Einar Rasmussen shipping company for offshore loading on and oil transport from Statfjord. This charter included an option
for a five-year extension. The first cargo had been loaded on Polytraveller by 4 December 1979.

Concerns about the ability of the ALPs to cope with weather conditions proved groundless. History shows that this buoy type had an availability of 99 per cent in the summer and 93 per cent during the winter months.

This positive result contributed to the installation of a third ALP in 1984. Of the same type as the B buoy, it was to serve production from the Statfjord C platform. Each buoy could be connected to all three of the production installations via a flowline system, and was operated independently of the others.

The original plan was that shuttle tankers would be moored to the loading buoys by taut hawsers to avoid collisions between ship and ALP. As the crew gained experience with these operations, however, the absolute requirement for such taut mooring was dropped. It eventually became possible to convert to dynamic positioning (DP), a system which allows a vessel to maintain station without the need for physical mooring. All deviations from the specified position are registered by a computer, which activates the necessary array of propellers and thrusters to return to the correct location.

When serious cracking was discovered in the A buoy after a few years of operation, the decision was taken to remove it to land for repairs. Development work had been under way for some time to make loading operations simpler and safer. It was now proposed to replace the A buoy with a new design known as the Ugland-Kongsberg offshore loading system (Ukols). This would be clearly cheaper as well as saving time and reducing risk.

The revolutionary feature of the system (see the illustration) was that connection to the loading hose was made via a moored underwater buoy. This meant that ships could load under even worse weather conditions than before, which naturally also increased availability.

Illustration showing UKOLS (Ugland Kongsberg Offshore Loading System) at Statfjord. Illustration: Equinor

Just two years later, similar cracking was observed in the B buoy. The decision was once again taken to replace the ALP with Ukols. Statoil’s application to dump the redundant buoy in the deep Nedstrands Fjord north of Stavanger was rejected, and it had to be taken to land in 1990. Its big rotating head was lifted off and installed at the Norwegian Safety Centre (now Nutec Sotra) for use in safety training. The 185-metre steel cylinder which formed the support column was towed to Sandnessjøen in northern Norway for scrapping.

It has not proved necessary to replace the C buoy. This functioned well right up to 2003. Since then, the structure has offered a back-up in the event of problems with the two Ukols units. Plans called for it to be removed from the field in 2012.

Sources:
Report no 90 to the Storting (1975-76) on the development and landing of petroleum from the
Statfjord field and a gas gathering pipeline.
Mobil. Statfjord unit offshore loading study. October 1978.
Lindøe, John Ove. From Sea to Shore. Stavanger 2009.
NTB (Norwegian Wire Service), 26 July 1989, “Statfjord B lastebøye til 400 millioner må skiftes ut”.