The Frigg gas field spans the UK and Norwegian sectors of the North Sea. The initial projection that approximately 40% of the reserves lay in the British sector proved to be accurate. In 1974, the final agreements were signed between the Norwegian and UK governments. From Stavanger, Norway, Elf (now Total E&P Norge A/S) operated all five platforms in the Frigg Field (two in Norwegian waters and three in British waters). From Aberdeen, Scotland, Total (now Total E&P UK Ltd) operated the MCP-01, the 32” pipelines and the onshore terminal at St Fergus.MCP-01
The Manifold Compression Platform MCP-01 sits 173 kilometres (104 miles) north-east of Aberdeen, half-way between the Frigg Field and the Scottish coast. It was built in Sweden by Doris Engineering and installed in the summer of 1976, becoming operational the following year. It is a concrete gravity type, meaning that the platform is kept on the seabed 94m below the surface by its own 386,000-tonne weight. All but 13,500 tonnes comes from the concrete gravity base, which is 100 metres (328 feet) in diameter and 102 metres (335 feet) tall. The perforated Jarlan wall was designed to protect the central core and has approximately 1250 holes to reduce wave impact. MCP-01 was used as a pigging station and, later, a compression platform. Pigs are tools that remove liquid that accumulates in the pipelines and were also used to check for any internal damage or corrosion to the pipeline. They are built to exactly fit the diameter of the pipe, but otherwise differ depending on their function. Cleaning pigs will have thick, abrasive brushes whereas inspection pigs contain sensitive electronic equipment to accurately record the condition of the pipeline. In the 1970s, it was not possible to pig a line as long as Frigg in one operation. At MCP-01, the worn pigs received from the Frigg field were replaced with undamaged ones or refurbished and sent on down the pipeline to St Fergus.
Changes in the quantity and quality of the gas led to the installation of compression facilities in 1982. Gas pressure decreased by two-thirds between leaving the Frigg field and reaching St Fergus. The compressors increased the capacity of the pipelines from 31 million standard cubic metres per day to 42 million cubic metres per day. From 1992, MCP-01 was ‘not normally manned’, i.e. no permanent presence onboard but with routine maintenance teams visiting regularly. Decommissioning of MCP-01 was originally planned to take place around 2024 once the pipelines passing through the platform are no longer expected to be in use. However, studies suggested that it was not possible to guarantee that an acceptable standard of safety could be maintained until the platform was fifty years old, even with an appropriate level of maintenance work. Plans for decommissioning were brought forward, with assessments indicating that the most viable strategy was to remove the topside facilities and as much of the steelwork for recycling as possible, but leave the concrete substructure in place. In January 2004, the process began to establish a Decommissioning Programme to be submitted to the Department of Trade and Industry (dti) Offshore Decommissioning Unit for approval started. Programmes of stakeholder and public consultations began in 2004. Removal of the topside began in 2006.
FRIGG TRANSPORTATION SYSTEM
The twin pipelines transporting gas from the Frigg Field to the processing plant at St Fergus are 362 kilometres long (225 miles) and 32 inches (81 centimetres) in diameter. In places the pipes lie in over 150 metres (492 feet) of water. At the time of their construction, the pipelines were the largest ever used at those depths. To withstand the incredible pressures and prevent corrosion, the steel walls of the pipes are ¾ inch thick and reinforced with between 2 and 4.5 inches (5 and 11.5 centimetres) of concrete. A combination of the great length of pipeline and short weather window (the period when the North Sea is calm enough for work to be carried out) meant that the work began in 1974 and was not completed until 1977. Major works were carried in the summers of 2004 and 2005 to bypass MCP-01. The pipelines ran through the base of the platform but needed to be rerouted by almost 2 kilometres, as they remain in use after the platform has been decommissioned.
ST FERGUS GAS TERMINAL
The St Fergus Terminal cost almost £100million to build and had an initial workforce of 160. Today, the terminal is much larger, having been extended since its initial construction in the 1970s. Phase 1 construction took place in 1977 and Phase 2 a year later; during the 1980s, the discovery of gas in the Alwyn and Bruce fields necessitated Phase 3.
The main function of the terminal is to separate the gas from any condensates before the gas is metered and distributed. The liquid condensates are then sent to facilities in Cruden Bay and Mossmorran. Originally, St Fergus supplied gas to just one customer, British Gas, who had their own neighbouring facilities where the gas underwent final treatment before being sent south (such as having the artificial ‘gas’ odour added). As of 2007, gas arrives at the terminal from approximately 20 different offshore fields. On average, 70 million standard cubic metres of gas, about 20% of the UK’s requirement, is received and processed there daily. The Phase 2 and Phase 3 facilities process gas from the Norwegian and UK pipelines respectively.