Oil & Gas UK Economic Report 2014

During the project’s design phase, a number of different concepts were considered. The conditions prevailing WoS can be very demanding, so Total opted for long distance subsea tie-backs to a gas processing facility on mainland Shetland, with all offshore equipment being placed on the seabed. This meant that the longest and deepest tie-back system ever contemplated, at that point, anywhere in the world had to be developed, posing a huge technical challenge. Once the wells were drilled at the site of the reservoir, the tie-back lines to Shetland had to be laid. These 143km pipes feature flowlines for monoethylene glycol (MEG), which are essential to ensure the gas flows at the very low temperatures prevailing on the seabed. Also incorporated at this stage of the development were in-line ‘tees’, which will allow third parties to access the infrastructure. This tie-back system pushed the technology to the limit. The size and length of the tie-backs had not been attempted before and issues such as degradation of control signals over such extreme distances had to be overcome. Furthermore, the wellhead was too deep for divers to work safely. The entire installation had to be carried out using remotely operated vehicles (ROVs) which, in turn, had to overcome the extreme currents on the edge of the UKCS and the Atlantic Ocean. Total met the challenge through extensive testing before the offshore installation work started. For two months engineers simulated the whole installation process, which proved pivotal to the project’s success. Another critical technical hurdle was the design and installation of the project’s umbilical, which runs for 127km from Shetland to Laggan. The umbilical gives life to the subsea

installation and comprises steel tubes for hydraulic control fluids and high voltage (three Kilovolts) power and fibre optic cables that will control the wells. The depth at which it had to be laid, too deep for diver maintenance and repair, meant that the umbilical had to be laid in one piece. This required the whole 2,800 tonne weight to be transported on and deployed from the installation vessel at the very limit of its capability. Meeting this combination of technical challenges in an extreme environment also led Total to develop a bespoke pipeline management system to monitor in real time the fluids in the system. This allows the company to anticipate failures, plan summer maintenance and predict the size and frequency of liquid ‘slugs’ 12 , which will need to be dealt with at the newly built Shetland Gas Plant, located alongside the existing oil terminal at Sullom Voe. The Shetland Gas Plant is the most visible part of the Laggan-Tormore project. It is also where the environmental and social impact has had to be most clearly addressed; the site selected for the gas plant was covered in upland peat bog. Total recognised that destroying the peat would have a detrimental environmental impact, could concern the local community and would require 23,000 lorry loads to remove it. Instead, the company chose to excavate the 672,000m³ of peat and store it in two specially constructed stores that form part of the site’s landscaping. The peat will be monitored and preserved in these stores and then reinstated following the eventual decommissioning of the gas plant. It is the first time such an exercise has ever been undertaken anywhere in the world.

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12 Liquid ‘slugs’ comprise natural gas liquids (NGLs) which have condensed from reservoir gas as it flows along a subsea pipeline, due to the cooling effect of the sea water.

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ECONOMIC REPORT 2014