North Sea

GAS

Installation challenges The depth of water at Laggan- Subsea-to-shore Tormore precluded the use of divers so Heerema Marine Contractors’s Thialf heavy-lift vessel was used to transport and install the production systems on the seabed. Subsea work was sub-contracted to innovation Aberdeen-based Specialist Subsea Services (S³), who provided and operated remotely controlled underwater vehicles (ROVs) for the was produced on 8 February 2016, subsea installation work. via a semi-autonomous system The project has a field life lying on the seabed connected by a expectancy of 20 years. As with 143-km subsea export pipeline, any production facility, wellhead known as the Laggan Tormore pressures naturally decline over export pipeline (LTEP). The LTEP time and the pipeline export flow links the gas fields with a new will fall below a critical value. At £800mn gas plant located in this juncture compression boosters , next to the will most likely be installed at each oil and gas terminal. The facility is wellhead and one for each export able to process 500mn cf/d – pipeline in order to boost enough gas to supply two million production and recovery from the households or about 8% of the UK’s reservoir by reducing backpressure energy needs. After processing, the on the wells by increasing the flow gas is piped through a second rate in the export pipelines, thus 243-km newly constructed extending field production. Shetland Islands Regional Gas Export (SIRGE) pipeline which In the pipeline connects with the Frigg-UK Two sets of new pipelines were pipeline and then into the UK’s gas constructed for the project, the first pipeline network in . being the LTEP network linking the A combination of the extreme fields with the newly constructed weather experienced in the Shetland gas processing plant. A Atlantic Margin, including second pipeline system exports the hurricane force winds in winter; processed gas from the gas plant to water depths of some 600 metres, the UK mainland. The entire where temperatures are commonly pipeline network required 200,000 –1°C; typical current speeds as high tonnes of high-grade steel. as 0.64 m/s; and the large volumes Internally the pipes are protected of gas to be developed made it by a three-layer polypropylene unrealistic to employ a traditional anti-corrosion coating, internal platform or a FLNG (floating LNG) flow-efficiency coating and The Laggan-Tormore project west of vessel to collect, process and store concrete-weight coating. The gas ready to be offloaded onto LNG pipeline network is regularly the Shetland Islands is the first tankers. Instead, a subsea-to-shore inspected and can be cleaned by a subsea-to-shore development in UK development concept was chosen remotely controlled pig. Nic Newman – a UK first. The LTEP comprises three waters. reports. Essentially, the Laggan and pipelines and a communication or Tormore fields are each serviced by control umbilical. On the seabed a semi-autonomous subsea this network consists of two eveloped by Total E&P UK production system consisting of a 18-inch diameter flow-lines that (operator, 60%) in set of six-slot manifold templates carry gas from the fields to the D partnership with SSE E&P lying beneath 600 metres of water Shetland gas plant for processing UK (20%) and DONG E&P UK on the seabed. Each manifold and an 8-inch diameter pipeline (20%*), the innovative Laggan- template measures approximately carrying MEG (mono ethylene Tormore project is located in blocks 30 metres wide, 40 metres long glycol) from the gas plant to the 206/1a and 206/5a, in water and 21 metres tall, weighing in at Laggan-Tormore field’s production depths of 600 metres. Total field 900 tonnes. At present, five subsea sites. MEG is used to prevent the reserves are put at 1tn cf of gas and wells are in operation out of a condensate from freezing at the condensates – a substantial 12-well capacity. start of its journey, much like addition to the UK’s known These subsea installations antifreeze is used in a car. When reserves. represent two major engineering the gas arrives at the processing The £3.5bn project comprises achievements for the UK. The first plant, the MEG is removed and the development of two deep is the installation of a subsea pumped back to the wellheads, to offshore gas fields – Laggan and production system with remote start its journey again. Gravity Tomore – the first to be developed controlled wells, and the second is causes the contents in the gas Installation of import in the Atlantic Margin, a region the longest tie-back between pipelines to separate into gas in pipeline at Orka Voe, Shetland Islands that until now was known offshore wells and an onshore the top half of the pipe, whilst the Source: Total primarily for its oil output. First gas terminal. heavier gas liquids settle along the

32 Petroleum Review | August 2017 North Sea

the SIRGE pipeline system, which then links with another pipeline to deliver the processed gas to the , near Peterhead in the north-east of Scotland for final processing. The plant can process up to 500mn cf/d of gas, which is more than Laggan-Tormore is likely to produce. This is in anticipation of future successful exploration in the West of Shetland region, an area estimated to contain as much as 17% of the UK’s remaining yet-to-find gas reserves. Development of the nearby Edradour and Glenlivet fields by Total and its partners, starting in Gas processing plant, 2017 and 2018, will necessitate Shetland Islands bottom half. The critical element, second gas compressor came Source: Total the control umbilical, monitors and online. It took a huge effort to get installation of new mercury controls all the subsea to this point – taking some 42mn removal facilities at the Shetland infrastructure. Both the MEG person-hours over two years, gas plant, and will ensure pipeline and control-umbilical are employing 800 personnel on production is kept at peak rates buried under newly laid stone to temporary contracts for the until at least 2020. protect them whilst the gas export construction, and currently Looking ahead, Laggan-Tormore pipelines are exposed to the employing 70. By May 2015 will have a key role to play, elements. Laggan-Tormore was producing at allowing future gas finds in West Construction of the Shetland gas a peak 90,000 boe/d. of Shetland waters to be easily processing plant began in 2010 and The gas processing plant transmitted to the UK mainland was handed over to Total by lead separates the well fluid into gas, oil via the existing infrastructure. ● contractor Offshore and water with the three fluids Engineering & Operations in being discharged separately. The *DONG’s entire oil and gas business, including its interest in Laggan-Tormore, was December 2015 for testing the associated liquid hydrocarbons are sold to Ineos in late May 2017 for $1.05bn, complete range of command and exported to the Sullom Voe oil and plus a contingent payment of $150mn control systems. As noted earlier, gas terminal. The treated gas from relating to the Fredericia stabilisation plant and a contingent payment of up to $100mn first gas arrived in February 2016 the plant is exported to the Frigg subject to the development of the Rosebank and in April the same year the UK pipeline in the North Sea via field.

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Petroleum Review | August 2017 33