From the North Sea (NT): Perenco and Tullow Oil have launched decommissioning plans for the Thames field and its associated satellites in the UK southern basin, which add up to the largest such project in the sector to date in terms of the number of fields involved.

In addition to the satellites in the so-called Thames complex - Bure Oscar, Bure West, Thurne, Wensum and Yare – there are six longer distance tiebacks - Arthur, Gawain, Horne, Orwell, Wissey and Wren, making a total of 12 fields.

Thames was developed with three small platforms. While Wensum was drilled from the field centre and Horne and Wren were jointly produced from a wellhead platform, the other satellites are all subsea tiebacks, in Wissey’s case to the Horne/Wren platform.

The long and short of it

The closest tieback to Thames is Thurne (4.6km) and the furthest Arthur (29km). From Thames, gas was exported to Bacton by a 90km, 24in pipeline.

Arthur, Gawain and Orwell have three wells each and the other satellites one each. Thames has five platform wells, including two water disposal wells, as well as one subsea well located beneath one of the platforms.

The platforms and subsea production equipment - xmas trees, manifolds, templates, protection structures - will be removed, while the operators recommend that the pipelines and umbilicals, which are trenched and buried, be left in place. Due to the strong seabed currents in the area, drill cuttings have long been dispersed.

Work on some of the fields begins straight away and all the programmes are due to be completed by 2018. Rigless plugging and abandonment of the Thames platform wells is scheduled to be completed by Q1 next year.

The cost of decommissioning this extensive network of fields has been put at £221.5mn. The Thames complex is put at £96.1mn with platform removal accounting for £43mn and the well p&a £40m. Elsewhere the well p&a is easily the major item, accounting for £33mn out of a total bill of £44.1mn on Arthur, £25mn of £31.8mn on Gawain and £14mn of £19.5mn on Orwell. Another £30mn has been estimated for Horne/Wren (£21.5mn) and Wissey (£8.5mn).

Thames was brought onstream in 1986 by Arco. The final tieback was Thurne in 2008, which employed a refurbished subsea tree from the abandoned Deben field. The official cessation of production took place in May this year, although some of the fields had already been shut in for a period - Orwell, for example, since 2009.

Thames, Arthur, Bure fields, Gawain, Wensum and Yare are operated by Perenco and the others by Tullow, which is still seeking to offload most of its Southern Basin assets.