Perenco files decommissioning plan for four Amethyst topsides

Oil and gas company Perenco has submitted its draft decommissioning programs for four Amethyst gas field topsides to the UK authorities.

The Amethyst gas field is located in the United Kingdom Continental Shelf (UKCS) centred on Block 47/14a and extending into Blocks 47/13a, 47/9a, and 47/15a in the Southern North Sea, some 40 kilometres east of the Humber Estuary and the Easington Terminal on the Yorkshire coast.

The field consists of several separate gas accumulations – Amethyst East covers the A and B areas while Amethyst West covers the C area.

Discovered by Britoil in 1970 and 1972 respectively, Amethyst East and Amethyst West have been producing gas since 1990.

In 2012, the field operatorship was handed over from BP to Perenco. Perenco explored all avenues for continuing production and concluded that due to high operational costs and a reduction of gas production, continued operations are uneconomical.

The cessation of production documentation is in development and a draft document was submitted to the OGA in February 2020. Approval for cessation will be gained before any decommissioning takes place.

The Amethyst field comprises of four normally unmanned installations (NUI) – A1D, A2D, B1D, and C1D.

The Helvellyn development, owned by Alpha Petroleum is located in block 47/10a. It consists of a single subsea well, tied back to the Amethyst A2D platform via pipeline PL1956, where it is co-mingled with Amethyst gas, and exported to the Dimlington Gas Terminal.

Helvellyn well is currently still producing, however, a separate COP request will be made by Alpha Petroleum. Alpha Petroleum is responsible for the decommissioning of Helvellyn and will be covered under a separate DP.

According to the plan, the preferred order of decommissioning is C1D, B1D, A2D, and A1D, starting with hydrocarbon free (HCF) operations, followed by removal of the platform topsides using a jack-up barge (JUB). The sequence is subject to optimisation.

Depending on the availability of jack-up barges, following completion of the HCF phase the platform will either remain in lighthouse mode or the topsides will immediately be removed and the jacket will remain in a ‘dismantlement interval’ phase.

The four platforms of the Amethyst field were installed in 1989 (A1D and A2D) and 1991 (B1D and C1D) meaning the subsequent pairs are similar in configuration.

A1D and A2D topsides are the larger of the four platform topsides and consist of a cellar deck, mezzanine level, main deck and helideck, with a crane and vent boom while B1D and C1D consist of a cellar deck, main deck, helideck, and crane.

Perenco has excluded several activities from the topsides decommissioning programme which will be part of the jacket and pipeline decommissioning programmes. The activities in question regard jacket dismantlement and seabed clearance and verification.

Decommissioning of the pipelines will be dealt with in a separate DP, however, jacket risers (including the Helvellyn riser), j-tubes, and caissons will be partially removed before the removal of the topsides. Pipelines will be air-gapped and flushed during the HCF campaign, before the removal of the risers.