First for Frigg – a Rigwatcher change-out
The AtoN had to be lowered onto a 2m steel pillar mounted onto a 20m tower in the sea, which was not visible to the pilots.
There can be few Aids to Navigation (AtoN) as remotely located as the three Rigwatcher units marking the decommissioned Frigg Field on the UK/Norwegian median in the North Sea.
Three concrete gravity based structures were decommissioned in place, leaving the remnants of the pillar legs and sub-sea caissons as the only visible sign of TOTAL’s once busy field. These remnants are marked for navigational safety by specially designed air portable Rigwatcher self-contained Aids to Navigation which are monitored and maintained by the Northern Lighthouse Board (NLB) under contract to TOTAL E&P Norge.
The Rigwatchers are designed for a deployment of up to four years remote operation. The first Rigwatcher was deployed in July 2007, with the second and third following in 2009. In order to bring the change-out schedule in to line all three were planned for replacement in 2011.
A full set of back-up units are stored on warm stand-by in NLB’s Marine Support base in Oban, on the west coast of Scotland. These underwent a stringent service and upgrade in preparation for a mobilisation in September, when a narrow window of opportunity became available.
Coordinating closely with TOTAL and Bristow Helicopters, who were to conduct the underslinging operations, NLV Pharos mobilised under the command of Captain Sean Rathbone with the three Rigwatcher AtoNs on board on 21nd September to arrive on station at Frigg at first light on 23rd, when forecasting predicted a six hour weather window when swells would be at or below two metres and allow safe lifting operations. Conditions at site were challenging but within safety parameters, so it was decided to proceed.
The helicopter, a Super Puma from Bristow’s Aberdeen base, flew directly to Frigg to carry out a final visual inspection before transiting to a nearby FPSO helideck, which was held clear as a “virgin deck” throughout the operation, to fit the lifting array. It then rejoined Pharos and transferred the three new Rigwatchers to the pre-installed docking poles on the structure using remote lifting hooks before uplifting the in-situ units to Pharos. Following a refuelling stop at the virgin deck the helicopter returned to Aberdeen.
In total the operation, which had never been fully attempted at sea before, was completed within half an hour. The close cooperation between ship and Bristow’s aircrew allowed the operation to proceed even though it was at the limit of feasibility due to the sea state. The pilot, Captain John McIntyre, had to engage a suspended ball and chain into the crook of the lifting arm on top of the AtoN while having no fixed visual reference. To achieve this three times from a moving deck was difficult enough. The AtoN then had to be lowered onto a two metre steel pillar mounted onto a 20 metre tower in the sea, which was not visible to the pilots who were reliant on the vocal con of the air crewmen, again with no static visual reference. Recovering the old units was an equal challenge.
The Rigwatchers on station will be monitored remotely by the Northern Lighthouse Board and the returned units will be prepped for warm standby – ready to do it all again in four years’ time.
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