The 25,000-tonne Hutton Hull tension leg platform (TLP) – manufactured 40 years ago and the first of its kind to be built – has been taken to shore in Scotland as part of a complex decommissioning project. Sophie Barnes reports.

The Hutton Hull TLP – known as the Pillars of Cromarty Firth – was first installed in the Hutton oilfield on the UK continental shelf (UKCS) in 1984 by Aker Offshore. It is not only the world’s first TLP, it remains one of the largest of its kind to date. During 2008, it was being moved to Mexico but, during the towing operation, it broke free of its mooring chains near Scotland. As a result, the TLP was towed and moored in the Firth – where it has remained ever since.

Bob Buskie, chief executive of the port of Cromarty Firth, said: “Since 2009 the legs have been safely and securely anchored here, becoming a distinctive part of the Cromarty Firth. Now its life has come full circle as it has been transferred to the port’s Queens Dock where Messiah Decommissioning will begin decommissioning operations onsite.”

Before the decommissioning of the TLP’s structures could commence, Hutton Hull had to be towed from where it stood idle for the last 12 years.

Early issues

Assisting in the project is MBM Consultancy. Speaking with HLPFI, company founder Mark Bambury discussed some of the issues that arose before any actual project work could get under way.

“After it was moored in Firth they cancelled the contract for Mexico. They closed the company down for that project and then a holding company became the owner, but legally it was not sold or transferred,” he said. With a number of other transfers and deals under its belt, it took a year for the team planning on purchasing the TLP to find out who legally owned Hutton Hull.

When MBM became involved to help with the engineering plans to move and decommission the structure, the company found that there were no drawings.

Lack of drawings

Bambury said: “Once the Hutton was purchased, MBM got in touch with Mark Darley, global marine and offshore director at Lloyd’s Register (LR). This was to gain access to any drawings or data of the Hutton. LR believed it had a box of drawings in an archive in Southampton.”

Much to the team’s disappointment, it turned out to be the wrong vessel and no drawings were available.

With no documents, plans or drawings to assist the project, MBM had to re-engineer the vessel hull and perform stability calculations from scratch.

“We had the a basic idea of dimensions and where the bulkheads are. So we began to recreate a whole new model of tanks and the main body. We conducted surveys to get the main draughts at the four corners, at approximately 100 m apart, to define overall displacement and weight of the TLP,” explained Bambury.

“By looking at the displacement, we estimated that it weighed around 24,800 tonnes – without ballast. This is primarily the steel lightship weight. From that we developed the towing and mooring plans.

“From starting the investigations to bringing Hutton Hull into a moored berth in Queens Dock Invergordon, took approximately three months… it was a very rewarding challenge.”

The operation saw the massive 100 m x 100 m Hutton hull relocate approximately 4 km into the dock at high tide.

From starting the investigations to bringing Hutton Hull into a moored berth in Queens Dock Invergordon took approximately three months… it was a very rewarding challenge. – Mark Bambury, MBM Consultancy

Port involvement

The port of Cromarty Firth has also been involved in the planning and execution of the structure’s transport. Its Queens Dock is an ideal facility for the project – it provides a sheltered deepwater location and offers great protection for underwater works.

Buskie said: “The port continues to hold all the relevant decommissioning and waste management licences to ensure the project is completed to the highest health and safety standards, which will be monitored by independent auditors and under a permit from SEPA. The project is expected to recycle around 95 percent of the structure, which is mainly made up of steel.”

For the physical decommissioning, MBM had to first undertake 3D scans of the structure. This will enable the MBM naval architects to recreate the steel structure, and determine weights and the centre of gravity, which is crucial information for any heavy lift operation.

“We have to re-engineer everything from scratch to ensure every lift is executed safely. We need to work out where to cut each section and how to lift each unit, ranging from 50 tonnes up to 250 tonnes,” said Bambury.