Brent Spar disposal to take place in Neptun catamaran cradle

Float-over can be reversed for platform abandonment

Neptun was responsible last year for the mating of the 38,000-ton Hibernia topsides with its concrete base. [38,199 bytes]

In the forthcoming disposal of the Brent Spar, a key role will be played by Neptun AS. The Oslo-based company is responsible for all marine engineering and operations for the Wood-GMC consortium, and will perform a substantial part of the work involved in the project.

Brent Spar is to be sliced into pieces and used as the foundation for a deepwater quay at Mekjarvik, near Stavanger. The huge storage facility, which has suffered damage to some of its tank walls, will be towed to a deepwater inshore disposal site at Vikeneset, not far from the Erfjord where it is currently parked. There, it will be positioned within a cradle which will be used to raise it in the water for the cutting operation.

In the assessment by Det Norske Veritas of the Brent Spar proposals put forward as alternatives to deepsea dumping, Wood-GMC was one of two consortia scoring highest in the confidence assessment of the proposals, and one of only two consortia whose proposals were not judged to display any intolerable level of risk. According to Shell, each proposal had to pass the hurdle of technical feasibility to be considered as a viable solution.

For the job, Neptun will provide a catamaran, marine spread and mooring. Two barges linked by a connecting box will make up the catamaran. The cradle will be formed by suspending a base on wires from hydraulic jacks on either side of the catamaran. Brent Spar will be floated into this cradle and secured.

First, the topsides will be cut off, skidded onto a barge, and brought ashore for scrapping. The cradle will then be jacked up in stages to allow the remainder of the structure to be cut into five slices. These will then be transported to Mekjarvik, four on transportation barges, the last still suspended from the catamaran. Here, they will be placed in a row on the seabed using A-frames, installed at the bow end of the catamaran, two on either side.

Hibernia

With its expertise in marine operations, Neptun has also been involved in most of the float-over GBS deck installations performed to date on the Norwegian shelf, including the Statfjord C and Gullfaks C matings.

Last year, the company was responsible for the lift-off, transportation, and mating of the 38,000-ton Hibernia topsides off Newfoundland. Unlike most bidders for the job, it was able to offer a solution based on the use of existing barges rather than special newbuilds, thus cutting costs.

The job involved a very tricky ballasting operation due to the combined effect of substantial tidal variations and shallow water depth at the topsides assembly pier. Two large barges were converted with the addition of huge combined ballast/tidal deck tanks. The operation was completed almost exactly to the day the schedule had been set three years before.

Repeat principles

Brent Spar is Neptun's first abandonment assignment, but despite its unique nature, not necessarily the last. The same principles used in float-over installations can also be used for the float-off of decks on redundant installations. The company has already performed engineering studies for such deck removals involving North Sea platforms, including Conoco's Kotter and Logger platforms in the Dutch sector.

Neptun's solution for Kotter and Logger was based on the same basic concept of a catamaran arrangement involving two barges and a connection box. This structure would be floated under the deck and tied to the jacket in such a way as to eliminate vertical movements. The topsides weight would be transferred to jack frames, installed on either side of the catamaran, and the topsides would then be cut free of the jacket.

Once the catamaran was supporting the entire weight of the topsides, a quick release system would free it from the jacket and it would be floated out.

After the topsides had been transferred to a barge at an inshore site, the catamaran would return and lift the jacket using jacks on either side. Suspended beneath the catamaran, the jacket would be taken to a deepwater quay for disposal.

One advantage of such a method is its applicability in waters too shallow for heavy-lift barges, which would not be able to operate in the 27-meter and 33-meter depths at Kotter and Logger, respectively.

Neptun's expertise in transporting and installing large structures at sea was also brought into play for the Oresund crossing between Denmark and Sweden last year. In this case, it again used two barges to construct a catamaran. In turn, each of the two 20,000-ton pylon caissons for the high bridge were transported from the building dock to the installation site suspended from strandjacks on either side of the catamaran. Each was then lowered onto three landing pads at the bottom of each caisson pit, and grouted into place.

Marine lifts of structures weighing more than 10,000 tons have traditionally been performed by means of pumping ballast water from the lifting vessel. The active lifting and lowering of the Oresund caissons using wires are believed to be the single heaviest lifts to date. The company is currently involved in the tow-out for installation of the tunnel elements of the crossing, which are ballasted into place.