Decommissioning and abandonment specialist Cutting Underwater Technologies (Cut) assisted Heerema's Thialf with the removal of the Norpipe 37/4A booster platform's eightlegged steel jacket as part of Norway's Ekofisk cessation project. The 5500t platform stood in 85m of water.
The contract award to assist with removal of the jacket was made to Cut Norway and was managed through Cut's Norwegian office whilst technical, operational, equipment and manpower support was provided by Cut headquarters staff based in Aberdeen. The company developed new equipment and methodologies in order to successfully complete the work.
The 37/4A platform, along with its sister Norpipe 36/22A, was installed in 1974, becoming operational the following year. The platforms acted as booster stations between the main Ekofisk platform and the onshore reception facilities at Teesside in the north of England. Both platforms remained active until their final abandonment in 1983.
Two phases, one offshore and one carried out in the harbour at Merjarvik in Norway, were completed during Cut's mobilisation. All the offshore and inshore work was carried out from Heerema's Thialf SSCV.
The topside modules had already been removed during an earlier phase of the workscope, prior to Cut completing eight severance cuts using its patented diamond wire cutting technology on the jacket legs and piles. These cuts allowed the jacket to be removed from the seabed in one lift before being transported inshore.
Offshore, the first two cuts were carried out at 26m below the sea surface on the main risers' attached to legs A1 and B1 prior to both sections being recovered to deck. Thereafter a further single cut was made to both risers to reduce their size and weight.
The 37/4A jacket was then prepared for removal. Trenching operations were carried out in the vicinity of the jacket to expose the piles to a distance of some 2.7m below the surrounding seabed. Cut's specialist 64in DWCM (diamond wire cutting machine) was then attached to the exposed jacket piles and the cuts performed. The DWCM's were to be powered from the surface (utilising a Cut- supplied hydraulic power unit, control panel, multi-core umbilical and associated hose reel), lowered into position using the vessel's crane, and located at the target cut site with the aid of an workclass ROV.
The eight cuts were carried out successfully on the primary (48in OD x 1.75in w/t) and insert (36in OD x 1.5in w/t) piles, many being grout filled, at an average time of around five and a half hours with the last two cuts being carried out simultaneously.
Once severance was completed, the jacket was lifted by Thialf and secured to the vessel with a specially-developed clamping system. It was then transported to shore where it was placed on the Heerema H627 barge being supported, on the barge, by its four inner legs. The project plan now called for the jacket to be reduced in size by removing the top third of the structure prior to removing the bottom section ashore for final reduction and recycling.
The reduction cuts were all carried out at a height of 62m from the barge deck and Cut had to develop a number of project specific modifications to complete the cuts as required by the client. Chief among these was the requirement to perform horizontal cuts such that the removed upper section would ‘sit' square on the ground. As the Cut DWCM's are self clamping the natural tendency is for the machine, once the clamp is activated, to bring itself into a 90° angle in relation to the target. However, as the jacket legs stood at some 7-8° from the vertical any cuts using a standard configuration would create pressure point problems when the upper section was removed to land.
Thus, to overcome this effect, special saddles were designed and built that corrected the angle of approach such that when the clamps were activated the DWCM sat at the correct orientation. Additional modifications were also made to allow the cutting of the two launch runner assemblies located on legs A2 and A3.
Furthermore, as the cuts were to take place ‘in air' a special 12-nozzle cooling system was designed and fitted to the DWCM to ensure adequate cooling was being supplied to the diamond wire.
Finally, a one-off, umbilical lifting saddle was designed and manufactured which offered support to the umbilical as it was being lifted into position.
All the cuts performed during this section involved severing three layers of steel:
Average cutting time for the two launch runners was eight and a half hours, while completion of the remaining jacket legs averaged some six hours, forty minutes.
Mark Chalmers, Cut senior project engineer, observes: ‘We are delighted to have completed this, the first of the eight-legged platform removals on time, within budget and, most importantly, without incident. This was accomplished in part because of the excellent working relationships developed with Heerema, especially in the pre-planning phase prior to mobilisation. I am convinced that our experiences on this workscope will allow us to develop savings over the remainder of the operations.'
The Cut Group is the sole global operator of the patented diamond wire cutting system which was developed by parent company TS Tecnospamec of Genoa, Italy. OE