Cutting Underwater Technologies (CUT), a specialist provider of diamond wire cutting services to the decommissioning and abandonment industry, has reported the completion of a major project for Heerema BV.

The contract award to assist with removal of the Norpipe 37/4A Booster Platform’s eight-legged steel jacket was made to CUT Norway AS and was managed through CUT’s Norwegian Office whilst technical, operational, equipment and manpower support was provided by CUT Headquarters staff based in Aberdeen. CUT developed new equipment and methodologies in order to successfully complete the work which is part of the large-scale Ekofisk cessation project situated 180 miles south-west of Norway.

The 5,500 tonne 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 the Heerema owned and operated Heavy Lift Vessel, “SSCV Thialf”.

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 26metres 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’s eight legged jacket, which stood in some 85metres sea water depth 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 2.7metres below the surrounding seabed. CUT’s specialist 64inch 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 HPU (Hydraulic Power Unit) Control Panel, Multi-Core Umbilical and associated Hose Reel), lowered into position using the vessels crane, and located at the target cut site with the aid of an WROV (Workclass Remote Operated Vehicle).

The eight cuts were carried out successfully on the Primary (48” OD x 1.75” w/t) and Insert (36” OD x 1.5” 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 “SSCV 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 62metres 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 amongst 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 90º angle in relation to the target. However, as the jacket legs stood at some 7 – 8 degrees 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.

Tags: Cutting Underwater Technologies