EDGE TECHNOLOGY Laura McShane, Decom Engineering, Northern Ireland, explains how the subsea cutting specialist completed a challenging decommissioning workscope offshore West Africa. he Chinguetti and Banda fields lie in water depths which vary between 280 m and 830 m, about 90 km from the Mauritanian capital, Nouakchott. When the field was considered no longer economically viable, the decision was made to undertake abandonment and decommissioning of the 15 designated wells, consisting of three major phases. Phase 1 and Phase 2A, disconnected the FPSO mooring lines and recovered the risers, allowing for removal of the FPSO from the field, as well as plugging and abandoning the 15 wells. Norwegian subsea and offshore wind contractor, Havfram, was contracted to perform Phase 2B of the decommissioning work, which involved the removal and disposal, or rock dumping, of all remaining infield assets. Havfram commissioned subsea cutting specialist, Decom Engineering, to perform cutting operations on 15 termination heads (TH), six umbilicals, hydraulic flying leads, chains, and fibre ropes. The THs are designed to terminate the flowlines to the subsea manifold. They provide an interface to the ROVCON tie-in connection tool which is a collet connector that provides the mechanical connection to the inboard hub. The production THs are insulated to prevent cool down of production flow during a production shut down. They are insulated in the annulus between carrier pipe and flowline with polibrid. There are three off drill centre (DC) manifolds with TH connections. All DC Manifolds have similar configurations, except for DC-3 which has a pigging loop that would remain connected during manifold retrieval efforts. Cuts on DC-1 and DC-2 manifolds (six cuts each) needed to be as close as possible to the hub. For DC-3 manifold (three cuts), the cuts needed to be close to the flange side (i.e. at the connection to the flexible pipe to the termination head and away from the manifold and inboard hub) so that the weight of the TH protrusion would help
balance the weight of the pigging loop module, which remained connected to the manifold during retrieval. Decom Engineering were contracted by Havfram to supply equipment for both the subsea cutting of the previously mentioned products and the on-deck cutting of umbilicals. The C1-24 chop saw provided by Decom had a number of modifications custom-designed for the specific cuts to be made during the project. To achieve the cuts desired on the three manifolds, the clamping method was modified to allow the saw to be clamped onto the manifold securely during the cut, and remain securely clamped to the manifold once the cut had been executed. It was essential that once the cut was performed the saw would remain clamped in position to prevent possible damage to the ROV or the tool.
ROVCON cutting As previously mentioned, there were three manifolds to recover, the first of which was DC-3, requiring three off ROVCON cuts. A 2 t uplift buoyancy module was rigged to the saw, which would allow the saw to be clamped onto the ROVCON and disconnected from the crane. The average cut time for a ROVCON was 35 mins. Once the cut was complete the blade was fully retracted. The upper clamp could then be opened – releasing the cut ROVCON whilst remaining securely clamped to the manifold. When the clamp was sufficiently open, the ROVCON dropped from the clamp arm onto the seabed. The slack was now taken up on the crane, the lower clamp was opened, and the saw released itself from the ROVCON. Once the first ROVCON with a 10 in. internal flowline had been cut, the saw was repositioned on a centre ROVCON with a 6 in. internal flowline and the process repeated. If this was the second cut, the saw was unstabbed and recovered to deck
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