Utilising the company's own versatile and modular Skate IV jack-up platform, Seacore deployed its hydraulically operated stem mounted trenching grab onto a cantilevered finger pontoon that travels the length of one side of the jack-up.

With this arrangement, Seacore is able to accurately excavate a 2m deep, 1.5m wide trench in the seabed in up to 22m of water, well within the maximum 14m water depth at Girvan. The spoil is precisely side cast in a windrow parallel and up to 10m away from the excavation, ready for reuse as backfill. Once a 24m long section of trench has been excavated, the platform is jacked down, moved and repositioned for trenching to continue on the next section of the pipeline. Concurrent with the repositioning of the platform, the polyethylene outfall pipe, complete with concrete ballast collars, is slowly fed off the stinger frame on the end of the platform, and guided into the previously excavated trench. A bottom dump barge immediately follows on placing a covering of beach gravel prior to the attendant Multicat support vessel using a seabed profiler to place the excavated spoil back into the trench to complete the pipe protection.

This sequence is repeated to complete the construction of the outfall section of pipe prior to the installation of the final 15m long diffuser pipe work assembly, surrounding gabion scour protection mats and diffuser protection modules. The diffuser components are positioned using a combination of Skate IV 'sonboard pedestal crane and diver assistance.

Working round the clock in two 12 hour shifts, Seacore completed the 1100m marine outfall works in 11 weeks, which the company says is very good progress in such exposed conditions and with unpredictable weather.

Within the works, Seacore had to treat 290 linear metres of trench route with drill and blast prior to excavation, which was performed from the jack-up without diver intervention. 'One of the major advantages of our fully integrated trenching system is that we can do all the drilling, charging the holes, blasting and excavation directly from the jack-up without putting divers in the water, said Seacore project manager Les Lugg. 'We had our explosives consultant, ABCO Divers from Belfast, to design a drill pattern and charge to produce a steep sided trench and give us the fragmentation we require, so the excavated spoil would not be moved by the tidal currents and stay in the windrow after excavation ready for backfilling.'

MJ Information No: 19416