This paper reflects on the operational experience gained with the new Coflexip Stena Offshore tranching plough and backfill plough during its first six months or so of operation. The equipment has now carried out a very active season of work in the North Sea with three major EPIC type construction project completed successfully.
The experience is also reviewed and discussed in light of the predictions made for the equipment. In general, the experience is very favourable and it is considered that it shows modern techniques can be capable of reducing many of the uncertainties which have been associated with offshore pipeline trenching in the past.
In this respect, the successful operational use of the ploughs' several unique features has been of particular interest. These include the functions which allow multi-passing of single trench, and the air buoyancy system which reduces submerged weight. The successful use of a single Trenching Support Vessel for both control and towing of the ploughs has also been noteworthy.
The new trenching system consists of the Multipass Plough (MPP) and Backfill Plough (BFP) and was supplied to Coflexip Stena Offshore Ltd in March 1995. It went almost directly into operational service on a series of three major EPIC pipeline construction projects in the North Sea. To date, the work of the plough spread has consisted of trenching support to the pipe lay activities of the Coflexip Stena Offshore Apache reel ship The MPP is illustrated on Figure la
The trenching system was supplied to Coflexip Stena Offshore by Soil Machine Dynamics Limited who already have a long track record in the development and manufacture of subsea trenching equipment, including many of the most successful recent pipeline ploughs. The plough was delivered with a standard 18m deep, 35 degree share and, whilst retaining similarities with many of these earlier ploughs, the specifications for the MPP and BFP include a number of new features and improvements.
Most significant among these improvements is the low ratio of structural weight to strength (97 tonne in-air weight versus 250 tonne bollard pull capability) which results from optimised structural design. The low weight also helps to restrict ground bearing pressures so that the plough can remain stable on soils with shear strengths as low as 5kPa. The ploughs are both further equipped with an air buoyancy system capable of reducing submerged weight to give a 3kPa shear strength capability when required.
The MPP is also fully equipped to carry out excavation of a single trench in multiple passes. In this respect it is unique in current North Sea operations. The technique (know as multi-passing) allows a deeper trench to be excavated in strong or more difficult soils than would otherwise be possible in a traditional single pass. In many circumstances this can actually result in a more rapid completion of the trenching programme. The contingency facility to carry out rectification trenching also exists in cases where the first pass trench may be out of tolerance.
