A New Trenching System for Deep Water Pipelines

Abstract The deep water trenching system has been developed through a period of 4 years. The system represents a new approach to trenching gas- and oil pipelines for water depths from 10 to 500 meters. Advanced techniques have been used to make the system feasible. The trencher system consists of a remote controlled trencher machine, operated through an umbilical cable from a mothership. The operations are diverless. The trencher has two basic modes of operation:Navigation modeTrenching mode During navigation the trencher is "flown" down towards the seafloor, powered by eight propellers. The pipe is located by the trencher sensor package. In the trenching mode the trencher is clamped to the pipe. The weightless trencher tracks the pipe using eight self-powered wheels. A highly by efficient trench-size suction-cutter combined with a dredge pump forms the trench. This mass handling unit works in a wide range of bottom materials from sand to soft and hard clay. The trencher system is designed for a maximum trenching speed of 500 meter/hour and a maximum power consumption of 1200 kW. The project concluded with a deep sea performance test. This test successfully proved the capability of the complete system. A test pipe of diameter one meter, lying at 320 meter water depth, was trenched at a speed up to 500 m/h in soft clay material. The sonar profiling equipment on the trencher was used to inspect the pipe in the trench. The inspection verified a pipe location one meter below sea floor. The test was not disturbed by any serious technical difficulty. A few problems of a debugging character were easily solved. An A-frame will be built to facilitate trencher handling in the final operational system. The experience until now indicated an encouraging reliability of the complete system. Only rough trenching-operation cost estimates can be done at the present time. As an example for 300 km of pipeline operation per year, the captial and running costs amount to $ 13 mill. Introduction Pipelines on the seafloor are exposed to loads from the sea and from the activity of man. On the other hand, pipelines can interfere with the fisherman's activity if the lines are lying over the sea bottom, It has been commonly accepted that trenching of pipelines will substantially reduce hazards to the pipe and hazards caused by the pipe, Hence the trenching of a pipeline can be required by the authorities. Highly specialized equipment is required to perform the deep water trenching operation. The development of the trenching system for 10 - 500 m water depth started in 1974, with the initiative and support of a Norwegian Governmental committee, the "Deep Water Pipeline Project Committee". The objective of this committee was to provide Norway with competence in deepwater pipeline technology. While the initial development was of a conceptual nature, a comprehensive pre-engineering and model test phase was conducted in 1975-76 with support from the Statoil-Mobil Group, the Petronord group and Norwegian research funds.