An extensive network of undersea pipelines and cables exists. For the majority of these some form of trench is required for protection from fishing activities, stability and/or thermal insulation. The prediction of the performance of the tools used to form the trench is based on geotechnical data obtained as part of the seabed investigation, however, the geotechnical parameters determined may not be of direct relevance to the failure modes imposed on the soil by the trenching tool. This paper describes the typical data provided and discusses its applications to prediction of trenching tool performance with particular emphasis on submarine ploughs. Modifications and improvements to existing investigation techniques are also discussed.
Trenching and burial of offshore pipeline and cables is often required for reasons ranging from protection from fishing gear and anchors to providing thermal insulation and upheaval buckling resistance. To achieve the required trench, a wide variety of submarine trenching tool are available. A number of factors are important in the assessment and selection of the correct trenching equipment for the proposed work including a good understanding of the geotechnical properties of the soils and rocks along the route.
The cable industry utilises cable which can run for considerable distances on the seabed and has previously adopted an approach based on investigating the seabed soils in a qualitative manner by use of a scaled down cable plough. This is in contrast to the oil industry which uses investigation techniques having their origin in onshore geotechnics. Typical techniques include cone penetration testing (CPT) and sampling by means of a driven tube. Such technique can provide a detailed vertical stratigraphy, but over a limited area.
This paper summarises geotechnical investigation techniques commonly used for subsea pipeline and cable routes. Some of the problems encountered in interpretation of data are discussed, with suggestions made for some improvements in investigation techniques, based largely on existing equipment.
Emphasis in this paper is placed on submarine ploughs, which are one of the most widely used trenching techniques. They have the particular advantage of being essentially a passive process producing a well defined, stable trench. A minimal number of moving parts are required maximizing reliability. The plough is normally pulled by a ship or barge mounted winch. For pipelines, a V shaped trench may be cut before (pre lay) or after (post lay) placing the pipeline. Cable ploughs cut a narrow slot through the seabed and the cable is placed within the plough share, which provides temporary support to the sides. Ploughs work most effectively in sand and clays giving good performance rates with minimal maintenance time. Their capability in rocks is highly dependant on fracture spacing and strength. While there is little experience of ploughing in rock, on most projects on which rock has been encountered, the plough has generally exceeded the expectations.
Other widely used techniques include jet tools, mounted on a sled, seabed tractor or a free swimming ROV. Jetting was the earliest technology used for forming submarine trenches.