A Seafloor Burial Assessment Survey System (C-BASS) has been designed and constructed by Williamson & Associates, Inc, to complement conventional geophysical surveying and sampling technique by providing data on the geotechnical characteristics of the top few meters of seafloor sediments. When used in burial feasibility assessment for pipelines and cables, the additional information provided by C-BASS can improve the reliability, optimization and cost effectiveness of the pipeline design and installation.
C-BASS is special purpose remotely operated vehicle (ROV) that is towed on the seafloor along an intended pipeline or cable route in water depths as great as 2000 metres. System sensors include a hydraulically driven mini-cone penetrometer (CPT) for in-situ soil strength measurement, towed direct current electrical resistivity (TDC) for measuring soil density, a high resolution acoustic subbottom profiler for determining sub-surface layering and a video camera for qualitative analysis of seafloor geology. A wide-band hydrophone is incorporated to characterize drag noise as the sled moves across the seabed, a strain gauge to monitor tow tension, a conductivity/temperature cell to make seawater resistivity corrections, and other sensors to monitor sled attitude (pitch, roll, heading, altitude and depth) and condition of the various sub-systems.
This paper presents theory and design details of the C-BASS and field methodology developed from more than 2000 line-km of continuous data acquisition and 500 CPT tests successfully completed by the system during the past 18 months.
Seafloor pipeline and cable installation projects normally include a pre-installation geophysical survey that produces bathymetry, subbottom profiles and sides can sonar imagery along the route. In addition to the geophysical data, seabed core or grab samples are often obtained and analyzed. This information is used to generate a variety of plots and maps to assist in route selection, design of the pipelines and cables in areas where fishing activity, vessel anchorage, ice gouging or other hazards may exist It is cost effective to be able to reliably assess the burial feasibility along a given route early in the engineering design process.
Geophysical techniques used in route investigation surveys lack the resolution necessary to provide detailed information about the upper few meters of the seabed, and the acoustic principles upon which they are based cannot be directly correlated to the engineering properties of the sediments. Core sampling and cone penetometer test can provide soil strength data for discrete locations, but the continuous data on soil parameters evolved from the need to make route engineering decisions during the route selection survey, if conditions necessary to adequately and economically protect a pipeline or cable are not present along a pre-planned route, development work should be initiated to find an acceptable route-but to be practical, this determination needs to be incorporated in the field during the actual survey operation.
In 1996 Williamson & Associates, Inc. presented a concept for an acquisition system that would provide information on the mechanical properties of marine sediments continuously along a survey route ? the Seafloor Burial Assessment Survey System (C-BASS). Cable & Wireless Marine Ltd.
