In response to the demand of the telecommunication cable market, new tools have been recently developed and introduced for burial assessment purposes, which are now available to the pipeline industry. The CPT has become the reference tool for obtaining geotechnical data required for burial engineenng. It is used m combination with electronic profiling techniques capable to provide an accurate stratigraphy of the subsoil over the target burial depth (a few metres). E-bas techiques are based on the seismic refraction method which characterises the sediments by their compressive seismic velocity Vp or on the measurement of the electrical resistivity. E-bas techniques are implemented via bottom-towed system
The paper reviews the equipment and techniques in use and outlines their technical capabilities and limitations.
In parallel to these developments, major advances have been made in data processing and integration procedures. The paper emphasises:
the last improvements in processing techniques used to produce the velocity or resistivity fields,
the integration process of the different sources of data: bathymetry, side scan sonar, seismic reflection, seismic refraction/resistivity, geotechnical data (CPT and sampling/coring).
The main engineering phase of the burial assessment consists of determining the maximum burial depth attainable by ploughing and the associated towing forces. Cable and pipeline installation data have been back-analysed and correlations have been established between these quantities and CPT data.
Route selection and burial assessment of a pipeline or cable require specific data which include:
water depth and seabed topography,
anomalies on and in the seabed (e.g. geohazards, existing pipelines or cables, other obstructions),
differentiation of strata with significantly different geotechnical properties affecting burial operations,
geotechnical parameters of importance for burial assessment.
Standard offshore surveys for pipeline or cable route selection are aimed at providing the first two sets of data. They consist of bathymetric, side scan sonar and seismic reflection surveys covering a wide corridor centred on the theoretical route Shallow seabed sampling (e.g. grab sampling, gravity coring or vibrocoring) is carried out from the geophysical vessel at more or less arbitrarily chosen locations for ground-truthing geophysical data.
The information provided by these standard route selection surveys is insufficient and inadequate to assess the burial feasibility of the pipeline or cable. Burial assessment should address a number of aspects, such as:
can the pipeline/cable be buried and If so, to what depth?
what is the most appropriate burial method (e.g. ploughing, jetting, trenching, others)?
what is the anticipated performance of the recommended burial method (e.g advancement speed)?
m case of ploughing, which is the most commonly used method, what are the expected pulling forces? How are these forces affected by the ploughing speed?
is the stability of the trench safe (in case of an open trench solution)?
is the burial tool stable on the seabed?
what is the anticipated wear of the burial equipment?