This chapter discusses the collection of higher quality soils data for pipeline projects and new equipment that has been developed for that purpose. Having been obtained, the data must be interpreted and presented in a manner that maximizes its value. Suggestions are made on how this can be achieved. Examples of the current trend towards "high technology" soils investigations for pipelines are also given.
High technology soil investigation methods can now be used on marine pipeline route surveys as a matter of routine, in the way that they are used for the investigation of production platform sites.
The Cone Penetration Test (CPT), for many years the favoured method for in-situ measurement of soil parameters in offshore soils investigations, can now be performed without the need for a drilling ship or special deployment equipment.
This chapter describes some of the new equipment developed for this purpose and how, when combined with improved sampling equipment, it can lead to a much higher quality of geotechnical input to the design and construction of a pipeline.
To be of most value, the data needs to be correctly interpreted and presented in a form that can be easily assimilated by the pipeline engineer. Some guidelines on these aspects are given and some typical projects are briefly described.
In the past the tendency with soil sampling on pipeline projects was to add it, as an after-thought, to the specification for the ‘geophysical’ route survey: so if the survey vessel being used could not handle a vibrocorer, the only soil sample recovered was by gravity corer or grab sampler. The end result was that the evaluation of soil conditions for all aspects of the pipeline design, installation and operation was entirely based on the interpretation of the shallow seismic profiling and handfuls of soil at infrequent intervals along the route. Little wonder that claims for additional trenching time due to unexpected conditions are common, that sour still surprises people and that pipeline supports sink into the sea bed.
Faced with this problem, the oil industry rightly claimed that even if it did upgrade its specifications for soil investigations, there were few options available for obtaining improved data without reverting to expensive geotechnical drilling vessels. There has therefore been a concerted effort to fill that need by some companies in the geotechnical field, and a number of independent seabed testing units have now been developed for in-situ soil testing and sampling, including Fugro's "Seabed Wison" and "Seasprite" systems.
The "Seabed Wison" is marriage of two well proven tools, the "WISON" downhole cone penetrometer test (CPT) system, that has been used for many years in soil investigation borings for offshore structures such as production platforms, and the tried and tested vibrocorer. Developed in conjunction with the vibrocoring specialists, Alluvial Mining Limited, the unit is basically a standard rectangular based "4 metre" vibrocore frame with the penetrometer unit occupying the space normally taken up by the vibrator and core barrel.