Table 7.25.

Offshore drilling vessel with in situ testing, sampling and coring

Offshore site investigationMethodApplicationsLimitations
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  • Rotary open-hole drilling or coring systems from a dynamically positioned, heave compensated vessel or mooring system;

  • Deployment of sampling, coring and in situ test equipment through the drill-string to the seafloor/drill depth via the seabed frame;

  • Deployment of ‘over-the-side’ seabed surface testing equipment, including seabed CPTs, drop corers and vibrocorers.

  • Offshore sites from 20 m to >2000 m water depth;

  • High-quality sample recovery and preparation for testing in onshore laboratories;

  • Allows collection of data of/and below glacial features that may cause obstructions (gravel, cobbles, boulders) to seabed surface testing equipment;

  • Allows downhole geophysical logging of boreholes;

  • 24 h operations.

  • Difficult to recover samples and seabed CPT data in certain glacial deposits containing cobbles or boulders, resulting in damaged equipment, requirements for coring and bumpover boreholes;

  • Nominal 10–100 m per day rate of penetration and can be significantly less in difficult glacial and periglacial deposits;

  • Weather dependent (vessel specific).

Cost factor: high
Relevance to glacial/periglacial depositsThis is the only option for intrusive data collection offshore in water depths greater than those accessible by JUBs. Many types of vessels or mooring systems can be used for an offshore site investigation and all will have specific advantages and disadvantages.