abstract
Offshore seabed sampling tools: Collection of high quality samples of offshore cohesive sediments is essential for accurate and reliable characterization of their mechanical properties. Without such data, any subsequent analytical and numerical analyses of offshore geohazards, such as submarine landslides, can be highly unreliable (i.e., they may misrepresent critical engineering properties). Significant research progress has been made on developing methods for improving sampling equipment for onshore practice. Numerical and experimental research (e.g., Hvorslev 1949; Baligh et al. 1987; Clayton et al. 1998) has highlighted the importance of sampler geometry (e.g., sharp cutting angle, small area ratio, and zero inside clearance ratio) and operation (e.g., fixed piston vs. free piston sampler) in order to obtain high quality "undisturbed" samples of cohesive soils deposits. And yet, many of these important aspects have not found their way into offshore practice. Research needs to be conducted on how to apply the results found for onshore sampling operations to the more challenging offshore environment (e.g., deep water, very soft sediments, vessel-based sampling operation, etc.). The US research team will partner with ICG/NGI to foster their seminal research on onshore and offshore sampling equipment and operations. NGI is currently leading a research effort on the design of a new continuous seabed sampler that incorporates many of the research findings noted above (Lunne and Long 2005). Dr. DeGroot will team with NGI on this effort. Specifically, UMass Amherst will provide the laboratory and human resources (research students) to conduct extensive advanced laboratory tests (triaxial, CRS consolidation, direct simple shear) on samples collected with the new sampler in order to evaluate its performance and, as appropriate, develop design improvements.
Development of portable nondestructive sample quality measurement equipment and test procedures for offshore drilling operations: Evaluation of sample quality is considered essential for cohesive sediments collected for laboratory measurement of engineering properties (Hight and Leroueil 2003, Ladd and DeGroot 2003). NGI pioneered the use of laboratory reconsolidation volumetric strain as an indicator of sample quality (Andresen and Kolstad 1979; Lunne et al. 1997). However, in spite of its great value (it is significant to note that it is rarely used in US practice, which is a technology transfer problem), this method is an a posteriori measure, i.e., one does not know a sample’s quality until a laboratory specimen has been trimmed and set up. Current NSF sponsored research being conducted at UMass Amherst and NU is focused on development of a nondestructive measure of sample quality using shear wave velocity. Field testing at onshore research sites by the UMass Amherst/NU team using bender elements shows that shear wave velocity is a viable parameter for nondestructive evaluation of sample quality immediately after sampling (e.g., Landon et al. 2004). Research needs to be conducted on how to implement such a tool in the more challenging offshore drilling environment. Both NGI and COFS have proven expertise in the use of bender elements (e.g., Dyvik and Madshus 1985; Ismail et al. 2003). Together the US Team, NGI and COFS will work towards developing a robust tool that can nondestructively measure shear wave velocity of offshore sediment samples immediately after sampling, i.e., on the drilling vessel. This will allow for "real-time" decisions to be made on sampling operations while the vessel is offshore and hence offers potentially significant cost savings. It also affords a systematic procedure for screening samples prior to setting up costly and often time consuming advanced laboratory tests for measurement of design parameters.
