A Critical Appraisal of the Benefits of and Obstacles to Gaining Quality Data with Offshore Seismic CPT and P-S Logging.

Abstract

Soil stiffness is a key criterion in the design of OWT foundations. The necessity for the development of accurate in-situ small strain moduli profiles has led to an increased interest in using Seismic CPT (SCPT) and P-S Suspension logging. This paper describes both methods, weighs up the relative benefits (and limitations) and identifies the obstacles experienced in the data processing in order to gain reliable results. The accuracy of the techniques is evaluated through the site investigations for wind farm developments and compared against shear moduli derived from advanced laboratory tests on the soil element. The relative performance is made more complicated by the location of the investigation, often including placing additional equipment on the seafloor (in the case of SCPT) and having to work with heave compensated systems. P-S waves were found to be affected by the borehole conditions in terms of borehole diameter, irregularity of its walls (rugosity and/roughness), cavitation issues, fractures and ambient noises (especially at shallow depth), as well as borehole stability. The analysis of the results confirmed that both in-situ testing methods have promising potential in the evaluation of soil stiffness with higher characteristic values being derived than those by laboratory testing, leading to potential foundation cost reduction. There are distinct benefits achievable by performing seismic tests in-situ.