Application of Remote Vane Results to Offshore Geotechnical Problems
Authors: Kraft Leland M. , Ahmad Niaz M. , Focht John A. 
Published: October 2008

A remotely controlled vane probe for in situ measurement of undrained strength of clay was first used offshore in 1970. Since that time the McClelland Engineers' Remote Vane, which is in its fourth generation of development, has been used at many Gulf of Mexico sites to penetrations below the seafloor in excess of 400 ft and in water depths of more than 1000 ft. To identify and quantify soil disturbance effects on strengths measured on samples recovered with a 2–1/8-in. diameter wire-line sampler and to consider the relationship of these effects on foundation design, the Remote Vane data accumulated from these Gulf of Mexico sites were reviewed, synthesized, and compared with the laboratory measured strengths of recovered samples. Factors included in this evaluation were: water depth, penetration, vane size, vane rotation rate, degree of soil saturation, measured methane content, soil plasticity and liquidity index, relative soil strength, and geologic history.

Analysis of data demonstrates that Remote Vane measured strength is not a constant multiple of the strength measured from laboratory tests. The ratio of Remote Vane strength to miniature vane strength, which is a useful parameter in assessing the in situ strength profile, ranges from one to more than three. Nevertheless, the results of the analyses demonstrate that when Remote Vane data are used with strength data from laboratory tests, the geologic history of the deposit is more readily and consistently identified and a more accurate assessment of the in situ soil strength profile can be determined.

INTRODUCTION

The shear strength of offshore sediments is an important parameter in foundation design for offshore exploration and production facilities. The undrained strength measured from a recovered sample is affected to some degree by sample disturbance. When a sample is removed from below the seafloor, the overburden pressure, on it is released and the sample tends to expand. The tendency toward volume expansion is resisted by the development of negative pore water pressure.

Disturbances due to the sampling and handling operations may further decrease the effective stresses in the sample (see Fig. 1a). Wireline sampling may cause more sample disturbance than will occur when the sampler is pushed into the soil. Qualitative representations of in situ and sample stress state profiles versus penetration below mudline are shown in Fig. 1. The deformations in the sample caused by these changes in effective stress state, even if the sample volume remains constant, also tend to affect the measured properties of soils.

In highly gas-charged sediments recovered samples "grow" from the sampling tube. Both the dissolved and undissolved gas phases, which are principally methane, expand and cause the samples to "grow" as the stresses are reduced with sample recovery. The amount of growth maybe great as shown by the photograph in Fig. 2, which illustrates the condition of a recovered sample. Sample disturbance due to significant "growth" precludes obtaining meaningful strength and stress-strain information on these recovered samples using "conventional" laboratory testing techniques.