Comparison of Methodologies for Statistical Evaluation of Characteristic Soil Properties for Top Hole Design

Abstract

Evaluation of characteristic values of geotechnical parameters is associated with uncertainties inherent to the geological processes that change soil strata. Statistical analysis of soil data allows one to deal rationally with these uncertainties. The present work addresses some normative recommendations and literature models for statistical characterization of undrained shear strength and submerged unit weight in offshore soils, providing more information to conductor casing design. Three methodologies were selected for the analysis. The NORSOK G-001 (2004) standard recommends the use of mean values computed conservatively. Lacasse et al. (2007) propose that characteristic value should be the mean value minus half a standard deviation of the parameter under analysis. DNV-RP-C207 (2012) suggests different methodologies for dependent and independent soil variables, though both methods of calculating the characteristic value involve linear regressions. Using data from geotechnical investigations that characterize eight oil wells located in two Brazilian offshore basins, the selected methodologies were applied to obtain the characteristic values and compared to each other. The analysis is carried out with data from seventeen CPTu tests associated to the eight wells abovementioned. It is noted that the NORSOK recommendation leads to the highest characteristic values, which are assumed tending to the mean value of the dataset over the well depth. The values obtained using Lacasse et al. (2007) methodology are more conservative and stand as a lower bound of the NORSOK methodology. The models suggested by DNV perform differently when applied to the geotechnical parameters. The dependent variables methodology fits both undrained strength and unit weight accurately. Analysis shows that undrained strength is better described using the methodology for standard deviation proportional to the depth while, for the unit weight, accurate results are obtained by using constant standard deviation. The lower bound procedure proposed by DNV provides, in general, non-conservative results in the first meters and more conservative values along the depth when compared with the other methodologies. Regarding all the formulations addressed, differences between them increase for wells whose CPTu data present higher dispersion. This larger dispersion suggests applications of different statistical-based approaches in order to reliably characterize offshore soil data. The datasets analyzed comprise different levels of scattering and soil heterogeneity, and comparing the statistical recommendations brings additive information for the designer to set the characteristic values of soil properties, aiming for the decision-making process on top hole drilling applications, e. g. conductor casing design. To the authors’ knowledge, few papers perform this comparative analysis.