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The geotechnical characterisation of offshore wind farm sites requires measurement or estimation of the small-strain shear stiffness G max of the subsoil. This parameter can be derived from shear wave velocity V s measurements if the bulk density of the soil is known. Since direct measurements of V s are generally not available at all foundation locations in a wind farm, correlations with cone penetration test (CPT) results are often used to determine location-specific stiffness parameters for foundation design. Existing correlations have mostly been calibrated to onshore datasets which may not contain the same soil types and stress conditions found in the North Sea. The distinct geological history of the North Sea necessitates a critical review of these existing CPT-based correlations. They are evaluated against an extensive database of in situ V s measurements in the southern North Sea. The importance of modelling the stress-dependent nature of V s is highlighted, and a novel stress-dependent model for V s from CPT data, which leads to an improved fit, is presented. As the small-strain stiffness is used as an input to foundation response calculations, the model uncertainty of the correlation can introduce significant uncertainty into the resulting foundation response. This transformation uncertainty is quantified for each of the correlations evaluated in this study and shows important variations. [ABSTRACT FROM AUTHOR]

Trenching and backfilling is one of the most practical and cost-effective methods for protection and stabilization of offshore pipelines. Defining the geotechnical properties of backfill material resulting from mechanical backfilling or jet trenching is an area of substantial uncertainty and the resistance against pipeline uplift provided by these backfills needs to be characterized accounting for these uncertainties. This paper compares the properties of sandy backfill material and the available calculation models for uplift resistance against a database of more than 300 controlled pipeline uplift tests. The model uncertainty for uplift resistance calculations is derived from a back-analysis of the uplift tests. The uncertainties on uplift resistance and mobilization distance are correlated and characterized using a joint probability distribution. The selected distributions are applied to an example uplift resistance problem. When compared against this probabilistic formulation, the factors applied to uplift resistance in pipeline analysis can be refined to lead to a more cost-effective solution. Key words: pipelines, uplift resistance, trenching and backfilling. L'excavation et le remblayage est l'une des methodes les plus pratiques et rentables pour la protection et la stabilisation des pipelines en mer. Definir les proprietes geotechniques des materiaux de remblai resultant d'un remblayage mecanique ou d'excavation par jet est une zone d'incertitude importante et la resistance contre le soulevement de pipeline fourni par ces remblais doit etre caracterisee compte tenu de ces incertitudes. Cet article compare les proprietes du materiau de remblai de sable et les modeles de calcul disponibles pour la resistance au soulevement contre une base de donnees de plus de 300 essais de soulevement de pipelines controles. L'incertitude de modele pour les calculs de resistance au soulevement est derivee d'une analyse regressive des essais de soulevement. L'incertitude sur le soulevement de resistance et la distance de mobilisation sont correlees et caracterisees en utilisant une distribution de probabilite conjointe. Les distributions choisies sont appliquees a un probleme par exemple de resistance au soulevement. Comparativement a cette formulation probabiliste, les facteurs appliques pour elever la resistance a l'analyse du pipeline peuvent etre affines pour aboutir a une solution plus rentable. [Traduit par la Redaction] Mots-cles: pipelines, soulevement de resistance, excavation et remblayage., Introduction Offshore pipeline uplift Offshore oil and gas pipelines can be protected against natural and anthropogenic hazards through trenching and backfilling (Jordan and Cathie 2004). Contrary to other trenched and [...]

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