Thomas Langford

Offshore Pipeline and Riser Geotechnical Model Testing: Practice and Interpretation

Offshore developments may typically feature a number of subsea structures for which pipelines and risers play an integral role. Especially for deepwater projects, very soft clays will be encountered at the seabed; these can be difficult to characterize by standard in-situ and laboratory testing. This means that geotechnical model tests are increasingly used to investigate the complex interaction between the seabed soils and the risers or pipelines. The prototypes for these model tests are typically at or close to the actual dimensions used by industry and often include specific coatings for the pipeline or riser section being investigated. The models may be loaded or displaced statically or cyclically in different directions to evaluate the different mechanisms involved. Although it is a challenge to recreate the undrained shear strength found at the seabed, experience and theoretical knowledge may be combined to give good agreement between the shear strength level in the test tank and on site. This may then be verified by in situ testing in the test tank and compared with high quality data from the field itself. The model test results themselves serve as input to pipeline or riser design which can incorporate geotechnical, structural and hydrodynamic effects. This paper describes the typical procedures involved in model testing and investigates the interpretation of the data using theoretical and empirical methods. The impact of results on pipeline and riser design within a project is also considered, where the focus is on very soft clays which are often encountered in offshore projects, especially in deepwater.© 2007 ASME

Pile-Soil Interaction Under Cyclic Loadings for Offshore Wind Monopiles

Offshore wind structures are subjected to cyclic loadings from wind, wave and current which must be accounted for when designing foundations. The effects of cyclic loadings can govern the dimension of the foundations. However, less standard and calculation procedure for designing foundations under cyclic loadings is generally accepted in practical design.A parametric study at the generic clay profiles were carried out by using the conventional beam column approach and the finite element analysis. In addition, a monopile foundation at the layered soil profiles in Korean West Sea were designed taking into cyclic loading effect account.Both design results show that the bending moments and shear forces along the pile by using beam column approach are larger than the results from the FE analysis. The FE analysis gives less pile displacement and rotation than the beam column approach. The reason for these differences can be partially explained by the soil reactions in the upper soil profiles. For the clay layers the lateral soil resistance is generally higher in the FE analyses compared to the cyclic p-y curves. However the lateral resistance in the silt sand layers is greater by using the standard cyclic p-y curves with a drained friction angle than using the constitutive models applying cyclic properties.Based on the results, a practical approach of monopile design for the specific offshore location can be recommended by calibrating and simplifying cyclic p-y curves from the results of FE analyses taking into account the cyclic stress-stain response of the soil.Copyright