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.