NGI has over 10 years of experience with large-scale model testing that investigate the complex interaction between seabed soils and risers or pipelines. Such model tests are of particular importance for projects in deepwater where seabed soils are commonly very soft clays, for which the characterisation of such soils using standard in situ and laboratory testing is difficult.

Overview of test rig placed on large test bin (5.0 m x 1.75 m).

The prototypes for these model tests are typically at or close to the actual dimensions of infrastructure offshore and often include specific coatings for the pipeline or riser section being investigated. The models can be loaded or displaced statically or cyclically in the vertical, axial and/or lateral direction with respect to the model axis.

Axial pipe model (200mm  diameter)

The principal test apparatus is a hydraulically controlled MTS rig incorporating two hydraulic actuators. NGI has other rig configurations and can fabricate alternative loading systems, as required. Throughout the test the load-displacement behaviour of the model is recorded and pore water pressures are monitored around the perimeter of the pipe. Video recording of the test and typographic surveying of the clay surface can also be performed.

Typically, large volume clay samples are obtained offshore using a box-corer. The samples are homogenised by mixing at an increased water content. The resulting clay slurry is then consolidated over a period of time evaluated from oedometer testing, followed by a suitable swelling period during which the stresses are released. Consolidation and subsequent swelling typically takes around 3 months.

Lateral pipe model (500mm diameter)

NGI is involved in various types of model test for both industry and research projects, including:

• Initial penetration of pipelines into seabed
• Axial load-displacement response of pipelines
• Lateral load-displacement response of pipelines
• Cyclic load-displacement response of risers
• Pullout testing of pipelines and risers

 
 
 
Further information is given in the following publications :

Langford, T.E., R. Dyvik. & R. Cleave (2007)
Offshore pipeline and riser geotechnical model testing: practice and interpretation.
Proc. Conf. on Offshore Mechanics and Arctic Engineering (OMAE), San Diego, California, USA.

Langford, T.E. & C.P. Aubeny (2008)
Model tests for steel catenary riser in marine clay
OTC19495, Offshore Technology Conference, 5-8 May 2008, Houston, Texas, USA.

Langford, T.E. and V.M. Meyer (2010)
Vertical cyclic testing of model steel catenary riser at large scale
Proc. 2nd International Symposium on Frontiers in Offshore Geotechnics (ISFOG), 8-10 November 2010, University of Western Australia, Perth, Western Australia.