The behaviour of the surface layers of marine sediment under the cyclic loads transmitted by an oscillating submarine pipeline, affects the response of the pipeline as the sediment acts as a restraint and helps to dissipate the work performed by the hydrodynamic forces on the pipe. In this field experimental data is of the utmost importance to calibrate parameters to be used in engineering models, due to the intrinsic difficulty to interpret and model theoretically the reaction from the generally loose surface layers of highly non linear constitutive nature.
Soil-Pipe Interaction under Oscillating Loads, is a key topic of free span assessment for submarine pipelines exposed to hydrodynamic loads. The hazard for offshore pipelines may be significant where either irregular seabeds or sediment instability and erosion give rise to critical configurations where pipeline lengths are suspended and exposed to on-bottom currents. The unsupported span may respond to cyclic excitation caused by the regular shedding of vortices or by wave induced loads. Under unfavourable conditions, resonance accompanied by large amplitude vibrations of the pipeline may occur. In this situation the welds may be exposed to fatigue damage. Analysis procedures for free span assessment have reached a good quality level, in particular as regards structural modelling. The boundary conditions are now being taken into account when evaluating the static equilibrium configuration and dynamic characterization of the suspended lengths of pipeline (Ref.1,2,3,4,5). It is important for the fatigue life evaluation of a span to take into account the restoring force of the adjacent pipe section interacting with the soil, which contributes to increase the natural frequencies and consequently to reduce the occurrence of resonant oscillations in the operating life of the pipeline (Ref. 6). Furthermore, the pipe-soil interaction introduces an energy removal mechanism in the free span.