Subsea facilities located in earthquake-prone regions of the world can be subjected to severe excitation and have to be designed in a two-tier manner against both ELE and ALE level earthquakes. The return period of these earthquake levels can be 100–300 years and greater than 1000 years, respectively. These facilities may typically consist of rigid pipelines, sliding PLET structures on skirted or hybrid mudmat foundations, connected via rigid spools or jumpers to manifolds on suction pile foundations, with flexible flowlines hanging off goosenecks.

In recent practice Subsea 7 have developed a novel methodology for modelling and simulating the seismic response of such complex clusters in which the so called “system effects” are intractable and dominate the response of certain critical components. Chief amongst the latter are rigid spools and jumpers that span large distances between PLEM, PLET and X-Tree structures without touching the seabed.

In the nonlinear implicit direct integration dynamic FE analyses in Abaqus each of the cluster models in a project is subjected to time history accelerations of seven representative earthquakes, as per ISO 19901-2 requirements. Hysteretic damping characteristics of soil-structure interfaces are modelled with special kinematically hardening elements, calibrated to the site-specific seabed geotechnical properties. Specially developed post processing scripts are used to automatically extract the vital information from the large amount of data produced and perform the unity checks of various components to their respective codes of practice.

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