Simulating interaction between sloped seabeds and preload capacity of jack-ups

Abstract

In the current energy market, where increasing demand for renewable energy is met with larger turbines which require larger installation vessels, understanding operational limits of such vessels becomes invaluable. For the installation of wind turbine generators at great heights, a stable working platform is offered by jack-up (JU) vessels, which use legs to elevate the hull and thus crane of the vessel above the waves. The operation taken into consideration is the preloading process where load is increased on the legs to ensure enough foundation capacity for later operations such as lifting of wind turbine generators. Traditionally, vessels have only been positioned above flat seabeds, but with the increased demand in the wind sector, unfavorable locations with sloped seabeds also need to be considered.
In order to capture global behaviour of a JU on a sloped seabed, two modelling methods are combined to create a framework; soil-structure interaction and structural finite element model. This framework implements a sub structuring approach by cutting and simulating a single leg on a sloped seabed using soil-structure interaction modelling. The results of the first method are foundation stiffness values and reaction loads during preloading of the JU while the second method takes those results as input to calculate internal loads of the JU.
The results show that the proposed framework is able to fulfill the task of combining two models and obtain meaningful results that can inform the JU during preloading phase. Then with this framework, the results show that sloped seabed induces an extra set of moments and lateral loads due to the asymmetric seabed causing an eccentric reaction load. These moments are then redistributed through the JU to other legs and their footings.
These results imply that for a JU on a sloped seabed, a framework combining geotechnical and structural domains can be applied to show the increased load due to the sloped seabed. Additional to the results, a sensitivity study is implemented for a variation of soil type and soil-structure geometry, the latter providing a more realistic seabed slope and larger reaction loads.