Offshore Wind Farm (OWF) Jack-Up Foundation Behavior in Crane Operational Conditions
Authors: Zhang Xi-Ying , Yu Qing , Kramers Job , Hofstede Hugo 
Published: April 2023

When an offshore wind farm (OWF) jack-up installation vessel performs a heavy lifting operation using its crane, the foundation loads may approach or, in rare circumstances, exceed the capacity generated from the preloading. Safety margins and acceptance criteria, which do not exist in the current industry standards, are under development through a reliability study funded by the Society of Naval Architects and Marine Engineers (SNAME) J-REG Joint Industry Project (JIP). This paper presents the first step studies on the spudcan foundation behavior associated with crane operations.

Coupled Eulerian-Lagrangian method was applied to assess the large deformation foundation performance. A new generation OWF jack-up with a high preload and crane lifting load was used in the current studies. The spudcan foundation was subjected to the loads associated with a full jack-up operation cycle. This includes penetrating into the seabed under a prescribed preload, unloading to the design working load, and operating under the heavy crane lifting force and the environment loads due to wind, wave and current. The resulting load-displacement curves define the soil-structure interaction behavior of the jack-up foundation under crane operations.

The foundation behavior was determined for the low risk (e.g., sand), medium risk (e.g., clay) and high risk soil profiles (e.g., layered soils with associated punch through risk). A 3-Dimensional numerical model was calibrated with the centrifuge test in terms of the soil flow mechanism, cavity depth, and bearing capacity at different soil depths. The results show that the spudcan reaction force increases rapidly with the increase in penetration depth during the crane operational stage, indicating the outcomes when the load would exceed the original preload. The soil bearing capacity failure and further spudcan penetration would be a concern based on the magnitude of the crane load. It is found that the spudcan follows the same trend in the penetration versus reaction force curve as the initial preload.

In the clay soil, sliding and rotation of the spudcan foundation start to build up during the operational stage. This is caused by the horizontal and moment loading on the spudcan due to the environmental loads and the heavy crane force. In sandy soil, spudcan sliding and rotation are negligible because of the large soil capacity even though the spudcan penetrates at a relatively shallow depth. When the spudcan penetrates stiff clay overlying soft clay, its reaction force merges into the original preload curve during the reloading stage. This means that the unloading and operational stages do not affect the spudcan vertical performance. The spudcan fails with rapid penetration and large rotation, indicating a typical punch through failure.

It is necessary to assess the OWF jack-up foundation stability when the crane load is close to or in the worst case exceeds the operational limit. The stability check may include, but is not limited to bearing capacity, sliding and further penetration. In the next step, the results will interact with the structural model to determine the load and resistance factors through reliability analysis. This will provide the technical basis for the upcoming SNAME Bulletin on OWF jack-up considering crane operational conditions.