All jackups, such as those used as Wind Turbine Installation Vessels (“WTIVs”) must undergo a process known as preloading before elevating the hull to the operating airgap. This is done to ensure that the seabed on which they are installed is capable of supporting the unit during elevated operations. Ideally, the soil at the sites of WTIV operations would be uniform with ever-increasing capacity with leg penetration, but that is not the case all the time. Instead, some sites have soil profiles with weaker sub-layers below stronger upper layers, with a regression in the capacity of the sub-layers of soil. At some sites, the soil capacity decreases with increased leg penetration. These sites are commonly referred to as having “punch through potential.” Herein, these are also referred to as “difficult sites.”
If during the preload process at a difficult site, all legs penetrate completely through the regression section of the soil, the degree of confidence in the integrity of the foundation for all operations in the elevated condition is high. That is not the case if/when one or more of the legs do(es) not reach the predicted leg penetration and the achieved preload reaction is just below the local peak in soil capacity. This scenario creates a condition in which a slight increase in load (beyond the achieved preload reaction) can cause the leg to enter the soil regression zone during elevated operations.
While it is a given that operators must ensure that the leg reactions in the elevated condition never exceed the achieved preload reactions to avoid possible foundation failure, a thorough risk assessment demands that we ask the following question:
What would happen if/when, while performing crane operations on a 4-legged unit (after having properly completed preload operations) at a difficult site, for some unforeseen reason (poor weight control, more wind/current/wave load than expected or extending the crane boom more than originally planned), the reaction on a leg that stayed at the shallow penetration gets to be higher than the capacity of the soil at that penetration (thus entering the regression section of the soil)?
This paper seeks to answer that question, noting that if that were to happen on a 3-legged jackup, a leg run will certainly ensue, leading to a punch through event (“PTE”). But, is it possible that in that same situation, a 4-legged jackup only has minimal extra penetration on the overloaded leg and reaches equilibrium relatively quickly by virtue of load redistribution (from the overloaded leg to the other legs), never having a leg run or a PTE?
