During coming off location operations for jackups (the transition from the elevated mode to the afloat and then the transit mode), the hull is jacked down into the water to obtain a buoyancy force great enough to retrieve the legs. In locations with soft clays, leg penetrations in excess of 15m are common. When the legs are stuck, it is rare that all legs become unstuck simultaneously, and whether planned or not, single-leg jacking becomes necessary to pull the legs.
One effect of single-leg jacking is that the hull inclination relative to the leg induces leg bending moment that gets transferred to the hull as a combination of a vertical couple (i.e., differential chord load on the pinions) and a horizontal couple (i.e., upper and lower guide loads). The magnitude of the loads for a given amount of jacking on a given leg is a function of the leg’s effective stiffness, which varies with water depth and soil stiffness.
This paper presents the results from a study intended to increase the understanding and quantify the degree of internal leg loads that are developed for a given amount of single-leg jacking in different water depths and with different soil rotational stiffness values. This is done so that these internal loads are properly accounted for when establishing environmental limits for the coming off location operations.
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