Analyzing self-elevating units in the elevated and afloat modes is fairly well understood, and standards exist for use in the development of limiting environmental criteria, as listed in the Marine Operating Manual (MOM) or as determined by site-specific assessments. However; the transition phase from the afloat mode to the elevated mode, commonly referred to as "going on location" (GoL) is not as well understood. During the GoL phase, the motions of the unit can cause severe loading as the legs impact the seabed when they are lowered. As a safe operating limit, MOMs typically list a single wave height or inclination angle for all conditions, regardless of water depth, soil conditions, wave period and/or direction. While these limits have proven to be safe, they are inherently conservative.

A common approach for establishing safe GoL operating limits is based on the conservation of energy principle. Perhaps due to the expected large pitch/roll response angles near resonance, industry standards usually focus solely on the rotation response at the pitch natural period while ignoring the contribution of heave. It is also common industry practice to equate the pre-impact kinetic energy to the energy absorbed by the leg and jacking system during impact. While these assumptions may generally produce conservative results, the fact that the impact load is directly related to velocity (through kinetic energy) and not motion indicates that the critical period may actually be slightly lower than the rotation natural period. Similarly, it is easy to see that the critical period for impact loads may be one with relatively small rotation and large heave.

From the above, it can be inferred that the parameter of interest for determining GoL leg impact loads is downward spudcan velocity (DSV). It is, therefore, the objective of this paper to illustrate the relationship of DSV and impact load, while answering the question as to whether or not this relationship is independent of oscillation amplitude or wave period. For simplicity, the analyses focus on a single water depth, soil stiffness and direction and are based on a generic jackup suitable for 300 ft water depth.

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