Every Jack Up unit needs to load the supporting soil to the maximum amount of force expected to be exerted under the most extreme conditions- usually, this will be when the unit is in its Storm Survival mode. By carrying out this preload, the risk of a foundation shift or failure during a storm is significantly reduced. However, it is possible that soil failure or a leg shift could occur during these preload operations. To reduce the risk of this, which could have disastrous consequences, the hull needs to be kept as close to the waterline as possible while still avoiding wave impact. If possible, preload the rig at positive draft during good weather. This method consider the best industrial practice.
While normal preload operations are taking place, it is essential that the weight of the hull, deck load, and preload are all kept as close as possible to the geometric center of the legs. This is so that each leg experiences equal loading. However, it may be the case that single-leg preloading is desired, in order to increase the maximum footing reaction of that particular leg. To do this, the preload tanks need to be selectively filled or emptied, depending on their relative position to the leg currently being preloaded.
The preload takes the form of seawater pumped into tanks inside the hull. Once it has been pumped on board, it will then be help for a certain amount of time. Preloading is only complete once there is no settling of the legs into the soil during the holding period, while also achieving the target footing reaction. How much preload is required will depend on the required environmental reaction, as well as the type of Jack Up unit being used.
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