Jacking, which is one of operation modes for jack up, is a specific way to raise up a hull of a jack up rig so this article will describe about the basic of jacking and preloading operation.
Mat jack up units (figure 1) jack the mat to the seabed, in accordance with the ballasting procedure. When the mat is fully lowered, the hull is then jacked out of the water. After this is complete, the unit will then proceed to preload operations. It is vital that all independent leg units also perform preload operations before being jacked to the design air gap. The majority of independent leg units lack the capacity needed to fully elevate the Jack Up unit while the preload weight is still aboard. In these cases, the next step will be to jack the hull out of the water until a very small air gap is achieved- this should be at the very most 5 feet. Once this has been reached, then preload operations can continue.
However, some more modern independent leg units (figure 2) possess elevating systems that are indeed capable of lifting the full weight of the hull at maximum preload weight. With these units, the preload will be loaded onto the unit before the hull is raised out of the water. Once the entire preload has been loaded, the Jack Up unit is slowly jacked out of the water to the same small preload air gap as other units, under 5 feet.
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.
In the event of soil failure or a leg shift, the affected leg loses some of its load-carrying capacity, and rapidly starts to move downwards, causing the hull to descend into the water (Figure 3 – Maersk Victory Punch-Through). Furthermore, some of the load that the affected leg was carrying will be transferred onto the other legs, which could overload them as well. The failed leg will continue to penetrate into the soil until the soil gives enough support, or until the hull is sufficiently under the water to provide enough hull buoyancy to halt the penetrations. When the hull is off-level, the legs will all experience increased traverse load and bending moment transferred to the hull, in the most part from the guide. As guide loads increase, some braces will start to experience large compressive loads. Special procedures need to be followed to minimize any structural damage that can occur under these conditions.
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. Usually, mat units do not require much preload.
Four-legged independent Jack Ups (Figure 4) also require little to no preload water, because they preload two diagonally opposite legs at the same time using just the weight of the hull itself. The unit is jacked to its preload air gap, before two legs are lifted slightly off the seabed. The unit therefore settles on the remaining two legs. The hull is then jacked back up to the preload air gap, and the process is repeated until all four legs are preloaded to the target footing reaction, and no extra penetration occurs.
Three-legged independent units (Figure 5), on the other hand, require the most preload water. If they are incapable of jacking with preload, then the water will be pumped aboard once the unit has been jacked to its preload air gap. If a significant amount of settling is experienced, then the full preload will need to be dumped before the hull if jacked upwards again. The process must then be repeated until there is no more settling.
In the case of units that are able to jack with full preload, the preload will be pumped into the hull while it is still sitting in the water. The hull will then be gradually jacked up, occasionally stopping briefly at predetermined draft points. This process goes on until the hull has reached the preload air hap, and has held the preload for the required holding period. When the preload has been held for the predetermined amount of time, all of the preload water is ejected, and the Jack Up unit can be elevated to its operating air gap.
Jacking to Operation Air Gap
After preload operations have been completed, the unit can be jacked up to its operational air gap (Figure 6). When this is carried out, it is vital that the level of the hull and elevating system load and characteristics are carefully observed, as well as the Rack Phase Differential (RPD) in the case of trussed-leg units. Care must be taken to keep these figures within design limits. When the unit has reached its operation air gap, the jacking system will be stopped and the brakes set, along with leg locking systems if these are present. Once this process is complete, the unit is ready for operations.
While normal operations are taking place, there is no particular difference between the various types of Jack Up unit. Neither are there any special precautionary measures which need to be taken, other than observing the design limits with regards to operation and equipment. However, in the case of units with large cantilever reach and loads, it is important to ensure that the maximum footing reaction remains below a predefined percentage of the reaction achieved during preload.
Jacking to Storm Survival Air Gap
During operations, it is important that the weather is constantly being monitored. If any storms which exceed the intended operating condition environment are predicted, then operations must be halted, and the Jack Up unit switched to its Storm Survival mode. In this mode, all equipment and stores are fully secured, and extra care is taken to ensure all weather and watertight enclosures are closed. Should cyclonic storms be predicted, then the extra precaution of evacuating personnel from the unit is taken.
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