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Jacking is a crucial process for ships that are equipped with jack-up rigs. The jack-up rig is a type of mobile platform that is used in the offshore oil and gas industry. It consists of a barge-like hull with legs that can be extended or "jacked up" to lift the platform above the water. Jacking operations are used to raise the platform to the required height above sea level, providing a stable base for drilling or other operations. The jacking process is typically done using hydraulic cylinders, which are controlled by a computer system to ensure precise and consistent movement of the legs. Safety is a paramount concern during jacking operations, and proper procedures must be followed to ensure the stability and safety of the platform and the personnel working on it.

The Operation of Jacking

The jack-up unit is ballasted to the seabed by slowly lowering its mat onto the ocean floor. When the deck is fully submerged, the hull is then jacked up from the water. After that, preloading operations can begin. To ensure that the jack-up unit is stable and secure throughout the preload process, all independent leg units must begin their respective preload cycles while the preload weight is still on board. When this happens, the hull is jacked out of the water to create an air gap of five feet or less and then preloading can be continued.

However, some advanced independent leg units have elevating systems that can lift the entire weight of the hull at maximum preload weight These units are preloaded with a load of steel pipes before the boat is lifted out of the water. Once preload has been achieved, the boat is raised from the water using air pressure and jacked up to a small air gap of 5 feet or less.

Preloading

Each jack-up unit must load its supporting soil to the maximum amount of force expected to be exerted under the most extreme conditions; for example, when the unit is in storm survival mode. To reduce the risk of foundation failure or leg shift during a storm, preloading the foundation can be carried out. However, there is still a possibility that soil failure or a leg shift could occur during these preload operations. To reduce the risk of water flooding the hull and causing it to sink, the hull needs to be kept as close to the waterline as possible.

When the soil beneath one of the vessel's legs fails or shifts, the leg loses some of its load-carrying capacity and starts to move downward. Furthermore, if a vessel's leg penetrates the soil, the weight of the ship will be transferred to that leg and other legs as well. If a leg fails, water will rush into the hull and cause it to sink deeper into the soil. Until the soil gives enough support or until the hull is sufficiently under water to provide enough hull buoyancy to halt the penetrations, the vessel will continue to sink. When the hull is not level, the legs of a barge will experience increased loads that are transferred to the hull. When guides are being lifted during offloading or loading, some braces may be subjected to large compressive forces.

To ensure equal loading on each leg during normal preload operations, it is crucial to keep the weight of the hull, deck load, and preload as close as possible to the geometric centre of the legs. Sometimes single-leg preloading is desired to increase the maximum footing reaction of a specific leg, this can be achieved by selectively filling or emptying the preload tanks depending on the location of the leg that is currently being preloaded.

A ship is equipped with tanks that are filled with seawater. Once the water is on board, it can be used to meet the ship's needs for a certain period. Complete preloading is achieved only when there is no settlement of the legs into the soil during the holding period and when the target footing reaction is achieved. The amount of preload required will depend on the environmental reaction required and the type of jack-up unit being used.  Usually, mat units require less preload than jack-ups.

Four-legged independent Jack Ups 

Four-legged independent Jack Ups do not require preload water because they can preload two diagonally opposite legs at the same time by applying the weight of the hull to those legs. The unit is lowered onto the seabed until it reaches its preload air gap; then, the two legs on either side of the gap are lifted slightly off the seabed. The unit settles on the remaining two legs. After the hull is jacked back up to the correct preload air gap, it is lowered onto the bearing surface until no further penetration occurs and all four legs are preloaded to their target reaction.

Three-legged independent units

Three-legged independent units require the most water preloaded into them. If they cannot be jacked up with the necessary amount of water, then water must be pumped aboard once they have been jacked to their preload air gap. To prevent settling, the full preload must be dumped before the hull is jacked upwards again. The process must then be repeated until no further settling occurs.

When jacking a ship with a full preload, the hull is pumped full of water while it rests in the water. The hull is then gradually jacked up as it settles onto its new keel. Jacking stops periodically at predetermined draft marks. The preload water is held in place until the hull has expanded to reach its preload air gap and holds that position for the period required. Once the holding period has elapsed, the rest of the water is ejected from the hull and it can rise to its operating air gap.

Operation Air Gap

Once the preload operations have finished, the unit is raised to its operational air gap, it is important to closely monitor the level of the hull, elevating system load and characteristics as well as the Rack Phase Differential (RPD) in case of trussed-leg units during this process. It is important to keep these figures within the design limits. Once the unit reaches its operational air gap, the jacking system is halted, the brakes are set, and leg locking systems are activated if available. After that, the unit is ready for operations.

During normal operations, there are no distinctions between the different types of Jack Up units, nor do any special precautions need to be taken other than observing the design limits for operation and equipment. In the case of units with large cantilever reach and loads, the maximum footing reaction mustn't exceed a predefined percentage of the reaction achieved during preload.

Storm Survival Air Gap

During operations, the weather must be monitored continually. If storms are predicted to exceed the operating environment, operations will be halted, and the jack-up rig will switch into its storm survival mode. In storm-warning mode, all equipment and stores are secured, and extra care is taken to ensure all weathertight enclosures are closed. Extra precautions are taken if cyclonic storms are predicted. If storms are predicted, personnel may be evacuated from the unit.

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