Leg punch through which is one of the biggest dangers of a Jack Up is happened when a jack up leg or more legs rapidly penetrates into formation underneath a spud can(s). This will result in damage in legs, loss of balance of a rig, and potential harm people life. Whether higher environmental loads or larger water depths, this increases demand on Jack Ups, leads to higher elevated weights, and creates more pronounced consequences of a punch. As depth increases, so does a typical soil’s bearing capacity. Soil strength also sees a rapid reduction when a soil layer is underlain by a weaker layer. The weaker soil eventually gives way as the spud can reaches the interface and the jacking system can’t cope with the speed at which the support of the leg moves downwards; the leg isn’t able to maintain the hull level. The hull then sways after it rotates and bends the legs. Relative to the supports, a weight shift occurs and to maintain equilibrium the required footing reaction increases. This process will only end when any hull buoyancy (from the hull now in the water) or the soil’s bearing capacity allows an equilibrium to be reached.
Figure 1 – Soil Bearing, Apvandenberg.com. (2008).
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.
Figure 1 – Mat Footing Jack Up Rig
This article describes basic data with regard to the jack up rig elevation system, guides, pinion chord and leg fixation.
Firstly, let start with the jacking system , to lift and lower the hull, every Jack Up will have a special mechanism and the most basic of these is called ‘pin and hole’ (Figure 1); discrete leg positions will be used for the positioning of the hull. In the market today though, most Jack Ups will use a ‘rack and pinion’ (Figure 2) system to allow for continuous operation.
Figure 1 – Pin and Hole (Hercules offshore,2018)
Figure 2 – Rack and Pinion (sagta.com,2018)
A jack up rig which is one of offshore mobile drilling units is widely used in shelf offshore drilling exploration and production. The unit consist of a buoyant hull fitted with jack up legs which can be raised up or down into the sea bed.
These videos are very fascinating to see because this is how shipyard launch brand new jack up drilling units into the sea. Some major components of a jack up rig such as hull, jack up legs, accommodation are fabricated when the unit is on land. One major component which is derrick will not be installed until a jack up is into the water.