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).
This article describes about basic information of jack up legs – Cylindrical Legs vs Trussed Legs, Three-Legged Jack Up vs Four-Legged Jack Up and Three-Chorded Legs vs Four-Chorded Legs.
Cylindrical Legs vs Trussed Legs
Since the hull needs to stay above the storm wave crest, withstand certain pressures, and transmit different loads between the footing and hull, every Jack Up unit will have legs of some sort. Similar to the footings, there are two different types in trussed legs and cylindrical legs.
Cylindrical legs can vary slightly but the basic premise involves hollow steel tubes. However, some units will have internal stiffening and others may have rack teeth or small holes to allow the hull to move up and down the legs. Generally, these cylindrical legs are used on units that stay shallower than 300 feet of water depth. With the newer units that are designed to work in environment deeper than 300 feet, they tend to use trussed legs and this is because trussed legs require less steel for the same resistance and same elevated response.
Figure 1 – Cylindrical Leg Jack up (dsboffshore.com, 2018)