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.
Punch through can occur in Jack Ups of all designs, and the damage normally reaches jacking units, braces, and chords. When a punch through occurs, the resulting accidental loading can cause a shearing of the chord, bucking of the braces, and punching shear and joint damage. The actions before, during, and after the punch through will entirely depend on the extent of the damage (and the size of the punch through). Since punch through is serious occurrence, high-quality management is essential. Some level of risk can be minimized with modern rigs since they offer effective punch through management systems and a better guide design. One example is shown in figure 2 which is Maersk Victory Punch Through Incident in 1996,
For a given amount of ‘leg run’ (leg bending as an indicator of seriousness), there are various factors that will determine the consequences of a punch through. For example;
- Depth of Water – Leg bending tends to be more severe in deeper water
- Leg Spacing – Leg bending will generally decrease with greater leg spacing
- Elevated Weight – Increased leg bending will result from higher elevated weight (the amount of preload)
- Air gap – Leg bending increases with a larger airgap because it takes longer for buoyancy to activate and aid the recovery
To reduce the extent of a punch through, there’s a process called ‘Swiss Cheesing’. As long there’s regression in the soil strength versus depth (penetration) curve (assuming it’s over a large span), holes can be drilled through the critical soil layers in an attempt to decrease soil strength. If done correctly, the legs will reach past the critical depth in a more controlled way (and at a reduced load level). This process is most effective where the regression is close to the maximum footing reaction of the Jack Up.
Apvandenberg.com. (2008). April 2008: Partnership of A.P. van den Berg and Keppel FELS – A.P. van den Berg. [online] Available at: https://www.apvandenberg.com/news-archive/april-2008-partnership-of-ap-van-den-berg-and-keppel-fels [Accessed 29 Oct. 2018].
Members.home.nl. (2012). Maersk Victory Punch-Through – Oil Rig Disasters – Offshore Drilling Accidents. [online] Available at: http://members.home.nl/the_sims/rig/m-victory.htm [Accessed 29 Oct. 2018].
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Bork, K. (1995). The rotary rig and its components. Austin: Petroleum extension service. Division of continuing education. University of Texas at Austin.
Davis, L. (1995). Rotary, kelly, swivel, tongs, and top drive. 1st ed. Austin: Petroleum extension service. Division of continuing education. University of Texas at Austin.