Soil Rapid Penetration (Punch through)

escveritas
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Soil Rapid Penetration (Punch through)

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Before a jackup is installed at a site, a prediction of the spudcans’ penetrations into the seabed as a function of the imposed loads should be made. The actual load-penetration response is recommended to be monitored on site, and this response should be compared with the predictions

There are basically two types of soil rapid penetration that could occur:
  • Spudcan penetration in single layer soil
  • Spudcan penetration in strong soil overlaying soft soil: which could be sand over clay, or strong clay over soft clay
Normally soil backflow starts immediately after the widest cross-section of the spudcan is below the ground surface, and any unsupported sand above the level of the spudcan flows freely into the hole above the spudcan. However, predicted penetrations in sand will often be sufficiently small that only partial penetration occurs and backflow is not relevant.

Punch-through potentially occurs in ground exhibiting post-peak reduction in bearing resistance, in which exceeding the peak bearing resistance results in excessive and uncontrollable spudcan settlement and loss of hull trim. Punch-through failure is possibly the most hazardous of all the geohazards for jackup foundations, and is responsible for more than half the events accounting for loss of life, injury, structural damage, etc.

Prediction methods may be used to evaluate such risk. Other methods can be used if they are justified.

Mitigation Method – Perforation Drilling

Spudcan skirts conventionally project downward from the rim of the footing. Such skirts can increase the jackup’s penetration resistance against punch-through and spudcan-soil rotational stiffness. However, skirts should be sufficiently long to be effective, but a skirt extending appreciably beneath the tip of a spudcan cone can increase hydrodynamic drag, making towing of the unit difficult [Ref.16]. Thus, a skirt’s benefits need to be considered against problems arising from its length.

Perforation drilling has been performed to perforate the stiff layer with drill holes and remove the soil. It has had varying degrees of success for predominantly clay conditions rather than sand ground conditions. In clean sand, it is difficult that the drilled holes will remain open and that the sand is not mixed with the underlying clay. The ability to reduce the punch-through risk by perforation drilling depends on the percentage reduction in the ratio of the reduced peak to the minimum post-peak bearing resistances. A reduced ratio of unity or less means that the punch-through potential has been completely eliminated, although this may not be either achievable or necessarily required.

The perforation drill hole distribution commonly used is an equilateral triangular grid within the spudcan footprint. A greater reduction in the punch-through resistance ratio may be achievable with an increased density of perforations located within and outside the spudcan periphery. The actual hole pattern may not be as regular as planned due to drill bit positioning control, hole collapse, hydraulic fracturing and linkage between adjacent perforations due to excess drilling (water or air) pressure, etc. The expected reduction in the punch-through resistance ratio should be re-evaluated on completion of the drilling operation. The drill hole “perforations” are typically constructed by open flush rotary drilling methods using a 26-inch
bit sometimes coupled with a 36-inch hole opener, which forms a hole of the bit diameter (or hole opener diameter if it is used) or perhaps a hole slightly greater due to drill string flexibility and soil wash-out.

The drill hole spacing (center-to-center) determines the efficiency of the perforation drilling scheme. Experience suggests that the efficacy of perforation drilling reduces once the spacing falls below about 2.3 hole diameters. However, increasing the hole separation results in fewer perforations and less total soil removal. If an inadequate number of perforation holes are drilled then the process will not succeed in removing the punch-through risk.

To date, perforation drilling has been achieved using the rotary open hole water flush method. If insufficient flush rates are used then the disturbed soil may not be removed from the hole. On the other hand, if excess drilling pressures are applied then this may lead to hydraulic fracturing and linkage between adjacent perforations. Both scenarios significantly reduce the effectiveness of the operation.

By using reverse circulation (i.e., using an airlift to remove cuttings from the base of the hole up through the center of the drill string to surface) the effectiveness of the excavation, and hence overall process, may be increased as the material removal efficiency will improve. Since there is little experience with this method for perforation drilling, it is not possible to confirm its efficiency and reliability on vessels so far used for this operation. Experimental data suggest that factors such as perforation distribution pattern in particular, perforation depth, drilling methods, etc., may significantly affect the reduction of peak bearing resistances.

Remedial Action during Punch-through

Leg runs and punch-through usually occur on a single leg at any one time, although occasionally more than one leg may simultaneously suffer such an event. On completion of the punch-through and leg settlement, the hull may be partially in the water, listing with the legs inclined. The legs may be resisting large bending moments at the hull interaction points (i.e., the guide wear plates and jacking pinions).

The implementation of recovery procedures, following a punch-through, should be carefully planned and executed with expert advice sought where damage may have occurred, or may occur during the recovery process. If inappropriate action is taken during the recovery operation then greater structural damage than caused by the punch-through may result. The recovery plan will depend on the post punch-through situation, the particular jackup design and possibly other factors.

Where a punch-through has occurred and not all the legs have been forced through the strong layer at the end of the preloading operations then unless the unequal spudcan load vs. penetration behavior can be adequately accounted for by the geological model and installation method, the safety of the installation.

After a jackup is removed from a site, spudcan footprints are left in the seabed.

Image

The spudcan footprints are ground conditions that experience:
• Changes in physical profile of the seabed (existence of depression); and
• Changes in soil properties.

The soil beneath a depression may be highly non-uniform due to back flow of remoulded soils during and after spudcan penetration and extraction, and reconsolidation of the soil. If the subsequent positioning of another jackup is very close to or partially overlapping the footprints, the slope of the footprint and the varying soil strength inside and around the footprint results in an eccentric/inclined soil reaction on the spudcan. This can
cause a spudcan to slide towards the footprint and hence leads to overloading the leg. Thus pre-loading of jackups near existing footprints can result in uncontrolled penetration, slewing of the rig and even excessive structural stresses in the legs, which might even lead to catastrophic failure. The footprint feature is dependent on various factors such as footing shape and size, soil types and strength, previous spudcan size and penetration, and the elapsed time of previous spudcan operation and after extraction.
should be reconsidered
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