Soil-Structure Interaction During Preloading Of Jackup Modu'S In Different Soil Conditions
Authors: Rapoport Vladimir , Devoy S.D. , Alford Jake , Huthnance W.B. 
Published: October 2008

A new procedure to assess foundation stability of independent leg jack-up drilling units during preloading is presented. This procedure allows assessment of the potential for rapid leg penetrations, taking into account changes in spud can loads due to increase in ballast and shifting of center of gravity.

FOUNDATION EVALUATION

Prior to MODU installation, specifically for an independent leg jack-up drilling unit, rig foundation stability can be evaluated by analyzing site soil data and developing a curve of soil resistance versus spud can penetrations. During preloading (or preliminary assessment) leg loads are calculated as a function of increasing water ballast, taking into consideration exact fixed and variable loads.

These leg loads are then compared with the soil resistance curve to evaluate penetrations and potential for punch-through.

At this time, it is assumed that leg loads during preloading depend only on the amount and location of water ballast. However, even during simultaneous preloading (all three legs) leg penetrations are not simultaneous. As a leg penetrates, the rig center of gravity moves toward the lower site of the unit with a corresponding increase in the leg load on the lower side. At a location with a punch-through potential or with a low rate of clay strength increase, this leaning can cause an increase in the leg load resulting in uneven and deeper penetration on one leg and, potential for rig physical damage.

PRELOADING

The conventional preload method is to elevate the unit at its preload air gap (about 2-5 feet above the surface of the water) and then to simultaneously add ballast to all of the preload tanks.

The elevated weight of the unit soon exceeds the rating of the elevating system to level the unit and the unit is allowed to settle. As the unit's legs are penetrating, the possibility exists that the unit may settle with differential leg penetrations resulting in the rig reaching equilibrium in and out-of-level position. A representative out-of-level tolerance for an independent leg jack-up unit might be in the range of 0.3 degrees.

When this out-of-level situation occurs, it is standard to dump preload ballast water until the weight of the unit is reduced to the value at which the elevating system can level the unit and raise it to the preload air gap.

Unfortunately, the results of this procedure can be disastrous. Leaning instability, that is the movement of the center of gravity toward the lower side of the unit with a corresponding increase in load on the lower side of the unit, can exacerbate soil problems. Simply allowing the unit to reach its out-of-level tolerance can increase the loading on the lowered leg by as much as 5 percent; and a punch-through at a 5 foot air gap with maximum preload onboard will result in leg load increases of more that 150 percent. These leg load increases, of course, result in greater values of punch-through before an equilibrium position is achieved. This equilibrium position would be one where the soil would support the load applied to the punch-through can, including all out-of-level loadings.