Summary This paper presents two case histories of jackup rig mobilizations to locations identified as high risk for uncontrolled leg penetration or punchthrough. The punchthrough risk is defined through an overview of the soil analysis work completed for each location. The effectiveness of remedial operations, known as "swiss cheese" drilling, in mitigating the punchthrough risk for the subsequent jackup mobilizations is reviewed. It is concluded that "swiss cheese" drilling can be effective in reducing punchthrough risk. Advance planning is important, and recommendations are provided to assist in effective punchthrough mitigation. Introduction Prior to commencement of drilling operations, a jackup rig requires a suitable foundation with requirements ultimately determined by the soil properties immediately below the mudline. During a mobilization, the jackup takes on seawater as a temporary weight or preload to simulate the storm reaction and offset loads associated with drilling, thereby causing the spud cans and legs to penetrate the seafloor to an acceptable foundation. The soil bearing capacity at final penetration must offset the maximum rig weight that may result during operating and storm loads. In some locations, hazardous soil conditions make it difficult to mobilize a jackup rig safely and achieve leg penetration to an acceptable foundation. This hazard exists when a strong soil layer having insufficient bearing capacity to support the rig overlies a weaker soil layer. Failure to recognize and mitigate this hazard can lead to severe consequences associated with uncontrolled leg penetration or punchthrough. Punchthrough occurs when a stiff layer overlying a weaker layer gives way under the can load and uncontrollable penetration occurs into the weaker layer. Uncontrolled penetration is exacerbated as the load on the penetrating leg increases because of the weight offset (P-delta effect) and to leaning instability as the jackup rig departs more and more from the vertical. A punchthrough may result in loads beyond the structural design of the leg and can cause significant structural damage. An industry practice known as "swiss cheese" drilling can be used effectively to weaken or degrade the thin, hard clay layer and allow controlled penetration in these environments. Although not common, "swiss cheese" drilling typically consists of drilling 30 to 40 holes, each having a 26- to 36-in. diameter, through the hard clay layer in each planned spud can footprint. The holes reduce the effective bearing capacity of the hard layer and allow controlled penetration into the weaker underlying layer during the preloading exercise. "Swiss cheese" operations are typically completed using the jackup rig being mobilized on location. In general, the operation is completed in the afloat mode with the cantilever extended to position the rotary just beyond the transom. A four-point mooring system is used to maintain position and achieve the target pattern of holes in each planned spud can location. Upon completion, the rig is repositioned over the planned footprint and preloaded for final penetration. The Raya B and Tapis F platform locations offshore Peninsular Malaysia were identified to have unacceptable risks of uncontrolled penetration or punchthrough. The decision to "swiss cheese" the respective footprints at these two locations presented unique challenges compared to other known "swiss cheese" operations in the region. A support vessel equipped with a coring unit and four point mooring system was selected to conduct the operations prior to each rig's scheduled mobilization as a result of jack-up rig specification issues. Limited equipment specifications and operational constraints of the coring unit challenged engineering to develop an optimized "swiss cheese" pattern for maximum bearing strength reduction.
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