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.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
3 November 2004
- Revised manuscript received:
15 November 2006
- Manuscript approved:
25 November 2006
- Version of record:
20 March 2007
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