BRIEF SUMMARY OF "DUAL RATINGS" PAPER
by J. Vazquez
November 14th, 2018
This is a brief summary of a presentation for a technical article I contributed to this summer, along with a few good people from NORTHERN OFFSHORE and ABS. The article’s official title is Alternative Operation Scheme for a Jackup Drilling Rig to Meet Target Soil Bearing Pressure Requirements. The article was prepared for the Abu Dhabi International Petroleum Exhibition & Conference (ADIPEC). The authors of the paper are: Tony Beebe, Jose H. Vazquez, Joseph Rousseau, Lars Samuelsson, Pete Cunnion and Clay Coan.
I should mention that it is my understanding that NORTHERN OFFSHORE has filed a patent for the concept, but I am not part of that patent application – so I don’t have any details on that.
The paper and presentation describe the particulars of the approach taken to allow NORTHERN’s CJ46-X100- D jackup to operate in the Arabian Gulf, where there is a requirement to have a maximum bearing pressure of 8.5ksf, when it was originally designed to operate in the North Sea with full-preload bearing pressure of 11.0ksf.
Rather than introduce larger spudcans to reduce the bearing pressure, knowledge of the prevalent conditions for jackup operation in the Arabian sea (or Persian Gulf) led NORTHERN to the creative solution of having two Operating Schemes for its jackup -- one for the North Sea and one for the Arabian Gulf. Specifically, the fact that the environmental conditions in the Persian Gulf are much milder than those for the North Sea allows for what appears to be a de-rating of the rig to achieve the target reduction in the maximum bearing pressure.
The figures below illustrate how wind & wave relate to overturning moment, which in turn is related to leg reactions of the jackup and how a reduction in wind/wave and/or water depth can lead to a noticeable reduction in the required maximum bearing pressure.
The following paragraph is a slightly modified quote from one of the presentation slides (with the change being made to use English units rather than Metric units).
… this solution is analogous to driving a car that is designed to go up to 120mph in a school zone with a self-imposed speed limit of 20mph. Having the imposed limit makes sense, but it doesn’t mean that if your car was designed to go faster than 20mph, it can’t be used to drive in a school zone.
The dual operating scheme is not a de-rating of the jackup. It is a fully-documented commitment to the client that the rig will operate within the self-imposed limit.
From the same slide, the figures below are included to convey the above point (reflecting English units)
The paper includes a section describing the roles played by NORTHERN (owner), 3DENT (consultant) and ABS (Classification), as illustrated in the figure below, from the presentation.
The paper and presentation end by showing a list of benefits, as illustrated in the figure below.