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Jack up stability
17 July 2012
There is a discussion ongoing in our office about the stability of a jack up when in dry dock. Basically, the questiion is, if the jack up is in dry dock, and sitting on the cans, can the cantilever deck be jacked right out whilst maintaining stability?
Thanks in advance for any comments....
6 answer(s)
modu
Student
Students
Total Posts: 1
Join Date: 01/01/20
Hi every body
One question I m curious to know why do you want to extend out cantilever in dry dock ? 
In design and manufacture of jack up I think ( honestly I see it in some clips) they put the cantilever on its position when the jackup is afloat .
And at the end please share your practical experience ( did you do that and what was your method to do it ) I will appreciate that if you explain more about conclusion 
Thanks
alex.craib
Director
SPREAD Associates
Total Posts: 2
Join Date: 19/04/10
Thanks for all the comments gents - very much valued and appreciated
Roy
Drilling Superintendent
Addax Petroleum
Total Posts: 6
Join Date: 27/12/11
Alex
Unless the JU has a suction anchor design on the spud cans, the drilling operation and yard situation should be similar.
On hard seabed, you would have a fulcrum, just as you have in a dry dock.
JDDrouin
Project Quality
Myspread Users
Total Posts: 109
Join Date: 06/05/09
Alex,

This is way outside my field of expertise, but, for what it's worth, the rig designer will have almost certainly considered a highly similar case in their various case load calculations (in fact, if they haven't, the rig shouldn't be accepted or operations should be suspended because this one's really basic AND critical).

All other factors being equal, the significant changes will be the position and loading on the cantilever vs the load on the remainder of the rig. In other words, as the aft legs act as a fulcrum point, at what point does load on the cantilever exceed the load on the rig.

Hope that helps,

James
Augusto
Consultant [retired Shell staff]
SPREAD Associates
Total Posts: 277
Join Date: 02/09/05
I would be very careful going to the extremes mentioned as everything is against you.
(There are studies of Jack up failure modes in elevated position”¦). After all, the jack up is no longer attached to the seabed/cased well with riser/BOP top tension helping. Furthermore, the deepwater mooring system has some spring not available in shallow moorings (taught mooring).

We have to assess the foundation stability/reliability (soil investigation first), eventual differential leg settlement (moment fixicity, punch through, scour,”¦), overturning related to leg preload, individual leg jacking behaviour (rack/pinion, racking phase, ”¦), etc.

Storm stability, wind load - beware of extra leg length (maybe wise to remove the extensions) - become difficult to predict.

That plus Damaged stability, boat impact (leg bent chords), Jack up crane lifts required, personnel access to the hull, etc recommend to lift the jack up hull to clear the water leaving the air gap at the minimum ie the Jack up hull as close to the seabed as possible.

West Gamma example from 3rd Quarter of 1990 may teach some lessons.

PS Some spud cans can be repaired without drydocking”¦
cnduncan
Drilling Consultant
SPREAD Associates
Total Posts: 25
Join Date: 10/04/11
Alex,
The correct answer is to consult the jack up marine operating manual.

The annecdotal and logical answer is that it should be fine.

Jackups are designed to operate on seabeds with a variety of strengths. The design must assume that the rig gains no stability from the legs being stuck in mud. On the NW Shelf of Western Australia, thereare many instances where the seabed is rock and only the points of the spud can penetrate.

You can then look at the cantilever load chart and see that at full extension, with the derrick centered, there is still some load capacity for the cantilever. The value is design dependent.

So we can jack up on a hard bottom and skid the cantilever out to full extension and then pull XXX on the blocks all within the design envelope. This is similar to what one could expect in shipyard so it should be OK.

One thing to check however would be the support mechanism for the spud cans. I have seen them supported on frames and sitting on the points. You would be wise to ensure that the support frame if used is rated to take the additional loan on the aft legs as the cantilever extends. It should be Ok but check first.

Regards Craig Duncan
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