Leg slips on MODU

what is the sign of leg slip in jack up and how to recognize that the legs are in good/right position ?
and how to calculate RPD ?

I could write volumes on “leg slips”, AKA “rapid penetration”, or “punch trough”, but what specifically are you referring to? If you can describe an actual situation, or scenario, it would help.

As a Rig mover for many years I have spent countless hours watching levellers for the slightest sign of settlement that could lead to catastrophic “punch though”.
(“Bobble watching” should have been made an Olympic sport for it’s excitement)

RPD (Rack Phase Differential) was not understood before sometime in the 1980s.
The older rigs were not as much affected, since the legs were sturdy and designed with large safety margins. (Before computer programs made it possible to “accurately” calculate stresses)

It became a problem with the introduction of high tensile steel legs and may have caused leg damages a long time before it was identified as the cause of the problem.

RPD is measured, not calculated. Originally measurement was done manually, but later done electronically. Accumulated RPD was displayed on a screen in the control room and could be adjusted by “single cord jacking”, before reaching critical levels.

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honestly i asked this question to improve my knowledge about it.
i dont have any idea about it
i am barge engineer trainee i search but i did not find any thing and asked for help but nobody help me

WHAT CAN I DO TO LEARN ABOUT “leg slips”, AKA “rapid penetration”, or “punch trough” ?

why do you watch bobble ?
what is it sign of ?

AKA = Also known as.
There is no book that would fully prepare you for the experience of a “leg slip”, especially if it results in a full blown “punch through”, not just a “rapid penetration” situation.

OK. let me try to explain the difference:
Leg slip is when the rig slowly develop a list while preloading, which is detected by watching the bubbles on the inclinometers. It can be mitigated by stopping preloading, or if necessary, dumping preload from the leg that is penetrating and/or leveling the hull by jacking, if still within allowed jacking weigh.
Rapid penetration is when the leg slip happens quickly, but does not cause any serious damages to the legs, guides or jacking structures. Mitigation is the same, but preferably to bring the hull down into the water enough to take stress off everything while you inspect for damages.

Punch through is when the situation gets out of hand causing serious damages and the list exceeds the limits for jacking. Usually this calls for evacuating all non-essential personnel and emergency procedures to be implemented to get the hull level and back in the water.

How to reduce the risk of any of this happening?

  1. Make sure that the site survey has been properly done, studied and understood and the person in charge is an experienced Rig mover.
  2. Make sure the airgap is kept as small as possible and never over 5 ft. If that is not possible, do an “individual leg preload”. NOTE: Watch the tide cycles to ensure that can be done safely before taking the decision to try it.

I don’t know of any book that can fully teach you all the tricks and pitfalls you need to know to be a good Rig mover.

My advise: Watch how experienced Rig movers do it, with your eyes open and your mouth shut, unless you have a sensible question to ask .


thanks for your complete description and your advise