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Weight of steel in a drilling rig

Weight of steel in a drilling rig

(OP)
Not sure if this is the right group to put this question, but I do not see a navel architect group.

I am engaged in an exercise to evaluate some old drilling rigs and I need to approximate the total weight of steel. I am told that the correct term for what I am after is the 'lightweight', essentially the weight when built in the shipyard. The problem is that the lightweight is not indicated for any rig that I can find on the internet.

What is indicated is the gross tonnage, which for conventional vessels is based on volume and can be used as a measure for cargo. Apparently if you know the type of vessel, you could approximate the weight by using some factor. However a drilling rig is such an unusual structure that old sea captains I have spoken to are not sure what gross tonnage even means when applied to a drilling rig.

Can anyone point me in the right directions?

RE: Weight of steel in a drilling rig

Gross tonnage is basically useless in this case.

The lightweight of the rig includes the steel weight as well as the weight of the equipment.

Clearly the lightweight varies with the size of the rig.

The lightweight is what the scrap value would be based on.

For the older generation of drilling semisubs (operating displacement around 20,000 to 25,000 tonnes) you could expect a lightweight around 11,000 to 14,000 tonnes. For the modern large vessels (operating displacement 40,000 to 64,000 tonnes) the lightweight would range from 20,000 to 32,000 tonnes.

The document where you will find the lightweight of a rig is the STABILITY BOOK or MANUAL. But you will not find these on the internet.

And even in the stability book you will not usually find the lightweight divided into steelweight and other components.

If you were to post details of the rig design/type I could probably give more specific advice on the lightweight.

RE: Weight of steel in a drilling rig

(OP)
Thank you for the info. My question relates to jackups only. Coincident with submitting my question, my attention was directed to the following paper.

http://www.offshore-mag.com/articles/print/volume-...

Please note that in the formula in the paper, 3,233 should be 323.3.

When I use this formula on the two rigs under consideration, the answers are quite different, which may be because they are different designs, which is your question to me. So if you look at the attached pdf you will see the designs.

Thanks again.
Barry Dillon

RE: Weight of steel in a drilling rig

A good source of information on the weight of these rigs is the heavy transport companies which perform "dry tow" movements of the rigs using their semi-submersible heavy lift ships.

The F&G L780 II will weigh around 8,300 tonnes.

The Mitsui 300C will weigh around 11,000 tonnes. So the formula seems to work well for this one.

It is not surprising that there is a difference as the "hull" volume of the F&G L780 is considerably smaller than that of the M300C (this is 40% larger).

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