Settle a bet for me please


#1

A semi submersible drilling rig on 12 point spread obviously moves with the wind and current. I’m trying to settle a bet with a coworker on the amount the rig can potentially move (sway) in high winds, seas, and or current. Ocean Depth of 1k meters or more.
A) Less than 20’ radius
B) More than 20’ radius

Thank you for answering.


#2

It depends on the water depth. The deeper the water the more the rig is allowed to move, even while drilling. In high winds the tension in the moorings may be reduced so that it will move even more - and it will always be more than 20’,


#3

Correct, I forgot the depth. Let’s say at least 1k meters of Ocean


#4

That’s fairly shallow water, but still workable with anchored semi’s. My first year in the oil field the anchored rig I was on used an 8-point in 1,500 feet of water more or less. A 1% limit there still gives you 15 feet in each direction (surge and sway). At 3,000 feet or more your looking at limits of 30 feet in any direction.

The twelve point or sixteen point spreads were mainly post-Katrina and it was more of a redundancy issue to keep rigs that had been down-manned from floating around the Gulf of Mexico when all of their moorings had broke free. I doubt if the footprint would change much


#5

So you’re saying 20’ is normal and acceptable movement.


#6

It all depends on water depth and your subsea equipment limits (the angle between the flex joint and BOP being critical). If you don’t have it within acceptable limits then they can key-seat the inside of your stack when your drilling, which is very very bad. Past very, very, bad.

Make sure you know the limits down there and any guidance on safety factors, but 20 feet of offset in 3,000 feet of water is only 0.38 of a degree down there. I’ve been conditioned by different parameters in dynamic positioning, but 20 feet is not so bad for normal drilling activities on that sort of water depth.

All of that being said, many clients will have more conservative ‘artificial’ limits and you will need to go by these potentially much lower limits depending on the company operating the rig and/or the client.


#7

The limiting angle at the flex joint used to be 1.5 degr., with a “rule of thumb” for max. allowed offset at 2% of water depth.

Of course I’m talking about back in the days when anything over 600 ft. was regarded as deep water.
The biggest problem was drilling with a floater in shallow water, which required very high tension in the anchor lines to stay within the limit.

I found this article for modern day limitations: https://www.netwasgroup.us/risers-systems-2/td-qsmsi-t.html

If I remember right, the drillship Chancellorsville drilled a well in only 45’ of water by blowing out a “cellar” in the seabed to sink the BOP into, but had VERY big problem keeping stationary enough to complete the well, even in benign waters.

SEDCO had a different solution to enable floaters to be used in shallow waters. The SEDCO 600 and 601 was designed to sit on bottom and drill with surface stack. This was possible because the Drill floor was placed over the stern, not over a moon pool in the centre.
Here they are together at PPL Yard in Singapore:
Semi-submersible04
This enabled them to also drill in deep water with surface stack, something other floaters could not do: http://www.offshore-mag.com/articles/print/volume-65/issue-4/drilling-completion/technologies-wedded-to-renew-life-in-older-rigs.html


#8

Since your on a drilling semi with a spread point mooring system, make sure you know your own vessels limits and the limit of your own subsea equipment.

Rules of thumb are good, but it’s your gear on your rig that counts. How much surge and sway your vessel can handle is only part of the issue. Another issue is if that motion will still allow the driller to drill or conduct whatever operations are ongoing on the rig floor.

You hopefully have a good Marine Operations Manual, it’s usually the same manual with all of your stability data. Those limits of service in the manual might be for vessel safety, and the BOP / drilling systems may have other, lower limits. And then specialized tools may have further limits so good enough for one operation may be a no-go for another operation.


#9

Gentlemen you’re missing the question. I’m a lowly 1600 ton master who has a wager against a peer. One of us says a submersible swings on surface less than 20’ and the other says more than 20’ is possible.

I’ve searched everywhere I can imagine for the acceptable surface movement of a submersible but have had no luck. If anyone can point me in the right direction where I can find my own answer I’d be appreciated.


#10

They’ve already given you basic guidelines, barring tighter restrictions on any given gear.

TL : DR

~ 1.5% the water depth is the maximum radius of allowed surface movement while attached and drilling. Maybe for an extra safety factor say 1.0% as the safe working limit.

In 1,000 meters of water that’s 15 meters (49 feet) of allowed position deviation at the maximum limit and 10 meters (33 feet) at the safe working limit.


#11

You apparently missed this part of the reply.


#12

Over 20 feet is an easy one with the evironmental forces. Even the Mars TLP moved 100 feet laterallly during Katrina.Caternary gives way to the environment to a certain degree…depending on moorings.


#13

One way or another it gives way. Hopefully in a manner that allows for a restoring force as opposed to breaking.


#14

A submersible should not swing at all, as it is a bottom bearing type unit when drilling. . . .a SEMI submersible CAN swing more than 20’ when the depth allows it, as the movement is a function of the angular displacement of the flex joint on the top of the stack. Hope that answers your question and settles your bet.


#15

B, In a Loop Current.


#16

If your looking for an actual expected footprint for a given water depth, catenary, and tension combo I doubt you will find much on the net. Your best chance for published numbers would be the vessels capability plots or station keeping study.

I’m unsure how submersible rigs from another century came up in regards to a semi-submersible in a thousand meters of water, but please do take care to not rest your semi on the sea floor out there.