Drill ship or MODU question

I have been wondering about the stability of a drill ship or a MODU at different points. My limited experience has brought me to these questions. The derrick makes them look incredibly top heavy and when they have all of the drill pipe or casing up there I assume it only gets worse. I have never been on one except for a crew change but I have always assumed that they ride fairly well considering their job and size.

How is the stability worked out on them? Is it different when they are attached to the bottom? Im assuming that there must be some form of tension on the drill string, not much though. I figured this tension would create some form of weight but I have no idea where it would be centered or where it is “attached” so I am not sure how it would effect the ship.

Also, when you have a large length of pipe downhole how does that calculate into stability? Is it at the attachment point on the ship?

Generally how much can they roll while operating without it causing problems with the operations? With the derrick being in place, it seems like their ability to roll and recover would be limited and the farther the roll the more the derrick would come into play. How much can they roll when not drilling or when they are underway? I assume they are fairly light for what they are when they are transiting large distances.

I know some of this is very dependent and generally random but they are things I have been wondering about nonetheless.

Christ! I used to know all the answers to your questions but suddenly I am left feeling unsure if they would now be correct. I must be coming down with Alzheimer’s or Mad Cow Disease or sumptin?

I do know for sure that drillers are always yelling to the bridge that the drillstring is rubbing and to transfer ballast. Any BCO or DPO has their hands full maintaining a drillship or rig at proper drilling draft and keeping the drillstring upright going through the rotary.

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[QUOTE=c.captain;168615]Christ! I used to know all the answers to your questions but suddenly I am left feeling unsure if they would now be correct. I must be coming down with Alzheimer’s or Mad Cow Disease or sumptin?

I do know for sure that drillers are always yelling to the bridge that the drillstring is rubbing and to transfer ballast. Any BCO or DPO has their hands full maintaining a drillship or rig at proper drilling draft and keeping the drillstring upright going through the rotary.

.[/QUOTE]

I appreciate it, I figured they had to be fairly on top of their game having to deal with the operational needs of many departments. I guess that is why they make so much more than their OSV counterparts (So the rumor says anyway).

This is what extended periods of time sitting next to one doing nothing leads to, wandering thoughts.

realize that on a drillship for all the weight topsides there is double the amount of ballast in the hull so really they are not unstable at all. Semis are a different animal entirely since the straightforward hydrodynamics of a monohull are thrown out the window. As far as motions are concerned a semi hull is very friendly but as far as righting arm and damage stability, they are not very forgiving at all. You can get into a lot of trouble very quickly in a semi.

In general the eye is not a good guide to vessel stablity. I’ve never seen anyone say that container ships look top heavy but they often have problems keeping GM within margins.

[QUOTE=c.captain;168615]Christ! I used to know all the answers to your questions but suddenly I am left feeling unsure if they would now be correct. I must be coming down with Alzheimer’s or Mad Cow Disease or sumptin?

.[/QUOTE]

Maybe years of alcohol abuse, ah I mean use?

[QUOTE=RespectMyAuthority;168619]Maybe years of alcohol abuse, ah I mean use?[/QUOTE]

and all the dozens of prescription drugs as well…

How is the stability worked out on them? Is it different when they are attached to the bottom? Im assuming that there must be some form of tension on the drill string, not much though. I figured this tension would create some form of weight but I have no idea where it would be centered or where it is “attached” so I am not sure how it would effect the ship.

[B]The stability is worked out the same way it is on a conventional vessel. All weights are input into a stability computer with their location. This can change dramatically from day to day or not much at all depending on the operation. From this you get your variable deck load and deck reserve capacity. You will get a stability margin and stability conclusion if it is OK or not. Light ship additions must be recorded and tracked closely. On a semi during different stages of the operation you can be very close to your max capability. It is different when you are attached to the bottom due to riser tensions. You get the riser tension information from the NOV screen and input it daily. The downward pull is factored using a constant VCG but keep in mind the LCG and TCG of this force is zero directy in the middle of the vessel. All the pipe and casing that is put in the derrick is also calculated. We call this area the setback. VCG is high there, on my vessel it is around 75m.[/B]

Also, when you have a large length of pipe downhole how does that calculate into stability? Is it at the attachment point on the ship?

[B]For Pipe in the hole is factored in the hookload as a weight. The VCG moves up and down. We use and average of the VCG constant at around 62m although it could be more or less depending on the location of the top drive.[/B]

Generally how much can they roll while operating without it causing problems with the operations? With the derrick being in place, it seems like their ability to roll and recover would be limited and the farther the roll the more the derrick would come into play. How much can they roll when not drilling or when they are underway? I assume they are fairly light for what they are when they are transiting large distances.

[B]The marine operations manual sets limitations to vessel movements depending on the operation will determine the limits. Some operations can be continued with a lot of heave, roll and pitch while other more delicate operations required a smaller safety margin. Keep in mind we sometimes have to suspend operations prior to reaching these limitations. They are only guidelines. On long transits we completely offloaded all of our marine riser, about 3600mt to a ship in order for us to pump up to transit draft. Then we met the ship at the destination and loaded it there. At our transit draft of 9.8m we were able to average 9 knots on our on power. If we had kept the riser onboard the draft would have been more with cross bracings in the water. The cross bracings create a lot of drag and slows us down to 4 to 5 knots. The ride was surprisingly very good, but this depends on each individual vessel. Some vessels roll quick and some roll slow. What makes me nervous is when the roll to one side, hang there for a few seconds, then roll back. If you have a lot of Free surface in long longitudinal tanks the pitch can be scary if the tanks are not full. Found this one out the hard way during DP trials in the south china sea. Rewrote the ballast manual with this knowledge.[/B]

It is not as difficult as you might think, but it is very important to be accurate with this information. With a little training I could probably show C-Captain how to do it.

Thanks @Capt. Lee Makes sense that they would have a computer program like the rest of us out here. I could imagine attempting to do it without would be very time consuming and difficult, not to mention the room for errors to be made.

One day I would like to get on one and see how the whole system works. The process seems like it would be interesting with a large learning curve coming from the industry that I am currently in.

Thanks again for all of the replies!