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.