In previous articles, I wrote about the safety improvements automation could make to the nuclear industry and to oil/gas drilling by replacing their manual mode of operation. Here I am continuing that series by showing why full automation could improve the safety of offshore drilling in and transportation from the Arctic Ocean.
In the next articles, I will continue this series by showing what automation can do for oil sand processing/pipelining.
Here I will not discuss the environmental damage drilling in the Arctic can cause nor the risks to the crew due to ice and insufficient winterizing. I will only focus on what optimized process control could do to protect the ships from storms and hurricanes by automatically stabilizing them while drilling.
Drilling ships have to pass through the Bering strait, which is open only during the summer. The storms in the Arctic Ocean are powerful. In the past, waves up to 40 feet high and wind gusts of 89 mph have been reported, and these storms are becoming more powerful as the planet is warming. Just last December, a Russian drilling ship working near the island of Sakhalin capsized, and 49 of the crew of 67 were lost.
Therefore, to keep the ship in place is critical. As can be seen in Figure 1, movement would break the vertical pipes that connect it to the well if the dozen or so anchor rodes (shown in Figure 4) were not automatically and correctly controlled. I will explain why the present semi-manual methods of stabilizing are inadequate, and will describe the automatic optimization strategies that should replace them.
The Stabilization Control Envelope
The ship is stabilized by a dozen or more anchor rodes made from chains which are several miles long (Figure 2) and weigh over 100 tons. This dynamic stabilization allows the ship to move in three dimensions—to yaw, pitch, roll and drift—without breaking the flexible pipes that connect it to the well. This is because its position is held inside a three-dimensional "safe envelope" that never moves further away from the well than a safe distance.
The present method of semi-automatic stabilization is to pull in the rode chains that are facing the direction of the wind to increase their "pull," while "paying out" the rodes on the opposite side of the ship.
The relationship between the force generated by this pulling that changes the "rode catenary" and the push of the storm is well understood. While the wind force is not constant, its strength and direction constantly changes, theoretically it can be perfectly balanced if the rodes are heavy enough and if the manipulated winches (Figure 3) are strong and fast enough. In other words, what we need to apply is classic multivariable envelope control (Chapter 8.6 in Volume 2 of the 4th edition of The Instrument Engineer's Handbook).
The limits of this envelope are the allowable distances from the well. One method to implement that control is to use my "Hungarian Puli" algorithm. The puli is a livestock herding dog that keeps the herd together and moving in the right direction by going after only one sheep at a time. This is always the one that is moving in the least desirable direction (closest to the limit of the control envelope). So in this application, we "go after" the rode that is able to pull the ship away from the closest limit of the envelope.
The setpoints of this multivariable control loop are the distances from the borders of the envelope, and the manipulated variables (the control valves) are the winches. It is desirable to put positioners on these "valves," which will compare the actual pay-out lengths and tensions in the rodes with the required lengths, and will reposition them if necessary.