How Does a Drillship Work?

When I first began work in the offshore industry, I was completely staggered by the complexity and expanse of skills involved in offshore drilling. We all know that a drillship goes out and drills wells, but can you name all the steps required to drill that well? Here are a series of fun videos that give an overview about how a drillship works. (I also welcome any comments from people in the industry who wish to correct me or add important details.)

We begin with a general overview of the drillship process.

Overview of Deep Water Drilling

So the general drilling process consists of these steps.

Arrive on site and set DP stationkeeping.
Run down conductor pipe
Run down casing pipe
Cement in casing pipe
Land BOP and connect riser
Start drilling in earnest until you hit target depth and spread
Remove the riser and get the well ready for completion

The first stage (steps 2 -4) essentially creates a good foundation to attach a BOP on (oversimplifying). When we do that first stage of drilling, we are nowhere near deep enough to risk releasing any oil. This is important because we can safely leave site at any point without risking environmental damage. And yes, drillships will leave site if the weather gets too bad to hold station.

At the second stage, we use a riser and BOP to contain the wellhead as we drill deeper. With deeper depths, we now get the massive pressure differentials. The BOP has several safety measures to help control that pressure in case of problems. Now, if the drillship must leave site, it will disconnect the riser from the BOP and leave the BOP to contain the well system.

Of course, the drillship will come back for the BOP. Are you kidding? That BOP is major equipment. It is over 50 feet tall and costs tens of millions of dollars. Of course they want it back. And it is no simple matter to lower a BOP to the sea floor. Something the size of a building doesn’t easily fit on a ship. And you don’t just toss it over the side to lower it. This animation shows an excellent example sequence for staging and lowering a BOP.

Landing a BOP

BOP = Blowout Preventer. Used to close off flow from the well in the case of a blowout.

LMRP = Lower Marine Riser Package. Additional valves, pipes, and tools for well control, but not complete shutoff necessarily.

If the drillship does an emergency disconnect, the LMRP will disconnect from the BOP and return to the surface.

Riser Pipe

Drilling mud pumps down the center of the drill pipe. It comes out the drill head and returns to the wellhead. But why doesn’t it just bleed into the ocean? Enter the riser. Think of the riser as a bundle of pipes serving various purposes, all hooked together. At the center is a large tube for the drill pipe. The drill pipe sits inside this tube. Drilling mud goes down the drill pipe and comes back up in the annular space between the drill pipe and the riser tube wall. In addition to that, the riser also contains pipes for hydraulic power, electrical power, communications with the BOP stack, and several other items that you need to ask a drilling expert about.

Even more amazing, the riser is slightly flexible. I know you don’t imagine pipe to be flexible, but when it spans hundreds of meters, that pipe has some give. Plus the riser has special flexible joints at the top and bottom. The riser provides a contained system to get from the seafloor to the drillship. And then we have special heave-compensating equipment to pull some tension on the riser, which is necessary to prevent the riser from buckling (very bad). Of course, the vessel at the top still heaves up and down with the ocean waves, but your riser doesn’t change length. Instead, the riser tensioner adjusts up and down, constantly compensating for the ocean waves. Add it all together, and you get an incredibly complicated system!

The simple process of adding riser segments, lowering them down is filled with specialized equipment. Examine this video.

Conclusion

As you can see, offshore drilling is extremely complicated. It requires a huge range of specialized equipment, everything tailored to a specific task. Hopefully this oriented you on the overall process and helps you appreciate the massive challenge of deepwater drilling.