When we had our orientation of the ship, we were shuffled along many different parts of the boat and had all these ship terms and drilling terms thrown at us. It was overwhelming, to say the least. I know a lot of the science team felt overwhelmed by the terminology as well! I would imagine a drilling specialist would feel the same way sitting in on a science meeting. Through the kind heart of the Operations Superintendent, Kevin Grigar (whom you’ll meet in a future blog), and others, I’ve got a much better understanding of how cores are drilled on the JR and I’m going to share this knowledge with you, because I like you.
Depending on the material you are drilling in, you need to use a different drill bit on the end. For example, you could use a cheap plastic shovel to dig holes in sand, but not for digging a significant hole in your backyard. With soft sediment, such as mud or sand, a drill bit called an ACP is used. The APC stands for Advanced Piston Corer and uses built up pressure to shoot down through sediment similar jamming a drinking straw through a piece of layered cake. Once the piston shoots down into the sediment, the drill bit rotates and drills down around it, repeating the process over and over until it starts to fail. This usually means it has hit hard rock and a new approach is needed to retrieve core.
The next most aggressive drill bit configuration is the XCB, which stands for eXtended Core Barrel. It’s used when the sediment is too hard, or has become too lithified (sediments compacted to rock by pressure) to use an APC configuration. The XCB uses the same drill bit, but has an added (extended) cutting piece on the front called a cutting shoe.
So far at our target site we’ve drilled and cored 4 holes and we are drilling the fifth hole now. Letters are assigned to describe the holes, A, B, C, and D have been drilled and now we are drilling the E hole. “A” hole was a test to see what kind of material we were going to be drilling in, they push their way down using fluids. This is called a “jet in” and they went to 25 meters below the seafloor. They determined that the material at the bottom was suitable (squishy enough) to use an APC drill bit (the straw – piston method). So they proceeded to use the APC drill bit and began coring until the sediment became too hard and then they switched to XCB coring. They made it to a depth of ~200 meters before they had a recovery of about 24%. At this point they decided they needed to go to the big-daddy-extreme drilling tool, the RCB drill bit.
If you want to do some serious drilling, you gotta go with this guys, the RCB (Rotary Core Barrel). It’s quite the effort to use the RCB as the drilling team pulls up the drill string (the long connected pipes that extend from the ship to the seafloor) pipe by pipe and re-fit it for the RCB drill bit. When you refit the drill string, it’s referred to as changing the “bottom hole assembly” or BHA.
The ship moved 20 meters to begin drilling the next hole, C, using the RCB drill bit. Since they had already cored to ~200 meters, they could skip coring that part. When you just drill and don’t core (bringing up the rock) this is called “washing down”. The drill team washed down to the depth they left off and resumed coring. When they got to about 400 meters, the drill bit got stuck. They crew tried several maneuvers to get free and none were working. You could really feel the stress circulating through the ship. It’s not a good day when you get stuck. By dropping the drill bit, they were able to get loose and pulled up the drill pipe, pipe by pipe to start a new hole. hole D.
With the prior knowledge of the past three attempts, the team now has a better idea of material and fluids to use to drill without getting stuck. They slowed the rotations per minute of the drill bit and were especially sensitive around 400 meters where they got stuck last time. We made it down to about 1,000 m before we encountered some problems (poor recovery and drilling mud in our core tubes). The Operations Superintendent made the call that we needed to start a new hole and case all the way down to 1,000m below the sea floor.
Currently we are drilling Hole E, which required a re-entry cone (basically a funnel) to go in and out of the hole to be able to change drill bits. We will also have to support the walls of the hole from caving in on itself in a process called "casing-the-hole". Casing takes a long time to do, you can think of it as cementing the walls as you go down in depth. Even though it takes a long time (we are a day or two away from being finished!) it’s a very effective way to drill to maximum depths. As I said this expedition wants to drill to 2,000 meters below the seafloor to get to the basement lavas they are hoping for. This would be a record depth for the JOIDES Resolution and truly great for understanding the process of continental crust formation!
No matter what depths the drilling reaches, the scientists will still have core to analyze and sample, advancing the understanding of plate tectonics in this area. Please do stay tune for the continued adventure! For science!
Catch-ya next blog!