So, we’re here to drill and to core, but what does this mean and how do we do it? To the non-initiated, the technical world of drilling seems pretty alien – ‘drill string’, ‘tripping’, ‘running’, ‘stingers’ and ‘releasing the chaser’ are all part of the lingo, and it takes a while to catch on!
Luckily, on Expedition 364, we have an experienced team of drillers (from DOSECC), who, alongside the ESO Drilling Coordinators do their utmost to drill and core to target depths. This involves constant adaptation, modification of plans and fabrication of new equipment to respond to rapid changes in the nature of the hole and the drilling environment. Part of the ESO role is also to communicate with the Science group regarding progress. Numerous excel spreadsheets, whiteboard drawings, scrawls and demonstrations later and we’re all up to speed….
Returning to the detail of how drilling works….ESO Drilling Coordinator Graham Tulloch provides his ‘Drilling 101’ (he’s well practiced at this….).
Drilling essentially involves putting a cutting bit on the end of an ever-increasing number of drill rods to make a deeper and deeper hole. The bits can be either open hole or coring (what we are really interested in) depending on what we need to achieve. There are other things too that help us along the way, casing, mud and even paper!
The first step is to make a secure link to the sea bed to allow us to re-enter the same hole should we need to bring all of the pipes back to the deck. We either put a heavy frame connected to the platform by wires to guide the pipe on the sea bed or join together a number of large diameter tubes or pipes from the platform and then either push or bore them into the sea bed (depending on the strength of the sea bed) a few meters to give it stability.
The drill pipe is then connected together and lowered, when the first drill pipe touches the sea bed it is said to have” tagged” it, and we proceed to penetrate the surface, or ‘spud in’, this is the official start of the borehole.
As I said before, to drill a borehole we must select the correct drill bit for the task in hand. If we do not need to see core on the deck we can use an ‘open hole’ bit (like the big ones at the back of the top right photo), this works in the same was as a drill we use at home to make a hole in a piece of wood. If, however, we want core, we use a coring bit: these bits drill a hole but leave the middle untouched, allowing it to be caught in another tube and brought to the deck of the platform. The material we drill away to make the hole, the cuttings, is brought to the surface by continually pumping liquid (drilling mud) down the string. To get difficult cuttings back we can add special paper to the mud which thickens it and allows heavier cuttings to be removed.
I like to use two words to describe adding pipe or removing pipe, running or tripping, some people use only “tripping” but add “in” or “out” depending on which direction the pipe is going. Running pipe is when a pipe is screwed onto the previous one and lowered through the drill floor to the sea bed or the bottom of the borehole, the collective noun for a number of pipes screwed into each other is “a string”. Pipes are usually 3.05 m or 6.1 m long so you can imagine that when a borehole is several hundreds of meters deep how long it takes to add all the pipes, even if we are not coring.
In long boreholes, or problematic ones, we can ‘case’ the hole by lining it with a pipe of a larger diameter than the bit we are using and then drilling through that. Obviously we have to drill a larger hole first and so it is not done routinely as it is time-consuming and therefore expensive, however it provides a stable drilling/coring environment, one that will not fill up with loose material ‘caving in’ from above, for example loose sand. Picture digging a small hole in sand at the beach and how it always falls in around the edges. No matter how much you clear it, it keeps falling in. However if you put a tube in the hole, the sand is held back and you can dig deeper. That is how casing works.
So, once we have selected the correct bit and joined all the pipes together we can start drilling or coring. Core is collected in other tubes or core barrels, as these are 3 meters in length we only advance the bit by this amount. Rather than tripping the “string” to get the core, we send a tool, the overshot, down the pipe on a wire which latches onto the top of the core barrel which allows us to pull the two of them back to the deck.
A clear plastic tube (you can see it’s a bit like a Russian doll this game; tubes within tubes) is extracted from the core barrel and cut into 1.5 meter sections. This is where the tube and its core stop being my responsibility as I pass it to one of the ESO curators to process, and continue through the offshore core flow.
Images top left: Drill pipes GTulloch@ECORD_IODP, top right A selection of drilling and coring bits. TBralower@ECORD_IODP, bottom: Different diameter pipe and casing CLowery@ECORD_IODP.