Memoirs of a GDC intern

On this day there was this problem with the rotary table drive shaft that we had to attend to. The drive coupling had a rubber grease covering that burst hence was subjected to high friction and had been eaten away so much that the metal loss lead to noise and vibrations that could be audible.

That is the shaft from the DC motor to the Rotary table gear box-a set of bevel gears that drive the drill string via a Kelly and master bushing. The rotary table has two main functions, to rotate and give torque to the drill bit and while tripping in and out-it suspends the weight of the drill string (using slips) to allow connection and disconnection of pipes and collars. The DC motor driving the rotary table is controlled by adjusting current to it from the drillers console to vary its speed depending on the formation type, bit size and many other factors. When opened, it is a steel box with a ball-type bearing assembly fitting into a huge diameter housing inside the frame. There are also the master bushings and a turn table fitted with a large diameter ring gear that mates with the bevel gear to change the horizontally rotating motor motion into vertical motion. There are seals in the system to ensure oil does not leak thus creating a closed oil sump for lubrication. I remember having the duty of monitoring this oil level by measuring it with a dip stick and adding it if it fell below the required level. Normally, though heavy, the rotary table unit is very easy to remove and I even remember when we removed it as the shaft had to be taken to NMC for machining.

 

The kelly bushing in place

Something to note with the rotary table drive mechanism that I had to draw in my notebook was the centrifugal motor before it. Seemingly, that AC centrifugal motor has to run before the main DC motor as it has to suck in large volumes of air to the DC motor so as to cool it down.

Also, I was told by Chris who was in charge of electrical systems at Rig 2 that DC motors are prone to create sparks so the cooling air also picks up and extinguishes these sparks as it blows around while cooling the motor windings. To further prevent fires in this area, the air is supposed to be clean hence the trunking that draws in fresh air from a safe area.

The master bushing is the rugged steel cylinder locked into the turn table with a center tapered middle section that fits very well with the bowls that in turn have a taper to match the one on slips. It was amazing how the two just clicked into each other to hold the pipes during trip in and out while I was at Rig 4 in my last week.

On top of the master bushing, it had to interface with the Kelly bushing that has hexagonal slots to match the Kelly profile hence help the rotary table drive the drill string. Kelly bushings have pin drive mechanisms on edges with drive pins that fit into drive holes on the master bushing.

As the driller turns the rotary table clockwise, it is attached to the Kelly which has a string of pipes below it, couples with collars, stabilizers, bit subs etc. all the way to the drill bit. As the drilling bit goes deeper into the formation, the Kelly slides between the Kelly bushing rollers allowing the bit to remain in contact with the formation and continue cutting. Once the bit has cut around roughly 10m, it becomes necessary to add a pipe so the driller raises the stem until the connection is about a meter above the rotary table so that the drill crew sets the slips to hold the weight of the stem while the crew disconnect the Kelly to add a new section of pipe.

While adding extra drill pipe, the weight of the drill pipe is normally suspended to prevent the whole string from falling into the well and secure the pipe so that the new joint can be fitted and tightened. To secure the pipe, a pair of slips is put between the pipe and bowl so that the weight of the drill pipe forces the back of the slips tightly onto the bowl, this force acts as a wedge hence locks the pipe tight in place.

A new pipe can then be added, tightened, pipes raised, slips removed and string lowered back into the well. Some of the crew members actually called the bowls-bushings and I was told it is still correct to refer to them that way. Above the Kelly, is a swivel and rotary hose. The swivel has a bail-just like a bucket handle that hangs from the hook on the travelling block. The swivel allow the attached Kelly and drill string to rotate, at the same time the rotary hose conducts drilling mud into a curved pipe called the gooseneck which attaches to the swivel and carries mud to the swivel via a wash pipe. The rotary hose is flexible…you can’t even believe it is steel reinforced. Just don’t let it fall on you as it can break your back. That is what Mr. Oyomno, our supervisor told me while insisting on the importance of PPE’s.

The wash-pipe is a high pressure passage way in the swivel that conducts high pressure drilling mud into the Kelly and drill string. There are very strong main thrust bearings that support the weight of the whole drill string as it rotates.

I remember on this afternoon we really worked a lot on the dismantling of compressor pipes as we disengaged them using big harmers and arranged them aside. This was a very tiresome exercise although it was a great muscle building exercise….finally, it was 4 o’clock, tea time, after which we changed our clothes and our van arrived to take us to town.