First posted Jan 2013, updated just now.
It’s been a long time since I’ve done a rig move and as i’m sort of re-lauching my blog I thought I’d share some details about how and why we move these things, Why? well that’s easy, drill more holes! Get more Oil/Gas. Actually it’s not just that, a lot of rig moves will be to work on and repair sub sea assets, more about that if I ever find out more about that.
This particularly move was a re-entry, meaning we were putting the rig back on a location that had been drilled/worked on before. Sounds easy? Sort of, That is if you get it on the correct location, it’s easy to tell once you send a ROV down because you will see the assets on the sea bed, Tolerance on this sort of move is about 1 meter, on the other hand if it’s an open location, (meaning nothing on the sea bed) tolerance is around 5-10 meters.
For those of you who don’t know, a Semi-submersible (half above water, half below) drilling rig is held in position by 8 or 12 large anchors, around 12-15 tons, these are spread out around the rig evenly, So after the normal safety talks and meetings it’s time to start, we normally use 2 or 3 AHV’s (Anchor Handling Vessels, Big Boats) These vessels all have Nav systems onboard, basically a laptop where they can see the rig, the anchor positions and themselves and the other AHV’s all live, if they move in the water they move on the screen.
For the first part of the move the AHV will come in close to the rig and we will past down the PCP (Permanent Chasing Pennant), This is a collar permanently attached to the anchor chain and a 150 meter cable with a socket in the end, once passed down to the AHV it is attached to the their work wire.
Here you can see a PCP wire cable running down the leg and into the water, when it appears again, it’s the short bit of chain and then the collar.
The AHV’s work wire running down the deck. I took this shot while we were just standing by, if we had been working I would not be allowed on the back deck.
Once attached, the AHV will proceed out to the anchor following a line on the nav screen.
Now another thing we had to do on this rig move was De-ballast before any anchors were recovered, why I hear you ask? Either you asked that question or were so bored you have already gone onto the next blog, but i’ll tell you anyway.
So what happens is, the anchor is lifted off the sea bed, brought up to the stern roller of the AHV, checked out and then the rigs large winches will haul in the anchor chain and store it in the chain lockers, but what do we do with the anchor then? we need to put it somewhere, so we rack it on these things.
Just under the fairleads (the wheels with the anchor chains going through) is the cowcatcher, (not sure if thats the real name for them) this is where the anchors are racked, they hook under the CC and the winch pulls tight and they don’t move. Now some of you will see the problem here, I’ll show you another photo of the rig we just moved at transit draft, meaning it’s de-ballast.
Now you see the problem, if we don’t de-ballast, the fairleads and the cow catchers are under water and we can’t see them to rack the anchors. You can see here how the rig floats in the water when it’s at working draft by the marks on the legs, this is now at transit draft, up on the pontoons ready for towing.
Now to get the rig up to transit draft is just a matter of pumping out all the water and up she comes, now you are waiting for me to say, Ah but it’s not as easy as that, and no it is really that easy. Oh but there is one sticky bit, the whole process of de-ballasting can take up to 12 hours and during a 3-4 hour period when the cross braces are coming out of the water is the critical time, during this time the rig is very unstable, no helicopters can land and no cranes can work.
Once all the anchors are racked and a boat is on the tow bridle we can start the move, here is a racked anchor. You can see one racked and the other chain where that anchor has been removed.
It’s as simple as that, all in a days work, or 10.
Any questions please ask away, leave a comment below.