Is this post rigged, or some information on oil platforms.

I noticed, over the past week, that there is some increasingly technical talk about the various ways that we get oil out from underwater in the Gulf of Mexico. And while I suspect that most of those who comment on this site are vary familiar with all the terms, some of the more general readership may not be. Since this is going to be a fairly hot topic in the near term, as the impact (spelled out in earlier posts and comments) spreads from the oilfields to the refineries and then up the pipelines to the local distributors and gas stations let me therefore explain just a bit about some of the different words that are being used here - with reference, where I can find them - to pictures of the different types of structures that are being used. And if I miss some, please chip in either to ask or answer. This is replacing a chat I was going to have about Horizontal Drilling, but that will be along at some time in the future. Earlier technical posts are listed at the end of this one (with a comment on the levee question).
Just as on land, we need some form of drilling rig if we are going to drive a bit down through the rock and find us some oil. But, unless we are working somewhere like the North Slope where we can wait until the sea freezes over and drive ice roads out to build islands to drill from, we are going to have to find a different way of providing the infrastructure support for that bit. And here is the first distinction - because a drilling rig, in the offshore sense, is more of an exploratory tool, going out to find oil, rather that developing the known fields and bringing in production. That latter task is left more to production platforms, which can be sited where best to drain the field, and may not even use the initial holes drilled by the exploration rig.

As you may have read the Minerals Management Service keeps track of these and is currently reporting that 471 platforms remain evacuated and 33 rigs out of totals of 819 platforms, and 134 rigs that were working in the Gulf - pre-Katrina. Some of these operate in shallow water, there are also significant developments in the deep water such as Thunder Horse. But let's start at the shallow end.

In really shallow water, or the bayous, you might launch your bit from a drilling barge, where the derrick can be assembled once the barge has been towed to the right place. Barges can be flooded to rest on the seabed in relatively shallow operations.

However this is very susceptible to bad weather, and so, as one moves further offshore, then one might use a self-erecting tender from a barge, but would more likely move to something which could get the drilling floor stabilized and up above the waves. These are the jack-up rigs. Typically they have three legs, that are raised as the rig moves around. Then when it has reached the desired site, the legs are lowered to rest on the sea floor and the entire structure jacks itself up out of the sea, and, hopefully, above the waves. (You can get a paper model to cut out and assemble of this rig). They can do this to work in depths to around 500 ft. The discussion about damaged rigs gave links to the different rigs that have been damaged, and some of these have photos of the rigs in better days.

As one goes out to deeper depths, then one will look for a more substantial vessel and so one comes to the Semi-submersible. These are built to either sail themselves, or to be towed out to the site, with the assembly floating, and then fluid is pumped into the bottom tanks to partially submerge the vessel and thus stabilize it. One can get some idea of the size of these from some of the photos shown where, (thanks to Ed Ames), wikipedia covers the subject..

Since these are floating there has to be a way of holding them in place. One way is to have them dynamically positioned, using thrusters to hold them in position, such as these. Note that it takes about 3 years to build such a unit. The alternative is to have the rig attached to anchors on the sea bed using cables, or tethers. (And for those interested in natural gas production, note that the same rigs are used for both).

The connection between the well and the platform now becomes more flexible and special connecting pipes called risers are designed to reach from the blow-out preventer (BOP) at the top of the well, but on the seabed, and the platform. These must allow the rig to rise and fall with the tides and so models of behavior have to be written to design ways of allowing this.

An alternative is to use a drillship to do the exploration. The one referenced, for example, found the Mars field. It is the platform for that field that was recently shown, after suffering through Katrina. The drillship has the rig mounted in the middle of the ship, and can thus move around somewhat more easily than the others. It is generally held in position by dynamic positioning while drilling.

Once the field has been established, then a larger production platform can be brought out and placed where it can, using directional drilling, reach the best places to extract oil from the field. It is these large structures, such as that the Yastreb from which Sakhalin Island oil finally began to flow this weekend, or the Thunder Horse, or Mars platforms. Although the former was due to produce by now, it has been delayed by damage from Dennis, while the Mars platform was extensively damaged by Katrina. One of the problems with using these large platforms for Deepwater recovery is that they focus collection and so when these two are disabled, for example, they take about 400,000 bd out of production. And once they are damaged they are not so easily replaced.

Demand for rigs has been so strong that, as the International Herald Tribune reports

"If a customer comes today with an order, he'll have to wait until 2009 for delivery," Choo said in an interview last week. "That's how busy we are. If he's willing to pay more, he can get a rig by 2008 from our American shipyard."
The article notes that while it may only take 2 years to build a rig, the yard can only work on 8 at a time, and thus current deliveries are for 2009. But the value of the market is such that other yards are now eyeing the opportunity since
According to analysts, about 350 rigs worldwide are more than 20 years old and due for replacement or upgrade. Around 70 rigs have been built in the past 10 years and approximately another 50 are now on order.

By the way, for those interested in levee failure, it was actually more the flood walls that failed, and they failed because when the water overtopped them it fell vertically down over the open wall, hitting directly at the base of the wall, and eroded the foundation of the wall, in the same way as a waterfall has a big hole in the rock (the swimming pool?) where it falls. Thus the wall ultimately lost its footing. This does not happen as easily with the more gradual slope of a levee since the water flows across the levee without the higher impact velocity, and though it will, in time, eat its way down, if there is a strong core this can take quite a while.

The question was, in part motivated by the reality that doing a 3-D rendering, as some of you know, can turn into a candle-burning exercise, and thus if I started adding the illustrations the number of posts would drop quite a bit. For now, therefore, I will continue just referring to other folks pictures rather than providing my own.

This is part of an ongoing weekend series on technical aspects of oilwell (and natural gas) drilling. Previous posts can be found at::
the drill

using mud

the derrick

the casing

pressure control

completing the well

flow to the well

working with carbonates

spacing your well

directional drilling 1

directional drilling 2
As ever, if this is not clear, or if there is disagreement then please feel free to post, and I will try and respond.

Thanks very much for this explanation.  It helps.
For those wanting to keep track of shut in, the main MMS page for releases is here

Great series - many thanks. My background and degree are in the social sciences, so the background technical info is really helpful.

Heading Out:

I just want to say that I've found your series of tutorials on well drilling technology extremely informative and well-written. As an engineer with some rudimentary knowledge of geology,  I've had a rather vague idea of how these things were done, but now I'm a whole lot better informed on the subject.  Keep em coming!

Hey folks,

Does anyone have any information on the damage in the GOMEX area in regard to what might be written off and uneconomic to repair, i.e production that may have been lost altogeather. Do insurance companies pay for the complete replacement of the rig itself or the associated costs of redeveloping a field ? At what point is it better to take your brand new oil rig, bought with insurance money from a hurricane in the GOMEX fields and ship it to say... Saudi Arabia or Nigeria ? Could some of the shut in production be over the horizon now in economic terms ?

Any thoughts ?


Great weekend post as usual.  I think you touched on this after Katrina but can you find some info. or just speculate on what happens to the wells and connections when the platforms or rigs are ripped off site by storms.

The companies are required to insert storm packers (something like a rather large cork) into the wells when a storm approaches so that even if there is a problem the well will remain sealed.  Unfortunately as some of our correspondents have noted this does not help when the rig is displaced and the risers get bent out of shape.  Then it gets harder to re-establish the well and it may be cheaper to redrill.  One then has to decide if there is enough oil left to justify that investment.

And at present  the debate on repair has also to consider how long it is going to take to get a new rig, as opposed to trying to fix, (and recertify all the parts of) the old one.

So are the risers attached to the platform?  Or is there a long flexible connection from the top of the riser to the platform?  It sounds as if the risers are coming up out of water at or near the level of the platform so that when the platform shifts it rides over the risers bending them in the process.  The bends can occur anywhere between surface and seafloor?  And likeley different places for each well going to the platform making repostioning and replacement of bends technically difficult (impossible) when there are multiple wells angling up to a centerpoint.

Am I visualizing this correctly?  If not, set me straight.

HO, one more thing.

If my previous post is correct than platforms don't need to be sunk, set adrift, etc. only shifted significantly off attachments.  This would cause major bends in wells, restricting or pinching off flow?  I'm trying to get a handle on what consititues major damage to a platform.  Might intact platforms with minimal surface destruction be sitting on top of non functional wells now if 50 foot waves moved them laterally?

Before a bad storm they will disconnect the risers and raise them.  But if the platform moves then they may need an ROV (Remotely Operated Vehicle - one of those little mini-sub deals) to go back and a) find b) if necessary repair the BOP and structure so that they can reconnect.  These structures have to be fairly tightly aligned to hit the holes.   There was a post at about the time of Katrina that spelled out all the problems, but unfortunately this morning I can't put my hands on it.  Sensibly I was quoting one of our insiders down in the Gulf, who is now stuck with having to do this (or arranging the same) for their rigs.
Okay I think I've got the picture now and understand the alignment issues even if no damage to casings.  If you have a series of risers they all have to align with there respective casing.  Thanks for the clarification.
Thanks HO.  I have only rudimentary knowledge about these technical aspects and your posts really help.  I also have noticed some new contributors to TOD of which some are adding greatly to the debate.  This is a very positive development.
just a quick question on mobile rigs for exploring. when they move from one location to another how fast are they moving? are there tugs that reposition them? then once they are in position at their new location, how long does it take to set up to drill? just curious.
Some are towed and some are self-powered.  A Drillship might for example do 15 knots.

A Semi submersible might do 10 knots.

A Russian jackup can do about 13.5 knots, and a Japanese one about 14.

So they all seem to move at about the same speed if they are self-propelled.  

There is a really interesting piece on towing jackups here .

After someone suggested to me that all those lost rigs were properly insured (which didn't sound likely to me), I did some research, and learned that a great many of them (especially the big ones) are uninsurable, and as a result "self-insured". I'll bet I can guess who gets to pay for the rebuilding of "self-insured" platforms...