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18 3/4" surface wellhead
23 December 2020
Looking for experience with 18 3/4" surface wellheads. These modified subsea style compact wellheads are on the market and offer many benefits over conventional surface wellheads. Please share experience, good or bad, if you have used this type of wellhead.
5 answer(s)
Managing Director
Vartech Eng
Total Posts: 22
Join Date: 06/07/15
Hi Edvin,

As mention by Scott, Plexus was on of the pioneers with the Unihead compact system. Back in Baku, while you were in Turkey, a special design 18.3/4"x 15M unihead was using for the tiebacks of SDA; one wellhead did fail while testing BOP. I arrived to SDA team while they were recovering from that incident (HIPO).
Back in Egypt in 2010, we used with GDF a 18.3/4" 15M  Plexus well head (in rental), in a HPHT exploratory well; eventhough we used a MLS (Vetco), there was no issues with the space out (three points - TD, MLS, WH), and everything went OK. I tried to bring those Plexus rental WH to Colombia in 2012, but there was some commercial issues.

Reynaldo Vargas
SPREAD Associates
Total Posts: 164
Join Date: 05/03/08
Hi Edvin,

The original Plexus were the first Company to offer modified subsea wellheads for use on surface wells way back in the mid-80's.

All manufacturers have offered them for a couple of decades ('compact' or 'unihead' is quite often the description) and they are in very common use - safer and much quicker than 'conventional' land/JU/Platform wellheads.

I've used them numerous times over the years, but don't have experience in that particular size.

Yes, they can be a pain if the casing gets stuck in the wrong place but in that instance you are basically just reverting to a 'conventional' slip and seal assembly.

In my experience, a casing patch often does not have the pressure rating required to match that of the casing that has just been run and in any case, will take you longer to actually do than using the contingency system.

All the best!

Drilling Consultant
Crescent Petroleum
Total Posts: 3
Join Date: 23/12/20
Elman, thanks for sharing. What type of wellheads are you using?
For the stuck pipe scenario, I would prefer to use an external casing patch rather than emergency hangers and pack-off's. Any reason you are not considering this?

ps. I was in Baku in the late nineties drilling the Shah Deniz exploration wells. Great memories.
Total Posts: 22
Join Date: 01/09/12
We are using them for years now without any issue on offshore platforms. Really good for safety (BOP in place), integrity, space and simplicity. 
The only painful piece is a contingency part in case you get stuck off bottom and need to repeat steps of conventional hangers to set the emergency seals.
Drilling Consultant
Total Posts: 3
Join Date: 01/07/19
I have a really funny experience in the upgrade of the surface BOP stack on a jack-up rig. Upon the request of Client we have upgraded the stack from 10Kpsi to 15Kpsi. I reviewed the scope and recommended to project preparation team to order a flange to transit from the top of the BOP stack to Bell Nipple. The boys decided I was wrong and ordered an incorrect spool designed to fit 15Kpsi flange. When they asked me to do an FAT, I was shocked at the yard. They had really forgotten that the top flange of 15Kpsi BOP stack has a 10Kpsi annular BOP flange. So, we had to order a new adaptor flange.
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