Hi Myles, Jamie, Christian
Just a couple of comments. The post from Myles relates to my post and responses received back in September / October last year "re-entering old abandoned wells", so you could have a look at that.
A major reason to leave the wellhead in place is for CCS where the well may have to be re-entered to re-cement with slurries resistant to CO2.
Or you could try and make a case for leaving it for future recoil tests of the LMRP and marine riser system.
As an aside to the specific question, different governments have varied views on what can be left on the seabed, being related to the maximum depth for commercial fishing or other users of the sea, and influenced by whichever international standards that country may have signed up to.
The maximum depth for commercial fishing is around 600m.
Accordingly, what is deemed as "deep water" is varied as well. For example, I believe that Norway works on 600m, Angola's presidential decree has a cut-off of 400m, Brazil is 800m, GoM used to be 800m as well, most of west Africa is 700m, but Vietnam is over 1000m. With the UK's goal setting regime, depths are not specified.
It sounds to me that there must have been a good reason for leaving wellheads in water depths of 160m or 190m - that's appears to be a little too shallow!!
Hope this assists
Talking specifically about subsea wellheads - a few years back we were asked to attend a meeting with an operator to offer up how 1) we can find the exact locations of previously abandoned subsea wells and 2) reconnect to the casings and conductor previously cut at 15' below seabed.
The reason for re-entry was never disclosed to us. Obviously a difficult task to reattach.
We have just finished a P&A project in Malaysia where the wellheads were left in place after reservoir and intermediate abandonments and that was around 160m water depth.