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Clearing the PGB of a large cement patio
01 June 2020
Does anyone have experience clearing large amounts of hard cement from the internal framework of a PGB?  We have a wellhead to recover and I'm concerned about dropped objects when we get the PGB back to surface so I'd like it to be as clear from cement as possible before recovering it.

The cement is 20 years old, the well has been plugged for almost as long.  I've considered both an ROV mounted HP water jet and similarly mounted percussion hammer but while each looks to be effective over time neither looks to be particularly quick. I'm also considering just having at it with a rock-bit on drillpipe.  Any pointers out there as to what may be the best way to get this done?

12 answer(s)
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 491
Join Date: 10/01/05
Great evident case study Allan.

I can take us back much further. To what I always suspected are the root causes to these issues.

Late 80’s one N Sea operator had  11 semis drilling subsea wells. 

Majority  of supervisors knew why it is so important not to drill a Y shaped top hole on the seabed in environments where you cannot jet. North Atlantic etc. 
Ie can accelerate failure of wellhead ssbop marine risers systems and top up cement jobs response reaction and cure time and money would be needed 

One particular well drilled in this period was so badly Y’d that when SSBOP was installed bop wobbled about. So much that hazard and risk further assessed resulted in well had to be respudded. At Significant loss and added expense. 

Rather than get to the root causes and take a more proactive and preventative way to resolve the problem?.  

‘Titus top up systemS evolved‘ that then became the Standard company norm. That others then also adopted. Management incompetence. 

Note: Despite evident well facts that 70-80 wells had also been drilled by that operator in this period that did not need Cement top ups.  

Whether wells needed or not top up Cementation was, has and is still being used to my knowledge? 

30-40 years later? and entering into abandonment of topped up wells. Where I suspect the issue resides! 

Because we predicted this 30 years ago that topped up wells will probably need a lot of ROV cement evacuation to remove Car parks of cement on the seabed from these wells that we observed :). 

Where  future  P&A hazard and risks we did inform operators about. 

My own experience of having to P&A a particular Y drilled top hole well ( that we did not spud) where 4 top ups resulted to fill in the Y cavern at the seabed. Fact was the main contributor to the added 4 days required when abandoning vs expected standard norm. 

Watch this space. West of Shetland In particular we suspect? ROV pilots shall get plenty of time in the chair excavating cement :) When final P&A of these wells Comes around. . 

I suggest RP2 initiates a separate learning evidence gathering sharing thread on final well abandonment of wells that had top ups cementation and those that did not. 

Then based on evident conclusions, assure best practices and learning is then translated and sustained into future subsea wells to be drilled and cemented?  

Ie drill perfect cylinders, from seabed,  avoid Y shaped caverns,  assuring best drilling practices are used / applied in the first instance? 

Finally there is a further discussion needed where CAN’s ie conductor anchor nodes, are now being used in areas where we know cement cannot be brought to the seabed. Eg First 10-15m of formations are  too weak to take a normal weighted cement slurry. 

Where either a light weight slurry is needed an/or a foundation pipe is required ie a 6m Diameter x 6-8-10-15m long CAN dependent on subsurface conditions that exists. 

Offering a better solution to the problem vs Y shaped wells, top up cementations And or repeatedly failed jetting jobs. 

Where every year we are noting more CAN’s being run for all the right reasons? 

Further food for thought for today’s drilling engineers. 

Total Posts: 3
Join Date: 21/12/08
Had a similar issue on in 2013 in the North Sea. Big crater filled with XS cement above subsea structures. From memory it took the oceaearing ROV crew  a few hitches to remove offline. Done with progressively larger jack hammer attachments and rig welder “rake” tools for removal after breaking up. The ROV pretty much worked 24/7 as long as breakdowns/weather permitted (by all accounts the trainees loved it as they normally don’t actually get that much time in the seat).
 In the end was all removed offline with no NPT. 

After that we used the ROV to attempted to make a hole in any observed crater round the conductor to prevent this issue reoccurring. As it was caused by the crater rim rising above surveyed seabed/bop/structure heights.
Consultant [retired Shell staff]
SPREAD Associates
Total Posts: 278
Join Date: 02/09/05
Thinking out of the box!
I would check the details of yhe connection Permanente guide base/wellhead housing. In the designs I was familiar with, unlocking them was practical,

If the guide posts were still an hindrance, I would remove them either by unbolting or cutting.
Managing Director (
Relentless Pursuit Of Perfection Ltd.
Total Posts: 479
Join Date: 10/01/05
We received additional input:

Yes there is a hammer deployable on ROV - Total used it WOS last year. Ask Oceaneering
SPREAD Associates
Total Posts: 161
Join Date: 05/03/08
Hi Warwick,

As mentioned by other posters, I'd suggest looking an the use of explosives as an option - if regulations still allow it on the UKCS.

This was a very common way of recovering Subsea Wellheads in the 70's & 80's when cut and pull equipment was not as good as it is now.

One thing was for sure - the explosive charge would shake off any cement pad that was present!

Another possible option is instead of using a bit on DP, use a bullnose on 2-3 stands of the largest OD DC's that you can handle, with a down Jar on top of the DC's and then DC's (as needed for weight to fire the down Jar) and DP back to surface.

Use the down Jar to hammer the living daylights out of the pad to break it up. You'll obviously need to do some string modelling to make sure you don't spaghetti the string as it's in open water.

All the best!

Drilling Engineering Specialist
Total Posts: 2
Join Date: 05/05/20
Thanks for the replies.  This is a well that was plugged 20 years ago but the wellhead was left intact due to surrounding producing infrastructure.  Recovery of the wellhead is planned to be with the MOST tool which I'm not concerned about at all.  The PGB will be recovered beforehand with a strip-over tool, this is the piece I'm concerned about primarily for safe handling of the PGB at surface, loading on the boat, offloading at the key and then onwards to its final resting place.  The ROV footage implies that the PGB lock ring is relatively clear so not overly concerned with actually getting it off the wellhead.

We had divers in the water back in march and they managed to cut the PGB strakes out of the patio without too much difficulty but the rig doesn't have a SAT spread so this will be ROV/surface manipulation.
Senior Consultant Decommissioning / Subsea Operations and Construction
Total Posts: 1
Join Date: 02/06/20
Hi Warwick - I'm presuming this PGB is in less than 200 m water depth since you are talking about using divers. I would suggest looking at a grit entrained jetting system whcih can be deployed by both divers or WCROV.  They are generally used for internal cutting of  well heads / tubulars  pipelines  and will eat concrete 

The only Issue will be accessing the various parts of the PGB and that might need some tooling modifications. 

Suggest Proserv , Oceaneering, Underwater cutting services
D&C Project Coordinator / Decommissioning
SPREAD Associates
Total Posts: 37
Join Date: 25/01/16
Have a look at using explosives.  Assuming you have recovered the Christmas Tree and tubing, the say there is 9 5/8" to 20" conductor sitting 30" structural which is landed on the PGB.  Need to have a greatly vented pulling too where a charge is set in a position that is just above the 30" connector because a charge won't cut the 30" but can shear the 9 5/8 to 20" was sizes.  Consider cutting and pulling the 9 5/8" casing say 50 ft below the mud line and make sure there is no gas in that annulus, and if there is then placing a cement plug in and outside the 9 5/8" to block gas migration.  Should be two barriers with the first being 150 ft above any water or gas zones. etc 
Total Posts: 6
Join Date: 08/11/07
Morning Warwick,

Your question appears to be regarding the potential dropped object hazard of recovering a SSWH with PGB attached.

One way to eliminate this hazard is to recover the cut-off wellhead/conductor/PGB to a suitably-sized open top CCU spotted on the seabed adjacent to the well.  You don't mention water depth or MODU type, but you will be able to determine what is the safest and most economical way to recover the basket to a work boat and back-load it for clean-up and disposal in town.

From my previous experiences, proving the wellhead cuts and demonstrating that the fish is free is critical to this task.

Trust this helps

Tim Clay

Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 491
Join Date: 10/01/05

Comment from abandonment cutting wellhead and recovering guidebase etc. Using a semi sub or drillship! 

Using a LWI vessel would be more of a challenge! 

Car parks and cement around the TGB / Pgb can be a Big problem. 

Particularly if top up cementation ‘titus systems’ etc were used.

People were warned and advised to look into the future  wrt future permanent abandonment. 

Plan A would be to use the MOST tool that latches on to the HPWHH or similar as already described.

0ne must keep cutting until you see the pressure drop on the tool. Only Then you know the conductor is 100% circumferentially cut.? Not continuing to the pressure drip is the most common failure. 

Cut deep enough so you can have a 2nd attempt shallower if 1st cut fails as I often the case.

The major headache problem is then to break the PGB and  pipe from the car park and iceberg of cement that may exist. Note:,This might not be an issue due to nature of subsurface conditions. Seabed’s are rarely the same! This can play in your favour.

However metocean effects May be a valued aide to recovery. Eg Heave pitch roll with compensator now locked can potentially generate forces and impact needed to work PGB and wellhead free. 

We have then observed icebergs of cement On intital release that raised moonpol handling concerns.

What resulted was that cement has generally Fallen off before we get the unit to the moon pool or on the deck of the LWI vessel. 

Working it free is likely the biggest hazard /  risks 

Plan B is release MOST tool after PGB and wellhead casings are cut.

 RIH with MOST tool again with a surface jar to add more force acceleration to work and break free from seabed shallow cement. 

With a LWI vessel you really need a mast so you can run pipe and a force multiplying assembly to break cement bond free. 

If you only have a crane? 

Or some type of device on the heave compensates crane a Subsea jack hammer or conductor hammer simple to hammer the cement into submission to break pipe and Pgb free. There are options. 

Use the devices that are often used for installation the purpose and goal this time simply to break the bond to the cement at the seabed!  

If all of the above fails.
plan C is you need to work a solution to excavate  cement around PGB with suitable ROV or other subsea tooling. And getting a diamond cutter to cut through wellhead.
and seek dispensation if rules state you have to cut X metre below the mud line. 

Particularly west of Shetland where some operators made top up celebration the standard ( whether each well needed a top up or not!) vs solve the cementing problem in the first place. 

This will cost in abandonment many tens to perhaps hundreds of millions of extra dollars to recover! If they don’t  come up with a more bullet  proof system to cut and recover as suggested above. 

Where additional excavation and Novel new technologies may be warranted. 

Or not as each case may be! As stated metocean effects can be a powerful natural set of conditions to break release and pull steel and cement off a seabed. 

If you are in a flat calm benign sea or ocean the you have to generate force impact effects to break Pgb and well head free.

in the past ( 80’s 90’s) plan C / D, we used SEMTEX Sam! Not sure if you would be permitted explosives anymore!  


Watch this space. 
Well Abandonment Manager
Total Posts: 10
Join Date: 30/09/13

Hi Warwick,

I have an example from around 2010 in the UK where we were called in to provide a solution to a UK client for cement patio break up and removal around a PGB and 30" x 18.750" subsea wellhead - after all attempts with a bit and fishing bumpersub assembly failed. Continuous damage to bumper subs.

Basically we designed and operated an AWJC system from the rig to run on a 'bucket connector' which swallowed the 18.750" wellhead. From the connector we had a split frame hydraulic driven table where we had a 'Linear Rail Manipulator' system with radial and Z axis grid path and the ability to run the AWJC nozzle and cut the cement patio into small blocks. The cement was removed very effectively.

Not cheap but effective, quick and saved a lot of rig time.

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