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Drilling conductor section through template (semi)
04 May 2015
Putting together detailed programme for drilling 42" x 26" hole through a (4 slot) Aker template. Would anyone be able to provide any best practice guidance, particularly around the following subjects:
  • Minimising washout around the wellbay insert such that returns come up the sleeve (to be removed by a CTS system) rather than into a crater and cover the template
  • 26" Bit / 42" HO configuration. Normally use a 10ft pony b/w in conventional wells but concerns around the 26" being off-centre within the 42" when the well is spudded and the HO subsequently sitting down on top of the well bay insert. Best option looks to be dropping or significantly shortening the pony such that the well spuds with the 42" HO already inside the well-bay insert sleeve but wonder if there is good practice that has been developed in this out there?
4 answer(s)
Drilling Superintendent, Thistle / Heather
Total Posts: 3
Join Date: 27/05/14
Thanks for all the replies on this. We've now batch-drilled these 7 wells. Decision was made to drop out the pony between the bit and the hole opener. Commenced drilling at very controlled flowrate and WOB, staging up as we went. Hole openers all just lasted one well, tried one 36" bit, the remainder 26" bits.

Achieved all objectives, 26" bit was more successful, optimised parameters as we learned, good cement returns seen at seabed without any signs of washout round the Well Bay Insert on the template. Very tight tolerance b/w hole opener and WBI but managed that.

Always good to provide feedback so thanks again.
Note from moderator: Thanks Malcolm, always good to close the circle. Good luck with the rest of the programme.
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 361
Join Date: 10/01/05


The riserless mud recovery system was actually originally designed to pump cuttings away from a template or a coral reef etc. So this is an option if one believes (sand/silt) cuttings piling up over template could be an issue.

First check the site survey stratigraphy data for clay and sand sequences. Any glacial, debrite zones also present etc? If no data exists, consider a penetration test on 1st well to planned conductor depths, i.e. to differentiate between clays and sands, hard zones etc, is a worthwhile exercise in my view.

If the first few metres were penetrable then as intimated a foundation pipe below each slot would have made perfect sense.

Once you have the shallow sections soils data, you can then accomodate a plan to best suit the needs and requirements present. e.g. Clay dominant void of stringers is far easier hole to drill than sand dominant or if some thin glacial deposit surprises are present. Then everything must be changed and optimised to suit as these two scenarios are like apples and oranges.

e.g. To avoid a Y shaped washout at each wellhead and to assure cement to seabed. This can be easily mitigated through a common sense best practice approach. i.e. allow BHA to self penetrate then stage pumps up. (Remember initially bottoms up is very short and very quick. Only as a function of section length is pump rate increase needed.)We know this works as we have both ROV snapshot of caverns at the seabed when PLF is used, and a pretty decent cylinder when best practise as recommended are applied.

In clays, cuttings will disperse, higher ROP is possible at far lower pump rates. If sand dominated sequences exist then this is where cleaning, pump, and ROP problems have to be carefully managed. e.g. In a 42" hole a 50bbl sweeps as commonly stated, in a 42in hole does not compute.

Also avoid drilling with a slick assembly BHA (this is a no no in my book), use at least 9 1/2" collars (twice as stiff and rigid than 8") If not BHA will wander and if you hit a stringer BHA could kick the hole sideways and on next stringer go off in a completely different direction and this is why when and how tripping, drilling reaming problems etc etc and/or a conductor hang up could possible result. Thus stiffness and rigidity to drill a useable straight hole as written and recommended in the 60's-70's e.g. Lubinski! avoids such typical top hole problems.

The 3rd party will also likely tell you a motor is the optimal assembly, again no data set has ever convinced me to this. e.g. If the motor is above a hole opener, just compute for yourself the peripheral and rotating speed of the HO cutters. Where in certain formation types, stringers etc, cutters can skid and you will be tripping for a tool failure. You also have no control over rotational speed and pump is limited by motor capability. 

The complete BHA can also be made up shore side, RIH, POH and sent back top beach all as one. This we learned is a far cheaper (rig time savings are significant) and safer handling on rig floor etc. note: Some modern rigs are more constrained how much BHA wt they can handle on skids, pipe conveyors etc so bear this in mind and check with drilling contractor what they can and cannot do.  

Therefore Applying lessons learned, the section should take 4-6hrs to rotary drill with an optimally designed rotary assembly. Projects data can be supplied to substantitate just this.

On Conoco's Heidrum platform they concluded drilling four conductors holes (back to back) and then running 4 conductors (back to back) was optimal is such a batch sequence operation. 

Check the hole openers and nozzle properly!

Note: Most HO are pretty gash so go to the yards and check them out for yourself is my best practised advice. Nozzles installed also generally make no hydraulic optimisation sense!! i.e. This section being essentially a jetting exercise!  

I would run a 26" bit a 36" hole opener about 2-3m (6-10ft) a short pony (at times we have run an 11" here), a 36" stab, (inclinometer), 9 1/2" DC's etc above this.

To avoid big rat-holes note a 3m (10ft) 26" lead below 36" HO is a good practise in conjunction with this.

The 42" hole opener then on top for 36" conductor length needed. Considering two joints of 36" conductor, as max bending moment is generally about 10m below the mud line, then cross over to 30" is also a common configuration.

Ref best practise HO report as attached.

Documents uploaded by user:
hole opening .pdf
Managing Director (
Relentless Pursuit Of Perfection Ltd.
Total Posts: 413
Join Date: 10/01/05
Hi Malcolm

I have asked my former Shell colleagues for their thoughts when designing the Central Cormorant UMC and hope that they will report back.

I know that one of the things that we did get involved in (on the Pelican wells) was "dredging" when we found that cuttings ended up all over the wellheads. I can't exactly remember the name but it was something like "Mud Hog" and the ROV needed to be spec'd  up to be powerful enough (pump) and the discharge lines needed to be long enough.

Not much help on BHA design but at least a contingency plan in place,

Kind regards

Subsea Engineer
Applied Drilling Technology International
Total Posts: 6
Join Date: 11/02/10
Could the template not have a large funnel below each well slot that would be just hovering above the virgin seabead?
This would force the majority of the returns up through the slot rather than going out to the sides.  Extra cost added to the template I know, but just a thought.

I guess the major downside of this is that you would have very little vision below the wellhead to check if a top up job was required.
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