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.