Private Forums
01 July 2020
Hi all,

I wish to pick the brains of the forum.

HPHT subsea well in North Sea in only 80m WD in around Q3/Q4 coming into winter time.
Really cries out to use a jack up rig but I'd like to ask you all for any feedback you can give me that disqualifies even trying this well with a mid-water semi.
  1. What is the min WD you could run with a Below Tension Ring (BRT) RCD?
  2. Running with Above Tension Ring (ATR) RCD reduces the slip joint stroke available and not wise during the harsh winter conditions in the North Sea were unlatching for bad weather WOW NPT would increase somewhat.
  3. Has anybody used a mid-water semi in the North Sea (UK, Norway etc...) at the same time of year and what was your experience?
  4. Any advantages in using a semi at this time of year over a jack up?
  5. Any disadvantages in using a jack up instead of the semi?
Many thanks

Ron Mackay
16 answer(s)
Drilling Supt
China National Offshore Oil Corporation
Total Posts: 11
Join Date: 24/09/09
at this water depth, no need to use Subsei Platform. usally we use a jack up rig.
Drilling Supt
Neptune Energy
Total Posts: 6
Join Date: 18/05/15
The contact with the drilling contractor normally states the rig is on contact at the 500m zone or midway between locations. The rig would not move unless there was a weather window to get on location. So there is little or no risk to the operator getting on location. Getting off location is a risk for a jack up. Normally needs 1.5m waves or less, and the wave period can be an issue from smaller jack ups or where periods are very long like the west coast of Australia.
Brad Muir
Wells Management Consultant
SPREAD Associates
Total Posts: 1
Join Date: 24/06/20
Jack up definitely preferable but of course the big risk is getting onto location and jacked up on the front end and down and away post well ops. I’ve used semis in 75-85m WD on the Grand Banks where weather conditions are similar to the UKCS / North Sea. Biggest issue we had to deal with was heave response vs stack height and the surge response impact on maximum anchor tension. The surge induced anchor tension that was predicted from the mooring analysis in the 10 - 12 sec sea periods required us to go K4 chain. 
SPREAD Associates
Total Posts: 153
Join Date: 05/03/08
Hi Ron,

Just one thought about JU vs. Semi.

As many have noted, a JU is obviously preferable from many viewpoints.

The key risk is clearly WOW to get a JU on or off location.

Given the water depth, even a DP semi will likely need an eight anchor spread (which we've just had to do in a project offshore Mexico).

Now, it's been many years since I've been in the NS, so I don't know what the restrictions are with respect to weather criteria for anchor handling or moving & pinning a JU.

But I found that the limitations on weather for handling anchors on the recent job (200MT bollard pull AHTS's from the NS), they weren't that different from what the wave height restrictions USED to be for moving a JU into location.

So perhaps an argument could be made that the weather window required to get a JU on or off location is actually smaller than that for a Semi in these water depths?

Might be talking out of my backside, but just thought I'd throw that out there!

All the best!

Rig Intake Advisor
Relentless Pursuit Of Perfection Ltd.
Total Posts: 14
Join Date: 06/01/07
Myles, Louis, Ramon, Emil, Peter and Graeme,

As per my earlier post. Thanks very much for your time to respond and valuable insight. It's very much appreciated and much to consider going forward.

Many thanks again guys

Drilling Supt
Neptune Energy
Total Posts: 6
Join Date: 18/05/15
I am planning a 4 well, 90m WD,HPHT subsea development using MPD in the North Sea now. I did consider both Jack up and semi until the borehole result showed it was okay for a jack up. Many advantages using a jack up including cost (at least 50% cheaper), WOW time, well control monitoring, wellhead fatigue and others. If you are running a Xmas Tree you need to select a jack up that can do this, a lot say they can but those are the sales guys. I have more information to back this up if you want.
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 471
Join Date: 10/01/05
Short answers to 3,4 and 5 Ron.

Lot of comments already afforded on the MPD parts.

From an operability standpoint. In a worst case 10, or 50year North Sea storm, you don't want to be having a semi with further marine riser restrictions and constraints in these water depths. I have experienced Snr Toolpushers spending days and nights in such storm events in shallower water depths <100m in the moonpool with their fingers on the button ready to hit the disconnect button.  That's how critical this becomes. Worst case conditions often results just after the main storm has passed and as seas still continue to pick up. Particularly if there has been no major wind shift to dampen things down.

Safest operating  Plan to drill these more complex wells is also within the spring to summer period. 

Heavy duty new generation Jack up with its surface BOP and an most appropriate MPD system to suit your well should be the least complex / risked based solution.

Jack up main disadvantage is getting rig on off station in autumnal winter operating periods. This is reduced but never guaranteed in the more favourable operating months. 
Furthermore get the right people involved and put lots of time, effort resource into the pre and detailed planning required. Do the studies needed. To assure you fully understand all hazard, risks and uncertainties involved in these wells.   


Emil Stoian
Tech Manager
Total Posts: 1
Join Date: 02/07/20
Hi Ron,

Just to add a note to the excellent previous answers received, in addition to the higher costs the potential for having problems with pressure control in rough seas is very much there - this could prove very challenging.

While performing connections, the most repetitive and critical process while drilling with MPD, you will loose compensation of the drillstring which could cause swab and surge pressure on the bottom as the string moves up and down. There are different opinions on the magnitude of the oscillation but the phenomenon is very much there and it will have an effect on the well. 

MPD Consultant
SPREAD Associates
Total Posts: 3
Join Date: 28/12/15

Looking at an ATR job like you describe but on Mediterranean,  floater min depth capability 70m., different environment. 
Hose damage on the external side of the jacket. 

The BTR stack I have drawings are 30 m long. 

MPD Consultant
SPREAD Associates
Total Posts: 1
Join Date: 02/07/20

There are a lot of pros for the jack up if you can get away with it.

Much more in the way of up-front cost for the operators/rig/ service providers using the ATR and BTR; modifying slip joints, crossovers and the rig time for potential modifications. This is not to mention the online rig up time.

With the standard jack up set up, it is an off-the-shelf option with minor modification required for the low pressure riser height to suit the rig set up and this can be done offline, the  RCD would bolt straight to the annular in most cases.
Regional Deepwater Drilling Manager EMED
Noble Energy, Inc.
Total Posts: 18
Join Date: 24/06/14
Allan mentioned the 3-part slip joint.  I used one for ATR in 7,000ft water depth a while back in Indonesia with extremely benign metocean conditions, and even with a 3-part slip joint the overall heave capability was a little less than half what the standard slip joint for the rig provided.

Then you end up playing games as to whether or not you want to space out mid joint , or be a little closer to the top to allow for greater lateral excursion of the rig, however then you start to worry about the riser recoil because you are now very close to the diverter.

For the water depth you are talking about there is a lot to be said for a jackup.
Rig Intake Advisor
Relentless Pursuit Of Perfection Ltd.
Total Posts: 14
Join Date: 06/01/07
Hi Steve, Allan, Angus and Simon,

Thanks very much for your valuable insight. It's very much appreciated and much to consider going forward.
Sorry about my Q3/Q4 start date information being out by one year. Potential start date wouldn't be until 2021. So, gives us a little breathing space but just on a little.

Many thanks again guys

Well Engineering Mgr
Zenith Energy Limited
Total Posts: 8
Join Date: 13/09/07
Hi Ron

Have looked at the jackup versus semi questions a few times.

At 80m I think you are on the minimum WD limit, you would have to get a riser analysis including the MPD components,  and then look at the riser excursions limits and whether the rig's mooring system (or combined mooring / DP) can handle it. 

You could also get an analysis to look at hull movement - I had this done by MCS Kenny for rig choices elsewhere, using a 100 year weather return and the rig's RAO data, they can model operating uptime for a given excursion limit.  When you are on the limit with minimal permissible riser excursion in shallow water, you probably find your operating uptime with a semi is pretty low - the theoretical modelling may show you might have to stop operation or disconnect for 30-40% of the time in winter.  (Similar for trying to do subsea ESP well tests West of Shetland in winter on a  4th gen semi - a lot of pain). 

Putting the RCD on the semi riser changes the load conditions for the riser substantially.  It can now have up 3000 psi pressure trapped inside it under the RCD, compared to being free vented (or 500 psi if it has a surface diverter).  That means the riser analysis has to be done taking that into account - the standard riser analysis for a 3rd gen semi will not have looked at it. 
Most of the modern deepwater semis that come pre-equipped with RCD (or gas handlers) have much higher riser spec than older North Sea semis. 
Last time I looked at putting a rotating head on a semi the lead time for design fabrications and installation was 12 months, but that might have come down a bit due to lower activity levels. 

On the jackup side, look at the geotechnical aspects before moving forward, there are some areas of CNS / NS (Witch ground for example) that are not amenable to siting a jackup.  You will likely need a geotechnical borehole, so you'd need to move fast to get that sorted this summer season for drilling this winter.

One you have the jackup on location, then I think there is less weather exposure for general operations with a j/u.  If its a big j/u you also have the deck space to store equipment which makes it easier to manage vessel working conditions through winter.

Recently, the cost differential between jus and semis has been heavily in favour of jackups, but it depends on rig market conditions.  I think there is more weather exposure on the rig move with the jackup, but spending on market conditions you may get the drilling contractor to accept some of that risk if it enables them to keep a j/u unit operating through the winter. 

Simon Lucas

Senior Drilling Engineer
Springbok Drilling Ltd
Total Posts: 8
Join Date: 20/02/12
It looks like the previous answers have covered the topic well. My initial thought was that the water depth is so shallow you would never be able to maintain the watch circle without triggering the auto-disconnect! 
Operations Manager / HPHT Drilling Superintendent
SPREAD Associates
Total Posts: 4
Join Date: 08/04/18

I have experience with this water depth (82m) and using a semi-sub (a new build Friede & Goldman L-907 Enhanced Pacesetter) offshore Nova Scotia, Canada. The metocean conditions are on the same order of magnitude with the N. Sea. Storm events off the East Coast of Canada tend to be more severe but are overall of a shorter time duration. This was for an HPHT well but formation pressure vs. frac pressure window was sufficient to not to need MPD.

In any case the Canadian government only permitted this water depth with semi-sub for the summer months weather window. And that was only permitted with minimum strength K4 anchor chain. The anchor tensions required are so high to limit flex joint movement to within operating margins and percent water depth excursion from well center for that water depth that there is not sufficient anchor tension safety margin for storm conditions. The space out between the bottom of the slip joint and the top of the flex joint is very small, only about 2 plain riser joints if I recall correctly.

Even for this case of 82 m water and in these summer months, if the weather kicked up at all, the anchor pattern showed to be very stiff making rig motion characteristics very quick and almost snappy in reaction.

Heave conditions in 80m water depth will not be the same as in 200m for example. Maximum excursion from well center is operably greater than in 200m vs. 80m. 

Once on location and spudded, WOW time with a jack-up will be mostly crane operation limitations and supply vessel resupply related. Although I did experience a well in the Dutch sector of the N. Sea where the legs scoured so badly in a storm that the diverter overshot lost its seal around the BOP riser extension.

Thus even without the space out and relative position of the MPD considerations, I can't imagine any N. Sea regulatory body allowing a sem-sub to operate in the middle of winter in this water depth, especially for HPHT operations. Maybe for the exception where the semi-sub had azimuthing thruster assist for added station keeping capabilities. But that is going to be expensive.

Best regards,
Well Engineering Consultant and Instructor
SPREAD Associates
Total Posts: 41
Join Date: 23/03/16

For 80m of water your are shallow. The below Tension ring MPD riser assembly is about 25m long you then need to consider the height of the BOP and LMRP and space out the riser accordingly. You are probably at the minimum water depth limit.

If you are indeed looking at using a semi would still recommend a BTR as that still provides the rig with the same heave capabilities. 

The challenge with an ATR would be that the ATR requires a three section slip joint and will restrict heave movement so probably not the best solution for a North Sea well in the winter.

A jackup would probably be cheaper and the MPD setup is much simpler when compared to a floater as there is no heave to deal with. My recommendation would be to keep it simple and go with a jackup, as well control is simpler and less impact of weather related issues. 

Installation of a below tension ring system with the flow hoses and other lines takes time and the weather could cause significant delays with the deployment of the MPD riser.  
Posted by

Ron Mackay

Rig Intake Advisor

Relentless Pursuit Of Perfection Ltd.

Total Posts: 14
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