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Experiences with bow centralisers on semi-flush Q-125 pipe
23 March 2009
In context, the story goes like this. We are looking to use a standard GoM (sub-salt) well design around the world with a deepwater drillship. Rather than chop and change casing designs we see a merit in keeping casing sizes/grade/connection constant (even though all strings below 22in are either semi-flush or flush). An obvious downside is when a particular well design doesn't require all the casing strings and here, specifically, we have no need per se for centraliser subs and bows would do.

However, the 2 questions I have are:-
1. Have folk run bows on semi-flush/flush pipe and seen problems and, if so, what...i.e was the solution that subs had to be used or a change to the bow fastening ?

2. Conventionally 1 stop collar either is used. Given hard "Q" grade, is there any experience suggesting 2 stop collars might be advantageous i.e. double chance of stop screws 'biting/binding' the casing ?

Thanks
Mike
Hess, GoM
9 answer(s)
hossybhoy
Global Technical Advisor, Well Completions – BP Account
Weatherford
Total Posts: 10
Join Date: 07/08/12

Running spring bow centralisers on flush or semi flush casing is always going to have risk associated with it, NOT because of the centralisers but due to the risk of the stop collars slipping on the pipe. This can lead to "bunching" of the centralisers or if semi flush connections it can lead to the centralisers / stop collars being split and lost down hole. Set screw stop collars rely on the point loading of the set screw on the pipe to prevent slippage and the choice of screw material can make a huge difference to the load rating. WFT have also used an epoxy resin injected under the stop collar to increase the load rating of stop collars and there is also the option of "friction lock" stop collars out there.

Having been involved in many wells using tight tolerance well designs with flush or semi flush connections my recommendation would always be to go with centraliser subs even with the big cost implications of using these. We have fitted these to the casing joints onshore and repackaged the subs to prevent damage when being transported to the rig site so this speeds up casing running on the rig.

PaulHowlett
CEO
Sudelac
Total Posts: 107
Join Date: 10/04/08
Dave, Its hard to believe in 2019 we are still asking questions about the holding forces of stop collars with grub screws. No matter who the supplier of slip on or hinged stop collars are that rely on a number of grub screws/set screws to hold them securely in place on the pipe they all rely on the correct preparation, a process and correct torque to be applied on each and every one of them, so there is always a risk that some of these are not done properly. Whenever vendors come on and provide holding forces of grub screwed stop collars its based on test results when everything has been well controlled, including the push force applied during the test. Real life is often not like this testing, including the knocks and bangs and movements of pipe as it goes into the well including various things that are not casing or open hole either. Whilst I have supplied many many slip grub screwed stop collars over the years without reported issues if I was hiring a rig and installing casings with centralisers in my own wells I would not use stop collars with multiple grub screws on them. I would use something else in my wells.
ReneWerker
Drilling Supv
SPREAD Associates
Total Posts: 2
Join Date: 15/06/16

Hi Dave,

Centralizers …… my ‘favorite subject’ …… let me chip in my two pennies worth.

Considerations:

1.       Narrow (between casings and sub mudline profiles) clearances / tight tolerances.

2.       Swab and surge pressures – and effect of tight tolerances on such swab / surge pressures.

3.       the ‘need’ to centralize (obtaining good isolation across HC bearing zones is pre-requisite, good stand-off across the entire cemented zone is in my view NOT a good argument. Do not hesitate to ‘challenge’  the ‘need’ for running  centralizers )

4.       cost (think about the considerable all-in composite day-rate)

5.       centralizer / stop collar design (numerous, numerous different types and configuration – both for centralizers as well as stop-collars) and the ‘individual designs (or components) associated structural integrity .

6.       hole stability and well bore cleanliness

7.       evaluate alternative for obtaining stand-off (e.g. Protech composite systems)

 

In my experience :

a)      in deep-water operations, the running of centralizers in tight tolerance casing schemes can be challenging (read : running casing down to landing shoulder / setting depth and subsequently getting it properly cemented can turn into a genuine drama and very costly affair). In this context and my experience 13-5/8” x 16.0” is a ‘bugger’

b)      surge reducing equipment (e.g. diverter subs) works very well for those hole sections with tight tolerances / narrow clearances (often ‘self’ induced by running  [too many] centralizers).

c)       time spent cleaning a stable well bore, prior tripping out for casing is time well spent  

d)      suggest to take due consideration of your all-in composite day-rate, and use ‘best for well’ only. Once again, a cheap centralizer / stop collar combination can turn the ‘securing’ of a hole section (i.e. running and cementing casing)  into a nightmare

e)      I have good experience running centralizer subs (better clearances and simultaneously better structural integrity).

f)       Running  centralizers across couplings is not recommended. It compromises deflection / clearances and will turn installation into an ‘on-line’ activity ($$$). By default – in case of desire to run across coupling- these would have to be of the hinged type (prone to down-hole failure)

 

In summary : challenge the need to centralize, challenge the number of centralizers to be used, use ‘best for well kit’ only (centralizer subs is the better ‘piece of kit’) – it should eliminate time consuming and very costly train-wrecks, prior committing to running casing (especially in tight tolerance / narrow clearance wells) ensure wellbore is clean, and last but not least minimize swab / surge pressures. 

Regards,

Rene          

Scott_McNeil
Consultant
SPREAD Associates
Total Posts: 140
Join Date: 05/03/08
Hi Dave,

I have installed centralisers over couplings on large sizes such as 20" and 18 5/8" and not had any issues.

But I wouldn't do that on any smaller sizes of casing - my normal preference is to put the centraliser underneath the casing collar with a stop collar underneath the centraliser.

One reason for this is the centraliser can get pushed up against the collar and poorer quality centralisers will pop out the bows, creating more resistance and also junk.

However, my main reason is that I've twice had to pull liners when the collar was pushed up, such that the bottom of it was hard against the casing coupling and this effectively bowed out the centraliser around the coupling.

This increased the OD (coupling OD + 2 x spring blade thickness) such that it wouldn't fit through the previous liner hanger and the entire string had to be pulled.

All the best

Scott
admin
Managing Director (rp-squared.com)
Relentless Pursuit Of Perfection Ltd.
Total Posts: 441
Join Date: 10/01/05
Hello all

I would like to re-open this discussion.

At a recent workshop its was suggested that we consider installing bow centralisers over casing couplings as we ran the pipe.

Some reasons were give, but I have always been taught that this is bad practice (I can think of at least two reasons not to).

However, there may be others in our membership who have a different opinion/experience.

What do you ALL think (2600 members in 1100 organisations)?

Many thanks

Dave
admin
Managing Director (rp-squared.com)
Relentless Pursuit Of Perfection Ltd.
Total Posts: 441
Join Date: 10/01/05
We have received the following response via Peter Aird of Marathon, Uk ..

QUOTE ...

The following relates to Stop Collars but on the forthcoming GoM Deepwater Drilling I would be more than happy to look at the 'applications' side. If you so wish, please send me initially the details of casing sizes and weights with combinations and OH sizes you will consider for the project.

Pre-grooved Casing
On your response to the discussion forum question regarding grooved casing I need to clarify a couple of points:

The groove system originated on a joint exercise between Weatherford and Shell back in the 1990's and was specifically applicable to the thicker walled MUST casing, mainly 7-5/8" and some 10-3/4" used to combat Salt zone loads. They never really proceeded with the special Weatherford centraliser but retained the groove method to locate some pretty horrible stop collars with a bunch of set screws - the idea was always to achieve a low profile commensurate with the comparatively narrow annulus between 7-5/8" and i.dia. of 9-5/8" casing. Fitting was a pain, the screws had been end profiled machined but still didn't fit the groove, claimed axial holding loads were a figment of imagination.

You have already commented on the simple method we employed. There were a couple of teething problems but the final product now gives ca 45 tons axial holding force.

If the wall thicknesses you intend allow, (for the said grooves i.e. 4mm radius x 4mm deep), then there may be some approval from Shell required.

Centek conventional stop collars
We have regularly run our standard stop collars with M12 socket set screws on flush and semi flush casings, providing annular clearance was sufficient and have not experienced slippage and subsequent bunching. We use the same material as our centralisers and heat treat to ca 94 tons/in2 tensile, the reason being that investigations showed the 'Hoop strength' of the main body was not the limiting factor as to how much axial holding ability could be achieved - it was extrusion failure around the 'effective' diameter of the threads. It followed that changing tensile strength through heat treatment allowed for much better make-up torques of the screws and substantial real axial holding ability, before onset of slippage

It is of paramount importance that correct set screws are used - they must be of the 'forged' variety, cup point. Cheaper units are machined and have inherent weakness. To avoid splitting, lower makeup torques are applied therefore resulting in slippage at best at too low an axial holding figure. In practice, torque application is frequently uncontrolled and these cheaper set screws do not have sufficient leeway in their strength.

Note: API 10D allows the stop collar to slip 4" to achieve a ridiculously low axial holding figure - so you have a number of axial grooves damaging the casing surface, (in excess of levels allowed in API 5CT), for an axial holding figure that bears no resemblance to current field requirements - the Standard is inadequate!

Finally, assembly of stop collars onto the casing is just as important as the selection of the correct centraliser for the application. Too often this function is:
Delegated, without adequate supervision.
To the least technically qualified personnel.
Without the correct tools
Frequently under inclement working conditions.
Our practice is to send with each shipment a copy of the 'Installation Instructions', which further containing recommended tools and sources, (copy attached)

Centek heavy duty stop collars
A new customer had traditionally used a so called 'heavy duty' stop collar with extra width and extra set screws. Investigation could not duplicate the axial holding claims being made or confirm the level of casing scoring. The set screw diameters was in our experience too small and of too fine a thread pitch, which on high tensile set screws has a high risk of thread corrosion weakening the set screw.

We utilised our conventional product approach but with additional screws and product width, taking care that distances between set screws gave sufficient bending strength for the torque applied. Again, the whole product is heat treated.

Attached are sample data sheets for 7" & 9-5/8" and it should be noted that our reporting method is to state the axial holding load at start of slippage - not after 4" slippage.

Correlation of Axial holding for different casing materials
During the HD stop collar investigations the question arose regarding axial holding ability on harder materials.

Firstly, from an engineering standpoint the materials we use for casing are not particularly hard. There are various tables which correlate hardness e.g. on the Rockwell 'C' scale to material strengths. Our tests were carried out on 2 separate casing materials and it could be seen that the results were spot on a graph constructed from such tables, (copy attached). It was so close we felt reasonably confident in using this graph to estimate axial holding forces for various materials.

Hope this all helps

Andy Jenner
Centek, Germany

.. UNQUOTE
WilliamCurry
Drilling Engineer
Oil Search Limited
Total Posts: 2
Join Date: 18/05/06
Mike,

You have raised a most interesting set of questions, and received some equally interesting answers.

The discussions/answers you have already received about centralisers are quite valid, in that it is far preferable to pull a spring bow in the hole than push it. Spring bows can get torn up much more easily than the solid slip-on varietes, etc. I've always been tempted to manufacture and sell my own design of centraliser, as I feel the industry needs a better centraliser than what is currently available. If you are planning to "pull" any centraliser in the hole, then make sure the stop collar on the inside contacts the ring and not the bows. Run the strongest bows you can if you go for the sping bow type, as some centralisers can certainly look terrible if you ever have to pull a casing string. Some centralisers have very strong bows, but need to be pushed into the hole because there is no ring inside the leading end for a stop collar to contact.

The primary reason I am preparing a response is because I feel there is a design of stop collar that hasn't received the attention it deserves. Weatherford bought a California company called B&W many years ago. B&W, in conjunction with the needs of a few operators in the area who were drilling horizontal wells (some in excess of horizontal) back in the 1970's, created a huge range of centralisers, scratchers and stop collars, and devised methods of experimenting with and modifying all their tools. As soon as Weatherford bought B&W, most of this equipment disappeared from the market place. In all fairness to Weatherford, I'm sure the market place just didn't recognise or demand the need for such a wide range of equipment, especially due to the down turn in activity during the 1980's. Fortunately, one particular stop collar still seems to be available on special request, and that is the B&W DA50 Slim Hole Stop Collar. There is a non-slim hole version, but I believe the design got changed somewhere along the way so it now has a rather torturous OD profile. The slim hole version is however the same...as far as I can tell.
In two totally indepent tests of stop collar holding capability (both were sponsored by two different major oil companies, and conducted perhaps a decade apart back in the 1970-80 period), the DA50 proved to be the only stop collar with any significant holding power, producing three to four times as much holding power as any other stop collar (i.e. those with set screws, twisted nails, bolts, etc, and all installed by the company selling those items). The DA50 has a number of slips positioned around the diameter of the stop collar. The slips are activated with a spring metal band which is simply hammered into a recess. I'm sure the slips and ring are both expensive to manufacture, which is probably why the industry opted to use/buy other/less costly stop collar designs, especially the set screw variety, which every centaliser company seems to have.

For a flush joint casing string, I would have severe doubts whether any stop collar will stay where it belongs if hole condtions are in any way difficult while running the pipe, but certainly the DA50 should give the best chance of keeping your centralisers where you want them.

I bought some DA50 Slim Line stop collars a couple years ago from Weatherford, so hopefully they are still available today.

Good luck, I hope this has been of some help.
B. Curry
Companyrep
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 421
Join Date: 10/01/05
Comments received from a UK company centraliser representative.

A bow centralizer is more than suitable in such conditions so long as the following is taken into account.

Standard multipart construction bows of either welded or interlocked type are inherently weak. They are over-sized and tend to create drag rather than reduce drag. They are weak by build and must be compressed to fit the bore hole and activate the pre-load required.

Energy absorbed by radial forces acting upon the units, which are designed primarily for vertical wells, is greater than the means of retention at the end band section and they become easily damaged.

Other important points are the hinge pins as here the centralizer are only as strong as the actual shear of the pin.

Casing design is also important as squeeze down of the bows does and will lead to set of the bow once the yield of the material is exceeded.

To overcome all such difficulties this UK compnay manufactures single piece units with no welds or interlocking devices. Units are slip - on, extremely robust yet retain flexibility. Units are push fit in std casing design mode with zero start and running forces. Unit variations are available for sections with Under-reamed well designs where the units must squeeze down, pass through the previous set casing yet flex out to the open hole size. Such units are dependent on casing well design.

Such units have and are being run in the North Sea for a number of companies now, not to simply replace the ”œsubs” but to offer the correct solution.

The biggest problem with such casings is the stop collar retention. Various have been tried such as nails, set screws, bonded materials etc yet most all fail.

The placing double stop collars will not actually give double the holding force. The key element is torque at the set screw, when this screw is over torqued the body lifts away from the pipe and stresses the set screw thread or the body of the stop collar. Thread stripping then occurs and stop collars move. Like the centralizer the Uk company heat treat the stop collar to 39-41 rockwell hardness, set screws have a larger surface area contact and have penetration onto the pipe. Screws are high tensile in manufacture and torque make up is set at 35~/ft made up with an impact torque wrench. Heavy duty collars are also available.

Companyrep
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 421
Join Date: 10/01/05
As most malfunctions of conventional Bow centralisers can be attributed to Stop collar slippage, resulting in centralisers bunching, becoming damaged, and potentially leaving junk in well etc often with further consequential difficulties? Two would be preferred but as you have stated in large size casing we have always tended to run only one i.e. with the centralisers wrapped around a single stop collar. This will and has worked, most times!!!.

There is one manufacturer I know off that can however manufacture a stop collar to heat treated spring conditions, that snaps into a small pre-cut groove in the casing, with no grub or Allen screws required!. Thereby guaranteeing not to move at all due to the high axial load that can be exerted on this unique stop collar design.

The bows made by this manufacturer are also solid forgings, so this is a one part component, that will not fail or fall apart.

Stop collars and bow can be installed onshore, simply by slipping on via the pin end, saving rig handling time and safety risks with little concern to transit, transportation damage etc.

With a solid forged single 'bow' and snap in stop collar, that can and has been specifically designed for slim hole.

This would be my recommendation, particularly on a high rig rate spread, drilling subsea wells where consequential loss associated with any 'non fit for purpose item run' will be xxx's time more than their cost.

Regards,

Peter Aird
Marathon, UK
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