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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 ?

Hess, GoM
4 answer(s)
Managing Director (
Relentless Pursuit Of Perfection Ltd.
Total Posts: 376
Join Date: 10/01/05
We have received the following response via Peter Aird of Marathon, Uk ..


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

Drilling Engineer
Oil Search Limited
Total Posts: 2
Join Date: 18/05/06

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 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
Drilling Specialist/Well Engineer/Training Consultant
Total Posts: 339
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.

Drilling Specialist/Well Engineer/Training Consultant
Total Posts: 339
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.


Peter Aird
Marathon, UK
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