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.
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