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