Centraliser for deep-water drilling

Paul Boughton

Cliff Berry reports on a centraliser which has been designed and manufactured to meet the extreme challenges of drilling today's deepwater wells.

When drilling in water depths of 5,000 feet and beyond, it is not unusual to have as many as nine casing strings running through each other so the tolerances between casings, and between the centraliser and casings, are incredibly tight.

Centek's UROS-CT centraliser is specifically designed for use in tight tolerance casings, where the centraliser must compress virtually flat to allow travel through a series of tight casing strings but still be capable of fully expanding to the designed open-hole size. The UROS-CT is engineered to precise ring-gauge tolerances because every millimetre saved allows for greater expansion in the open hole.

The other problem with tight casing strings is securing the centraliser in position. Bolt-on stop collars are too proud of the casing so Centek uses low-profile, positive stop-rings.

A further benefit of the UROS-CT is that it fits straight on to standard casing joints, so there is no need to run an expensive pup joint and substantial rig make-up time is avoided. This centraliser significantly reduces insertion forces and drag, and the unit does not need to be pulled into the well as the design allows for normal run-in-hole (RIH) methods, so it runs into the well under its own mass.

Standoff is key to success once in the open hole and the UROS-CT provides the highest levels of standoff, better well clean-out during well conditioning, and improved cement displacement to ensure good zonal isolation as well as minimising corrosion later in the life of the well.

The forces acting on the Centek UROS-CT can be measured. When a centraliser enters previously set casing it has to overcome an initial insertion force. Each time a new joint is added there is an accumulative restart force and once all the centralisers are installed there is the running force, or drag, to contend with.

The UROS-CT substantially reduces all these forces because of its patented bow design, in which the high points of the bows are offset alternately. However, once out in the open hole the bows regain their designed outer diameter to maximise standoff, providing zero start and running forces and matching the performance of a Centek S2 centraliser.

With spread costs for a drill ship currently running at around $700 a minute, avoiding problems when running-in-hole in deep water is vital. While there is the temptation to cut costs by using cheap centralisers. High quality centralisers add less than one per cent to overall well costs. This bears no comparison with the expense of pulling a drill string out, possibly fishing for broken centralisers, and then putting it all back in again. A cost likely to lie on the wrong side of $5 million! And a cost that can be avoided by specifying the correct type of centraliser at the outset.

For more information visit www.engineerlive.com/iog

Cliff Berry is Vice-President Global Business Development, Centek Group, Newton Abbot, Devon, UK. www.centekgroup.com

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