Benoil Services Ltd

6.10. Cement Dart Guide

Being more complex, these always take at least a few days to produce to order, though we keep a range of cores and components in stock to facilitate this process. In complexity, they are another matter entirely compared to the cleaning darts and there is a great variety available. For a given tubing size there is often a multiple of choices to be made; all are the same price so that is not a deciding factor, but there is often a choice of pressures, and bores and allowance for wall taper. We guide customers through this when asked, but illustrate the process as follows.
For example, for 1.50" tubing you can use 12mm, 13mm, 16mm and 20mm bore darts. But a wall of 0.125" , for example, is probably a limitation and inclines us away from recommending the 20mm bore; this is because the metal OD of 26mm is close to the limit of the tubing bore (1.5" - 2 x 0.125" = 1.250" or 31.75mm) if one makes proper allowance for some 2-3mm for a weld bead; it is even closer if there are butt welds in the tubing, as you might lose 4-6mm on the diameter. A dart would just squeeze through a 4mm reduction (at 27.75mm) under pressure, but would stick at a 6mm reduction. But if the restrictions do not apply, you could choose this size, and might wish to if you need a wide bore; the size exists because they have been required. Generally one would choose from the 12, 13, and 16mm bore darts.
If you have a tool below the landing point of the dart, which might need a ball release to be passed after the cementing operation, the choice can be limited by such a tool and ball. One would not choose a 12mm dart, which cannot accept even a 1/2" ball, unless one can manage with a 3/8" ball. For this reason we now tend not to recommend this size except for a simple operation.
This means the choice is perhaps dictated by the available pressures. Please note these do change with time as foil stocks used for the burst discs are used up and change.
Experience enables appropriate choices to be made, but even then care is called for. A job was planned in its entirety based on new clean 1.75" tubing, on 24mm bore darts and a comprehensive tool set above and below the cementing region. A late realisation was made that a join would have to be made in the tubing offshore; this invalidated the choice of a 24mm dart in favour of a 20mm one; this in turn invalidated the assembled tool string because the chosen ball would not pass. It delayed the job while the tools were changed, but at least it was eventually successful and not a disaster.
All cement darts need some sort of landing point, but not necessarily a specific plug catcher. Such darts will land on any restriction which prevents the metal head passing. However, ideally it will be a narrow straight tube section, not tapered which could allow the dart to tip over and be bypassed. Ideally, too, it would have a 45 degree chamfer at the bottom, as the darts have a 45 degree chamfer on their heads; however, if the tube is parallel, the precise angle should not matter.

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