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.