To Mill or Not To Mill, that is the question!
Two (2) studies have been performed which collaborate with each other to explain why a mill groove is
good and what considerations need to be made when determining whether to use a mill groove.
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100306 - Influence of Backing Strip Mill Groove
100160 - Steel Sleeve Fit-Up Test
Study 100306 dated November 2021 was to examine the influence that an installed long seam weld
backing strip mill groove relief has on the long-term integrity of a fully welded Type B steel sleeve. Mill
grooves are commonly specified as a means for providing relief for the long seam backing strip to allow
the sleeve to provide tighter contact to the pipe. The mill groove also reduces concern of the backing
strip from shifting during sleeve fit-up, reducing or eliminating the risk of welding the long seam directly
to the pipe.
The study had two (2) 12-inch OD samples containing a dent directly behind the sleeve long seam weld.
This configuration, although not typical of field installations, would apply the greatest amount of stress
to the sleeve long seam weld. A single dent was installed by applying an indenter on the outside surface
of the pipe, and ten (10) pressure cycles were applied to form the pipe around the indenter. Following
the dent installation, an AE Type B Steel Sleeve was fit and welded to the pipe to reinforce the dent. The
samples were cycled to 100,000 cycles or failure, whichever occurred first.
The two tested sleeve configurations (with and without a mill groove, same wall thickness) both survived
the 100,000-cycle runout. After the test concluded, ADV sectioned and removed the Type B sleeve, then
inspected the inside surface of the sleeve mill groove and outside surface of the dent via magnetic
particle inspection. No relevant indications were identified. Therefore, the mill groove did not influence
the long-term cyclic integrity of the sleeve (at least to 100,000 cycles between 10 and 72% SMYS).
Accounting for a safety factor of five (5), results in an approximate cyclic life tested of 42 years in a Very
Aggressive pressure cycling regime and 1,155 years in a Light pressure cycling regime – both significantly
long periods when examining the 100,000 cycles each sleeve survived in this study.
However, in study 100160 dated February 2021 looking at the effect of fit-up of the sleeves. The mill
groove provided relief for the approximately 1/16 inch backing strip resulting in tighter fit. The gap
generated due to the backing strip in the non-mill groove sample results in a looser fit-up. The findings
were that a looser fitting sleeve resulted in an expected life 3.5x times shorter than a tighter fitting
sleeve once the annulus space became pressurized. Since this result only occurs when the annulus space
becomes pressurized, assuming the underlying feature remains stable this effect does not come into
consideration.
In conclusion, the tight-fitting repair sleeve performed significantly better due to the compressive
residual stress applied to the pipe body when the sleeve was installed. The presence of the compression
counteracts the tendency of the sleeve to expand during pressure cycling. Now the decision to use a mill
groove comes down to regulatory considerations. Both ASME B31.4 (Section 451.6.2.9 (c)) & B31.8
(Section 851.4) reference sleeve design outlined in ASME PCC-2, Section 206-3.1, which states “…and
should have a thickness equal to at least two-thirds the thickness of the carrier pipe.”. Considering
sleeve thickness less the 1/16 inch mill groove, any carrier pipe wall thickness greater than 3/16 inch can
accommodate a same thickness sleeve with a mill groove and comply with ASME PCC-2
recommendations. The information above will hopefully help you be informed when deciding whether a
mill groove is appropriate for your application.