Effect of Fit-Up on Steel Sleeve Life

Abstract

Type B steel repair sleeves are a recognized technology for repairing most features encountered on a high-pressure transmission pipeline. Ensuring load transfer between the carrier pipe and the steel sleeve decreases the stress in the carrier pipe and increases the life of the repair. By nature, a Type B steel repair sleeve includes a circumferential in-service weld that makes the repair sleeve pressure containing. Although the sleeve itself is pressure containing, understanding the life cycle of a steel sleeve once the annulus space is pressurized is critical to ensuring a properly installed sleeve.

This paper summarizes a testing program to compare the performance of a tight-fitting steel repair sleeve relative to a condition lacking a tight fit‐up, or one with a loose fit. Full-scale test samples were prepared using 24‐inch OD x 0.250‐inch WT, Grade X52 pipe samples repaired with either a tight- or loose-fitting Type B steel repair sleeve. The annulus space of the Type B steel repair sleeve was pressurized at the start of the test to simulate a worst-case operating condition.

The tight-fitting sleeve survived approximately four times as many pressure cycles as the loose-fitting sleeve. These results are supported by both the steady increase in the hoop and axial strain measured on the outside of the steel sleeve. The observed change in strain early in the cycling process is an indication that the loose sleeve was experiencing bending loads that contributed to the premature failure of the repair sleeve’s long seam weld.

Introduction and Background

There is currently no requirement within industry Codes and Standards that requires a specific fit-up tolerance of a Type B sleeve. Although not specified, it’s common to order sleeves to a tight fit of the nominal pipe outer diameter. This paper investigates the benefit of a tight-fitting sleeve. Although the sleeve itself is pressure containing, understanding the life cycle of a steel sleeve once the annulus space is pressurized is critical to ensuring a properly installed sleeve.

To better investigate the effect of steel sleeve fit-up, a testing program was designed utilizing steel sleeves manufactured and provided by Allan Edwards, Inc. Sleeves were later fully welded resulting in a Type B sleeve configuration. Sleeves were installed in a “tight” fitting and “loose” fitting manner. The tight-fitting sleeve minimized the annular gap by an additional manufacturing step that allows the sleeves to contain a tighter fit at the longitudinal weld area. The loose-fitting sleeve was manufactured without that additional manufacturing step resulting in slightly flared ends at the longitudinal seam weld. The sleeve was pressurized at the start of the test via a ½-inch hole drilled through the carrier pipe under the steel sleeve.

Test Setup

Two, 10-foot long full-scale test samples were fabricated using nominal 24-inch OD x 0.250-inch, API 5L Grade X52 pipe with end caps and pressure ports installed at both ends. The Allan Edwards, Inc. manufactured steel sleeves were nominal 24-inch ID x 0.375-inch WT, ASTM A572 Grade 65 material, one fabricated with a tight fit and one fabricated with a loose fit.

Each repair sleeve also included a 1-inch bosset, 90° from the sleeve seam welds. A ½-inch hole into the carrier pipe through the bosset on the steel sleeve. Once plugged, this setup pressurizes the steel sleeve. Two (2) biaxial strain gages were installed on the assembly, including:

One (1) biaxial gage on the repair sleeve located 1-inch from seam weld (90°) in middle of the sleeve.

One (1) biaxial gage on the base pipe halfway between the end cap and the steel sleeve located 90° from the seam weld.

Following the instrumentation installation, both test samples were installed into a testing chambers for testing. The samples were filled with water and pressure cycles between 100 and 867 psig (9% to 80% SMYS). All samples were cycled to a runout of 100,000 cycles or failure, whichever occurs first.

Test Results

The tight fit sleeve, representative of a typical installation, outperformed the loose fit sleeve by reaching an additional 3,552 pressure cycles.

The tight fit sleeve reached a peak-to-peak hoop strain of approximately 0.20% and an internal pressure of 795 psig prior to failure. The leak was identified along the long seam sleeve weld. The loose fit sleeve reached a peak-to-peak hoop strain of approximately 0.23% and an internal pressure of 765 psig prior to failure. The sleeve leaked at the sleeve seam weld.

What is especially important to note is that the hoop strain in the loose fit sleeve rapidly increases much sooner than that of the tight fit sleeve configuration. Almost immediately the loose fit sleeve hoop strain gage shows an increase in hoop strain, while the tight fit sleeve hoop strain gage only starts showing a notable increase in strain after 3,000 pressure cycles have been applied. An increase in axial strain was also measured by the gage installed on the loose fit sleeve; however, no appreciable axial strains were measured in the tight sleeve during cycling.

Another noteworthy observation is that the hoop strain in the loose fit sleeve is significantly less than the hoop strain in the tight fit sleeve (i.e., less than 50%). This indicates that the loose fit sleeve is not providing the same level of reinforcement (or restraint) as observed with the tight fit sleeve. This is extremely important when considering the level of stress reduction provided to the carrier pipe below the steel sleeve. The presence of a lag in load transfer results in higher stresses being generated in the carrier pipe.

Discussion

This study examined the performance of an Allan Edwards, Inc. Type-B sleeves considering a tight fit and a loose fit. The annulus space of the sleeve was pressurized at the start of the test, resulting in a worst-case scenario for the sleeve repair. The loose fit sleeve failed approximately 4 times sooner than the tight fit sleeve. This was likely due to the ineffective load transfer to the loose fit sleeve resulting in minor bending strain at the seam weld leading up to failure. Both the hoop and axial strain values at the sleeve seam steadily increased and the sleeve was able to bend axially which accelerated the increase in strain. The observed change in strain early in the cycling process is an indication that the loose sleeve was experiencing bending loads that contributed to the premature failure of the sleeve’s long seam weld. The life of the sleeve is also dependent on the quality of the welds, therefore, the results would be expected to vary from test to test.

The tight fit 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. The tight fit sleeve begins with a higher hoop strain than the loose fit sleeve, but slowly increases over cycles while the axial strain remains constant until approximately 4,000 cycles. Something to note is that the peak-to-peak axial strain does not increase as the sleeve approaches failure, therefore it is more resistant to bending axially and improves performance.

Conclusions

This study consisting of pressure cycling demonstrated that a tight fit Type B steel sleeve manufactured and supplied by Allan Edwards, Inc., inc. performs considerably better than an alternative steel sleeve design that lacks a tight fit-up (i.e., loose fit). While the fit-up of a Type B steel sleeve is not specified in industry standards, these results are an important consideration for pipe operators who use steel sleeves to repair pipelines, especially when using Type-B steel sleeves where the sleeve contains or may contain internal pressure during the life of the repair. It is possible that a tight fit sleeve might prevent the formation of thru-wall defects that will in turn will prevent pressure in the annulus between the pipe and sleeve and prevent the fatigue failures that occurred in this test program.