White Paper: Axial Shear Testing Evaluates Casing Spacer Integrity and Performance

Axial Shear Testing Evaluates Casing Spacer Integrity and Performance

Abstract/Introduction

Axial shear resistance is an important variable and determines the maximum amount of force that can be applied to a spacer before it begins to slide. Clock Spring Company, LP (Clock Spring) contracted with Stress Engineering Services, Inc. (Stress Engineering) to perform the shear tests on its derivative casing spacers product, evaluating and comparing Clock Spring performance against traditional casing centralizer and spacing solutions; the tests were applied on both Powercrete coated steel and PVC pipes with both concentric and eccentric loads.

 

Problem

Casing spacers are installed to protect the integrity of the cased pipe and are fixed on the carrier pipe to prevent contact between the carrier pipe and casing. During the installation process or once installed, should a casing spacer break or become dislodged, the effect can be excessive deflection within the carrier pipe. This can cause the two pipes to come into contact, which, in theory, can also lead to problems with the cathodic protection. Casing spacers that are not sufficiently secured to or become dislodged from the carrier pipe can result in the casing spacers congregating or bunching mid-span. This leaves the carrier pipe susceptible to contact with the casing and abrasion and coating damage when pulled.

 

Installing a carrier pipe within a casing allows for simple removal of the carrier pipe, should it ever require replacement and protection from geotechnical disturbances. If a leak were to occur in the carrier pipe, the casing should act as a storage vessel, limiting potential environmental effects. In addition, the external casing is often used as a method of protecting against third party interference and damage to the pipeline.

 

The two commonly used casing spacers / centralizers are the bolt on variety, which use various material such as plastic or steel, and banded wood skids. While the theory behind using casing spacers on a carrier pipe is sound, these types of traditional casing spacers are prone to break loss during a line pull. Therefore, shear tests are performed to report the likelihood of a casing spacer breaking or becoming dislodged.

 

The Test/Results

Stress Engineering conducted a total of eight tests; three samples were tested with a Clock Spring centralizer applied to a Powercrete coated steel pipe; three samples were tested with a Clock Spring centralizer applied to a PVC pipe; two samples were tested with conventional centralizers on Powercrete coated steel pipe.

 

Stress Engineering’s tests compared a Clock Spring centralizer to a traditional centralizer in concentric and eccentric shear applied on both abrasion resistant outer-coating (ARO) coated steel and PVC pipes, determining the resistance of each centralizer to an ideal circumferential shear load. A defect or protruding object was recreated to act as the trigger.

 

In the testing frame, force required to initiate sliding of a bonded Clock Spring was approximately 19 times greater than traditional bolted on casing spacers.  The testing also demonstrated that the shear capacity of a Clock Spring centralizer on plastic pipe was approximately 60 percent higher than the traditional method. In static resistance, the Clock Spring samples installed on ARO have 7 to 18 times greater resistances than the traditional centralizer on ARO. In short, the bonded layers of the CS centralizer provide a dramatic improvement over traditional methods.

 

The following graph details a series of shear tests, performed by Stress Engineering, to compare a Clock Spring centralizer to a traditional centralizer in concentric and eccentric shear.

 

Spacer Specification(s) based on this testing shows the following:

Spacer width: 3” wide

Spacer thickness: 0.75”

Spacing from Center: 10’

Conclusion

Based off the data and results presented by Stress Engineering, Clock Spring centralizers have stronger static capacities for both eccentric and concentric shear loadings.

Pictured Below: Clock Spring centralizer bonded to an ARO coated steel pipe tested in eccentric shear.

Result: required a load of over 5,300 lbf to initiate movement compared to 660 lbf for the traditional centralizer tested, validating the resistance capabilities of the Clock Spring centralizer.

The concentrated load tore through the sleeve, which is due to the high resistance of the bond that kept the centralizer from sliding.

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