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How to Test the Restoring Force of a Bow Spring Casing Centralizer?

Bow spring centralizer is designed for primary cementing and applied in vertical, deviated and horizontal wells where low starting force and high restoring force are required.

Restoring force is a key parameter that measures the quality of bow spring centralizers. Here, we'd like to briefly introduce what the restoring force is, devices and samples required for restoring force testing, test procedures and results.

Restoring force refers to the force exerted by a centralizer against the casing to keep it away from the wellbore wall. The restoring force value varies as installation method changes.

Samples & Test Stand Preparation

Restoring Force Test Procedures

  1. When performing the test, the horizontal deviation of inner pipe and outer pipe shall be within 5°.
  2. Before collecting the force data for the test, flex all bow springs 12 times.
  3. Apply an external force to the outer pipe, so that it will be transferred to the inner pipe vertically through the point of contact of the centralizer within the outer pipe.
  4. Apply load and record load-deflection reading at a minimum of 1.6 mm (1/16") increments until three times (± 5%), the minimum restoring force has been obtained, see Figure 2. The travel distance to obtain 67% standoff shall be determined for each test position.
  5. Repeat the process, test the centralizer until each spring and each set of springs have been tested in position 1 and 2 as shown in Figure 3.
  6. Calculate the total load at each deflection by compensating for the mass of the travelling pipe and attachments.
  7. Prepare the final load-deflection curve using the arithmetic average of the force readings at corresponding deflections. Restoring force shall be determined from this curve at 67% standoff ratio.
The picture shows the load-deflection curve for the restoring force test.

Load-deflection curve

The picture shows two casing centralizer test positions during the restoring force test.

Casing centralizer test positions

The minimum restoring force for a 67% standoff ratio shall not be less than the values shown in Table 1. See Determination of Restoring Force Requirements listed below for the derivation of the requirements.

Table 1 Casing Centralizer Specification – Restoring Force
Casing Diameter Medium Linear Mass Casing Minimum Restoring Force at 67% Standoff Ratio
mm inch kg/m lb/ft N lbf
89 3-1/2" 14.7 9.91 1761 396
102 4" 16.9 11.34 2019 454
114 4-1/2" 17.3 11.6 2064 464
127 5" 19.3 13.0 2313 520
140 5-1/2" 23.1 15.5 2758 620
168 6-5/8" 35.7 24.0 4270 960
178 7" 38.7 26.0 4626 1040
194 7-5/8" 39.3 26.4 4697 1056
219 8-5/8" 53.6 36.0 6405 1440
244 9-5/8" 59.5 40.0 7117 1600
273 10-3/4" 75.9 51.0 4537 1020
298 11-3/4" 80.4 54.0 4804 1080
340 1-3/8" 90.8 61.0 5427 1220
406 16" 96.7 65.0 5783 1300
473 18-5/8" 130.2 87.5 7784 1750
508 20" 139.9 94.0 8363 1880
  • The specifications for restoring force for bow spring centralizers are based on the centralizer being installed as per manufacturer recommendations and tested with lugs on the casing. If the centralizer is tested over a casing collar, stop collar, or with an integral stop collar, the actual results obtained from that test can vary from the specifications. It should be noted on the test report how the centralizer was installed and the type of holding device used during the test. If a centralizer is tested in this manner, the test can no longer be considered a specification test and the results may or may not meet the specifications set forth in Table 1
  • Casing diameter refers to the liner sizes.
  • Medium linear mass casing refers to the plain-end weight.

Determination of Restoring Force Requirements

The bow spring casing centralizer restoring force test shall be performed in strict accordance with requirements aforesaid.