The possibility of damage to risers and wellheads, which may be caused by the passage of a bow spring centralizer during the running of an inside casing string, has been the subject of much discussion and speculation. To address this concern, different centralizer geometries were evaluated through finite element analysis (FEA) analysis to show how surface stresses change, depending on the bow type. This, along with a series of testing performed in a controlled environment, revealed that the nuances associated with centralizer design will cause significant differences in the outcome of an operation.
The tests were performed with the aid of an overhead crane facility, which was modified to vertically run and retrieve a bow spring centralizer sub inside samples of plain casing and connectors. This system was designed to run and pull the centralizer sub at rates of 7.5 ft/min. (2.3 m/min) and 14.5 ft/min (4.4 m/min). Tests were performed using a curved-cross section bow on 11 3/4-in. (298.5-mm) close-tolerance centralizers subs to reduce surface pressure. The samples used during the test were 14.488-in. OD × 13.198-in. ID blank casing subs and two similar 5-ft (1.5 m) connector subs, which had been thoroughly greased internally. The tests ran 10 single trips in and out. After each trip, the centralizer sub was then rotated 22.5° to prevent marks from subsequent trips obscuring the results of the first. This process was repeated on each of other samples. Load and displacement data were taken over the whole stroke during all trips and laser measurements of the resultant scratches were obtained. There was little or no rotation resulting from the multi-trip sequence, and a thorough compilation of the data was accumulated and reported.
This paper provides a detailed report of the initiation, creation, and performance of these testing procedures. State-of-the-art laser-measuring technology it reveals details of the scratches created by the bow spring centralizers. The resultant findings reveal that there is more to evaluate during the selection of a centralizer than just standoff performance.