Erich F. Klementich, P.E., SPE, Oil Technology Services, Inc. (OTS)
An increasing number of users are finding it necessary to utilize "High Collapse" easing grades for deep set and intermediate size drilling and production casing strings. Unfortunately, the great variety of proprietary (non-API) high collapse easing grades has made it extremely difficult for drilling and completion engineers to logically select the optimum size, weight, and grade of pipe meeting the design objective, due to the wide variation in collapse resistance claims and the guaranteed pipe dimensional and mechanical properties supporting that claim. This paper provides a technically sound method for selecting and safely using high collapse pipe.
Several unbiased experimental and collapse data analysis studies have conclusively proven the claimed collapse resistance of declared "High Collapse" (HC) easing grades are essentially unsubstantiated and not achievable in the field. For declared high collapse pipe, the manufacturer simply claims high collapse properties without providing specific guarantees on pipe attributes above API Spec 5CT to justify the claim. Often, the pipe manufacturer's claim of "high collapse" properties border on the technically absurd. Some claim a collapse resistance greater than achievable from elastic collapse, which is akin to requiring rounder than round pipe; others whose claimed high collapse resistance values are allegedly based on "API collapse tests", are in fact misleading and not at all based on even an approximation of real world collapse conditions. Collapse failures of high collapse pipe have occurred at loads substantially less than the claimed high collapse resistance.
Much of the problem with exaggerated high collapse claims lies in the inadequate number of actual tests conducted to define the true minimum collapse resistance, i.e. a usable collapse rating of "high collapse" pipe. As shown in Table 8.1 of Reference 13,598 collapse tests, without a failure, are required to demonstrate a collapse resistance to a reliability of 0 - 995, at a confidence level of 95 %, the definition of the minimum API plastic collapse rating. With up to one (l) failure, 946 successful tests must be conducted.
One (l) manufacturer of a "HC-95" type product did provide test data on 1470 collapse tests. An analysis of this data showed there was very little difference in the minimum collapse resistance of this HC-95 pipe, tested at the mill, and API C95. The greatest difference, as would be expected, was at the higher D/t ratios in the plastic and transition collapse range. Furthermore, the API study of HC-95 pipe in user's inventory showed NO high collapse properties. The lesson is obvious: High collapse resistance claims must be supported by reliability and quality factors. Otherwise, it is only an unsubstantiated sales promotion, not engineering reality!
There are no mysteries concerning the dimensional or mechanical properties of the pipe body or the manufacturing processes of the pipe which are required to achieve maximum collapse resistance. The subjects of "high collapse" and collapse in general have been studied in detail by technical committees of the API Sub-Committee on Standardization of Tubular Goods. The salient conclusions were:
A technical (attribute based) specification for high collapse "95" grade, HC-95, casing could not be developed because of the irreconcilable differences in the design of the product used by different mills for making the "HC-95" pipe. Furthermore, a functional (performance based) specification could not be developed because of the substantial difference in opinion between the users and the manufacturers on the minimum testing requirements to qualify a (heat treat) lot of pipe as "high collapse" easing. As a result, the API technical work group recommended discontinuing work on developing an API high collapse grade, "HC-95" specification. This recommendation in turn was approved by the general committee. In other words, a consensus specification for high collapse resistant pipe could not be developed.
After thorough and extensive investigations, another API technical work group on collapse resistance in general concluded the added new requirements for API casing and the new manufacturing innovations, primarily - mandrel mills, quench and temper heat treating, and hot rotary straightening, have resulted in some increase in the minimum collapse resistance of API casing. However, that increase was not sufficiently large to warrant the adoption of new collapse resistance properties or new API collapse equations.