The impact of compression forces on casing-string designs and connectors

The effect of downhole axial compression forces on casing string designs an d connectors is a major concern to engineers designing high-temperature, hi gh-pressure, extended reach, and horizontal wells. Widespread concern regar ding the significance of compression loads on casing string design is relat ively recent. For decades engineers were either not aware that high-compres sion loads may occur or were not concerned about the effect of compression on the casing. This paper discusses the sources of axial compression forces in straight an d directional wells. Methods to analyze casing subjected to compression and bending loading are presented including the effect of compression and bend ing on the von Mises equivalent stress state of the pipe. Connector selecti on for casing applications where high axial compression loads are anticipat ed is critical. Compression ratings for API casing connectors have never be en addressed in API standards. The paper offers methods to evaluate API con nectors for applications with significant compression loading. Compression ratings for premium casing connectors are generally available, but the justification for these ratings varies substantially from one manuf acturer to another and the basis for the compression ratings may not be wel l documented. The paper discusses compression rating methods for various pr emium connector configurations including threaded and coupled, flush and ne ar flush. Desirable and undesirable premium connector features that relate to compression capacity are reviewed. Results from compression testing of p remium connectors are also presented. The paper benefits engineers involved in the design of casing strings for a pplications with high compression and bending loading.