The Impact of Compression Forces on Casing String Designs and Connectors

Abstract

Abstract The effect of down hole axial compression forces on casing string designs and connectors is a major concern to engineers designing high temperature, high pressure (HTHP), extended reach and horizontal wells. Widespread concern regarding the significance of compression loads on casing string design is relatively recent. For decades engineers were either not aware that high compression 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 and directional wells. Methods to analyze casing subjected to compression and bending loading are presented including the effect of compression and bending on the von Mises Equivalent (VME) stress state of the pipe. Connector selection for casing applications where high axial compression loads are anticipated is critical. Compression ratings for API casing connectors have never been addressed in API standards. The paper offers methods to evaluate API connectors 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 manufacturer to another and the basis for the compression ratings may not be well documented. The paper discusses compression rating methods for various premium connector configurations including threaded and coupled, flush and near flush. Desirable and undesirable premium connector features that relate to compression capacity are reviewed. Results from compression testing of premium connectors are also presented. The paper benefits engineers involved in the design of casing strings for applications with high compression and bending loading. P. 137