Failure analysis and avoidance for elastomeric diaphragms at high temperatures and pressures

Elastomeric diaphragms are used in the Smart Stabilizer/Weatherford rotary steerable systems (RSS) used in directional drilling down hole. The diaphragms need to operate correctly for the RSS to function effectively. The diaphragm separates the mechanical parts of the RSS, operating in hydraulic oil, from the outside environment. Failure of the diaphragm can lead to failure elsewhere in the RSS. Such failure is likely to have catastrophic consequences for the RSS and, associated with these, very high costs. The diaphragms are subjected to hostile conditions including high temperature and high pressure and the presence of chemical agents. Moreover, monitoring of the behaviour of the diaphragm in service is extremely difficult due to the nature of a drilling process. There is, however, a need for understanding the behaviour and failure mechanisms of such diaphragms; especially with plans to extend the depth of the drilling borehole. In this PhD project an outline design guide is developed for failure avoidance of the diaphragm down hole. Besides proposing an outline for such a design guide and identifying hypothetical modes of failure, detailed analyses of the normal behaviour of the diaphragm down hole and the failure due to fracture are also performed. A large part of the project is focused on gaining understanding of the normal behaviour of the diaphragm down hole. This is identified as being of a great importance in failure avoidance. Analytical, numerical and physical methods are employed here to allow for prediction of such behaviour. Those are supported by forensic examination of used diaphragms.Mechanisms of failure leading to fracture are proposed. Crack growth is investigated via the strain energy release rate approach. Three “ingredients” for failure are examined in the context of the conditions down hole: stress/strain characteristics, crack growth rate as a function of the strain energy release rate (including catastrophic tearing) and intrinsic flaw size or cut. The outcome is then verified in service-related tests on the actual diaphragms