Topp, David Anthony; (1992) Fatigue/fracture mechanics analysis of threaded tether connections. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The use of threaded connections for joining tubes and pipes is widespread within the oil and gas drilling industry. Such connections have more recently been employed for the joining of tethering elements for a new generation of offshore platform, the Tension Leg Platform (TLP). The platform design depends totally on the integrity of the tethering system and the threaded connection between tether elements has been identified as a critical structural component. The hostile environment of the North Sea leads to severe cyclic loading on the tethering system and fatigue is the most likely in-service damage mechanism. This study involves an analysis of the fatigue behaviour of large threaded connections of the type proposed for tethering applications and considers the implications for subsequent in-service inspection and integrity assessment. A simplified model for the prediction of the non-uniform load distribution within the connection is proposed and this is validated using finite element (FE) modelling of a complete connection. A methodology for the use of this model, in conjunction with simple FE sub models, for the prediction of dynamic stresses in preloaded and unpreloaded connections is presented. Fatigue initiation and fracture mechanics based crack growth models are proposed for this application and large scale tests, to provide experimental data for validation of these models, have been conducted. An inspection system was developed to enable fatigue crack growth to be measured during the test. It is likely that this system will be suitable for integrity monitoring of large scale threaded connections removed from service. The requirements for integrity monitoring during service, based on a knowledge of the likely fatigue behaviour, are considered for a tethering system and a methodology for defining service inspection intervals as discussed.

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