Analytical methods for design of internal composite repairs

6 Analytical methods for design of internal composite repairs 

Analytical methods are important for internal composite repair systems to ensure that damaged pipelines are properly analysed to evaluate the system against performance measures. Existing methods specifically focus on external repairs of pipelines with internal repairs still unexamined, even though the concept could potentially be applied to internal repairs. Therefore, the concepts and approaches that could be adopted for analysing unbonded and bonded repairs are considered below. 

Thin-wallcd and thick-walled cylinders arc forms of pipeline primarily used in the offshore industry to transport gas and hquid under pressure. Cylindrical pipes are also used as a stmctural load bearing member to restrain the motion of a platform, for example, in response to wind, waves and currents (to within a specified limit). When a pipe is exposed to a combination of internal and external pressure, the pipe material will be subjected to pressure loading and, as a result, will develop stresses in all directions. The resulting normal stresses induced by the pressure loading are functions of the pipe diameter, wall thickness and cylindrical shell boundary conditions (open-ended cylinder or closed-ended cylinder) (Kashani and Young, 2008). 

Two types of analytical models are available for dealing with cyhndrical pressure pipes. The thin wall method is the most commonly adopted solution. It is based on a simple mechanics approach and is only apphcable to vessels having a diameter-to-wall-thickness ratio of greater than 20 (Hearn, 1997). Using the thin wall method for an unconstrained, closed-end thin wall cylindrical pressure vessel, the circumferential hoop stress and longitudinal axial stress can be shown to take the form  

The second method is the Lame approach (Kashani and Young, 2008), which is based on displacement differential equations and is applicable to any cylindrical vessel with any diameter-to-wall-thickness ratio. The Lame method is often referred to as the solution for thick wall cylindrical pressure vessels. Equations for the hoop stress and radial stress in a thick-walled cylinder were developed by Lame in the early nineteenth century (Timoshenko and Goodier, 1969)  

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