Finite Element Analysis of Bending in a Threaded Connector for a 5 1/2-in. Marine Riser

Abstract

Abstract This paper describes the development of a new finite element modelling technique for performing nonlinear bending analysis of tubulars and its application to a threaded connector for a 5–1/2 inch production tubing marine riser. A finite element technique has been developed for analyzing bending loads applied to an axisymmetric geometry. The method uses a Fourier series solution. The first two terms of the series are solved simultaneously, allowing nonlinearities to be included since the method does not use superposition, which normally requires linearity. Existing methods of analysis require either a linear elastic assumption and axisymmetric approximation of bending loads, or a full three dimensional analysis. The new technique includes nonlinearities in mechanical properties, gapping, and friction. It is more accurate than the method where axisymmetric loads are applied so that pipe OD stresses are the same as those that would result from bending. The model is considerably less complicated to use than a three dimensional model and is also considerably less expensive. The method described above is applied to a 5-1/2 inch threaded connector. The connector is analyzed under make-up, tension, pressure, bending, and shear loads. Predictions include average and reversing stresses in the pin and box wall and at stress concentrations. These predictions can be used to evaluate the fatigue life of the connector. Introduction Fatigue analysis is important in determining the expected useful life of a marine riser. As with any tubular string, one of the weak links of the system is the joint connections. In the case of a marine riser, the connector is an area of complex stress distributions and stress concentrations. Full scale laboratory tests can go a long way towards determining the tensile strength and leak resistance of a threaded connector, but it is nearly impossible to measure stresses or strains at critical locations in the connector (at thread roots and seal ring grooves) or to test a connector until it falls in fatigue. However, fatigue calculations require that the connector stresses be computed. Finite element analysis has been applied as a useful tool in connector design and evaluation. It is most useful in determining stresses and deformations that cannot be measured in the laboratory and in determining the effects of connector variables and loads that are not easily controlled in the lab. In this case, accurate stress values can only be obtained with a finite element analysis. Due to the complex and nonlinear behavior of a threaded connector, existing finite element codes could not efficiently model a connector subjected to shear and bending loads. The objective of the work presented in this paper was to develop the capability to perform efficient nonlinear analysis of bending in tubulars, then apply the newly developed method to analyze a threaded connector for use in a marine riser.