Surrogate models for the stiffened catenaries: applications in subsea pipe laying

In this paper, the governing nonlinear ODE of the suspended stiffened catenary is reinvestigated. It is shown that strong nonlinearity arises from stiffened catenary length, which should be checked by an iterative numerical solution. The two concepts of stiffened catenary (guessed length) and natural catenary (known length) geometries of the suspended pipe, are compared and critically commented upon. In applying the theory to subsea pipeline installation, it is shown that natural catenary assumption, underestimates the installation stresses, particularly in shallow water and low laying depth. However, the true values of the stresses can be computed via stiffened catenary theory, in which the bending stiffness of the suspended pipe is not ignored. Thereafter, substantial iterative numerical solution of the governing nonlinear differential equation, in each load case is carried out. From these batch simulations, a surrogate expression is developed via optimization techniques. This model provides a correction factor by which, the accurate installation stress can be found. Moreover, the accuracy of results is verified by FEM analysis. It is concluded that for the initial estimation of the stresses, the simple natural catenary assumption, which is currently practiced can be used. However, the results should be corrected by the new surrogate expression, that has been produced in this paper. This can eliminate the underestimation of the installation stresses when a simple computational procedure is used.