1 Department of Naval Architecture and Offshore Engineering, Technical University of Denmark
Flexible pipes are often a technically attractive alternative to the traditional steel pipe. Often commercial utilisation of oil/gas fields depends on the use of flexible pipes. An example is when floating production vessels are used, where the flexible pipe follows the wave induced motions and thus resists the loading from the environment in a more suitable way.A flexible pipe is a composite structure, built up of several layers each having a specific function. Due to the flexibility, the response of the pipe in wave induced motions are governed by a complex hydroelastic set of differential equations.An important part in the design of flexible pipes is the end fitting. The purpose of the end fitting is mainly to connect pipe units, but it also combines and terminates the different layers of the flexible pipe. The end fitting system contains a bending stiffener and an end fitting. The bending stiffener is installed to prevent damages caused by bending concentrations at the connection. The bending stiffener is designed to optimise the curvature distribution from the flexible riser to the vessel and to minimise the support forces at the vessel.Today, when end-fitting system are designed FEM programs are normally used to determine the stresses in the end fitting system. It is a timeconsuming procedure due to large non-linearities and therefore costly. The purpose of this study is to generate design curves for the tension andaxial force in a flexible riser as function of different riser parameters.The idea is to divide the riser system of riser and end fitting system into a global system including the riser and a local system of the bending stiffener and the end fitting. The forces at top of the riser shall be used as input for the design of the local system.This report introduces a non-dimensional description of these forces, a force design basis. This description is non-dimensional curves where the tension and the bending moment can be estimated at top of the riser when input data like water depth, weight per length, inner diameter andpressure (for tension) and current velocity (for bending moment) are given.