Validated Methodology for Calculating Fatigue Capacity of Deepwater Umbilicals

Abstract

Steel tube umbilical designs are becoming more complex and are being deployed in increasingly severe environments. Umbilical designs can now accommodate up to 3-inch diameter steel tubes for chemical and hydraulic injection, up to four layers of armoring, multiple electrical cables and fiber optic lines. Large power transmission cables are also being incorporated in umbilical constructions. This is leading to ever increasing umbilical mass, radial loads, pressure loads and increased demands on the designer to demonstrate adequate fatigue capacity. A method has been developed for predicting the fatigue response of deepwater umbilicals, particularly for umbilicals subject to high tension and bending loads. The method involves predicting cyclic stress in the umbilical components (steel tubes or power cables) using an industry standard, general purpose numerical tool. It comprehensively accounts for the complex relationships between umbilical curvature, tension, inter-layer contact forces and tube stress variation. This paper describes the validation of the method against measured data obtained from full scale fatigue tests performed on two deepwater umbilicals. A key feature of the method relates to the construction and verification of suitable finite element (FE) models and the simulation of fatigue conditions. For the validation of the models, results from the full scale dynamic umbilical test cases incorporating bend stiffeners and loading arms are used, which demonstrate umbilical hysteretic behavior under cyclic bending loads. Hysteretic stress-curvature loops obtained from strain gauge readings for various tube locations within the bend stiffener regions for two umbilical constructions with significantly different lay-up configurations and fatigue stress regimes are used for comparison to strain predictions from the FE models. A significant advantage of the modelling and analysis methods described in this paper is that it employs the widely-used industry standard numerical FE tool, Abaqus, for the modelling and analysis of the umbilical. Key features and insights from the modelling, analysis approach and validation against the results of a deepwater umbilical dynamic test program are described. A robust and efficient method for the independent assessment of the fatigue capacity of deepwater umbilicals is demonstrated.