Proposed Methodology for Fatigue Testing on Umbilical Round Armor Wires

Abstract

Subsea umbilicals are a vital component for offshore operations, being responsible to provide hydraulics, fluid injection, power and/or communication services to and from subsea facilities. They can be designed for both static and dynamic applications and can be deployed in harsh environments where potentially damaging factors such as corrosion, tension, pressure, clashing, entanglement, among others are commonly present. For armored umbilicals, the tensile armor wires of these structures are responsible for the tensile strength and its fatigue life is a key design consideration for subsea umbilicals subjected to dynamic loading. Despite its considerable importance, there is no standardized methodology for conducting fatigue tests of tensile armor wires of umbilical cables. Some general guidelines are given in technical publications such as ISO 13628-5 standard and DNV’s recommended practice F401, but there is still need for more detailed information regarding fatigue testing of these components. This lack of methodology for fatigue testing of circular wires that comprise the tensile armor of these structures lead to the motivation for this study. The main advantage of using specific data from an adequate testing methodology of tensile armor wires — instead of using standard SN curves — is to reduce conservatism, mainly considering damage calculations for the overall structure. Additionally, with the increasing operating water depths, an excessively conservative design can lead to a considerably heavier structure which will induce higher topside stresses. Therefore, it is of great importance to have a well defined methodology to obtain SN curves that can be used in the design phase of umbilical cables which can lead to an optimized structure and also keep an adequate degree of conservatism. In this work, it will be presented several attempts to achieve an adequate methodology to obtain SN curves for circular tensile armor wire of umbilicals and the final methodology adopted will be described in detail. Tensile armor wires in a wide range of diameters and strength were tested in order to validate the proposed methodology. Additionally, the normalized fatigue results from several tests are also presented and compared to SN curves commonly used during the design phase of umbilical cables.