Design Life Prediction of Flexible Riser Systems
MARINFLEX 92 Conference on Flexible Pipes, Umbilicals, Marine Cables, London, November 1992
Flexible risers have been traditionally used to transport fluids at high pressure under dynamic loading conditions. Typically within the offshore industry they are used in a catenary type configuration to connect say a floating production system to the sea bed, or, either a fixed structure or another floating vessel. The risers themselves are designed to have a high axial stiffness yet be flexurally compliant to withstand the static and dynamic offshore loading. This loading arises from direction induced motion of the connected vessel.
A riser configuration is designed to withstand extreme wave and current loading conditions, typically the 100 year conditions, within the allowable integrity limits of the riser. Once the system configuration has been established, an additional and equally important requirement for full qualification is the prediction of its design life under operational loading conditions. Due to dynamic loading and the different layer structures forming the riser cross-section its design life is governed by combined wear and fatigue. The evaluation of stresses, slip and wear within unbonded flexible risers under these conditions have been addressed by a number of papers [1-6]. It is the intention of this paper not to reiterate these evaluation techniques but to present an engineering approach using them for predicting the design life of flexible risers.
A basic chart showing the input into the design life prediction is given in figure 1. From the frequency of occurrence diagram defining the environmental wave data the configuration experiences, all possible damaging load cases are analysed using a 3D flexible riser package, in this case Zenriser. The corresponding time history results are stored for subsequent life prediction using a post-processor referred to as ZENLIFE. In addition to the general equivalent cross-sectional riser properties available from the riser analyses the processor must also access the pipe geometrical description, material properties and the criteria against which the design life prediction is to be made. The processor can then evaluate stresses, slip and wear during each load case from which the design life for the riser can be assessed. This is expanded in greater detail throughout the paper.
The procedure outlined in this paper has been used on a number of cases ranging from existing risers already in use to the qualification of future riser designs. However, at present confidentiality prevents the disclosure of any of these results. Instead a demonstration case is presented - the design life assessment of a lazy 'S' configuration. For simplicity all wave loading was applied in a single direction with the riser configuration in its 'far' position. In practice a spread of wave directions consistent with environmental conditions would have to be applied in addition to all vessel offsets and rotations consistent with this wave loading.