The fatigue performance of unbonded flexible risers is severely reduced if a non-dry corrosion-fatigue environment becomes established in the annulus space that houses the steel armour layers. This is in contrast to the exemplary fatigue performance achieved when the annulus space stays dry throughout the service life. Flexible riser operations have shown that there are several mechanisms for the annulus to loose its dry-state. A flexible riser that is not tolerant to a non-dry annulus often needs to be replaced at short notice or shut-down due to the rapid onset of corrosion fatigue induced by CO_2 and H_2S. Field operators are demanding flexible riser systems that can survive with non-dry annulus conditions for the full service life of the field. The loss of production endured from replacing a riser or simply shut-down without replacement has severe implications on the profitability of a field, especially in marginal and deepwater developments. This paper demonstrates how recent advances in analysis are influencing the design of new flexible riser systems that can sustain life of field operation with a non-dry annulus. These advances are accomplished by various methods that include optimising bending stiffeners so that concentrated fatigue hotspots are eliminated and modelling bending hysteresis of flexible risers. Frequency domain techniques are also shown to be an efficient and accurate method for flexible pipe fatigue analysis, thus enabling considerable modelling detail of the seastate loading environment in an efficient manner. The application of new design methods and results are shown at various fatigue critical locations on deepwater flexible risers. The key conclusion of the paper is that life of field design of flexible risers can be achieved with onerous corrosion-fatigue conditions if proper rigour is applied in the fatigue design.
展开▼
