Establishing Operational Fatigue Limits for Short-Term Riser Operations

Abstract

Drilling and intervention risers are widely used for oil and gas production in deep as well as shallow waters in oil fields around the world for subsea operations. The risers come in a diverse array of configurations, some of which may be challenged by fatigue if operated in high currents or seastates. The suitability of the selected riser and the operating limits are assessed by conducting strength and fatigue analysis based on design codes such as API RP 2RD, [7], API RP 16Q, [9], and API RP 17G, [10].Typically, drilling and intervention activities are conducted for short periods of time but used repetitively. The codes are clear about the return period of the design environmental event which must be checked to insure safe operation with respect to strength; however, assessment of fatigue integrity can be more difficult to determine. The allowable fatigue operating environment should account for the ability to disengage, the time required to disengage, the damage rates in particular seastates, prior accumulation of fatigue damage, and variations in soil, tension and internal fluid weights.In this paper, an orderly method of establishing the allowable fatigue operation limits for drilling and intervention risers is presented based on Monte Carlo simulations along with a case study implementing the methodology in a shallow water environment. To illustrate this concept, a riser with wellhead and conductor system is assessed and is subjected to directional loading from several long-term seastates. The variation in effects is studied by doing fatigue analysis for different durations: 3 days, 1 week, 3 months, 1 year and 10,000 hours.