Axial fatigue design of sheathed spiral strands in deep water applications

Using the previously reported axial fatigue model of Raoof, and by carrying out extensive theoretical parametric studies on large diameter (i.e. realistic) sheathed spiral strand constructions which very nearly cover the full manufacturing range of lay angles (with the lay angle being the primary geometrical parameter controlling the strand axial fatigue performance), a new set of design S–N curves for sheathed spiral strands in deep water applications have been developed. These S–N curves provide a very simple means of predicting the axial fatigue life of sheathed spiral strands (under uniform cyclic loading) to first outermost (or innermost) layer wire fracture, both at (or in the vicinity) of the end terminations, as well as in the free-field (i.e. away from the detrimental effects of end terminations), with the sealed strands being subjected to simultaneous actions of a wide range of external hydrostatic pressures, such as those in deep-sea conditions with water depths of up to 2000 m. It has been shown that, although the fatigue life to first innermost layer wire fracture is largely unaffected by the magnitude of the applied external hydrostatic pressure, the fatigue life to first outermost layer wire fracture may be significantly reduced (cf. corresponding in-air conditions) under the influence of sufficiently high levels of external hydrostatic pressure. The proposed design S–N curves have been compared with others (which are based on tests under in-air conditions) and it has been shown that the application of such previously available in-air design S–N curves to sheathed spiral strands in deep water applications may lead to unsafe designs.