Efficient and accurate methods for assessment of riser collision become increasingly important as offshore oil and gas exploration moves into deeper water. Collisions may occur as a result of the environmental loading (current and waves), flow interaction and surface floater motions, or combinations of these effects. This paper provides a parametric study on the influence of these factors, especially the influence of the fluid interference on the minimum distance statistics. The conditional collision probability for two flexible risers in a tandem arrangement is also estimated for sea-states with a 100 year return period.
The drag coefficient of the downstream riser is reduced due to flow-field interaction according to the relative position of the risers. The reduced drag force results in a decrease of the relative distance between two risers when current is applied. This leads to an increase of collision probability. In this paper, the drag coefficient is obtained by Blevins wake model and used for both static and dynamic analysis. The statistics of the minimum distance between two risers are calculated according to the relative position of Finite Element nodes which are obtained from dynamic analyses. Based on a set of such dynamic analyses, the probability distribution function of the minimum distance is estimated. The probability of riser collision can then readily be computed.
