Impulse and Energy in Deepsea Riser Collisions Owing to Wake Interference

Abstract

Abstract The paper describes model testing of top tensioned, deep sea riser arrays, conducted by MARINTEK for the Norwegian Deepwater Programme. The originalpurpose of the tests was to determine criteria for on-set of clashing orcollisions between individual risers in current. Furthermore, force andacceleration measurements made possible the determination of the impulse andenergy involved in the collisions. The results show that the previously published theory based on turbulentwakes and momentum balance, predicts onset of collisions to an accuracy whichis in most cases within 10 to15 percent, referring to velocity. For riserswithout strakes collision energies up to 65 kJ, prototype, were measured. Withstrakes fitted to the risers the maximum measured collision energy was oneorder of magnitude lower. An approximate empirical formula for the collisionenergy of bare risers is suggested. The paper also discusses possible practical solutions to the problem ofcurrent induced riser clashing. Introduction Previous research projects and publications (Refs.1 and 2) have concludedthat collisions or clashing between individual risers may beome an issue ofconsiderable concern in the design of riser systems for future deep seafloating production systems. As part of the Norwegian Deepwater Programme (NDP)MARINTEK has conducted a series of model tests with top-tensioned, deep searisers to determine criteria for on-set of collisions, and to determine theforce impulse, effective mass, and energy involved in such collisions. Thepurpose of the present paper is to give a survey of these tests and theresults. Physics of riser collisions The static deflection of a riser due to the current drag is proportional tothe square of its length, other parameters kept constant. For deep sea systemsthis means that a relatively small difference in static deflection may lead tomechanical contact between adjacent risers. The situation is illustrated in Fig. 1. The downstream riser, R2, is sitting in the wake of an upstream riser,R1. The effective inflow velocity, and thus the static deflection, of R2 willbe reduced relative to R1.