Peak force coefficients on small-diameter spanning pipelines under waves

Abstract

This paper reports an experimental investigation undertaken to measure the peak force coefficients for small-diameter pipelines (cables and umbilicals) spanning over rocky, uneven seabed under regular wave conditions. The free spans formed by the uneven bed are investigated by modelling multiple regularly spaced free spans (referred to as multi-span) in a flume. Approximately 600 physical tests were conducted to quantify the effects of gap height to pipeline diameter ratio G and the span length to pipeline diameter ratio S on the peak force coefficients. The Keulegan–Carpenter number KC and Reynolds number Re p tested were in the range 20 ≤ KC ≤ 2000 and 5 × 10 3 ≤ Re p ≤ 10 5 , respectively. The ratios between the peak force coefficients for the multi-span configuration and a uniform-span configuration ( S → ∞) are demonstrated to be a function of S /[( S + C ) G ], where C is the contact ratio, defined as the distance between two adjacent free spans divided by the pipeline diameter. Combined with the peak force coefficients under uniform-span conditions obtained from the present experiments, this finding enables the peak force coefficients under a multi-span condition to be reasonably estimated. • Peak force coefficients for fully spanning small-diameter pipelines subject to waves are provided based on a number of physical tests with KC up to 2000. • Quantification method for the peak force coefficients under the effect of multiple free spans is proposed. • The scattered seabed roughness and the multiple free spans lead to significant reductions in the peak force coefficients.