ADV measurements of blockage flow effects near a model jacket in waves and current

Abstract

Design standards for drag loading on offshore jacket structures do not presently account for the reduction in forces arising from flow blockage effects in the event of combined waves and current. This force reduction is believed to originate in reduced mean flow velocity through the jacket, but this has never been directly measured. To address this, we conducted physical-model tests which measured the flow adjacent to a jacket structure in combined waves and in-line currents using acoustic Doppler velocimeters. Results confirm a dramatic reduction in the mean flow velocity up-wave and down-wave of a model jacket in waves and current, far greater than the flow reduction observed in current alone. These results unambiguously confirm the significant additional blockage (and hence reduction in structural loads) not captured in current offshore design standards.