A True Transient Hydrodynamic Force Prediction Method on a Descending and Oscillating Buoy

Abstract

Disconnectable turret-moored Floating Production, Storage and Offloading (FPSO) or Arctic Floating Drilling (AFD) units may operate in harsh environments for offshore oil and gas production. The turret buoy is designed to be disconnected from the FPSO or AFD to circumvent excessive loads on the mooring system in these harsh environments which could be driven by extreme waves or icebergs. Due to the large descending and oscillating motions after disconnect, the hydrodynamic force on the buoy is of transient nature (i.e. unsteady hydrodynamic force). To confidently predict the buoy motions during the disconnect event, it is necessary to understand and accurately predict the transient effects from the body’s instantaneous position and the water surface on the hydrodynamic force. However, no analytical or numerical methods have been successfully applied by current industry practice that accurately capture the forces on a descending buoy in this this transient phenomenon. In this work, we present a numerical analysis method developed by ExxonMobil Upstream Research Company (URC) to calculate the “true” transient hydrodynamic force on a descending buoy. A transient potential-flow mathematical model is developed in the time-domain through a transient free-surface Green’s Function by applying the instantaneous buoy position and velocity. The hydrodynamic force on the buoy is evaluated using a “fluid impulse” theory, which is computationally efficient. Computational results are presented and compared with model test results to illustrate the performance of the developed numerical approach on predicting the hydrodynamic force on a descending buoy for various disconnect scenarios.