The behaviour of a moored oil tanker in the Port of Leixões, Portugal

Abstract

The Port of Leixões is located in the north of Portugal. The operational conditions at berth “A” are affected by several factors resulting in a down-time of the berth of about 20%. In order to describe the behaviour of the moored vessel physical model tests were performed at the Faculty of Engineering University of Porto within the research and development program DOLPHIN. Numerical simulations of wave propagation including non-linear wave interaction and the generation of both sub- and super-harmonics were not carried out in the DOLPHIN program. Within this study a sequence of numerical models was applied to describe the moored ship motions and make a comparison with measurements from the physical model tests. An approach which combined a Boussinesq-type wave model with a panel model was selected as the appropriate approach for the present study. A Boussinesq-type wave model was selected, since diffraction around a breakwater, partial reflection from port structures and non-linear wave processes are important and relevant hydrodynamic processes to describe the wave field in the vicinity of the ship. The output from the Boussinesq-type wave model describes the surface elevations as well as the velocity field of the waves in the vicinity of the ship. The successive panel model takes the presence of the ship within the incident wave field into account and calculates the wave forces on the ship. The calculated total wave forces are a summation of the Froude-Krylov, diffraction and second order wave forces. Time-series of wave forces serve as an input for a ship simulation model, which simulates the ship motions and the mooring forces, taking into account environmental forces and non-linear interactions with the mooring system. The initial approach combined the Boussinesq-type wave model MIKE21 BW, with the panel model Harberth and the ship simulation model Quaysim. Due to numerical instabilities within MIKE21 BW an eddy with unreliable velocities was formed after longer period of simulation. This eddy was located in the vicinity of the ship. The subsequent computations with Harberth resulted in a continuous increase in second order wave forces on the ship. The first order wave forces on the ship did not increase continuously, but via an analysis it is shown that the calculated first order wave forces on the ship are not reliable. The finally simulated ship motions in Quaysim are not reliable as well. The unreliable wave forces on the ship as well as the unreliable subsquent simulated ship motions are a consequence of the pre-simulated instabilities within MIKE21 BW. The cause of the numerical instabilities in MIKE21 BW could not be discovered during this study. Since longer period of simulations are required to obtain statistical reliable ship motions a switch was made to an alternative Boussinesq-type wave model called TRITON. Adaptations in the model set-up in TRITON ensured that numerical instabilities over longer periods of simulation in the simulated velocity field were avoided/minimized. The emphasis in simulations with TRITON is on simulating and understanding the low frequency waves. The focus was on simulating the low frequency waves that caused significant moored ship responses within the physical model. The simulated waves in TRITON agree well with the measured waves in the physical model basin. It is shown that a standing wave is measured in the physical model basin, which caused measured surge responses of the ship. The obtained insight in the generation of these low frequency waves may be used when performing additional physical and/or numerical model tests. It is recommended to simulate the total Port of Leixões with a Boussinesq-type wave model, taking into account all relevant hydrodynamic processes for moored vessel response. In that case a bound long wave should already be imposed in the incoming generated waves, since the generation of primary waves only may lead to an underestimation of the total content of low frequency waves. The wave simulations, as performed with TRITON, are expected to be sufficiently accurate to serve as an input for vessel response computations. The recommended additional wave simulations and vessel response computations are expected to provide additional insight in the causes of excessive ship motions. New insight in the behaviour of waves as well as vessel response may lead to improvements of the operational conditions at berth “A” in the Port of Leixões.