Modeling Hydrodynamic Forces on Vessels during Navigation Lock Operations

Abstract

Navigation locks are some of the most complicated hydraulic structures engineers are challenged with designing. Evaluation of lock filling and emptying systems includes examination of pressures at critical locations within the culverts, time variance of the chamber water surface, and loads exerted on a transiting vessel's mooring system. Detailed analysis of the lock system, especially regarding the forces acting on a moored vessel during locking operations, is usually conducted using physical models. Longitudinal oscillations of the water surface within the chamber generate forces on the vessel. Safe navigation conditions during lock operations limit the acceptable magnitude of these mooring line (hawser) forces. The difficulty of measuring the oscillating forces exerted on the hawsers is that the mooring system is a mass-spring system. The frequency of the hydrodynamic forcing must be considered when designing a force-measuring device. The dynamic equation describing the moored system is a second-order, nonhomogeneous, ordinary differential equation for a damped system with external forcing. In mooring applications, the system is generally underdamped, and the displacement of the moored vessel oscillates with an exponential decay in amplitude. Application of the dynamic equation requires knowledge of the added mass and the hydrodynamic damping of the vessel within the confines of a lock chamber. This paper describes the hawser force measuring method in the physical model. Discussion is given to the dynamics of the mooring system and the interaction with the chamber water-surface oscillations triggered by flow from the lock culvert system. The added mass and hydrodynamic damping coefficients are quantified from experimental results. Analytical methods for predicting prototype performance and the interpretation of model parameters in the equation of motion are also presented.