The wave drift force on different tanker and barge models was measured in a wave tank test. A supertanker model, a small tanker and a barge model were tested. The models were moored in the head sea with two linear springs fore and aft. The spring constants were varied in the test series and the springs were pretension so that they never went slack. The tests included the regular waves, wave groups and the irregular waves. The motions of the floating vessel and the loads in the mooring lines were measured. The steady wave drift force on the model and the long period oscillating load on the mooring line due to regular waves, wave groups and irregular waves are presented as functions of frequencies, model sizes and shapes and spring constants. The added mass and damping coefficients for these models at different natural periods are computed from the free oscillation test in still waters and from the regular wave tests. Data are presented in no dimensional form using Froude's law. Since the wave drift force follows Froude's law, the data are directly applicable to a prototype situation and are believed to aid in the design of a single point mooring system.
Wave drift force is a second-order quantity. On a fixed object where linear theory is applicable, this quantity is, generally, small. However, for a moving object, especially in the case of a moored object, the wave drift force is particularly important.
The wave drift force has two components a steady component and an oscillating component. The steady component arises from the incident, diffracted and radiated waves and is present in regular or random waves. The oscillating wave drift force that is important has a long period and depends on the frequency difference in the wave components in a wave group or in a random wave. When the frequency of the oscillating wave drift force approaches the natural frequency of a moored system, the load in the mooring line increases due to dynamic amplification. If the damping in the system is small (e.g., in the surge motion of a tanker), the mooring line load due to the oscillating drift force becomes large. In this case the amplitude of the drift force is about the same order of magnitude as the first-order force on the mooring line.
Therefore, it is important that the steady and oscillating wave drift force be known accurately in the design of single point mooring systems. In naval hydrodynamics the wave drift force has been the subject of study for a long time. Both theoretical and experimental studies have been conducted to determine drift force on a ship heading in a seaway (called added resistance). The earlier theories are based on the conservation of energy or momentum principle (see references I and 2 for a review on this subject).
In recent years, linear wave diffraction theory based on three dimensional potential flow has been extended to second order to derive the wave drift force on a floating object. The technique e.g., developed by pinkster (5) is particularly suited for floating moored offshore structures including tankers.
