Experimental investigation of towing forces on gravity cage models in calm water and regular waves

This study investigates experimentally drag forces on gravity cage models in calm water and regular wave conditions to understand the impact of currents and waves on these structures. Two cage models, differing in the weight of their bottom rings, are tested in a towing tank under varying towing speeds and a range of regular wave conditions. Load sensors and cameras measure drag forces and cage deformation. Results show that drag forces increase with current velocity in calm water. In wave conditions, mean drag forces are generally higher than in calm water, with short waves generating larger drag than long waves. A heavier bottom ring results in greater drag force but can reduce the influence of wave height. Wave height and steepness also influence drag, especially for short waves, with higher waves leading to larger drag amplitudes. At last, the study acknowledges limitations related to the scaling effects of net material, manufacturing inconsistencies, and maintaining consistent wave conditions, which contribute to experimental uncertainties. It emphasises the need for further study of the fluid field around cages and improved experimental methods to address identified limitations and uncertainties. • • Towing tests are conducted to measure the drag forces for gravity cages in calm water and regular waves. • • Drag forces increase with current speed in calm water. • • As the wavelength increases, the average drag forces decrease and turn to be closer to the drag forces in the calm water. • • Wave height and steepness amplify drag, especially for short waves.