Feasibility study of a system for damping ship pitch motions and extraction of wave energy

Abstract

A system for pitch motion reduction of offshore vessels is investigated for feasibility. An air chamber is simulated in the bow section of a case ship open to water in the bottom and with a small opening in the top where the air is ventilated out.

The chambers dampening effect is investigated by combining new equations derived from the well-established theory of the air cushion of surface effect ships, with the equations of motion of the conventional ship. A commercial grade vessel response program is used to find the hydrodynamics of the conventional ship, while a general purpose program is used to simulate the physics of the air chamber.

Further the feasibility of extracting the energy in the oscillating flow in and out of the chamber is investigated, based on the theory behind the wave energy concept, oscillating water column.

Scatter diagrams and wave spectrums are utilized to investigate the results of the system in relevant sea states in the North Atlantic Ocean.

When comparing the significant response of the conventional ship and the ship with an air chamber, it is possible, with the largest AC configurations, to reduce the motions by over 50% in small sea states, 40% in medium sea states and 30% for severe sea states. The impact of the larger air chambers on the rest of the ships systems is investigated.For smaller AC configurations, that can more easily be installed with less impact on the rest of the ship systems, the reduction potential is lower and one might argue that the gain is not worth the sacrifices.

The potential for extracting energy from the air chamber is very high. Results as high as 5 MW are observed for the larger air chambers while the highest results for the smaller, more practical air chambers are just less than 600 kW.