Study on the wave impact forces below the deck of multi-column offshore floating structures

• Main Author: Liew, Mei Shi.
• Other Authors: Low Ying Min
• Format: Final Year Project
• Language: English
• Published: 2012
• Subjects: DRNTU::Engineering::Civil engineering::Water resources
• Online Access: http://hdl.handle.net/10356/48979
• Institution: Nanyang Technological University

An offshore disaster brings about severe impacts to human lives and capital investment. To avoid such unfortunate events, estimation of air gap is particularly important in designing floating structures to reduce undesirable wave impact that occurs in offshore disasters. In this Final Year Project, the objective is to investigate the impact of platform dimensional changes on platform motion and wave elevation and the air gap response of the platform. The study was carried out using computer simulation. A full study of the wave impact on offshore structures is very complicated as many factors are involved. To keep the study simple, only one geometric property of the platform is assumed to change, while keeping all other factors constant. However, that is not the case in the real world as many factors are interrelated and do contribute to the impact on platform motion and wave elevation. Hence, some of the simulation results obtained differs from theoretical knowledge. In the simulation process, a variation in 1% of column diameter, 5% in pontoon length, breadth and height was carried out separately to analyse the effects on RAO motion and wave elevation. Simulation results are assumed to draw the analogy of the Morison’s Equation. The results show that a larger RAO motion resulted mainly due to a reduction in column diameter, increment in pontoon length, breadth and height. There are several factors affecting RAO motion, including F-K diffraction force, radiation damping and added mass. Considering the few factors above, the primary source influencing RAO motion is the F-K diffraction force. Since a smaller column diameter, larger pontoon, breadth and height results in a larger F-K diffraction force for most frequencies, the same constraints for the geometric properties will bring about a greater RAO motion. As a result of a larger RAO, the air gap is also maximized. To conclude, a smaller column diameter or larger pontoon length or larger pontoon breadth or larger pontoon height will maximise the air gap, while all other factors remain constant. In addition, the air gap for wave spectra with RAO motions is significantly bigger than wave spectra without RAO motion. Hence, it is crucial to include RAO motions for a more conservative estimate of the minimum air gap required for the design of floating structures in order to minimize wave impact below the deck. It is recommended to include model testing for the validation of the simulation results due to the poor accuracy of numerical methods in the estimation of air gaps. In addition, simulation studies for varying multiple geometric properties to find out the combinatorial effects on RAO motion and wave elevation could be carried out in the future.