Plunging Breakers Impact Loading on Offshore Structures

Abstract

A plunging wave induces slamming impact forces due to its collapsing jet. Linear wave theory would not be applicable for slamming load prediction. This is also one of the most devastating force that an offshore structure could experience in its operational lifespan. There are design guidelines to predict the resultant design slamming load contribution as a function of the breaking wave celerity. Hence it is important to investigate and understand the factors that would have an impact of the slamming load contribution. Current industry design guideline for the slamming load coefficient doesn’t take into account for different cylinders geometries, cylinders cross-sections, wave parameters, impact rise time, types of mediums used. Instead, the recommended slamming load coefficient is based on a ‘smooth cylinder’. This would potentially give rise to scenarios where the design breaking wave load would be overly conservative, thus leading to unnecessary added construction cost and time wastage. The aim of this project is to investigate this slamming load coefficient to different geometries and wave parameters. Besides the traditional circular cylinder, squared cylinder will also be analysed and compared. A circular cylinder of different diameters, coupled with different breaking wave signals of varying intensities would be used in this case, with the main objective of establishing a semiempirical relation between the slamming load coefficient and the wave kinematics. This PhD project also aims to establish a separate breaking wave design guidelines for 2 other geometrical cylinders; the square cylinder and the square cylinder with a 45-degrees shift. These project will also investigate the sensitivity of different plunging jet maturity. It was found that the geometric cross-section of the offshore structure has a direct influence on the slamming load distribution and coefficient. De Wang Chia, found that the fullness (𝐶𝐶𝑤𝑤) of the offshore structure relative to the breaking wave propagation direction, has a direct correlation with the slamming load and slamming coefficient contribution. The fullness of the offshore structure would affect the air entrapment capability during the plunging jet contacting with the offshore structure. (De Wang Chia, 2019) concluded that the diamond-shaped cylinder that has the lowest fullness, would give the lowest slamming load and slamming coefficient, and with the square (flat wall) cylinder having the highest slamming load and coefficient contribution. For the circular cylinders, a larger radial cylinder would also have higher fullness and associated slamming contributions, with a larger radial cylinder starting to exhibit behaviour closer to a square (flat wall) cylinder. De Wang Chia tries to investigate past researchers ( (P.A Blackmore., 1984)& (Chan E.S W. M., 1988)) conclusion that the impact rise time would have a negative correlation with the slamming load. Breaking wave signals of varying breaking intensities were used for the purpose of achieving different rise times. It was shown that a lower rise time is indeed associated with higher slamming load contribution. However, it does not automatically equate to a higher slamming load contribution. This is due to the higher breaking wave intensities associated with higher wave celerity. The increased slamming load contribution derived from higher wave kinematics and not from the slamming load coefficient. It was concluded that the slamming load coefficient is not affected by the breaking wave intensities and rise time.