ABSTRACT

In this study, numerical studies were carried out on the wave impact loads on a circular cylinder by breaking waves. Computational fluid dynamics (CFD) simulations were performed based on the model test observations where three different waves, i.e. steep, spilling, plunging waves, were generated to measure the wave impact forces acting on a truncated circular cylinder. For the validation, the numerical simulation results were directly compared with the model test data. After the wave hits on the truncated circular cylinder in the steep wave, a wave run-up phenomenon is occurred in front of the truncated circular cylinder. In case of the spilling wave condition, the impact force peaks are changed in accordance with the truncated circular cylinder locations. In the plunging wave tests, impact force time histories by the model tests can be categorized three types. It can be found that only one peak of the impact force is occurred in numerical simulations. All of the phenomena as well as the impact force time histories by the numerical simulations are in good agreement with the model test results.

INTRODUCTION

Accurate estimation of an impact force is very important for the design of the offshore platform. The impact force occurs in heavy sea condition and high breaking waves. Moan (2005) reported accident rates of floating and fixed offshore platforms about number of accidents per 1000 platform-years. He also presented that the structural damages of various accidents cause a low fatigue life. Rosenthal et al. (2007) reported about Draupner wave in the North sea through MaxWave Project. They showed the existing high waves that were more than two times higher than the significant wave height, so called ‘rogue waves’. It is know that the severe sea conditions had caused the loss of more than 200 carriers.

The breaking waves are usually categorized as spilling and plunging waves. Especially, the plunging wave has a dramatic wave shape as overturning fluid of the wave crest and is important topic about the wave impact force. In the past, the model tests were performed about the strong impact force by the breaking waves. The representative study, Kjeldsen et al. (1986) dealt with experimental study about local wave forces on a vertical pile in irregular waves and regular waves. The model test results showed that the impact force caused by breaking waves exceeded the impact force in regular waves by 3 times. Interestingly, the wave breaking phenomena in research of Kjeldsen et al. (1986) were caused by phase superposition of waves and were occurred also in wave steepness 0.05. It was known that the wave breaking phenomena could be occurred any time by phase superposition of waves and could be occurred larger impact force on the object. In recent years, experimental and numerical studies about the wave impact forces were being carried out to understand of wave impact force characteristics. Representatively, Wienke and Oumeraci (2005) performed experimental studies about wave impact forces according to inclined angles of slender circular cylinder in large wave flume. Curling factors of the model test results were compared with 2-dimensional curling factors by Von Karman (1929), Wagner (1932), Cointe (1989) and Goda (1966). It was shown that the 3-dimensional curling factors by Wienke and Oumeraci (2005) were in agreement with 2-dimensional curling factors given by existed researchers. Wienke and Oumeraci (2005) reported that peak value and duration of impact force were affected by a run-up effect. The impact force could be estimated by 2-dimensional curling factor and the run-up phenomenon should be predicted for the time history estimation of the impact force.

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