Nowadays, the pile-supported structures have been applied in many offshore industries, especially in the wind energy field. As a new type of stationary pile-supported structure, high-rise pile-cap structures are attracting more attention. In the present study, a fully nonlinear solver based on Navier-Stokes equations is established for the investigation into a ten-pile cap structure. The wave loads are obtained by integrating pressure over the surface of each part of the structure. The effects of the interaction between the piles and the cap are investigated. In some cases that the cap is on the top, the wave loads on the piles are 30 percent larger than that without the cap. This is caused by the effect of impact. It means that the wave loads are underestimated significantly by simply using Morison equation. The relationships between the wave loads on piles and the gap between the cap and still, the wave height, and the wavelength are also discussed in detail.
In the recent decades, the pile-supported structures are commonly found in the costal and offshore environment, especially in the wind energy field for Offshore Wind Turbine (OWT). They are generally built by means of a group of piles in different arrangements.The long-term safety of the pile group-supported structures is still a major concern in coastal engineering. As a consequence, the prediction of wave loads on offshore pile group- supported structures is of great importance.
Nowadays, we have already had some typical types of pile-supported structures, such as monopile, suction pile, pile cap and so on (Ryu, et. al., 2012). As a new type of stationary pile group-supported structure, high-rise pile-cap structure is attracting more attention. It has lots of advantages, such as high stiffness, reasonable cost, easy mounting, etc. (Chen & Zhou.et al.,2016). Generally, for pile group-supported structures, the total wave force could be obtained by calculating each single cylinder pile with Morison equation (Morison, J.et al., 1950). However, the effect of different interference parameters is difficult to estimate, especially the pile group effect. Zdravkovich (2003) discussed the effects of the interference parameters on the overall drag forces for pile groups. Through some small-scale and large-scale experiments, Bonakdar et al (2012) and Bonakdar (2014) reviewed that the interference parameters such as the relative spacing between piles and the number of neighbouring piles could noticeably affect the wave loads on every single pile. Anew characteristic geometric scale called effective diameter is introduced by Qu et al (2017), and the total inline wave force on different complex pile-supported structures could be estimated by the unified empirical formulas.Moreover, the interaction between the cap and the piles should also be taken into consideration when the wave hits the cap. Therefore, the Morison Equation is no longer appropriate for estimating the inline wave force of the high-rise pile-cap structure. Meanwhile, the cap is much larger than the other parts of the structure and it will also lead to a larger lift force on the structure.