This paper presents the results of wave force tests conducted on three types of offshore support structures considering eight waves and three sea levels to investigate the corresponding wave forces. As a result of this study, it is found that the occurrence of shoaling in shallow water induces a significant increase of the wave force. Most of the test models at the shallow water undergo a nonlinear increase of the wave force with higher wave height increasing. In addition, the larger the diameter of the support structure within the range of this study, the larger the diffraction effect is, and the increase in wave force due to shoaling is suppressed. Under an irregular wave at the shallow water, the wave force to the long-period wave tends to be slightly higher than that of the short period wave since the higher wave height component included in the irregular wave has an influence on the shoaling. In addition, it is found that the influence of shoaling under irregular wave becomes more apparent in the long period.

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In this study, in order to investigate wave force and wave-induced overturning moment according to the support structure types and sea levels, wave force tests were carried out for the three offshore support structures of Monopile, GBS, and Hybrid under three sea levels of HSWL, MSL, and LSWL. As the results of this study, it was found that, as the wave height increased at the shallow water (LSWL), wave force increased significantly because of the wave nonlinearity, named wave shoaling. Also, as the structural member size became larger, wave nonlinearity effects on wave force at the shallow water (LSWL) were reduced because of the wave diffraction effect of the large-sized member. Accordingly, variations of wave force and wave-induced overturning moment according to the wave heights and sea levels were larger in the slender member and smaller in the large-sized member. Therefore, it was concluded that wave force and wave-induced overturning moment of the large-sized support structure were little sensitive to the wave height and sea level than the slender support structure. Maximum overturning moments occurred at the HSWL in the Monopile, at the MSL in the GBS, and at the LSWL in the Hybrid, respectively.

In this paper, for the case of “service life extension” with the same capacity for wind turbines, a structural safety evaluation was carried out to determine whether to extend the service life of the aged foundation. As a result of this study, it was found that the aged foundation satisfies the structural safety of material strength, ultimate strength, fatigue life, and serviceability up to the present. Although the in-service period has been over 16 years, it has been shown that the material properties of concrete have exceeded the design strength, and no significant material deterioration has occurred. Also, structural safety could be evaluated more realistically based on actual concrete properties. In particular, it has been shown that it has a fatigue life of 40 years or more, so service life can be extended. It is expected that the methodology used in this paper will be useful not only for structural safety evaluation of the foundation in service, but also for decision-making for extending the service life. Furthermore, a more technical approach should be explored by many researchers in the future.

In this study, the behaviors of a suction cap shaped as an arch were analyzed using finite element models. The fundamental structural behaviors of both flat-and arch-type caps were compared, and the advantages of the arch-shaped cap were explained. Moreover, the effects of geometric parameters and stiffener arrangements on the stress and deformation of the arch-shaped caps were investigated by comparisons of the changes in the behaviors of the caps. Additionally, the effects of boundary condition at the edge of the cap were analyzed to study the interactions between the cap and cofferdam walls; these results were used to derive the fundamental structural design of stiffened arch caps. Unlike flat caps, the results showed that ring stiffeners could improve the structural behaviors of arch caps remarkably, while the contributions of the radial stiffeners to the structural behaviors of the caps are dependent on constraints at the edges of the caps.