Experimental study on the effect of second order wavemaker theory on the response of a flexible large diameter monopile in irregular sea

Abstract

Motivated by the need for larger offshore wind turbines, large diameter monopile foundations are being developed. To ensure safe design, there is a need for model testing and validation of hydrodynamic load models. Scaled model tests with a piston-type wavemaker commonly apply first order wavemaker theory for irregular waves. This approach results in the generation of second (and higher) order spurious (also known as parasitic) free waves in the tank. In this study, the effect of superharmonic spurious waves on the response of a monopile with eigenfrequency close to three times the wave peak frequency is examined experimentally. The bending moment response statistics are not found to be significantly affected by the wavemaker correction. Different wave breaking patterns are observed for individual events, but our results do not indicate any clear relation between breaking waves and the wave generation technique.

Comparison of laboratory wave generation techniques on response of a large monopile in irregular sea

“…For both tests without and with correction, the quasi-static responses have COV of less than 2%. The COV for the total bending moment and for the contribution near the first natural frequency is less than approximately 11% (as in our previous tests [11] ). For the responses near the second natural frequency, the COV reaches 73% for the test without correction, which again is similar to our previous test campaign [11] .…”

“…The COV for the total bending moment and for the contribution near the first natural frequency is less than approximately 11% (as in our previous tests [11] ). For the responses near the second natural frequency, the COV reaches 73% for the test without correction, which again is similar to our previous test campaign [11] . The maximum COV is less than 52% for the test with correction.…”

“…Two irregular sea-states were tested with the two techniques, as summarized in Table 1. The results presented in this paper are from the steepest sea state with significant wave height 𝐻𝐻 𝑠𝑠 = 8.6 m, and peak period 𝑇𝑇 𝑝𝑝 = 11 s. To separate the bound wave from the spurious wave and compare the two wave generation techniques in the wave only tests, the wave elevations were measured over a length of 7 m with a step of 0.08 m (model scale) in the similar way as our previous test [11] . In the next step, model tests were performed with the monopile wind turbine.…”

“…Dadmarzi et. al [11] show that the spurious wave in the tank affects the breaking/slamming events. The repeatability test here suggests that removing spurious wave might increase the repeatability of the experiment in the slamming events.…”

“…Subharmonic spurious waves has been studied by Stansberg & Kristiansen [9] and Fonseca et al [10] for low-frequency horizontal motions of moored floating structures. The authors [11] previously studied the effect of superharmonic spurious waves on the response of a monopile with natural frequency of three times the wave frequency. The spurious waves changed the wave breaking process and can significantly affect the individual events, however, statistically they had minor effects compared to stochastic variations in the irregular sea states.…”