Wave Impact Experiment of a GBS Model in Large Waves

Abstract

For the design of offshore structures in harsh wave environments, model testing continues to be the recommended industry practice for determining wave impact forces on offshore structures. Accurate measurements of wave impacts in model tests have been a challenge for several decades. Transducers are required to accurately capture the short duration, high magnitude, and dynamic nature of impact loads. The structural model, transducers, and the transducer mountings need to be designed such that mechanical vibrations in the integrated transducer-mounting-structural model system do not contaminate the wave impact measurements. In this work, the dynamic oscillations in the measurements were controlled through the design and fabrication of transducers, their mounting and the GBS model. Wave crest probability distributions were developed that included fully nonlinear effects. These distributions were used as a benchmark to qualify the waves in the wave calibration tests. The highly stochastic nature of impact loads makes it challenging to obtain converged probability distributions of the maximum impact loads (i.e. forces or pressures) from model tests. To increase the confidence in the statistical values of wave impact loads, a large number of realizations were used for a given sea state. Variability of the maximum pressure due to wave basin effects (such as wait-time between tests) was examined with fifteen repeat tests using the same wave maker control signal. These tests provided insights into the random behavior of the impact loads.

OC6 Phase Ib: Floating Wind Component Experiment for Difference-Frequency Hydrodynamic Load Validation

“…The OC6 Phase Ib campaign incorporated load measurements for each of the column/heave plate components of the semisubmersible seen in Figure 2, as well as pressure measurements on the heave plate and near the still waterline, where it is expected that the nonlinear hydrodynamic loading is the highest. Prior examples of pressure measurement in a wave basin with a surfacepiercing structure often focus on water impact problems during extreme wave events (see, e.g., [18][19][20][21][22]). In contrast, measurement of pressure on structures in nonbreaking and moderate waves is less common.…”

Wave Front Steepness and Influence on Horizontal Deck Impact Loads

“…Given the rare occurrence of the critical wave impact phenomena, both approaches-experimental and numerical-can be quite time-consuming for a proper statistical robustness to be obtained. Thereby, many recent model test campaigns do include multiple 3-h realisations (seed numbers), with typically up to 20 seeds, and sometimes even more (see, e.g., [18,19]). However, e.g., in the case of wave slamming on columns, it is found in [18] that even with almost 100 realisations there is a statistical uncertainty to be taken into account, at least when using the frequently applied contour method reflected in Figure 1.…”

On the Importance of Considering Multiple Seastate Realizations When Estimating Design Loads in Multi-Directional Shallow-Water Waves

“…However, because neither of these approaches is exact, the resulting pressure and load estimates must be associated with a considerable degree of uncertainty which introduces considerable risk to the design process (Huang et al, 2017).…”