The effects of ocean variability and breaking waves on wave energy converter reliability and survivability

Graduation date: 2015The reliability and survivability of a wave energy converter (WEC) is highly dependent on the variability and non-linearity of the ocean itself. Ocean variability occurs on many time scales. Climate variability occurs decade-to-decade, and year-to-year in the number and intensity of winter storms. Seasonal variation of wave height and period will greatly influence system loading as the calm conditions of summer give way to the raging storms of winter. Real, energetic wave fields are directionally spread, irregular, and highly non-linear leading to lulls-and-bursts of energy incident upon a WEC. System loads will vary greatly from second-to-second, as the WEC absorbs each burst of energy. In order to design for reliability and survivability, the threats to the system must be identified and fully understood. In this dissertation I examine two possible causes of WEC failure: system fatigue, and breaking waves.\ud Over a WECs 10-20 year design life, it will be subjected to tens of millions of loading cycles. System fatigue will likely be the primary WEC failure mechanism. A time-domain simulation of WEC fatigue was performed for devices operating off the coast of Oregon in order to investigate the importance effect of ocean variability on the distribution of fatigue failures around the 10 year design life of the system. The analysis demonstrates that large energetic storm seas contribute significantly to WEC fatigue, and that year-to-year storm variability increases the variance of the distribution of WEC fatigue failures around the design life.\ud Intense impact loads produced by breaking waves may lead to unexpected single-event WEC failures. Yet little is known about the probability and intensity of wave breaking in deep water. A literature review was conducted examining the similarities and differences between shallow and deep water breaking waves with the intent of determining whether data obtained in shallow water breaking waves can be extrapolated to the deep water environment. It was concluded that the dynamics of the crest of a breaking wave are similar regardless of water depth and that valuable information may be gained by testing the response of deep-water systems subject to shoaling breakers. \ud Taking advantage of this finding, a custom wave measurement buoy, housing a tri-axis inertial measurement unit, was built and subjected to breaking waves in shallow water off the coast of Oregon. A large number of highly variable breakers were measured, and the characteristic signature of a breaking wave impact was developed. With this information, real-time detection algorithms may be developed and implemented in wave measurement buoys operating in deep water at the site of potential WEC deployments. Buoys capable of detecting the impact of breaking waves will help provide insight into the probability and intensity of breaking waves in deep water