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Title: Wave loadings and scaling effects on, and within, an Oscillating Water Column (OWC) caisson breakwater
Author: Pawitan, Krisna Adi
ISNI:       0000 0004 8497 5816
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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Even though wave energy has been established as one of the biggest sustainable energy sources over the last 70 years, the development of the wave energy harvesting device is proven to be challenging. Many types of wave converter device have been developed in the past to bring in some of this ocean energy while surviving the harsh conditions of the site. Among many types of wave energy converter invented, the Oscillating Water Column (OWC) has advantages due to its simplicity and low maintenance cost. Several OWC types have been designed and deployed from 1965 to date with many lessons learned. Most of deployed OWC designs, however, have focused on small, stand-alone structures. This thesis is focused on an OWC type wave energy converter (WEC) integrated in a vertical breakwater. This way, the OWC device can be placed in a very energetic location where a breakwater is most likely located. The integration of an OWC into the design of a new breakwater, furthermore, provides the opportunity for cost sharing between energy generation and coastal defence function. This thesis aims to fill two knowledge gaps in methods for the design of breakwater integrated OWCs. The first one is the wave load uncertainty of the device. There is an extensive literature for conventional vertical breakwaters. There are, however, few studies for the OWC installed one. This thesis proposes a new wave load model for the inner chamber of an OWC. The model can be used to estimate the wave loads acting on the rear wall and the ceiling inside the caisson chamber. The model considers three conditions of the OWC chamber: the closed chamber condition, the fully open condition, and the operating condition. Both regular and irregular waves are considered. The model is validated by means of large-scale experimental data from the 2014 "GKW OWCs" project in the Large Wave Flume (GWK) facility in Hannover, Germany. The validation was done for the three chamber conditions. The model was successful in predicting the in-chamber pressure generated, rear wall landward force, and vertical force. It predicted to within a factor of 1.2 for the lower wave steepnesses and over-predicted the forces for the higher wave steepnesses with a more conservative agreement factor between 0.4 to 0.7. Furthermore, the number of front wall impact events comparison between the physical model observation and the existing conventional vertical breakwater probabilistic design tools show that the existing tools can be used for the OWC installed vertical breakwater impact probability in the irregular wave condition. In addition to the wave load prediction, this thesis also investigates the water column behaviour inside the chamber over various wave conditions for both regular and irregular waves. In particular, "sloshing" is explored where the water column behaviour is not according to the idealised piston movement. The conditions under which 'sloshing' is likely to occur have been characterised by means of in-chamber video. Impact pressure measurements of up to 12 ρgHmo have been measured by means of pressure transducers within the chamber. Three different types of in-chamber wave impacts have been identified, characterised, and quantified: single impact, successional impact, and whole water column impact. The second major contribution the thesis addresses is the scaling effect related to physical model testing of this kind of wave energy converter in a laboratory environment. Unlike common breakwater, an OWC includes an air chamber inside the structure. Froude scaling used for the scaled-down experiment in coastal structure only maintains the gravitational force and the inertia ratio between the prototype and the physical model. Consequently, Froude scaling is not sufficient to scale the influence of air stiffness in the OWC chamber correctly. A series of small-scale physical model of 1:79 to the prototype design of an OWC installed vertical breakwater experiment was done in the long wave flume in the University of Edinburgh. These experiments were designed to be as faithful a scaled down version of the large-scale GWK tests as possible, for both the dimensions of the structure and the wave conditions tested. This is done in order to facilitate the most direct comparison between the small-scale and large-scale experimental results. It can be concluded based on the chamber pressure comparison that on the optimum operating condition, the smaller scale physical model will over-estimate the pressure recorded in the large-scale by a factor of about 2.45. Similar results are obtained for both closed chamber and fully-open chamber conditions. This thesis is expected to reduce the uncertainty in the design of breakwater integrated OWC by enabling the in-chamber loading estimation and more representative small-scale physical model test in a laboratory. By reducing these uncertainties, the wave energy harvesting development could be push forward.
Supervisor: Bruce, Tom ; Venugopal, Vengatesan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: wave energy converter ; OWC ; wave-structure interaction ; wave loading analysis