Wave-Structure Interactions on Point Absorbers - an experimental study

Morten Møller Jakobsen

    Research output: Book/ReportPh.D. thesis

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    Abstract

    We are facing a significant challenge when it comes to securing the energy
    needed to sustain our standard of living in the future. With the increasing
    global temperature and the climbing levels of carbon dioxide in our atmo-
    sphere we need to reduce the consumption of fossil fuels. The renewable
    technologies will play a key role in achieving this. The renewable energy sec-
    tor consists of a wide range of technologies yet the source with the highest
    energy density is still untapped. The extraction of energy from the ocean has
    shown to be costly and difficult.

    This thesis deals with the development of a wave energy device by determin-
    ing the loads on the device. The loads plays a key part in optimizing the
    power extraction, reducing the structural cost, and increasing the survivabil-
    ity. Experiments are carried out in small and large scale and compared to
    simulations and empirical functions. The WEC used in the case studies is a
    pitching point absorber (Wavestar).

    The central part of the thesis deals with the challenges, choices, and experi-
    ences gained during the Ph.D. The more in-depth technical details and results
    are presented in peer-reviewed publications and technical reports. The chal-
    lenges addressed in this thesis can be summarized as:

    1:Characterizing wave induced forces on the WEC during operation
    conditions. Morisons equation is used to characterize the wave induced
    excitation loads. Then comparison is made between the estimates of the
    coefficients in experiment and numerical models. Using a modification
    by Faltinsen to take into account the relative motion of the device, the
    contributions from drag, excitation and body motion are determined.

    2: Determining the peak pressure on the surface on the device during
    extreme events and in freak conditions.
    A great deal of work has been done to determine peak pressures on mono-piles worldwide, but only very little on spherical structures. In order to shed more light on the wave induced loads on a hemisphere the peak pressures are
    measured with the traditional drop test and during impact of so-called
    freak waves.

    3: Implementation and comparison of sub-optimal control in both sim-
    ulation and experiments. A long list of control schemes for wave en-
    ergy devices has been presented in the past. After outlining some of
    these methods a non-predictive proportional and differential controller
    is selected for implementation in the laboratory experiments. Through
    simulation the maximum extractable power is determined and the cor-
    responding control gains are used in the experiments.

    4: Ensuring a high comparability of the experiments and with numeri-
    cal models. Herein also accurately determine the waves produced in
    the tank. Scaling effects, unwanted cross waves, reflected waves and
    other disturbances may affect the results when comparing experiments
    with other experiments and numerical models. Methods are presented
    that reduces the influences in these comparisons. This includes work in
    determining the waves in the tanks accurately.

    5: A practical and reliable method to run experiments with combined
    waves and current. Loads on a WEC’s are affected by currents both
    directly and indirectly. The current adds to the particle velocity around
    the device and affects the shape of the waves. Work is carried out
    to find a way to make experiments that combines waves and current
    in a meaningful way. The method needs to be inexpensive, easy to
    implement and reduce the turbulence without distorting the incident
    waves in a detrimental way.
    Translated title of the contributionBølge-struktur interaktioner på punkt absorberer - et eksperimentelt studie
    Original languageEnglish
    PublisherAalborg Universitetsforlag
    Number of pages239
    ISBN (Electronic)978-87-7112-331-9
    DOIs
    Publication statusPublished - 2015
    SeriesPh.d.-serien for Det Teknisk-Naturvidenskabelige Fakultet, Aalborg Universitet
    ISSN2246-1248

    Bibliographical note

    Morten Kramer, Principal supervisor

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    • PH. D.-GRAD

      Hans Jakob Walnum & Morten Møller Jakobsen

      01/12/2015

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