The urgent concern about global warming from the emission of greenhouse gases has provided a strong impetus for engineers and scientists worldwide to research alternative renewable and clean energy. Wind power is one of the fastest growing renewable energy technologies. Onshore wind farms are, however, unsightly and they swallow up valuable land for agriculture and urban development. Already some countries, are considering constructing huge wind farms offshore to take advantage of the generally steadier and stronger winds found in the sea. Moreover, the wind turbines can be larger than those on land because they can be transported to the site by sea. In Japan, the offshore consist of a vast wind resource in deep water where use of conventional bottom-mounted wind turbines is not possible, and floating wind turbines are the most attractive. Thus, it is necessary to consider the effect of floater motion on the tower loading to check the serviceability of the wind turbines which are designed for the bottom-mounted systems.

In the current study, the tower loading is taken as the combination of wave-induced load and wind-induced load. Since their coupling is negligibly small, the analytical formulae are proposed for each kind of load independently. Sway-rocking (SR) model is used as the equivalent calculating model, and the reason why the fixed-foundation model can not be used is presented. For wave-induced load, the influence of each floater motion is investigated separately and their combination is carried out. The resonance of tower vibration will increase the tower loading. For wind-induced load, both along-wind direction and across-wind direction are investigated and their combination is performed as well. Finally, in the combination of wave-induced load and wind-induced load, the load reduction factor changes with wave period, different from the constant value given in IEC for bottom-mounted system. All the results have been verified by the dynamic response analysis of a fully coupled finite element model.

Chapter 1 is a review of current situation of offshore wind energy around the world and in Japan. It explains why it is essential to use floating wind turbine systems in Japan. The outline of this dissertation is also presented.

In Chapter 2, a literature survey of research and development on floating wind turbines is presented. An overview of the research work that has been undertaken pertaining to floating wind turbine technology thus far is carried out, and based on its conclusions and limitations, objectives of this research are presented.

In Chapter 3, two kinds of floating systems: tension leg mooring system and catenary mooring system are considered. Surge is the dominant floater motion caused by wave for tension leg system, while for catenary system pitch motion is significant as well as surge motion. Takahashi used the fixed-foundation model with acceleration acting on tower base to consider the influence of floater motion on the fatigue load. However, this fixed-foundation model is not verified, and in most cases it can't be used. In this research SR model is used as the equivalent model to calculate the tower loading. The stiffness, damping and equivalent wave force of each mode are recognized. A theoretical comparison between SR model and fixed-foundation model is performed with modal analysis and thus give a clear explanation why the latter model can not be used.

Chapter 4 uses SR model to predict the wave-induced load under regular and irregular wave respectively. Since the aerodynamic effect due to floater motion is negligibly small, which means the coupling between wave-induced load and wind-induced load can be ignored, the wave-induced load is investigated independently. The effect of sway (surge) motion as well as rocking (pitch) motion is investigated separately by locking the other mode. The combination of sway motion effect and rocking motion effect is calculated with complete quadratic combination (CQC) rule, and their correlation only depends on the damping and natural frequency of the system. For irregular wave, the maximum wave load can be calculated with the product of standard deviation and peak factor. It is noticed that the resonance of tower vibration causes the non-Gaussian feature and increases the tower loading. This effect will decrease with wave period, since the external exciting effect becomes weaker.

Chapter 5 gives details of the prediction of wind-induced load. Equivalent static method is adopted to estimate the maximum wind load on wind turbine towers. In both along-wind direction and across-wind direction, the analytical formulae are proposed for mean wind load and gust loading factor which contains standard deviation and peak factor of fluctuating wind load. The critical parameters in the standard deviation such as mode correction factor, aerodynamic damping ratio and size reduction factor are investigated to identify the dominant influence factors and their characteristics. A non-Gaussian peak factor which can be reduced to the standard Gaussian form for a Gaussian process is proposed. For floating wind turbine, SR model should be employed for the wind-induced load prediction, since the low natural frequency increases the resonant standard deviation, while the large damping causes significant reduction. Considering the wind response correlation of along-wind direction and across-wind direction, a loads combination formula is proposed to calculate the final design wind load on towers.

Chapter 6 presents the combination of wave-induced load and wind-induced load. Perfect correlation causes overestimation. In this study, the wind induced load and the wave induced load are assumed to be perfectly uncorrelated and the load reduction factor is introduced. It was found that the estimated reduction factor is less than the value specified in international design standard for offshore wind turbine, IEC61400-3.

Chapter 7 summarizes the conclusions of this study. An equivalent SR model is proposed to calculate the tower loading due to wave and wind. The evaluation formulae for wave-induced load are proposed, considering the influence of each floater motion separately and their combination with CQC rule. The resonance of tower vibration causes the non-Gaussian feature and increases the tower loading. The evaluation formulae for wind-induced load are proposed as well and the characteristics of critical parameters are identified. Finally, the load reduction factor is proposed in the combination of wave-induced load and wind-induced load.

本研究は、浮体式洋上風車のタワーに作用する暴風波浪時の荷重を評価する理論式を提案した。浮体式洋上風車は浮体動揺の影響を強く受けるため、従来の陸上風力発電設備のように固定基礎モデルを使用することができず、浮体式洋上風車のタワーに作用する荷重を理論的に評価することできない。本研究では、浮体式風車に作用する荷重に大きな影響を与える浮体のサージ運動とピッチ運動を表わすSRモデル(Sway Rocking Model)を提案し、浮体式洋上風車のタワーに作用する荷重の等価静的評価を可能にした。また、浮体の動揺に伴う変動波荷重と変動風荷重は複数の周波数にピークを持ち非正規過程となることに着目し、これらの非正規過程のピークファクターを推定する手法を提案することにより、浮体式洋上風車のタワーに作用する波荷重と風荷重を解析的に評価することを可能にした。さらに、波荷重と風荷重の組み合わせにおいては、最大波荷重と最大風荷重の相関が低いことに着目し、波荷重と風荷重の組み合わせのための低減係数を提案した。論文の構成ならびにその概要は以下のとおりである。

第1章は本論文の概要であり、世界と日本における洋上風力の現状を述べるとともに、日本の洋上風力発電の必要性を示している。

第2章では既往研究のレビューを行い、浮体式洋上風車に作用する荷重の評価手法に関する既往の研究成果と課題をまとめると共に、本研究の目的を明らかにした。

第3章は、テンションレグ係留およびカテナリー係留を用いた浮体に搭載する風車の挙動を記述する方法について述べている。波によって生じる浮体の運動は主にサージとピッチであるため、これら2つのモードに対する等価的な剛性と減衰を同定することにより、浮体式洋上風車のタワーに作用する荷重を評価できるSRモデルを提案すると共に、従来の固定基礎モデルを用いることの問題点を明らかにした。

第4章では、サージ運動とピッチ運動による波荷重を別々に評価し、組みあわせることにより浮体式洋上風車のタワーに作用する波荷重を評価する手法を提案した。不規則波中での風車タワーに作用する変動荷重は、波周期に対応する周波数と風車タワーの固有周期に対応する周波数にピークをもつ非正規過程であることに着目し、この非正規過程に対応するピークファクターの推定手法を提案することにより、不規則波中における最大波荷重の等価静的評価を可能にした。

第5章では、浮体式洋上風車のタワーに作用する最大風荷重の等価静的評価式を提案した。平均風荷重、変動風荷重の標準偏差およびピークファクターにより最大風荷重を評価する解析式を提案すると共に、標準偏差に含まれるモード補正係数、空力減衰率および規模補正係数が支配的な要因であることを明らかにした。また変動風荷重は風のスペクトルに対応する周期と浮体運動に対応する周期にピークを持つ非正規過程であることに着目し、この非正規過程に対応するピークファクターの評価手法を提案することにより、最大風荷重の等価静的評価を可能にした。最後に、風方向および風直交方向の風応答の相関を考慮し、タワーに作用する風荷重の組合せ式を提案した。

第6章は波荷重と風荷重の組合せ手法を提案した。完全相関を仮定した場合に過大評価になる原因を明らかにすると共に、無相関と仮定した場合に波荷重と風荷重の組合せを精度よく評価できることを示した。また波荷重と風荷重の組合せを考慮するために提案した波荷重の低減係数を提案し、IEC61400-3で示されている着床式用の値より低くなることを明らかにした。

以上のように、本論文では浮体動揺を表わす等価モデルを提案し、浮体式洋上風車のタワーに作用する風荷重と波荷重を解析的に評価することを可能にした。また変動波荷重および変動風荷重の非正規性を考慮したピークファクターの評価方法を提案することにより、最大波荷重および最大風荷重を解析的に評価することを可能にした。さらに、浮体動揺に伴う風車のタワーに作用する風荷重と波荷重の特性を明らかにすることにより、風荷重と波荷重を組み合わせる際の低減係数を提案した。

これらの研究成果は、浮体式洋上風車のタワーを合理的に設計するための理論的基盤を与え、浮体式洋上風車の支持構造物の暴風波浪時の安全性、信頼性、経済性の向上に貢献するものである。よって、本論文は博士(工学)の学位請求論文として合格と認める。