Dynamic Response of a Spar-Type Floating Wind Turbine at Power Generation

In this paper, dynamic response of a Floating Offshore Wind Turbine (FOWT) with spar-type floating foundation at power generation is presented. The FOWT mounts a 100kW wind turbine of down-wind type, with the rotor’s diameter of 22m and a hub-height of 23.3m. The floating foundation consists of PC-steel hybrid spar. The upper part is made of steel whereas the lower part made of prestressed concrete segments. The FOWT was installed at the site about 1km offshore from Kabashima Island, Goto city, Nagasaki prefecture on June 11th, 2012. Since then, the field measurement had been made until its removal in June 2013. In this paper, the dynamic behavior during the power generation is presented, where the comparison with the numerical simulation by aero-hydro-servo-mooring dynamics coupled program is made.

Computations of Arbitrary Thin-Shell Vertical Cylinders in a Wave Field by a Hypersingular Integral-Equation Method

We present a numerical formulation and computational results for the hydrodynamic loads on bottom-mounted thin-shell vertical cylinders of arbitrary cross-sectional shapes, including open bodies. Such cylinders may undergo prescribed or free motion or may be subjected to a wave load. The formulation is based on linear theory and a hypersingular integral-equation results from sectional contours of zero thickness. The method reduces the fully three-dimensional problem into a number of two-dimensional ones in the horizontal plane and is therefore much faster than the usual boundary integral method used for water wave problems. This traditional method of solution is also known to become ill-conditioned as the body thickness decreases. As an example of the current method, radiation and diffraction loads are presented for the cases of a circular and square closed and opened cylinders with the effect of the increased opening size discussed at different frequencies. The free-surface elevation associated with this method of solution is presented for some cases as well.

Tow Forces for Emergency Towing of Containerships

For a series of five containerships of differing capacities (707, 3400, 5300, 14000 and 18000 TEU) systematic computations were performed to estimate the tow force required in an emergency. Time-average ship positions with respect to the given waves, wind and current directions and the corresponding time-average forces were considered. Current speed was considered to include also towing speed. Directionally aligned as well as not aligned wind and waves were investigated. Wave height, wind speed, and wave and wind direction relative to current direction were systematically varied. Wind speeds based on the Beaufort wind force scale corresponded to significant wave heights for a fully arisen sea. Waves were assumed to be irregular short-crested seaways described by a JONSWAP spectrum with peak parameter 3.3 and cosine squared directional spreading. For each combination of current speed, wave direction, significant wave height and peak wave period, the required tow force and the associated drift angle were calculated. Tow force calculations were based on the solution of equilibrium equations in the horizontal plane. A RANS solver for current and wind forces and a Rankine source-patch method for drift forces in waves computed hydrodynamic forces and moments. Tow forces accounted for steady (calm-water) hydrodynamic forces and moments, constant wind forces and moments, and time-average wave drift forces and moments. Rudder and propeller forces and towline forces were neglected.

Development of a Time Domain Boundary Element Method for the Seakeeping Behavior of a Cruise Ship Under Combined Strong Wind and Extreme Irregular Beam Seas

A computational method for the seakeeping behavior of a cruise ship at zero speed and under severe wind and oblique wave loads is presented. The proposed methodology is a time-domain panel method where the transient Green functions used for the estimation and implementation of the free surface effects on the vessel’s motions are estimated assuming constant low lateral speed, instead of the common practice zero speed influence functions. For the evaluation of the overall hydrodynamic forces, the so called “blended approach” is followed in the sense that the induced hydrodynamic pressures due to the scattering and radiation phenomena are calculated over the linearized position of the body, ignoring any displacements with respect to its mean position, while the hydrostatic and non-linear Froude-Krylov forces are considered at the actual body location and taking into account the free surface elevation at each time step. For the validation of the proposed methodology, heave and roll motions, the drift velocity as well as lateral accelerations of the vessel were investigated for two cases of severe beam seas combined with a constant strong wind load and the results were compared against experimental model tests. The model tests were performed to investigate the vessel’s behavior under extreme weather conditions. The low lateral speed Green functions were estimated for a speed similar to the one that the vessel was expected to drift, an estimation based on the model tests, as well as for the case where the input speed corresponded to the half of the expected speed. Good agreement was presented for both cases, showing that accurate and computationally efficient numerical simulations of the vessel’s motions under severe wind and wave excitations can be obtained by using low lateral speed transient Green functions.

Towards a Lateral Line Sensor to Supplement Sonar in Shallow Water

Sonar provides vessels with a sensory system to detect and identify still and moving obstacles. In shallow water both active and passive sonar meet their limits. Acoustical methods exist, aiming at supporting sonar systems by means of digital signal processing, or, coming from the field of biomimetics, imitating echolocation principles of marine animals. This paper introduces a sensor system combining these approaches by the use of a vector sensor array applying Near-field Acoustical Holography (NAH) imitating the Lateral Line organ (LL) of fish; a passive method to supplement active and passive sonar. LL is able to localize obstacles due to their dipole-like water displacement by comparing low-frequency water accelerations distributed along the whole body. In contrast to pressure, accelerations are highly evanescent and do not propagate into the far-field. Thus LL does not suffer under reverberation or scattering. The performance of the proposed NAH-based LL-sensor is tested by a computer simulation of a source in absence and in presence of a disturbing source. The LL-sensor has proven to be more robust than pressure detection methods like beamforming and conventional NAH.

Near-Optimal Time-Domain Control of Small Buoys in Irregular Waves

Within the linear theory framework, smooth optimal control for maximum energy conversion in irregular waves requires independent synthesis of two non-causal impulse response functions operating on the body oscillations near the free surface, and one non-causal impulse response function relating the exciting force to the incident wave profile at the body. Full cancellation of reactive forces and matching of radiation damping thus requires knowledge or estimation of device velocity into the future. As suggested in the literature, the control force can be synthesized in long-crested waves by suitably combining the ‘full’ impulse response functions with wave surface elevation information at an appropriately determined distance up-wave of the device. This paper applies the near-optimal control approach investigated earlier by one of the authors (Korde, UA, Applied Ocean Research, to appear) to small floating cylindrical buoys. Absorbed power performance is compared with two other cases, (i) when single-frequency tuning is used based on non-real time adjustment of the reactive and resistive loads to maximize conversion at the spectral peak frequency, and (ii) when no control is applied with damping set to a constant value. Time domain absorbed power results are discussed.

Hydrodynamic Response of Multiple Fish Cages Under Wave Loads

Nowadays, marine aquaculture is playing an important role in meeting people’s ever increasing need for sea foods high in protein. The multiple fish cages are commonly used in large fish farms currently. Mooring systems in these fish cages should be strong enough to withstand extreme environmental loads. Thus, a reliable mooring system is required in designing safe structures. In this paper, a finite element model is built. The floating collar is simulated by the beam element, the twine of the net and the mooring lines are simulated by the truss elements. The geometric nonlinearity of the net model and the material nonlinearity of the mooring line are considered. In this analysis, the hydrodynamic forces estimated by Morison equations are applied to the model. The hydrodynamic response of multiple fish cages under the wave loads by considering the pretension of mooring line is carefully studied. The maximum tension in mooring lines under different wave propagation direction is also analyzed in this study.

Port Placement Theory in Consideration of Geographical Characteristics and Disaster Risks in Case of Ocean Space Utilization

In case of ocean space utilization, the factors which have to be taken into consideration in order to form an international basic container route are geographical factors which are the position on a global scale, the economic scale of port hinterland, the ocean climatic condition for setting the route etc. as well as factors from the viewpoint of transport such as the volume of container cargo and both size of container ships and container terminals. It is important to consider these geographical factors not only in order to study the port placement from the global point of view but also to devise the port policy. Although there are many studies on factors from the viewpoint of transport, there is almost no study on these geographical factors. Then, the authors made a new simulation model and analyzed these geographical factors of the international container ports in all parts of the world. As a result of analysis, the authors got the conclusion that there were three port placement patterns. The first type is ‘the Continent Base Port Type’, which it is located in the continent and has the large economic hinterland. Typical ports of this type are Antwerp, LA, LB and Shanghai. The second type is ‘the Ocean Base Port Type’, which is located in the ocean space where geographical predominance is high. This type forms route hubs. Typical ports of this type are Singapore, Malta and Kaohsiung. The third type is ‘the Tight Hinterland Port Type’, which is located in an island and has the tight economic hinterland where the economic activities density is very high. This type has characteristics that the distance between the ports is short and there are a lot of numbers of ports, which is unique and special in the world. Japan’s ports are classified in the third type. Furthermore, Japan has a characteristic that there are many large-scale earthquakes and has to consider earthquake measures to reduce disaster risks. The authors will suggest the most suitable port placement theory in consideration of these characteristics in case of ocean space utilization.

Noise Footprint: A Proposal Within the Framework of FP7 AQUO Project to Define a Goal Based Approach Towards the Reduction of Underwater Radiated Noise From Shipping

The Marine Strategy Framework Directive has officially stated as soon as 2008 the anthropogenic noise due to shipping were to be mitigated. The policy makers, the yards and the ship owners still strongly rely on the expert studies and guidelines to find the appropriate methodology to assess and then mitigate the acoustic pollution impact shipping on the marine biota. To address this issue, the project AQUO “Achieve QUieter Oceans by shipping noise footprint reduction” (www.aquo.eu) started in October 2012 for 3 years. The AQUO project was built in the scope of FP7 European Research Framework. It involves 13 partners from 8 European countries, mixes academic experts, industry representatives from yard, classification society and other acoustic and bio-acoustic specialized bodies. Addressing the anthropogenic noise pollution into the marine biota is an increasing concern which is logically paired with the increasing commercial maritime traffic. The IMO itself has recently issued a first draft guideline, to be followed up during the next MEPC, addressing this matter. In parallel to the different achievements that are effective or in progress by standardization bodies or other delegated or assigned work group, the AQUO project aims to finally issue guidelines to be taken as methodological tools. The multi-disciplinary team of this project aims to comply with expectations from the different stakeholders. First, an overview of the project is given. The objectives and the related project structure are detailed so as to better understand which axes are chosen and studied. Secondly, it is here proposed to share the recent outcomes of AQUO project. The current status of applied knowledge, related legal decisions as well as standards empowerment are essential to identify the remaining needs and consequent expected efforts. Drawing first the background rationales the noise footprint concept is then detailed. The process followed towards the main objective of mitigating the anthropogenic noise from shipping will be partly revealed and completed by the expected future work to be achieved by 2015.

Structural Analysis of Aquaculture Nets: Comparison and Validation of Different Numerical Modelling Approaches

The performance of three different numerical methods were compared and evaluated against data from physical model tests. A parameter study of a simplified net cage model subjected to a steady flow was performed by all methods, varying the net solidity and the flow velocity. The three numerical methods applied models based on springs, trusses or triangular finite elements. Hydrodynamic load calculations were based on the drag term in Morison’s equation and the cross-flow principle. Different approaches to account for wake effects were applied. In general, the presented numerical methods should be able to calculate loads and deformations within acceptable tolerance limits for low to intermediate current flow velocities and net solidities, while numerical analyses of high solidity nets subjected to high current velocities tend to overpredict the drag loads acting on the structure. To accurately estimate hydrodynamic loads and structural response of net structures with high projected solidity, new knowledge and methods are needed.