Planning and Design of Floating Offshore Architecture

Floating offshore structures are attracting increasing attention as a method for addressing problems such as rising sea levels due to global warming and the increasing global populations. However, unlike ground structures, floating offshore structures must consider the effects of waves. The movement characteristics of the floating offshore structure have been reported. However, no studies have compared variations in motion response characteristics according to the scale of floating structures or buildings atop them, so it is currently difficult during the initial planning and design stages to estimate the size of superstructures that can be designed for a given marine area. Therefore, with the aim of obtaining basic data for planning floating offshore structures, in this study we developed floating structure modules (a square 36m on a side) according to their superstructure and investigated the basic motion response characteristics for each. We furthermore derived tendencies for horizontal acceleration and inclination occurring in individual modules according to design waves for Tokyo Bay.

Study on Applicability of FEMA Formula in Prediction of Impact Load by Tsunami Drifting Vessels on Coastal Zone Structures

In this study, we consider and evaluate the applicability of the FEMA’s formula which is one of the main simple formulas for calculating impact forces through the comparison with our proposed numerical simulation model. We investigated the situation of collision with a thin structure such as a pillar by numerical experiments based on the particle method, in particular, we focus on the case where the drifting speed is fast in targeting the quay of Shimizu Port, Shizuoka prefecture. We compared the results of numerical experiments obtained by detailed simulations with FEMA’s formula and evaluated the applicable range of FEMA s formula for huge vessels. As results, it was suggested that the overestimation was occurred with FEMA’s formula for large vessel using the inundation height and its velocity. FEMA’s formula calculates the impact force of large vessel on the safe side if the drifting vessel speed is used: the condition of added mass coefficient changes the results, but these indicate the safe side in this study. On the other hand, the results of safety sides are fluctuated depending on the height of the action point of force. Therefore, we confirmed that there is a need to examine the height of the impact point when using FEMA’s formula for large vessels.

Quantitative Wear Estimation for Floating Structures by Using 3-D Geometry of Mooring Chain

For reducing the maintenance cost of floating offshore wind turbine structures, it is necessary to establish a quantitative wear estimation method for the mooring chains. In this paper, attempts have been made to improve the accuracy of the estimation method in terms of the mooring chain model. These investigations were performed about a spar-type floater moored with three catenary mooring lines at Goto, Nagasaki prefecture, Japan. Up to now, the mass-spring model had been used for the mooring chain in response analysis and the relative angle between two spring lines was considered as only a sliding angle without friction. However, there is also rolling in the motion between mooring links, which should cause less wear than by sliding. In this study, the detailed motion of the link in response analysis is calculated and applied to the wear estimation by using a 3-D model in MSC. Adams. This enables the wear estimation considering link motion closer to a real phenomenon. A Contact analysis between the 3-D chain model requires some contact properties (e.g. contact stiffness and friction). In this paper, these properties are calculated based on the Hertzian contact method and FEM analysis. As a result, the wear amounts overestimated by using the mass-spring model in the previous investigation, especially at the point located clump weight and touchdown point, decrease getting closer to the measurements. In addition, by tracking the contact points it is found that the major motion caused between links is the rolling. For future works, there remains a need for further validation and the consideration of elasticity between mooring links, impressions caused by proof load test and the effect of corrosion.

Characteristics of OWC Type WEC Dampers Installed on a Very Large Floating Structure

Oscillating water column (OWC) type wave energy converters (WECs) have been researched and developed. OWC WECs are relatively friendly to maintain them in operation because all of mechanical units are set above a sea water surface. In addition, a feature of an OWC device is similar to an air dumper system. Thus, it should be possible not only to harvest wave energy but also to reduce motion of a floating system at the same time. As well as WEC system should be used with other ocean renewable energies as a combined system. This paper describes hydrodynamic characteristics of OWC devices and wave fields around them of multi-OWC devices equipped large floating structures. For this research, the linear potential theory based in-house programme code was applied to calculate hydrodynamic performance of OWC regions and elastic motion behaviours of the structures. Besides, calculation results were compared with some experimental results of characteristics of OWC devices on reference papers published. Then we proved validity of the calculation method. We have quantitatively summarized how much the reduction effect can be seen according to the aircushion placement and the number of aircushions on the floating body. the paper investigated arrangement of OWC devices on the floating structure with several variations. Using the prediction method, effects of arrangement of OWC devices on the performances are investigated.

Numerical Analysis of a Floating Fish Cage With Feeding Systems

A modern marine-based fish farm normally consists of a feeding barge, several fish cages, and feeding tubes. Although many studies, both experimental and numerical, are available in the literature to investigate the global responses of the fish cages under wave and current conditions, research on the coupled system including both the fish cage and the feeding system is very limited. This paper presents a numerical study on the coupled system with a floating fish cage and the feeding system. The purpose is to study the dynamic responses of the coupled system under different environmental conditions and configurations of the fish farm. A numerical model is firstly established in the numerical program OrcaFlex, comprising of a feeding barge, a gravity-based floating fish cage with mooring systems, and a feeding tube between the barge and the cage. Time-domain simulations of this coupled system are then performed under environmental conditions corresponding to 1-year and 50-year return periods for a reference site. The deformation of the fish cage, the tensions in the anchor lines and in the feeding tube are compared under various conditions. Sensitivity studies on the solidity ratio of the fish net as well as the lengths of the feeding tube are addressed, and their influences on the responses of the coupled system are also discussed.

Impacts of the Madden-Julian Oscillation on South China Sea Monsoon

It is of great practical importance to understand the variability of the South China Sea (SCS) monsoon on intraseasonal time scales, since the anomalous enhancement of the SCS monsoon may exert serious impacts on the safety of offshore engineering and marine transportation. Our composite analysis shows that the SCS surface wind anomalies are considerably varying with the Madden-Julian Oscillation (MJO) eastward propagation. The SCS summer southwest monsoon tends to be stronger (weaker) in phases 5–8 (1–4) of MJO with the largest positive (negative) wind-speed anomalies when the MJO convection is centered in the western Pacific (far western Indian Ocean), suggesting the highest (lowest) probability of the gale over the SCS. The variation of the western Pacific Subtropical High (WPSH), induced by the variations of the local meridional circulation, is shown to play a crucial role in the MJO-SCS summer monsoon linkage. The SCS winter monsoon is also shown to be modulated by the MJO with strengthened (weakened) surface northeasterly in phases 5–6 (1–2). The extra-tropical East Asian trough and East Asian westerly jet associated with the local meridional circulation can well explain the changes of the MJO-SCS winter monsoon relationship. The opposite responses of the wind direction during the same phases of the MJO between summer and winter may be attributed to the discrepancy of meridional circulation related to the wintertime equatorward shift of the MJO convection. The present study indicates that the MJO could be taken into consideration when applying extended-range weather forecast over the SCS as the predictability of the MJO activity is up to 15–20 day currently.

Conceptual Design and Model Tests for a Mid-Water Floating Hyperloop Tunnel

Aviation has a significant impact on the global emission of greenhouse gasses. On the Northern Atlantic route alone there are over 2,500 crossings daily. This illustrates the high demand for connecting people. It is expected that this demand will only increase in the future, which will increase the emissions due to aviation even further. An alternative way for connecting people can be the hyperloop, which obtains comparable speeds while using a fraction of the energy. For intercontinental connections a tunnel would be necessary. In this study, a conceptual design of a mid-water floating hyperloop tunnel is made and tested on model scale at MARIN. In the present paper the results are discussed of model tests on a mid-water floating tunnel in Atlantic storm conditions at various wave directions and tunnel depths. The conceptual design of the tunnel is based on (nearly) available technology. One kilometer tunnel segments with a diameter of 11 m are connected to construct a tunnel length of > 5,000 km. Model basin tests are performed on scale 1:110, where a scale model of 140 m length is tested. The tunnel is designed as a neutral buoyant tunnel to reduce complexity and costs for the mooring system. The motions, deformations and mooring line tensions for the tunnel segments are measured by force transducers, accelerometers and an optical measurement system. Due to flexibility of the slender tunnel segments in combination with a soft mooring system, the tunnel tends to following the incoming waves for certain tunnel depths and wave directions. Only small motions and deformations are allowed for a hyperloop capsule to travel on high speed. The conceptual tests show first results on tunnel depth, structural and geometrical design of an hyperloop tunnel and mooring system.

Effects of Scale and Configuration of Air Chamber of OWC Type WECs on Air Chamber Characteristics

This paper describes scale effects and influence of configurations of oscillating water column type wave energy converters from model tests and theoretical calculations. Many researches regarding wave energy converters (WECs) have been conducted. The behavior of an oscillating water column of an OWC type WEC is complicated because of including wave-air-turbine interaction, and thus several issues remain. One of the issues is that influence of difference in scale between small scale experimental models and full scale models is unclear. It is important to understand its characteristics accurately to improve design technologies for such as complicated systems. In this study, we carried out forced oscillation tests using multiple scales and shapes of OWC models in still water, and measured the pressure inside the air chamber and the internal mean water level with a multi-line wave probe. The experimental models used have a box like air chamber or manifold type air chamber, and which scales were 1/1, 1/2 and 1/4.The difference of the two air chambers is an orifice or a duct to be inlet-outlet of air. As a result, the difference in scale and configuration of the air chamber affected the characteristics of the air chamber. In addition, as a result of numerical calculation using the linear potential theory and comparison with experimental results, the experimental results could be reproduced by numerical calculation. Besides, we could discuss the effects and the influences of the air chamber basically.

Configuration and Performance Analysis of Deep Ocean Mining Flexible Riser

Deep-sea minerals such as polymetallic nodule, hydrothermal sulphides and ferro-manganese crusts have for long attracted attention as an alternative source of metals to terrestrial deposits. To bring these minerals up to the land, flexible risers are needed. As the mining industry developing towards deep sea area, the conveying system is usually designed as a combination of steel riser and flexible riser. According to different transport requirements, various flexible riser configurations, such as steel catenary riser, lazy-wave riser and saddle-shaped riser, have been proposed. During mining operation, the riser bears gravity, buoyancy, wave and current force, therefore the assessment of structural safety and reliability is quite challenging. In addition, the riser response caused by the mining vehicle motion during working process in a large area should also be considered. To guarantee a safe operation and service life of the riser, it is necessary to carefully design its configuration and to analyze its performance. In this study, taking the saddle-shaped riser as our model, the influences of main design parameters on the riser configuration, tension and stress are examined. These parameters include the installation position of buoyancy modules, the buoyancy ratio and motion of mining vehicle. Firstly, the analysis model of the riser response is established based on FEM in which the nonlinear large displacement and deformation of the structure are considered. Secondly, through our FEM simulation, the distribution and variation of tension and stress along the axial length of risers with different configurations are presented. Finally, the impacts of the mining vehicle motion on riser response are discussed. Our numerical results show that a small change of the buoyancy position and buoyancy ratio may lead to a significant change of the riser configuration, but a little change of riser tension/stress. And the saddle-shaped riser has a good tolerance performance to the bottom-end excitation.

Numerical Modelling of the Interaction of Moving Fish Nets and Fluid

The significant difference in length scales between the flow around a moving fish net and the flow around each twine of the net prevents the resolution of the complete structure within a discrete fluid domain. In this paper, this issue is overcome by calculating the net and fluid dynamics separately and incorporate their interaction implicitly. The forces on the net are approximated using a screen force model, and the motion of the net is computed with a lumped mass method. Here, a linear system of equations is derived from the dynamic equilibria and kinematic relations. The net model is coupled to the CFD solver REEF3D which solves the incompressible Navier-Stokes equations using high-order finite differences in space and time. Several numerical calculations are provided, and the comparison of loads and velocity reduction with available measurements indicates the good performance of the proposed model.