The Markov approach to estimate fatigue damage for a monopile-based offshore wind turbine exposed to aerodynamic and hydrodynamic loading is investigated in this study. The focus of this study is on obtaining the rainflow-counting intensity from a peak-trough counting using the Markov method proposed by Frendahl & Rychlik. The fatigue damage estimated from the rainflow-counting intensity is compared to fatigue damage estimated from the original time-series using the rainflow-counting algorithm. The comparison is performed for different load situations. The study shows that the Markov approach performs the best for load situations where wave loading is dominating the response, making it interesting for load calculations of large-diameter monopiles and monopiles in parked or idling conditions.
Offshore wind turbines are prone to failure from fatigue damage due to their exposure to a significant source of quasi-periodic excitations from wind and waves. A detailed fatigue assessment is based on cycle counting (the rainflow-counting algorithm is widely used) from a large number of load simulations in the time-domain, typically in the order of a few thousand. Detailed fatigue assessment is therefore primarily performed in the final stage of the design process (Seidel et al. 2016), since it is inefficient for sensitivity studies or conceptual design phases where several designs have to be assessed. Simplified and/or reduced models of wind turbines or loads (Muskulus, 2015; Schløer et al., 2016; Ong et al., 2017) or calculations in frequency-domain (Ragan & Manuel, 2007; Seidel 2014; Ziegler et al., 2015) are commonly used to estimate fatigue damage in these situations.
Another method to compute the expected damage was proposed by Frendahl & Rychlik in 1993. Fatigue damage is estimated by assuming that the sequence of local extrema forms a Markov chain. This allows to obtain the rainflow-counting intensity directly from the spectrum of the input loads (referred to load spectrum throughout this paper) without the need for response time series in the time-domain. The fatigue damage can be estimated from the rainflow-counting intensity afterwards. The benefit of this method is the possibility to estimate the total damage from a load spectrum without the need to perform lengthy and computationally demanding simulations in the time-domain. The flowchart for both methods are shown in Fig. 1.
Chen, Zhe (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | He, Yanping (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Liu, Yadong (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Zhao, Yongsheng (Shanghai Jiao Tong University, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | He, Chong (Tongji University)
Due to the influence of the scale effect, the thrust of model wind turbines mismatch the target value under the Froude-scale condition in floating offshore wind turbine model tests. This needs to redesign model blades to satisfy the similarity requirements of the force. This paper presents the design methodology of multiple airfoils blade based on the blade element momentum theory and the map of the lift and drag coefficients. Then, compared with the single airfoil blade in numerical simulation, the thrust value of the multiple airfoils blade is closer to the target value.
National research institutions and scholars carry out different model tests of floating offshore wind turbines to study the performance and improve the efficiency of floating offshore wind turbines. The model tests of Hywind and Windfloat are the most representative among the model tests of floating offshore wind turbines. The Hywind model test was carried out in Marintek, in which the power of wind turbine prototype is 5MW and the scale ratio is 1/47 (Nielsen, Hanson and Skaare, 2006). The Windfloat model test was carried out in the UC Berkeley wave tank, in which the power of wind turbine prototype is 5MW and the scale ratio is 1/105 (Roddier, Cermelli, Aubault and Weinstein, 2010).
The thrust of the wind turbines under the Froude scaled wind is less than the target thrust value because of the scale effect in the floating offshore wind turbine model tests. To ensure the accuracy of the test, blades are redesigned to meet the target value. For example, NACA4412 is used as the airfoil to form blades in the model test carried out in the ocean engineering basin of Institute of Industrial Science, the University of Tokyo (Nihei and Fujioka, 2011). However, due to the pitch control system installed in the model of wind turbines, the cross section of the blade root zone changes, which means that the airfoil of the root zone is no longer the single airfoil. It causes the lift and drag coefficients calculated from single airfoil are not suitable. In the multiple blade, the lift and drag coefficients of different blade elements are calculated from different airfoils which reduce the effect of this problem.
The Marine and Oil & Gas industries have increased interest in Self Propelled DP Jackups vessel to enhance the overall operational efficiency. The self-propelled jackups vessel has numerous applications such as well intervention, wind farm installation, repair, accommodation, service etc. in this cost optimization process. However due to closed proximity of operation (i.e. near to hydrocarbon platforms or other offshore assets), it has gigantic risk during the Simops Operation.
The Simops Operation of Dynamic positioning and Jacking operation has a grey area in force calculations for DP system on board which use mathematical model. The input from position reference sensors, environmental sensors to calculate relative position based on position, heading, speed and rate of turn inputs for station keeping/ DP intended operation and finally issue thrust output command. The system continues to monitor the position by taking its feedback and comparing with calculated vessel position. However, many DP jackups does not have system for hydrodynamic force measurement of leg & spudcan. This is mainly due to drag coefficient of complex leg and Spud Can geometry. The drag coefficient plays vital role in force calculation and due to non-standard shape currently industries follow approximation method. This will increase risk in station keeping under different condition and failure cases.
The intension of this study is to find the gap in existing force calculation, which is affected by drag coefficient. During the CFD analysis evaluation the effect of marine growth, leg shape, spud can, leg transit speed under variable environmental conditions are considered. The software use for CFD analysis is Ansys Aim. The vessel data used for the simulation purpose are from most popular design of jackups vessel used in offshore O&G and wind farm industry.
The study will provide drag coefficient variation in transit time domain, which has large impact on force calculation estimation for DP System.
Carbon Capture Storage method is expected to be counterpart for global warming but it holds also a risk of CO2 leaking in the sea. CO2 hydrate formation has the potential of solving the problem. The aim of this work is to build CO2 hydrate formation model based on previous studies. In order to determine hydrate growth rate, matching with temperature rise of experimental result, interface mobility parameter was decided.
After several numerical calculations, the order of interface mobility parameter was identified.
As one of the methods to suppress the global warming, Carbon Capture and Storage(CCS) attracts much attention over the world.
CCS method is gathering CO2 gases from big emission source such as factory or thermal power plant and accumulating them under the seafloor.
By the method, it can be expected to reduce the concentration of CO2 in the atmosphere. In the other hand, CCS method holds also the risk of CO2 defluxion in the sea. If stored CO2 leak into the sea, it may make impacts to ecological system in the sea and CO2 concentration in atmosphere.
To prevent the CO2 defluxion from the seabed CO2 hydrate technology is taken into account. CO2 hydrate has cage structures that trap CO2 molecules in water molecules. Hydrate is stable under high temperature and low temperature condition. Though the areas in which CO2 is stored are unstable for hydrate structure, if accumulated CO2 gases arrived seafloor in which temperature become lowest, it makes hydrate structure. Then generated CO2 hydrates reduce the width of CO2 gases flow to the sea (Fig. 1).
To estimate how much CO2 hydrate can block the CO2 gases flow, it is necessary to calculate change of permeability under the sea floor by the CO2 hydrate generation.
As previous work, (Fukumoto 2013) used the Phase-Field Model (PFM), which simulates the growth of CH4 hydrate and calculated permeability change in porous media.
The objective of this research is to develop a PFM for the growth of CO2 hydrate, based on the model of Fukumoto (2013) in order to evaluate how CO2 hydrate formation prevents the gas leakage.
We propose new concept of simple DP system. Minimum data input from sensors are used for control the ship position and heading. Only GPS position (latitude and Longitude) and Gyro compass heading data are necessary. The prediction of the position and heading realized by means of simple extrapolation using recorded data are used for calculation. Therefore, no complex ship motion analysis is necessary in the system, it has features that it is easy to process and easy to implement. Proposed DP system have been installed on board Training Ship FUKAE MARU of Kobe University. Outline of proposed dynamic positioning system and its user interface on the navigation system, and the results of experiment in actual mission of the real ship are shown in this paper.
It is very difficult to keep a ship stationary at a certain point in the sea. Even if a ship anchors, it causes a whirling due to disturbance such as wind and tidal current, and the ship position and heading direction move every moment. In order to stop at sea, it is necessary to operate like the engine (main shaft propeller), rudder, and thrusters are used to cancel the disturbance forces, but the performance of the operation manually by human is limited in its accuracy. In recent years, a dynamic positioning (DP) system has been realized that performs operations such as position keeping with high precision by automatically, and is used on offshore support vessels that conduct ocean development, observation vessels necessary for ocean observation work, etc. They perform optimal control or model-based control (Balchen, 1980; Sorensen, 1996). In this paper, we propose design of a DP system that is practical with simple procedure and easy to equip on existing ships.
In the process, only data of GPS position and Gyro compass heading are used as input to control, and simple proportional control is performed. Any other sensors like anemometer and current profiler are not necessary. At that control step, instead of the data at the present, the predicted value of the movement after tens of seconds based on the recorded data for several seconds before are used. The movement of past several seconds reflects all trends of disturbance influence, and by using the prediction value, it is thought that control with disturbance taken into consideration without calculating for complex analysis based on hydro dynamics. Therefore, the system has features that it is easy to process and easy to implement. This control method is simple and straightforward and can be processed with a small amount of calculation, so today's PC can calculate in a very short time, repeatedly, one after another. The system can be realized with an appropriate accuracy at an inexpensive price as compared with the conventional system.
The Floating Liquefied Natural Gas (FLNG) Production, Storage, and Offloading Unit and the Floating Storage and Regasification Unit (FSRU) have appeared in the offshore liquefied natural gas (LNG) chain recently. Large-scale membrane tanks are expected to be used in some such facilities; however, special attention should be paid to partial loading in the membrane tanks because of the risk of sloshing. A distribution control method by which LNG is distributed and interchanged between the tanks to reduce the sloshing risk was proposed by Yokohama National University. In this operation, the sloshing risk is reduced by shortening the period spent in the partially-filled condition as much as possible. In this study, the operation was optimized through the use of a genetic algorithm (GA). Several cases involving different charging operations were examined, and the flexibility of this method was demonstrated. It was shown that a considerable decrease of the sloshing risk was achieved.
Interest in the production of liquefied natural gas (LNG) at offshore locations has grown remarkably, and the development of related technologies has accelerated. The floating liquefied natural gas (FLNG) production, storage, and offloading unit is one of the related technologies to be applied in retrieving natural gas from the ocean (Arai et al., 2012). FLNG has advantages such as its lower initial cost and shorter construction time than conventional natural gas facilities, and it can be widely applied to several types of gas fields in the sea. For these reasons, such systems have gained considerable attention, and some engineering companies are starting to design and build these new facilities, e.g., Shell’s Prelude FLNG (Shell Global, n.d.). Large-scale membrane tanks are expected to be used in some of the FLNGs because of their good space efficiency. However, it is necessary to avoid partially filled conditions as much as possible in membrane tanks in order to minimize the sloshing risk. In particular, the charging operations in LNG tanks take a long time, and there is a possibility of sloshing during the operations.Sloshing refers to the violent movement of liquid with a free surface inside a liquid storage tank that is caused by tank motion. Since membrane tanks have almost no internal structures, the liquid in the tank can move freely. Moreover, violent sloshing loads may occur at the corners of the tanks if the tanks have a prismatic or almost-square shape. A severe sloshing phenomenon is caused by the matching of the natural frequency of the free surface motion with that of the ship motion. In ships with very large tank breadths such as FLNGs, the rolling motion frequency is likely to match the free surface natural frequency. One way to avoid the occurrence of resonance is simply to prevent these frequencies from matching. The free surface motion’s natural frequency mainly depends on the tank breadth and liquid height.
Floating Liquefied Natural Gas Production, Storage and Offloading Unit (FLNG) and Floating Storage and Regasification Unit (FSRU) have appeared in the offshore LNG chain recently. Large scale membrane tanks are expected to be used in some such facilities; however, special attention should be paid to half-loading in the membrane tanks because of the risk of sloshing. A distribution control method by which LNG is distributed and interchanged between the tanks to reduce the sloshing risk was proposed by Yokohama National University. In this operation, the sloshing risk is reduced by shortening the period spent in the half-load condition as much as possible. In this study, the operation was optimized using a genetic algorithm. Several cases involving different charging operations were examined, and the flexibility of this method was demonstrated. It was shown that a considerable decrease of the sloshing risk is achieved.
It is the first part of a two-part study devoted to deriving three-dimensional (3-D) motion equations of a cantilevered pipe discharging fluid at sea. The 3-D nonlinear equations were derived based on the modified Hamilton’s principle. The Morison formula was used to calculate the external fluid-induced static drag force and viscous damping forces in the in-line (IL) and cross-flow (CF) directions. A double-van der Pol oscillator model was employed for the Vortex-induced vibrations (VIVs). The derived mathematical model employing wake coefficients in Facchinetti et al. (2004) and solution methods were validated by comparisons of blind numerical predictions with experimental results.
The ice-diminishing Arctic Ocean has inspired the world’s shipping industry to explore the feasibility of the historical Arctic routings, on the Northern Sea Route and Northwest Passage, at least as seasonal commercial operations. Both of the routes could significantly shorten travel distances between Europe, East Coast of America and Far East Asia. The feasibility of the Arctic routes is discussed, mostly based on the integrated outcome of the International Northern Sea Route Programme, which was conducted soon after the Russian declaration of the NSR as an international sea lane.
Responding to the world’s growing demand for oil and gas, Arctic resources have been given much attention by the energy and shipping industries. In addition, global warming has accelerated oil and gas development in the Arctic, particularly in its western region. The icediminishing Arctic has inspired the world’s shipping industry to explore the feasibility of the historical Arctic routes, the Northern Sea Route (NSR) and the Northwest Passage (NWP), as seasonal commercial sea lanes, at least. The NSR is a waterway from the Atlantic Ocean to the Pacific Ocean along the Russian coast of Siberia, lying mostly in the Russian Arctic waters, which markedly reduces the distance by 40%, comparing with the traditional route via the Suez Canal. Before the beginning of the 20th century the Northern Sea Route (NSR) was known as the Northeast Passage (NEP), or Sevmorput in Russian. Along the NWP, the saving in travel distance of about 5,000nm from Asia to Europe by avoiding the Panama Canal eventually should prevail within the shipping industry. The NSR was officially opened up in 1987 by Russia to the international shipping industry. Responding to this declaration, a comprehensive and multidisciplinary feasibility study of the NSR, called the International Northern Sea Route Programme (INSROP), started in 1993 and ended in 1999.
Huge earthquakes on the Nankai Trough which is located in the offshore of Shikoku island and Ki-i peninsula, Japan may occur in the coming years. Ships under tsunami attack in that bay may be uncontrollable, collision, drifting, grounding, etc. It is decided, to avoid such a dangerous situation, that mooring, arriving or leaving ships should evacuate to a safety area as soon as possible and be anchored there, (and let tsunami go away). The purpose of this paper is calculation of anchoring ships’ motions under tsunami attack using mathematical models, and therefore, discussed and evaluated for the safety guidelines. INTRODUCTION The Nankai Trough is a submarine trough located with south of Japan’s island of Honshu, extending approximately 900 km offshore along the south coastline. The trough outlines a subduction zone that is caused by subduction of the Philippine Sea Plate beneath Japan, which’s part of the Eurasian plate. Huge earthquakes have occurred at intervals of 150 years to 100 years in the trough, as shown in Figs.1-2. Moreover, it is estimated that there is a 50% probability of a tsunami being generated by an earthquake in this area in the next 30 years. Tsunamis may occur after an earthquake, and the tsunami arriving time to Osaka Bay is around 1h. Consequently, ships may begin to move uncontrollably, subjecting piers to tremendous sideways forces, and crash relentlessly against breakwaters. Ultimately, vessels are set adrift and run aground. That is a large number of important industrial facilities exist around Osaka Bay, several research projects concerning measures against tsunamis have been undertaken. The shallow Seto Inland Sea is 450 km long from east to west. Its width from south to north varies from 15 to 55 km. The average depth is 37 m and the greatest depth is 105 m.