A development program to further increase the competitiveness of the direct electrical heating technology has been initiated. By increasing the power frequency from 50/60 to 100-200 Hz, less current is needed to develop the same heat generation in the pipe. A test program is running with the purpose of studying AC corrosion on the cathodically protected steel pipelines at higher frequencies. This article presents results from initial tests, showing that corrosion on steel specimens are present at 60 Hz, but that zero or very little corrosion is present at 100 and 200 Hz. On the aluminum specimens, corrosion rate is reduced by 55 % from 60 to 100 Hz. and is somewhat further reduced at 200 Hz.
When a hydrocarbon production pipeline is shut down, it gradually cools to a temperature level where hydrates or wax may be formed. If hydrates and wax are formed in a pipeline, these may restrict the flow and may in a worst case situation block the entire pipe. Removal of hydrates is very difficult and potentially hazardous. One of the traditional methods for hydrate or wax prevention is to add chemical inhibitors to the well-stream. In continuous operation this method has considerable operational costs, (Hesjevik, 2006). Chemical treatment requires storage tanks, inhibitor regeneration plants, piping and pumps on the platform or vessel that occupy space and deadweight tonnage, as well as a sizeable subsea piping infrastructure. One of several alternative qualified methods to prevent hydrates and wax formation is direct electrical heating (DEH).
A 3-year development program for further increase of the competitiveness of the DEH technology was initiated in 2016. An innovative step by increasing the power frequency is proposed. Prestudies indicate clear benefits by going from a DEH-system operating at normal power frequency of 50/60 Hz to a system operating at a frequency ranging between 100 and 200 Hz, (Lervik, 2016). In a DEH-system, electric alternating currents of some hundred amperes are transferred from the steel pipe to the surrounding seawater. Thus, AC corrosion aspects must be considered. At present, DEH systems are only qualified for use at 50 and 60 Hz power frequency. One of the outstanding issues is related to AC corrosion.
In order to be economically viable PV-storage systems must fulfill certain performance criteria. The system control strategy has a large impact on the overall system performance and the system control software should be regulated in such a way as to maximize the self-supply ratio as well as the battery lifetime.
At KIT approximately 20 commercially available PV-battery systems have been analyzed with respect to specific performance criteria.
The following criteria were used to quantify the level of intelligence of the storage system control software: (a) whether the software contains a predictive module at all; (b) the success of delayed charging in reducing the time spent at high stage of charge (SOC) levels and (c) whether prediction errors lead to the battery not being fully charged so that the user's self-sufficiency is unnecessarily reduced. Since the different effects are not independent the goal is to quantify the effects on system performance in each case. This complements other studies based on simulation or those that look at different aspects like cell aging in an isolated context.
The present work outlines how the control software can influence the performance and especially the calendar aging of the PV storage systems.
Efficient and economic energy storage technologies are key elements for a sustainable future energy supply. Attractive and economical applications can be established if we succeed in providing persistent and cost-efficient storage systems for electrical energy.
Since electricity prices in Germany have increased in the last few years (European Commission, 2016) and storage prices are starting to decline, this will also soon become true for small German household applications. It can already be seen that more and more PV home storage systems have been installed over the last few years (Karais et al., 2016).
There are several critically important aspects affecting performance and as a consequence commercial viability: the system design (AC-coupling or DC-coupling) and dimensioning, the level of development of the system control software and the calendar and cycle life of the battery and electronic components. The better the system design and dimensioning is the better will be the performance of the system due to a high efficiency of the power electronics and the battery.
Nam, B. W. (Korea Research Institute of Ships & Ocean Engineering (KRISO)) | Ha, Y. J. (Korea Research Institute of Ships & Ocean Engineering (KRISO)) | Hong, S. Y. (Korea Research Institute of Ships & Ocean Engineering (KRISO))
A tension leg platform(TLP) may experience high wave run-ups and nonlinear wave forces in extreme waves due to smaller vertical motion responses as well as local wave amplifications. In this study, a Cartesian-grid based flow simulation method is applied to compute the nonlinear wave forces and run-ups on a TLP in high waves. The present simulation method solves the two-phase Navier-Stokes equations using finite volume discretization with a volume-of-fluid (VOF) method. As a simulation model, a TLP with four columns and a ring pontoon was considered in the CFD simulations. Present study focuses on nonlinear characteristics of wave forces and run-ups on the TLP under both regular and irregular wave conditions. CFD simulation results are directly compared with linear potential solutions. Discussion is made on nonlinear characteristics of wave forces and run-ups on the TLP.
Since TLP is a buoyant structure anchored by pre-tensioned cables (tendons) to the sea bottom, the natural frequencies of vertical motion of a TLP are away from typical wave frequencies. Small vertical motion response of a TLP allows the usage of dry tree system. However, nonlinear wave loads including second- and third-order forces acting on TLPs cause high-frequency resonant response like springing and ringing. In extreme wave, the TLP can be exposed to frequent wave-in-deck impact events due to smaller vertical motion responses and wave amplifications inside columns. Large drift motion induced by wave drift force is also important issue for the design of TLP.
The estimation of the nonlinear wave forces as well as prediction of the wave run-ups are quite challenging because these are basically highly-nonlinear wave-body interaction problems. Accurate estimation of the nonlinear wave forces and run-ups requires the careful consideration of nonlinear wave characteristics and wave amplification effect due to the platforms. Typical potential-flow based solution methods, which have been widely used in ocean engineering society, cannot predict nonlinear wave forces and run-ups accurately.
Shu, Minhua (Shanghai Jiao Tong University) | Chen, Ke (Shanghai Jiao Tong University) | You, Yunxiang (Shanghai Jiao Tong University) | Zhang, Xinshu (Shanghai Jiao Tong University) | Wang, Hongwei (Shanghai Jiao Tong University)
Based on the dynamic mesh method and RANS turbulence model, the unsteady characteristics of a three-blade conventional propeller was investigated under external axial excitations of different amplitudes. The results showed that significant responses appeared in the frequency domain of the bearing forces histories if the external axial excitations were introduced. The frequency of the response equalled to the excitation frequency and its amplitude increased proportionally to the excitation amplitude. The pressure coefficient at three different radial positions 0.4R, 0.6R and 0.8R all increased with the increase in the excitation amplitude near the blade leading edge, while the change was relatively small near the blade trailing edge.
As one of the three major sources of the ship noises, the propeller noise has an important impact on the total sound pressure level. A large number of practical measurements on ships show that the high total sound pressure level is commonly caused by some particular components with medium or low frequencies. These particular noise components are closely related to the unsteady bearing forces of the propeller near the stern (He, 1987, Wei, 2013). Systematic researches on the unsteady bearing forces will greatly benefit the studies on low frequency noise of the ships and may help to make breakthrough in noise prediction and controls. Thus the unsteady bearing forces of the propeller have become an important subject in naval architecture and ocean engineering.
Two methods are commonly employed in the research of the unsteady bearing force, i.e. the model experiments (Boswell, 1968, Jessup, 1989 and 1990) and numerical simulation. The unsteady numerical simulation of the propeller had experienced a long-term development from the potential flow method (Hoshino, 1993, Kerwin, 1978, Ma, 2014) to the viscosity CFD method (Guo, 2009, Hu, 2009, Koushan, 2008, Lesfvendahl, 2011). Nowadays, high-speed computer technology has brought a huge advantage for the viscosity CFD method in the unsteady calculation of propeller. However, they usually study the characteristics of the unsteady bearing force without any external excitations. Only a few works are found reported focusing on the propeller with external excitations (Abhinav and He, 2011, Spyros, 2012). Such studies are still in the beginning stage.
In multiple-electrode submerged arc welding (SAW) can be achieved with high welding speed and high deposition rate, the deflection of the arc due to the electromagnetic force generated between the electrodes affects the welding quality. However, the arc behavior in high welding current of large diameter wire is not clear. In addition, in submerged arc welding, the arc is completely covered with the flux and its behavior can not be observed. Therefore, in this research, the behavior is clarified by observing the arc exposed from the thin flux placed on the steel plate surface with a high-speed video camera for the four- electrode submerged arc welding used for pipe welding of UOE steel pipe. It was observed that each arc was dynamically deflected by the electromagnetic force generated between the electrodes.
Welded pipes such as UOE steel pipes and spiral steel pipes used for petroleum and natural gas transport are mainly manufactured by forming and welding steel pipes. In these welding pipes for energy transportation field, the plate thickness tends to increase from the viewpoints of increasing the operation pressure for the purpose of improving the transportation efficiency and getting the collapse resistance during laying down to the deep sea. Increasing the plate thickness decreases the formability and weldability of the steel plate, therefore further improvement in productivity is required.
Particularly in the UOE steel pipe, after forming the steel plate into a U shape, the butt groove is welded by submerged arc welding. This UOE steel pipe is called a SAWL pipe. In order to ensure high productivity, the multiple-electrode submerged arc welding process is used that have several electrodes.
Submerged arc welding is a method in which a wire is automatically feed into granular flux placed on the surface of a steel plate and an arc is generated between the wire and the base metal for welding. Welding is performed with a large current of several hundreds to several thousand amperes by applying a large diameter wire of 3.2 mm to 6.4 mm in diameter. Therefore, high efficiency welding is possible with high deposition rate and high penetration depth. Other features are that arcs are generated in the flux, it is unnecessary to take measures against arc light, that spattering and fumes are small, wind is not affected to welding quality, stable welding regardless of the worker's skill can be obtained. On the other hand, the disadvantage is that welding position is limited flat or horizontal, supply of flux and remove of slag are required, and weld heat input is increased. In addition, there are disadvantages that it is difficult to evaluate the stability of welding because the arc can not be observed.
Li, Fang (Wuhan University of Technology) | Li, Tingqiu (Wuhan University of Technology) | Jin, Qiu (Wuhan University of Technology) | Price, William Geraint (University of Southampton (UoS), Wuhan University of Technology) | Temarel, Pandeli (University of Southampton (UoS), Wuhan University of Technology) | Hudson, Dominic A. (University of Southampton (UoS), Wuhan University of Technology)
Faced with the convection-dominated problem on cut-cell grids involving phase interface, the high order schemes for convective discretization tend to produce unphysical oscillations in the vicinity of discontinuity or steep gradient. Based on the Total Variation Diminishing (TVD) concept, a flux limiter weighted high resolution approach is proposed by blending a low order upwind scheme and the Essentially Non-Oscillatory (ENO) scheme. The approach is simple and efficient for convection-dominated problem. Since the fluxes are restrained by the monotonic blending scheme, the approach maintains high order accuracy while suppressing spurious oscillations near discontinuity or sharp gradient regions. Water entry simulations of differently shaped rigid-body including multi-body are presented for immersed moving boundary problems, which are the representatives of the nonlinear convection dominated problem involving discontinuity or steep gradient. Pressure on free surface and velocity field are obtained with different convective schemes. It has proved that the approach can yield results consistent with measurements available and works effective in capturing high gradient with different flux limiters.
The convection-dominated gas-liquid and solid-liquid flows, such as water entry and wave breaking, are accompanied with pressure and viscous stress discontinuity from surface tension and large density gaps. Meanwhile, the gas-liquid interface and solid-liquid interface in these unsteady and nonlinear flow systems make the solution of high gradient problems extremely complicated (Hudson and Harris, 2006). The traditional high order convective schemes tend to cause unphysical oscillations near jumps or high gradient region. In recent decades, efforts have been made to build high order schemes free of unphysical oscillations, which are also called high resolution schemes (Gao et al., 2013). In fact, according to Godunov's theorem (1959), all linear high order convection schemes cannot be monotone and result in unphysical oscillations. To this end, nonlinear high order scheme with monotonicity property can make a high resolution scheme. One of the most effective high resolution schemes is the nonlinear flux limiter approach (Sweby, 1984).
This study used computational fluid dynamics (CFD) to analyze the performances of an air-blower wave power generation device with buoy wave energy converter, and simulated the process of waves acting on the double buoys to produce vertical kinetic energy and subsequently applying the energy to an ocean wave electricity generator set to generate electricity. Notably, we can compute the average power based on the movement and output force of buoy over a period of time. The parameters of buoys are the same with a diameter 2 m and height of 3 m in the vertical direction. Here the wave height is 0.6, 0.8 and 1 m, the wave period is 3 to 6 sec, and the range of gaps between the buoys is about 0.5 to 3 m. The power output of two buoys was calculated when various conditions of waves act on the buoys, and the numerical analysis of the front and back buoys was performed.
Development and application of renewable energy is a significant topic in the field of multi-application power generation such as wind energy, solar energy and ocean energy. The above motioned energy does not have to speed the fuel to generate electricity without the emission of air pollution. Many researches proposed the policy of carbon tax (Astariz and Iglesias, 2015; Leijon et al., 2006; Kim et al., 2011; Mendes et al., 2012; Retzler et al., 2006) which achieves an identical level of national emissions and employment at a fraction of the cost. It is going to receiving by many of national attention. Wave power devices attract many researchers to investigate, in which a series of methods used to capture the energy of the waves. Wave power has the following advantages: (1) infinite resources: the oceans are 70% of the Earth's surface, and not need to spend money on energy; (2) reliable power supply: 24 hours of continuous power supply, no lack of raw materials; (3) clean energy: no environmental pollution; (4) factory site is easy to find: land issues. Although the waves with the above-mentioned advantages, the current wave technology is still in the stage of research and development, in addition to small applications, still no commercialization of wave energy power plant. Therefore, the power generation efficiency of wave energy is still uncertain, it is generally estimated future commercial operation of wave energy efficiency of power plants. Now actively engaged were found in the development of major national wave as the United Kingdom, Japan, Norway and the United States.
This paper presents a numerical investigation on the significance of the role of the compressibility of the fluids associated with water entry problems using a multi-phase solver OpenFOAM, in which the water and air are treated as either compressible (compressible solver) or incompressible (incompressible solver). The models are validated by using the experimental data of a 3D plate dropping case, whereas the detailed investigations focus on 2D wedge dropping with different dead-rise angles and/or tilting angles. The effects of the compressibility are examined by comparing the results of the compressible solver and that of the incompressible solver. It is concluded that the free surface profiles during the impact are significantly influenced by the compressibility of the fluids, leading to different patterns of impacts (convective motion between fluids and dropping wedge); even in a case with large dead-rise angle, the incompressible solver may lead to incorrect predictions on the peak pressure and the force acting on the wedge surface.
Large impulsive pressure and slamming forces may lead to the damage of the offshore structure, and are of interest for the engineering purposes. Typical examples include breaking wave impacts on quay walls/breakwaters, slamming of the ship bow during extreme weather condition. The experimental (e.g. Miyamoto and Tanizawa, 1985; MOERI, 2013; Mai et al, 2015), numerical or analytical studies (either based on the potential theory, e.g. Zhao and Faltinsen, 1993; Zhao et al. 1996, or viscous flow theories such as Gao et al., 2012; Oger et al., 2007; Skillen et al., 2013) on the water entry problems, initiated by Von Karman (1929) and Wagner (1932), provides useful references for reliably predicting the slamming loads and exploring associated small-scale physics, such as the air trapping, spray and extreme free surface deformation. Significant advances have been recently made on computational fluid dynamics (CFD) modelling on such problems. Both single-phase (e.g. Gao et al., 2012; Oger et al., 2007; Skillen et al., 2013) and multiphase models (Kleefsman et al 2005; Sussman et al, 1994; Soulhal et al, 2014) have been attempted, and a promising accuracy was demonstrated on predicting slamming loads.
Wan, Yufei (CNOOC Ltd.-Tianjin) | Liu, Renwei (CNOOC Research Institute) | Wang, Wenguang (CNOOC Ltd.-Tianjin) | Du, Xiaying (CNOOC Ltd.-Tianjin) | Qu, Zhaoguang (CNOOC Ltd.-Tianjin) | Liu, Chunyu (CNOOC Ltd.-Tianjin) | Qian, Xin (CNOOC Ltd.-Tianjin)
There is large liquid inventory in long-distance subsea wet-gas pipeline because of complex topography. This may accelerate internal corrosion and produce hydrate, which bring safety hazards to pipeline operation. Therefore, it is imperative to determine the pigging period appropriately and make a reasonable pigging plan. Three common methods of calculating pigging period were compared, including minimum gas transmission efficiency method, maximum pressure drop method and maximum liquid holdup method. After analyzing their advantages and disadvantages, the maximum liquid holdup method was adopted to determine pigging frequency in this paper. As the pigging slug is too large to be handled by downstream slug catcher timely, four schemes were presented to reduce instantaneous slug: a) Increase the pipeline throughput for one day and then turn it down before pigging; b) Lower the end-point pressure for 3 hours and then raise it back before launching a pig; c) Increase the pipeline throughput for one day and then raise the pressure before pigging; d) Lower the end-point pressure for 3 hours and then decrease the pipeline throughput before conducting pigging operation. Finally, pigging period and scheme were determined according to calculation results and on-site operability. Analytical approach adopted in this article may provide some technical support for subsea wet-gas pipeline pigging operation.
Associated gas produced by an oilfield in Bohai Bay of China is transported to the central platform where heavy hydrocarbon and water is removed from the gas, via a 27km multi-fluctuations subsea pipeline. As the pressure and temperature drop along the pipeline, the subsea wet-gas pipeline is running with low liquid holdup (Xu and Gong, 2005, Chang, 2014, Esmaeilzadeh, Mowla and Asemani, 2009). In the later phase of the oilfield development, the reduction and fluctuation of oilfield production make the pipeline's actual running state out of design. Furthermore, low fluid-carrying capability together with complex topography increases liquid accumulation in the pipeline (Li, Zong and Zhu, 2016). Especially for the high pressure subsea wet-gas pipeline, the overstocking liquid will accelerate internal corrosion and hydrate formation (Minami and Shoham, 1996). The effective method of removing liquid accumulation is periodic pigging. In order to ensure safe and efficient operation of the pipeline, pigging must be conducted regularly. However, too frequent pigging brings high OPEX and operation hazards(Lima and Yeung, 1988). Consequently, reasonable pigging period and scheme are even more important.
Although floating offshore wind turbine (FOWT) can extract large amount of wind energy in deep water region, it often suffers from large amplitude motion in heavy sea states. The large amplitude motion can reduce aerodynamic performance of wind turbine and result in additional structural loads. Therefore, suppression of floating body motion in waves is an crucial task in designing a FOWT system. In this paper, heaving plate effect on the hydrodynamic performance of semisubmersible FOWT is studied by using a computational fluid dynamic tool. The 5MW semi-submersible FOWT model test has been carried out by Zhao (2012) in HEU. The benchmark case based on the above experiment FOWT1 without heaving plate and FOWT2 with heaving plate have been tested. This benchmark case is investigated in detail at the beginning to validate the numerical method. Then, a set of heaving plates with different size and shape are studied based on numerical simulation. Response amplitude operator (RAO) is analyzed in detail to investigate the heaving plate effect on the hydrodynamic performance of semi-submersible FOWT.
Offshore wind energy has been one of the most important renewable resources in the world. In order to fully utilize the wave energy resource, the critical technology of wind turbine has been developed extensively. Many wind turbines which have bottom fixed on seabed are well established products in the onshore industry and transported to appointed place in shallow water. However, the wind energy resource in shallow water can't meet the increasing demand, as a result, the further wind energy exploitation has been moved to deep water. Compared to shallow water, the sea area is large and the winds in deep water are strong and steady which is very suitable for wind energy development.
In deep water, the concept of bottom-fixed can't be adopted for economic consideration which makes floating wind turbine comes into the people's vision. Compared to TLP platform and Spar platform, the semi-submersible floating wind turbine has the highest ability on windresistance and large water plane area to insure its high stability. The detail design of a semi-submersible foundation came up with a Mini-Float design and was applied to a WINDFLOAT DESiGn. In 2008, a WINDFLOAT project is coordinate with the support by NCNS and Vergnet, their foundation is a semi-submersible form. However, this kind of floating foundation is easy to suffer the impact of wave resulting in its big motion, especially heave motion, which seriously affecting the productivity of wind turbine.