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Yin, JiaoJian (China University of Petroleum, East China) | Sun, Dong (Technical Testing Center of Shengli Oilfield Branch of Sinopec) | Yang, Yousheng (Ocean University of China)
Summary The pump dynamometer card is a direct reflection of the operating conditions of the downhole pump, which is important for the diagnosis of sucker-rod pumping systems. In this paper, we propose a novel diagnostic method based on the estimation of the parameters from the polished-rod load vibration signal of sucker-rod pumping systems in a vertical well. In this study, we deduce a new analytic solution of the 1D wave equation of the sucker-rod string, which can be used for the predictive and diagnostic analyses. The relationship between the polished-rod load vibration and the pump equivalent impulse load based on the analytic solution is studied, and the diagnostic parameter estimating method is proposed. Therefore, the pump dynamometer card calculated method based on the surface dynamometer card is realized.
When using bracings to build a joint between the jacket base and the tower, it becomes complicated both in force capacity and fatigue performance due to its complex joint form and large sizes, which also make the empirical formulas recommended in DNV rules or other specifications no longer apply when calculating the joint stress concentration factors (SCFs). Based on above-mentioned, this paper focuses on the joint SCFs by calculating FE models with different load directions and structural parameters, tries to give their change rules or values variation range which can provide reference for the fatigue design of the offshore wind turbine (OWT).
Jacket foundations are typical and reliable structures of marine engineering, also widely applied in offshore wind turbine (OWT) in recent years. There are different structure forms to connect the tower with the jacket base, using diagonal bracings to transfer load and support the tower is one form of these structures. The bracings and the tower form a joint which has much larger size than common joints of the jacket base, this results in difficulties when considering its fatigue properties. The stress concentration factor (SCF) of joints is hinge parameter especially in preliminary design stage to calculate the fatigue stress, because of the large size of the joints, the empirical formulas no longer suit for deriving SCFs.
Studies on the joint SCF s began in 1970s, measures to get the SCF s are considerable developed over the past decades. Measuring the hot spot stress directly from large number of model experiments is considered to be first-hand and accurate (Cheng, 1999), empirical formulas to calculate SCFs were subsequently formed (Li, 2014). However, model test is not suitable for joint with large sizes, also the huge expense is another limitation. With the rapid development of the FEM, calculating stresses of tubular joint become convenient. The shell element and solid element are both wildly used according to the actual needs (Shao, 2006), concretely, using shell element means less computing burden, but it cannot model the attachment structures like the welds, on the contrary, the solid element brings larger amount of calculation but can fit reality more (Zhang, 2008).
The suction bucket foundation will gradually become one of the basic forms of offshore wind turbines in the future because of its advantages of simple installation, high anti-overturning bearing capacity, material saving and reusability. The current research mainly focuses on the stability analysis of single bucket foundations, and there is little research on the inter-bucket effect and bearing characteristics of multi-bucket foundations. In view of the strength characteristics of the offshore wind turbine foundation, the bearing capacity of horizontal cyclic load is particularly important for the stability of bucket foundation. In this paper, the suction four-bucket foundation was subjected to static bearing capacity model test and series of multi-point cyclic load model experiments under horizontal load, and the foundation failure modes and joint action mechanism of the four-bucket foundation structure on the saturated sand foundation are explored. The innovation of this paper is to carry out model test research on the suction four-bucket wind power foundation, analyze the interaction mechanism and failure mode between the buckets, and explore the influence of multi-point cyclic load on the bearing capacity. So far, few similar studies have been conducted.
Offshore wind energy as an important renewable clean energy, reasonable use of wind energy can effectively alleviate the world’s current energy tension and environmental pressure problems. At the same time, the support system of offshore wind power generation is in a more complex environment, and subjected to a variety of environmental loads, how to ensure the safety and reliability of the support system is very important.
Suction bucket foundation is a new type of offshore platform foundation. Because of its advantages of low cost, convenient installation and reusability, it is especially suitable for application in offshore foundation (Wang 2008; Senders 2009; Liu 2016; Xu 2018). The offshore wind power foundation under complex marine environment has to bear the horizontal cyclic load from wind, wave and ice. Therefore, it is particularly important to study the horizontal cyclic bearing performance of the offshore wind turbine bucket foundation (Qu & Wang 2011). However, the current research results mainly focus on the stability analysis of the single-bucket foundation, and there is still no in-depth study on the bearing characteristics and joint working mechanism of the multi-bucket foundation (Liu 2009).
Xie, Yingchun (Ocean University of China) | Liu, Xuyan (Ocean University of China) | Sun, Yuanfang (Ocean University of China) | Xiao, Yucheng (Ocean University of China) | Liu, Guijie (Ocean University of China) | Leng, Dingxin (Ocean University of China) | Tian, Xiaojie (Ocean University of China)
The movement of a radar platform in complex marine environment will affect the radar operational efficiency and personnel safety. In this paper, the structure of a semi-submersible radar platform is optimized based on its hydrodynamics to reduce the environment effect. The hydrodynamics of the semi-submersible radar platform was studied in the frequency domain using ANSYS Workbench software. The structure parameters were optimized to maximize the maneuverability of the platform. A detailed relationship between each hydrodynamic parameter and the wave frequency was established to supplement the hydrodynamic database. Because of the difference between the bow and stern of the buoy, the first-order wave excitation force and motion response of the platform will be different. The analysis results showed that the variation of the pitch motion of the platform was obvious. Lastly, the habitability of the semi-submersible radar platform was evaluated according to human physiological tolerance. In general, the optimized semi-submersible radar platform has better hydrodynamic performance.
Offshore platforms typically operate in harsh marine environments, subject to not only hydrostatic pressure, buoyancy and gravity, but also environmental loads such as wind, waves and currents (Wang, S., Cao, Y., Fu, Q., & Li, H., 2015). Many researches have been addressed structural optimization and hydrodynamic analysis of offshore platforms. Birk and Clauss et al. (2001) studied the heave response of a semi-submersible platform shape optimization scheme. They optimized the shape of the buoy by increasing the volume of the buoy part at the connection between the pillar and the buoy and reducing the volume in the middle of the buoy to reduce the heave motion excitation while keeping the displacement unchanged. The results showed that the hydrodynamic performance was improved. Cermelli et al. (2004) studied the structural optimization by adding a heave plate to the bottom of the platform column, which increased the additional mass and damping coefficient of the platform itself, thus greatly improving the performance of heave motion. Huang and Mansour (2007) designed an H-shaped buoy structure, which added connected auxiliary buoy between two buoys of the conventional semi-submersible platform. The results showed that the heave and pitch motion in the regular wave period were reduced obviously. Lai et al. (2013) used numerical simulation method to study the hydrodynamic performance of a new-type semi-submersible support. Deep-draft buoys structures were used to resist the wave forces on the floating offshore, while damping structures were used to enhance the stability of wind turbine and reduce the heave amplitude. It was showed that the designed semi-submersible platform had excellent hydrodynamic performance. Garrido-mendoza et al. (2014) proposed a new-type heave plate on the semi-submersible fan platform and calculated its additional mass parameters. They found that the heave response of the platform was effectively suppressed. Domala et al. (2014) used the numerical simulation method to explore the optimal parameters affecting the hydrodynamic response, such as the main scale and the structural layout of the platform. Wang et al. (2015) designed a new-type semi-submersible lifting platform with asymmetric buoy and no transverse brace. The platform was more efficient in conventional sea conditions, but due to the asymmetric buoy arrangement there were obvious heave-roll and heave-pitch coupling responses near the natural period of heave. Jiang et al. (2016) used HydroStar software to quantitatively analyze the influence of different shapes such as column, brace and buoy on the heave motion and average drift force of the semi-submersible platform. Wu et al. (2017) developed a simplified algorithm for evaluating the hydrodynamic performance of mobile offshore bases (MOBs) in the preliminary design stage based on the assumptions of no navigational velocity and only random and irregular wave forces at high sea states. This study provided significant support for the evaluation of the hydrodynamic performances of very large modular semi-submersible structures. Gao et al. (2018) conducted numerical and experimental studies on the hydrodynamic performance of a new multiple column platform (MCP) with a center column and middle pontoon. Numerical simulations were conducted in both the frequency and time domains based on three dimensional (3D) potential theory. Moreover, a comparative study on MCP and two conventional semi-submersibles were carried out by using numerical simulation. Goncalves et al. (2018) studied the effect of vortex motion caused by section shape and surface roughness on hydrodynamic performance by pool test.
Tian, Zhe (Ocean University of China / Wuhan University of Technology) | Shao, Yinlong (Powerchina Huadong Engineering Corporation Limited) | Zhou, Lin (Ocean University of China) | Xie, Yingchun (Ocean University of China)
The safety of propulsion system which is regarded as the heart of ship receives more and more attention by the industry and academia. During the operation of the large vessel, the flexible ship hull could be deformed by the wave excitation more easily than before. The hull deformation would have an effect on the ship propulsion through the bearings. Meanwhile, the vibration from the propulsion system could be transferred to the ship hull by the bearings. A coupled effect occurs during these components of the vessel. If it exceeds the allowance, damages would occur which could threaten the safety of the ship structure and the life of crews. Therefore, this paper focuses on the vibration response of the ship propulsion excited by the dynamic forces. As the shaft is over 40 meters long with a huge physical dimension and many condition limits such as the sensors installation difficulties, structure welding difficulties are existed in the commercial large real vessel, a scaled shaft test platform is established to investigate the characteristics of shafts’ vibration. Various conditions of hull deformation and propeller thrusts force on the shaft to make a vibration through the supporting bearings. Based on the experimental study, several vibration characteristics of the shaft are concluded as well as some suggestions to improve the reliability and safety of the ship operating system.
More and more super large size of vessels are used to transport various goods and minerals from one side to the other side all over the world to promote the development of globalization. The safety of large vessels is received much attention by governments, organizations, shipyards and academic institutions. For the large vessels, it has a larger size hull and propulsive power which result in many problems in the propulsion system (Main engine damage study, 2012). The propulsion system is consist of main engine, supporting system (including shafts, bearings and many other transmission devices), propeller and auxiliary system. When the vessels sail on the sea, they could receive some wave excitations which make the shaft transfer huge torques and thrusts. The vibration of the shaft is much larger than before. In order to guarantee the power output performance during the ship navigation, it is hard to adopt isolation measures for the shaft vibration. When the vibration exceeds the allowance, damages would be occurred to threaten the safety of the vessels. As a result, the investigation on the shaft vibration is very important to improve the safety and reliability of the ships.
Structure-from-Motion is a technique for estimating 3D structures from 2D image. Due to different refractive index of mediums, the light ray will bend during the underwater imaging process, making the perspective camera model invalid. In this paper we set a camera fixed in a glass housing and use a virtual camera geometry relationship to describe the imaging process, it’s analyzed that the relative camera transform between two images can be recovered and the 3D object points can be reconstructed with absolute scale. Under different thickness of refractive plane, synthetic reconstruction experiments are conducted and the result demonstrates good performance of proposed method.
In many underwater applications, three-dimensional structure of underwater object is needed to gain measurements. One of the significance methods to gain the 3D structure is Structure-from-Motion, which can recover the structure of object from two-dimensional image sequences captured with different camera poses. However, underwater images are often obtained by a camera fixed in a glass housing, thus the light ray coming from water will meet three different mediums: water, glass and air. Due to the different refractive index, refraction occurs. The fact that light ray will bend during imaging makes the perspective camera model invalid for Structure-from-Motion with underwater images. In order to deal with the problem, some try to simplify the model by assuming the glass to be very thin (Treuvutz, Schechner, Kunz and Singh, 2012) or drive the effect caused by refraction into the change of focal length and radial distortion (Lavest, Rives and Lapresté, 2000), some use underwater calibration targets to do reconstruction(Kunz and Singh, 2008; Telem and Filin, 2010) though it’s not applicable in real operation.
Agrawal et al.(2012) analyzed the geometry of multi-layer refractive system and modeled it with an axial camera, dividing the calibrating process into axial estimation, glass housing thickness estimation and so on. However, the system can be used to estimate pose of the camera on the basis of the 3D-2D correspondences given.
Wang, Zhifeng (Ocean University of China) | Yu, Miao (Ocean University of China) | Li, Songtao (Ocean University of China) | Tao, Shanshan (Ocean University of China) | Dong, Sheng (Ocean University of China) | Yin, Zegao (Ocean University of China) | Yu, Tongshun (Ocean University of China)
The surface waves are simulated and extreme parameters are calculated in the Xinghua Bay in this paper. The typhoon process is simulated by the MM5 model. Then wave elements such as significant wave height, wave period and wave direction are simulated every 1 hour by SWAN model based on the double-grid nesting in the whole of Xinghua Bay. By comparison and validation, both the simulation wind data and wave data show a good agreement with the observation data. Extreme parameters of wind speed, wave height and wave period in 16 direction are calculated according to Pearson-III distribution at the point of −40m isobaths in the Xinghua Bay. The results show that the strong wind is in direction SE and the wind speed of 50-year return period is 35.61m/s. The strong wave is in direction E and the significant wave height of 50-year return period is 10.04m. The relative mean wave period is 12.82s. Furthermore, the SWAN model is used to simulate the wave fields from deep water to the shallow water with different wave levels and different wave direction. The wave elements of different return periods are obtained inside and outside the submerged breakwater. The wave height before and after the breakwater is compared. The transmission coefficient and the significant wave height after overtopping is obtained.
With the development of economy in China, the dynamic equilibrium of the coastal region has been disturbed and serious coastal erosion and accretion occurred (Chen, 2010). Surface waves may play significant roles in change of coastal climate. There are many efforts to minimize unwanted coastal erosion and deposit by using artificial structures such as groins and submerged breakwaters. Submerged breakwaters are effectively used to maintain tranquility inside the harbor basin, to reduce the siltation at the harbor entrances and along the approach channel, against coastal erosion, as a shelter for marine habitats or an eco-friendly structure. The level of coastal protection provided by the submerged breakwater depends on the various factors such as width and depth of submergence of breakwater; distance of breakwater from the shore, hydrodynamic characteristics of the structure, wave climate and angle of wave attack (Pilarczyk et al, 2003).The main function of these breakwaters is to protect the seaward area from the severe wave actions by attenuating the wave pass over the structure. Submerged breakwaters could absorb some of incoming wave energy by causing the wave to break prematurely, thus diminishing the transmitted wave energy (Chen, 2010).
This paper attempts to characterize the long-time extreme behaviors of drift ice in the northern Barents Sea from 1987 to 2016 using ice motion dataset from NSIDC and ice thickness datasets from NECP-CFSR and CFSv2. Three typical locations from west to east have been chosen for statistical analysis. Results show that the annual ice velocity extrema primarily ranges between 15 and 50 cms−1, with the simultaneous directions predominantly from W to SE. For first-year ice thickness, the annual thickness maxima approximately range from 90 to 170 cm, but with an obvious decreasing trend. Regarding the extreme parameters estimation, four fundamental extreme value distribution functions, namely, Gumbel, Weibull, generalized extreme value and Pearson type III distributions are selected to obtain different return values of ice velocity and thickness. All functions have passed the K-S test but with different goodness of fit at the three sites. In particular, Gumbel distribution possesses higher extreme values at longer return periods. Furthermore, the patterns of co-occurrence distribution for concurrent ice velocity and ice thickness are calculated and analyzed.
Drift ice is often driven by winds and sea currents; consequently it can potentially pose hazardous threats for ocean engineering, marine structures and offshore activities. However, in the Barents Sea the available literatures mostly concern about sea ice concentration and its interactions with thermodynamic-dynamic factors, such as atmospheric anomalies and inflowing Atlantic warm waters. By contrast, little studies have focused on peculiarities of extreme ice velocity and thickness conditions for offshore drift ice in the Barents Sea.
At present, measuring and tracking the ice drift has been a hot spot in the northern Barents Sea. Vinje and Kvambekk (1991) presented that drift ice near the Svalbard was about 20 cms−1. Løset and Carstens (1996) initially described the iceberg observation program in the western Barents Sea in 1987. In May 2003, in the northern Barents Sea some icebergs motioned towards a direction of 228° with average speed 14 cms−1 (Zubakin et al., 2005) and maximum was 24 cms−1 (Dmitriyev et al., 2005). In spring 2004, the largest speeds near the Novaya Zemlya were between 39.7 and 48.9 cms−1 (Dmitriyev et al., 2005). Nesterov et al. (2009) found that the average observed ice drift speed was 17 cms−1 and the maximum was 50 cms−1 in the northeastern Barents Sea. Further in western Barents Sea, the drift ice characteristics such as velocities and directions are compared with winds and tidal currents (Marchenko et al., 2011; Marchenko and Marchenko, 2015). Specially, their observable drift ice data is from ice trackers with short measuring intervals. Duan et al. (2018) estimated 100-year winds can reach 28 ms− 1 in the northern zone. Regarding the drift ice thickness, Forsström et al. (2011) collected the some discrete 44-points thickness measurements using drillings in the marginal ice zone during 1999-2008. King et al. (2017) presented two raw helicopter-brome-electromagnetic thickness data in March 2003 and 2004 in the northwestern part, indicating the modal thickness varying regionally from 60 to 140 cm. Besides, many researches have been focusing on developing new algorithms to quantify drift ice features using the scattered observations and satellite data (e.g. Kaleschke et al., 2016; Marchenko, 2018).
Wang, Ruimin (Ocean University of China) | Jiang, Zhenqiang (Powerchina Huadong Engineering Corporation Limited) | Tian, Zhe (Ocean University of China) | Lu, Hongchao (Ocean University of China) | Wang, Xujie (Ocean University of China) | Liu, Fushun (Ocean University of China)
Dynamic response analysis is becoming an essential part in the stage of structural design and is significant to the safe operation of floating platforms during the service life. In this paper, a method of dynamic response analysis for fully symmetric floating platforms is proposed, which is based on Laplace transform and complex exponential decomposition. Two numerical examples are used to investigate the performance of the proposed method. Compared with the traditional New-mark-β method, the proposed method is more efficient.
With the increase of population and the development of the society, the demand for resources is rising sharply. As a result of the extensive exploitation and consumption of land resources, people turned their attention to the vast ocean which contains abundant resources including petroleum, mineral and marine wind energy. Gradually, the coastal countries of the world have been engaged in the development of marine energy, and the design and use of platforms is an essential part closely related to oil exploitation. As oil production is moving from shallow water to deep-sea, traditional fixed jacket platforms are no longer suitable, which leads to the application of many floating platforms. However, considering the severe environment in deep waters, floating platforms may be destroyed easily, causing significant loss of life and property. Therefore, it is vital to carry out the analysis of dynamic response at the design stage to improve the service performance of floating platforms.
The time domain method can be used to obtain the dynamic response of floating platforms by solving the differential equations of motion in time domain by using a numerical approach. When the time domain method is adopted, all the matrices of the differential equation of motion are required to be known. Oglivie (1964) found that the retardation function can be obtained by the cosine transform when the hydrodynamic parameters are known.
The first order potential theory method can be used to analyze the dynamic response of floating structures depending on assumption of Gaussian process in frequency domain(Newman, 1997; Faltinsen, 1990; Faltinsen, 2005). The well-established theory for Gaussian process is used to obtain the response statistics. The method based on the linear theory suggests that the step of the wave is small, and the response due to the wave excitation is proportional to the wave amplitude (Faltinsen, 2005).
Gao, Shujian (Ocean University of China) | Wang, Bin (PowerChina Huadong Engineering Corporation Limited) | Liu, Fushun (Ocean University of China)
The Hilbert-Huang transform (HHT) has been used in many fields for time-frequency analysis of the non-linear and non-stationary signal. But when implementing the HHT to analyze the measurement signals from actual offshore wind turbines, due to the large rigidity and weak wave energy, the results will suffer a serious mode-mixing problem and contain a lot of noises which will lead to a bad result. In this paper, a new time-frequency analysis method based on the Hilbert transform is introduced, the Intrinsic mode functions (IMF) obtained by empirical mode decomposition (EMD) is redefined by introducing a combination of state-space model and energy gridding. To demonstrate the proposed method, one numerical example of three slow varying frequency components was used. Specifically, a field test on an offshore wind turbine located in the Putuo of Zhejiang Province of China is introduced, and measured data are used to verify the effectiveness of the proposed method. The result demonstrates that the proposed method has a better performance when implemented on the offshore wind turbines.
Due to the stability and availability of the wind sources in the marine environment, the offshore wind energy has become one of the fastest growing renewable energy in the world. But the cost of investment is much higher than that of onshore wind energy (Ren et al., 2018). And because of the impact of the harsh marine operation environment, offshore wind turbines are vulnerable to the process of exceeding wearing and accelerated degradation in for their key components in the offshore wind turbines is accelerated, making the structures vulnerable to be destroyed (Li and Peng, 2016). For example, there were nearly 1000 safety accidents of offshore wind turbines in 2014 (Seyr and Muskulus, 2016). Therefore, a real-time monitoring of the offshore wind turbine becomes more and more important.
Traditional signal processing methods, such as fast Fourier transform, require the signals should satisfy the hypotheses of stationary and linear. However, when analyzing non-stationary or non-linear signal, it is not enough to only obtain the information in time domain or in frequency domain. It is also necessary to understand the variation of spectrum with time. The traditional method is no longer suitable and effective signal processing techniques are needed to analyze the signals (Sejdic et al., 2009). Therefore, by establishing a joint function of time and frequency, the frequency energy intensity of the signal at different times can be described, this is also the source of the idea of time-frequency analysis. In recent years, with the increasing attention to nonlinear vibration of structures (Xing, 2000), the frequencies of the components of nonstationary signals and their time-varying characteristics can be effectively revealed by implementing the time-frequency analysis, and it also becomes a novel and practical technical for analyzing structural health monitoring in vibration systems (Alisaraei et al., 2016). Through the time-frequency analysis, the characteristics of the signal can be displayed, and by selecting a stable time period and taking targeted measures, the correctness of the modal analysis results is ensured (Liu et al., 2016), thus avoiding the occurrence of major accidents.