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Near the offshore production platform Vega A, the tanker FSO Leonis is moored to a steel column with circular section, 130 meters high and 10 meters in diameter, installed in 1988 in the Sicilian Channel, the connection is via a bridge structure with welded steel sections that make up the system SPM (Single Point Mooring). The structural system in steel box girders and column, with its cylindrical hinges, is calls-to the actions of the sea that induce cyclic stresses and fatigue. The paper presents the monitoring system of the structure connecting the ship to the column, installed on your system by structural in October 2009, is composed of strain gauges sensors, in fiber optic, and inclinometers biaxial that detect rotations of the column and the bridge. The Structural Health Monitoring (SHM) relies on the repeated observation of damage-sensitive features such as natural frequencies and mode shapes. The problem is that changes in temperature, relative humidity, operational loading, and so on also influence those features. This influence is in general nonlinear and also nonlinear is the system SPM-Yoke-Vessel. In this work, a technique based on kernel principal component analysis is used with both the sensor measurement (strain gauge data and inclinometer data); this method is used for eliminating environmental and operational influences on the damage-sensitive features. The results allowed us to evaluate the conditions assumed in the project and made it possible to define a program of inspection and maintenance of steel structures.
The ship Leonis is located in the VEGA field that is approximately 12 miles south of the southern coast of Sicily, off the coast of Pozzallo. The field includes the platform called VEGA-A for the exploitation of the oil field and a 110,000 ton floating deposit obtained from the transformation of the former oil tanker Leonis in FSO (Floating -Storage - Offloading). The float is moored at SPM (single point mooring) located about 1.5 miles from the platform and connected to it via pipelines. In Figure 1 the ship Leonis and the SPM (column and yoke) are shown.
Fulin, Yu (Shandong Jiaotong University, Harbin Engineering University) | Lingling, Ji (Shandong Jiaotong University, Harbin Engineering University) | Lei, Song (Shandong Jiaotong University) | Hui, Wang (Huaihe River Basin Water Resource Planning and Design Institute) | Shengbo, Wu (PLA Rocket Force NCO College Qingzhou) | Shaofei, Ren (Harbin Engineering University)
In this paper, we develop a RKDG-LS-GF method for solving air blast load. RKDG method is constructed to discrete the spatial and temporal domain. The level set method is used to track the dynamic interface. Ghost Fluid Method is applied to treat the interface of multi-medium flows. With Fortran program, the characteristics of far-field air blast load are solved. Combined with LS-DYNA software, shock response of hull structures subjected to air blast load was simulated. the results are compared with the analytic solution. The computations demonstrate that the present compressible two-fluid flows approach is a unique coupled method for numerical simulating complex two-fluid flows and fluid-structure interaction in air blast.
Air blast field is described by the Euler equations, so the key of solving air blast load is numerical solutions of the Euler equations. In this paper, we develop a new method by coupling Runge-Kutta Discontinuous-Galerkin (RKDG) method, Level Set method (LSM) and Ghost Fluid Method (GFM) for solving air blast load. RKDG method is a kind of high order accurate scheme and high-resolution shockwave-capturing methods. RKDG method is suitable and robust for solving compressible two-phase flow problem with large density ratios, large pressure ratios and strong shocks. LSM is used to track the moving fluid interface of the multi-phase flow field. GFM is applied to treat the physical quantity on both sides of the interface for the multi-phase flows. Temporal and spatial domains were discretized by RKDG Method. Based on the Euler equations, taking the fluid compressibility into account, the present calculation program is compiled with Fortran and introduced into the numerical simulation of air blast load. In this paper, one-dimensional and two-dimensional shock tube problems were calculated by self-compiled program, the results are compared with the exact solution to verify the accuracy. The calculation results demonstrate that the new method is stable and accurate for the numerical simulation of air blast load. With the self-compiled program, the air blast load near wall boundary was simulated. The results show that the program is suitable for simulating the process of shock wave generation, propagation, reflection and explosion products expansion. At last, shock responses and damage characteristics of hull structure subjected to air blast load are simulated with the explicit nonlinear dynamic analysis software of ANSYS/LS-DYNA.
Regional wave overtopping risk of Shuang-Chun coast which locates at the southwest of Taiwan was estimated. The estimation is based on a combination of a statistical model, a wave propagation model and empirical formula of runup height. The statistical model considered the dependences by using copula functions between affective factors to generate simultaneously offshore wave and surge conditions which were used as the input conditions of the CMS-WAVE model to simulate the wave propagation on a complicated coastal bathymetry area. Then, runup heights were estimated empirically. The results demonstrated that top elevations of the constructed seawalls at the study site are too conservative due to the perfect dependence assumption between wave and surge. On the other hand, the high overtopping risk areas were identified and also the risk level was estimated for the natural beach area.
Sea level rise due to global warming under climate change is becoming gradually a critical threaten to coastal community. One of the potential disasters induced by sea level rise is coastal flooding. Seawall construction along coast line was commonly used to prevent coastal flooding in some area. Although some negative side effects were reported due to seawall construction, it is one of the effective measures. More than fifty percent of Taiwanese’s costal line is protected by seawalls. Most of the seawalls were constructed during past several decades according to the given design parameters. However, more extreme conditions of the design parameters are expected for the mid latitude western Pacific Ocean in the future due to climate change. Meanwhile, coastal erosion trend is also contributed to the potentially higher risk of coastal flooding. Therefore, a detail assessment of flooding risk is necessary under climate change conditions before an adaptation measure can be proposed. Estimation of runup height of coastal water is the key issue in flooding risk assessment. Affective factors in estimation of runup height include wave height, period and storm surge etc. In general, perfect dependence between wave and surge was assumed to predict the runup height by using an empirical formula, which treats the contributing factors independently and generally induced to a conservative result. In order to improve the estimation, a joint probability method was used which accounts for the dependent correlation between affective factors. However, the approach is limited to the individual behavior of the two variables must be characterized by the same parametric family of univariate distributions. The limitation makes it difficult to apply in practical application. Quantifying the dependence among two or more random variables has been an enduring task for statisticians. In order to overcome the drawbacks of measuring the dependence between extreme value risk with traditional linear correlation coefficients, Copula method was proposed which presenting the theory and basic introduction to the nonlinear dependence measure. Since then, copulas have been widely used in the area of multivariate statistics. Hawkes et al.(2002) described the partial dependence between wave height and water level by using a bivariate normal (BVN) distribution and a mixture of two BVNs. De Michele and Salvadori(2003) applied a two-dimensional copula function to simulate the statistical dependence between storm rainfall intensity and duration. Garrity et al.(2006) compared the time series response based on measured data and Monte Carlo simulation response based on considering dependence structure between wave height and water level. De Michele et al.(2007) developed a multivariate model of sea storms using copulas. Callaghan et al.(2008) applied a statistical extreme value model based on copula function to develop a framework for determining the coastal erosion hazard on sandy beach. Yang and Zhang(2013) investigated the joint probability distribution of wind speed and significant wave height by comparing Gumbel logistic, Gumbel-Hougaard and Clayton copula function. Masina et al.(2015) presented a copula based approach for modelling the joint probability distribution between wave and water level which results highlighted the importance of taking into account all the variables involved in the flooding phenomenon. Jane et al(2016) proposed a copula based approach for predicting the wave height at a given location by exploiting the spatial dependence of the wave height at nearby locations. Chiang(2016) developed a copula based multivariate probability method in which the dependence structure between wave height, wave steepness and storm surge were considered. Then, the overtopping risk on seawall was estimated. In this paper, a risk assessment model of regional wave overtopping was proposed which include a multivariate statistics model to generate the affective factors of wave overtopping and a numerical model to simulate the wave propagation and overtopping on the seawalls by using the generated factors as the input conditions.
Engine mounts are components often neglected in scientific papers but very precious for onboard comfort. In this respect, they perform the very important function of absorbing and mitigating the vibrations of operating components that they are connected to. As soon as engines are started up, both at rest and when sailing, they produce annoying airborne and structural vibrations and noise. Air and structure borne noise depend on the intrinsic dynamic characteristics of both engine and engine supporting system. In recent years, a new double-stage mounting system, called “raft mounting”, has been developed in order to drastically reduce the structure borne noise transmission of superyacht medium speed diesel engines. In this paper, an investigation on the performances of a double stage mounting system installed on a superyacht is carried out.
Comfort on board luxury superyachts at present is the object of great attention by the most important yacht builders. At the same time Classification Societies, taking advantage of the experience gathered in the field of merchant and passenger ships, adjusted existing rules to meet yacht requirements as well. This activity resulted in a wide set of noise and vibration limit values proposed in different areas and situations (Boote et al. 2013). Low vibration and noise levels on a yacht are becoming the new key factors to attract potential customers. Besides passenger comfort, engine mounts are fundamental to preserve the hull structures to all these kind of damages caused by vibrations. The engine is connected to structure foundations through supports, connecting rods, mounts of different shape and layout, all designed to dampen and minimize as much as possible annoying vibrations (Biot et al. 2015). As a matter of fact, if the engine mounts are not properly designed, vibrations generated by main engines and generator sets are perceptible everywhere on board, significantly lowering the quality of onboard life and introducing also fatigue loads which could create non negligible structural damages (Pais et al. 2017). Most part of experimental data gathered in recent years show that, in the case of steel and aluminum light alloy vessels, the noise transmission through structures is usually more important than airborne transmission. The structure-borne sound influences the acoustic comfort at long distance from the sound source because of the low damping values of steel and aluminum light alloy. On the contrary, air-borne sound has significant relevance only in the proximity of the source. For the reduction of the structure borne noise transmission, an improvement of the mounting systems can be attempted by changing from a conventional single stage design to a double stage mounting system (also called “raft mounting”). Raft mounting is a quite new solution by which main engines are resiliently mounted on an intermediate mass, called sub-frame. This allows to significantly reducing the noise and vibration level. This sub-frame, in its turn, is again resiliently mounted to the ship structure, usually the thick keelson flanges. The scantling variables of the system are represented by the mass of the raft and choice of upper and lower stage mounts. The sub-frame can be quite large (up to the 50% of the controlled mass), and results in a taller foundation. Particular attention should be devoted to the raft installation in existing engine rooms because of the consequent weight increase and with regard to hull girder strength and engine room height.
For the purpose of engineering application in the design of offshore structure, an analytical approximation is presented. The solution is based on a Fourier series expansion and contains some dimensionless constants determined by regression analysis. For the regression analysis, all field quantities are represented in dimensionless forms and the required data are numerically calculated with a set of nonlinear equations formulated by tensor analysis. The set of equations is solved by Newton’s method. Thus, a numerical method to determine the constants is also made. Unlike the other numerical methods, the numerical method is valid for all waves including those in the solitary wave limit and in the deep water wave limit. Because there is a wave height limit, the constants can be presented with continuous and bounded functions of two dimensionless variables whose domain is also bounded. In the domain, sufficient data are calculated with the numerical method. Applying regression analysis to the data, the functions are presented with closed forms equivalent to analytical solutions. The functions provide an analytical approximation to the problem. Because of its simplicity, it is suitable for engineering application. Some waves in the breaking limits are calculated. The results are compared with the known results and found to be closely accorded. Applying regression analysis to the results, the breaking limit is also presented with a closed form. Results for fluid velocities are compared with experiment and agreement found to be good. Some profiles are calculated and compared with those calculated by the other wave theories.
A structure design is to determine structural configuration, material and dimension. In order to verify structural integrity, a structural analysis is definitely necessary. But the outputs of a structure design are considered as input data for a structural analysis. The outputs of a structural analysis such as stress, strain and displacement etc., are merely referenced data in a structural design. Because of the reasons, a structural design is a nonlinear process. There are two basic methods to overcome the non-linearity. One of these uses the conservative scheme in which design loads are greater than actual values and design strength and stiffness are less than actual values. Therefore, the integrity of an actual structure can be guaranteed when we use the scheme for a structure analysis and the result satisfies integrity requirements. The reason for using the scheme is that a structural analysis is simple and the determination of a structural design is easy. If approximations satisfy the scheme and are simpler than exact solutions, they can be acceptable to a structural analysis. The other is to use analytical solutions instead of numerical solutions. Structural designers prefer analytical solutions instead of numerical solutions because it is possible to determine a structural design without a structural analysis when analytical solutions are available. Therefore, it is obvious that approximations should be made as simple as possible, analytically and close to the exact solutions. The objective of this study is to provide an analytical approximation accorded with the scheme.
The coast of Yantai in China has often been ravaged by severe coastal storms together with high elevated water levels, resulting in severe beach erosion and inundation hazards. This study is undertaken to collect essential field data on beach transect profile, beach slope, maximum wave runup level and sediment grain size to assess coastal erosion hazard on Yantai Beach. There are 20 permanent beach transect profiles surveyed every month with RTK-GPS, and three sediment samples were collected from the dune/seawall, beach berm and swash zone. This study has found that the sandy beach backed by the vertical seawall is often flooded at daily high tides, while the beach backed by the sand dune is generally stable. A conceptual model is also applied to estimate shoreline setback distance in n years.
The coast of Yantai is on the north coast of the Shandong Peninsula, the south of the Bohai Sea. Yantai consists of 13 local council areas, of which ” councils are on the coast. The topographical breakdown consists of about 50% plain, 40% hilly, 7% mountainous and 3% basin. The urbanized area is about 2,644km2, and its population is 7.0 million in 2016. The total length of the coastline is about 910km, of which the cumulative length of sandy beaches is about 130km. The total number of sandy beaches is 25, of which the longest sandy beach is 17.3km and the shortest one is 260m.
The coast of China is often exposed to tropical cyclones together with storm surges and large storm waves (Sousounis et al, 2008), resulting in great losses of coastal economy and human lives. For example, the 2007 cyclone generated large coastal waves and elevated coastal water levels by 1.5~2m in Laizhou Bay, 1.1~1.6m in Longkou, and 1.3m at Yantai Port, resulting in heavy losses of US$120 million coastal economy and 193 human lives (You et al, 2017). The storm-generated high surge levels and large coastal waves are direct causes of coastal erosion (see Fig.1). Most recently, the local government of Yantai has initiated the 5-year coastal plan (2016-2020) to address coastal erosion problems. This five-year coastal plan has been financially supported partially by the national Blue Bay Remediation Action Plan to combat coastal pollution and beach erosion problems in China. The national action plan started in 2010-2014, and will continue for 2016-2020. There are about 66 coastal bays and estuaries selected nationally, and each of the 66 projects is funded for US$90~150 million. On the coast of Yantai, Zhifu Bay and Laizhou Bay are remediated under the national blue bay action plan.
In this paper, the forced oscillation of a circular cylinder, known as the Radiation problem, close to the free-surface is studied for a range of Reynolds (Re) and Keulegan-Carpenterr (KC) numbers, using a semi-analytical linear solution and a fully nonlinear Navier-Stokes solver. The fully non-linear solution is obtained using the OpenFOAM package and the Radiation forces and coefficients are compared against the linear potential flow solution for when the cylinder oscillating vertically (Heave mode), horizontally (Surge/Sway mode) and rotating around its centreline (Pitch/Roll mode). It is shown that even for relatively small amplitudes of oscillations, there are considerable differences between the hydrodynamics coefficients obtained from the nonlinear model and the ones from the linear solver. The discrepancies are then studied in details to understand the sources of the non-linearities.
Although, the problem of oscillating cylinder in an infinite medium has been widely studied across all relevant parameters; for example see Morkovin (1964), Berger & Willie (1972), Williamson (1996), Rafiee & Fiévez (2015), flow past a cylinder close to a free-surface has just recently attracted attentions (Reichl et al. (2005)). Flow past a bluff body in the vicinity of free-surface has a large number of applications, e.g marine renewable energy devices, offshore structures, submarines and risers. It is also expected that the existence of free-surface can result in considerable changes from the infinite-medium reference cases. Such expected differences are due to interaction of the free-surface with the bluff body’s motion, resonance of evanescent waves between the free-surface and the bluff body at particular frequencies, deformation of the free-surface, creation of submerged jets from the free-surface to the ambient air, impact of the falling jet on the bluff body and possible interaction of vortices with the free-surface. It is obvious that none of the aforementioned phenomena occurs in the case of bluff body oscillating in an infinite medium.
Yun, Jong Seok (Kongju National University) | Han, Ki-Jang (Kongju National University) | Ahn, Hyoil (Kongju National University) | Falcon, Sen Sven (Kongju National University) | Kim, Kee-Dong (Kongju National University) | Choo, Yun Wook (Kongju National University)
This study aims to investigate on static behavior of guardrail supporting piles subjected to lateral load using three-dimensional finite element analysis. The numerical model was verified by a field static load test. Then a parametric study was carried out and the parameters include slope angle, pile location from slope edge, and pile rigidity. The yield bearing capacity and its corresponding soil reaction force distribution for all cases were discussed and compared with the previous study.
Guardrails barriers are a typical safety system preventing vehicles from running out of roadway and/or falling into the slope at the roadside. The guardrail generally consists of a protection cross-beam and supporting in-line piles (Wu and Thomson, 2007; Sassi, 2011; Pajourh et al., 2017). The guardrail piles are generally installed in vicinity of the slope edge of the embankment because the roadside area is very limited. In Korea, the design of these piles including the barrier system practically solely relies on engineers’ previous experience and numerical analysis. The final design is afterward validated with a full-scale car-crash test carried out in a certified testing site. Then, the final design of the pile as well as the barrier system are constructed and inspected by a field static test to check whether the piles would meet the criterion. The criterion is that the static bearing capacity of the pile should meet 90% of dynamic bearing capacity measured by the carcollision test. This static load test at the construction site is performed for every sample pile selected from in-lined piles at every 1 km (MOLITK, 2016). Among these procedures, any geotechnical consideration is not involved. Moreover, the behavior of piles installed for the support of guardrail system has not been well understood.
In this study, the static capacity of guardrail supporting piles subjected to lateral load was investigated to improve the current design practice using three-dimensional finite element analysis. The lateral capacity of piles is generally estimated by semi-theoretical methods. These methods assume a form of lateral soil pressure distribution along the length of the pile. This lateral soil pressure distribution has been updated by analytical and/or experimental studies (Brinch Hansen, 1961; Broms, 1964; Petrasovits and Award, 1972; Chari and Meyerhof, 1983; Prasad and Chari, 1999). Thus, this soil reaction pressure approach can be also used to the static capacity evaluation of the guardrail supporting piles.
Recently, improvement of fuel efficiency is a big theme in the shipbuilding industry. And also, the HCS, a system with good fuel economy performance is required. It is not easy to evaluate the fuel economy performance of the HCS. The reason is that the influence of disturbance such as wind and waves is greater than influence of fuel efficiency performance by steering of the HCS. We got various data such as rudder angle, navigational instruments, institutions, weather, etc. from training ship Fukae maru sailing using the HCS and evaluated the performance of the HCS from the viewpoint of the influence on fuel consumption.
For vessels of a certain size or larger, it is common to navigate using the function of Heading Control System (HCS). Therefore, HCS has been developed for performance improvement from the initial development to the present day. HCS has a long history. HCS has been put into practical use around 1920, about 10 years after the practical use of the gyrocompass (Fossen, 2011; Ohtsu, 2015). Currently, HCS has excellent performance, so it is often used in ocean survey sailing (Lee 2009). The HCS currently in use is generally controlled by PID control (Amerongen, 2009) or adaptive control using T, K model (Nomoto, 1957; Fossen, 1999), and it performs optimal control for the sea condition, wind or loading conditions. Most of these systems are developed based on the concept that fuel efficiency can be improved by minimizing the steering amount. However, it is not easy to evaluate the influence of steering by the HCS of a ship sailing while being affected by external disturbances such as wind and wave, tidal current, and the like on fuel efficiency (Terada2009: Brink, 1981).
EXPERIMENT FOR HCS EVALUATION ON REAL SHIP
The HCS named PT − 21, Yokogawa Electronics Instrument Co., Ltd. installed in Fukae maru, we can select three modes with different steering characteristics. The basic mode is “economy mode”. Economy mode is the mode with the lowest steering amount among the three modes. The second mode is called “Course keep 1”, which has higher keep heading performance than economy mode. The third mode is called “Course Keep 2”, which has a higher keep heading performance than Course Keep 1. The economy mode is a mode that focuses on fuel consumption performance by reducing the steering amount as compared with other modes.
Once the gas hydrate is formed, immeasurable damage will be imposed on pipelines especially for long-distance, high-pressure, deep-water subsea wet gas pipeline. Therefore, the normal solution is strict control and precaution the hydrate’s formation. The thermodynamic inhibitors, such as ethylene glycol (MEG), methyl alcohol (MeOH) always be injected into wet-gas pipeline. Injection suitable dosage inhibitor helps to lower down hydrate formation temperature to prevent hydrate’s formation in the stage of commissioning and continuous production. However, there is hardly any reports on hydrate’s precaution when in shut-in. In this paper how to economically and safely avoid the formation of hydrate in the phase of shutdown is studied. Based on a subsea wet gas pipeline ZB which lying in the Bohai bay of China, and with the help of dynamic multiphase flow simulator OLGA, key parameters along the pipeline are computed, which show that hydrate will be formed after several hours’ shutdown. For a safe operation, measures should be taken to both the planned and emergence shutdown: To the planned shut-in of the ZB, the regular practice of elevating dosage of MEG doesn’t work as to the topography. To the emergence shut-down, method of venting and burning is chosen against the hydrate’s formation, which can be also used in planned shutdown when others method is failure. In order to cut down the venting gas amount to the greatest extent, the law between venting rates and the total amount of burned gas is depicted and analyzed. Finally, an optimal venting rate of 2000 Sm3/d is achieved for pressure relieving after shutdown. The results and the analytical approach adopted in the paper provide some technical support for the similar offshore wet gas pipeline and other multi-fluctuate onshore wet-gas pipeline.
A large amount of experimental and theoretical studies have been conducted by Researchers, and some measures for prediction, control, and elimination of gas hydrate when pipelining wet-gas in deep-water (Guo, Sun, and Gao, 2016, Li, Wang, and Rao, 2014), cold-environment (Zhao, Deng, and Liu, 2014) have been developed since the Davy first discovered the gas-hydrate in 1810, and Hammer Schmidt found that the gas hydrate would plug the gas pipeline in 1934. Hydrate is a combination product of high pressure, low temperature and free water, which will be formed in the process of drilling (Guan, Ren, and Sun, 2014), recovery, processing, and transportation (Bi and Jia, 2009, Guang and Tian, 1996). Once the three factors encountered simultaneously in nature gas pipeline or pressure vessel, the gas hydrate will be formed to plug the gas pipeline resulting in dramatic damage to the oil and gas industry (Hou, Lei, and Zhao, 2013, Yu and Zhang, 2005). Therefore, the normal solution is strict control and precaution the hydrate's formation in the stage of commissioning and operation (Li, Wang, and Zhou, 2014, Ng and Robinson, 1976). There are several ways, including dehydration, insulation, heating, inhibitor injection and depressurization, to avoid the formation (Notz, Burke, and Hawker, 1991). Among them, inhibitor injection method is extensively used in offshore oil and gas industry when consideration of limited platform space, expensive installation and procurement fee, which has a significant different with onshore (Xu, Li, and Chen, 2006, Yi, Zhou, and Zhu, 2012). Injection suitable dosage inhibitor help to lower down the gas hydrate equilibrium temperature and prevent the formation of hydrate to benefit the high pressure wet-gas pipeline's safe running in the cold environment.