Chen, Songgui (Tianjin Research Institute of Water Transport Engineering, Hohai University) | Chen, Hanbao (Tianjin Research Institute of Water Transport Engineering) | Zheng, Jinhai (Hohai University) | Zhang, Chi (Hohai University) | Duan, Zihao (Tianjin Research Institute of Water Transport Engineering) | Huang, Shuo (Guangzhou Institute of Energy Conversion)
Experiments of the wave impinging forces on seawalls reefs were conducted in the large wave flume of the Tianjin Research Institute of Water Transport Engineering. Wave forces, including the horizontal and uplift pressures, on seawalls were measured at different water levels and wave conditions. The force values were compared with those obtained by empirical equations. The experimental results show that the wave force on seawalls was determined by the incident wave height, the wave setup on reefs and the wave-induced flow after breaking. A rectangular distribution for the horizontal pressure and a triangular distribution for the uplift pressure were found. Correction coefficients were proposed based on the relative distance between the seawall and reef edge and the relative water depth at the reef. The calculated correction values were demonstrated to be in good agreement with the experimental results.
The water depth around coral reefs is generally a few hundred metres or kilometres. The terrain of the coral reefs is similar to a hill, and when waves reach the reef flat from the deep water, they will inevitably pass a large gap in the reef edge with a strong non-linear characteristic. Coral reefs are exposed to harsh marine environments with variable water depths, tropical cyclones and prevalent monsoons. Under the effect of typhoon waves, storm surges and other extreme waves, the reef wave-protection structures will be strongly affected. The wave forces on seawalls directly affect the safety of the wave-protection structures. Although the current equation of wave force on a seawall of a rubble-mound breakwater was given in the Code of Hydrology for Harbour and Waterway (JTS145-2015), its application conditions clearly stipulate the following: “The slope of the bottom is i<1/10, and the water depth of the structure is 1.5~5.0 H; the fore slope of the structure is i<1/50.” However, the steep slopes of coral reefs are often 1:10~1:0.5; then, the existing norms about water depth and the bottom slope do not satisfy the special topographical conditions of coral reefs. Therefore, the relevant norms are no longer applicable.
As the offshore oil and gas explorations move to deeper waters, the taut wire mooring system which utilizes the synthetic fiber ropes as the main sections of mooring lines, is widely utilized due to the excellent mechanical properties of fiber ropes. However, the dynamic stiffness of mooring lines decreases due to the damage resulting from the mooring installation and service. Consequently, the dynamic stiffness degradation of mooring lines further affects the dynamic responses of the taut-wire mooring system. Nevertheless, less researches were conducted on the taut-wire mooring systems whose fiber ropes are under different damage levels. Therefore, it is necessary to investigate the dynamic responses of the taut-wire mooring systems with damaged lines. In this paper, taking an FPSO with HMPE mooring lines as an example, dynamic responses of the HMPE mooring system with damaged lines are investigated. First, the dynamic stiffness equation of damaged HMPE ropes is obtained by performing experiments of HMPE ropes. Then, the equation of dynamic stiffness is utilized in mooring analysis. By performing time domain analysis of mooring system with different damaged levels of HMPE ropes, the FPSO's offsets and tension responses of mooring lines are obtained. According to these results, suggestions about inspection and evaluation of mooring lines are proposed. These investigations can not only contribute to capture nonlinear dynamic responses of the damaged HMPE mooring system, but also aid the industry to monitor and determine the integrity of mooring system. The present study is of great benefit to the safe operation of taut-wire mooring systems.
As offshore oil and gas exploration and production increase in the Gulf of Mexico, West Africa, Asia, Brazil, et c., taut-wire mooring systems have become attractive as alternatives for catenary mooring systems due to their excellent properties such as higher mooring stiffness, smaller footprint and higher platform payload. Polyester ropes are commonly utilized in taut-wire mooring system (TMS). Currently, the deepest Spar mooring in the Gulf of Mexico which is Shell's Perdido Spar and the deepest moored floating production storage and offloading (FPSO) which is Petrobras’ Cascade and Chinook utilized polyester ropes as main sections of mooring lines. However, with numerous discoveries of natural resources in deeper waters, the question of whether polyester ropes can be utilized and provide enough stiffness to maintain acceptable platform offsets in ultra deepwater regions has been raised (Davies et al., 2002; Chi et al., 2009; Vlasblom et al., 2012). Based on the fact that HMPE fiber ropes have shown higher stiffness than polyester ropes on the condition of equivalent minimum breaking load, they are considered as alternatives to polyester ropes (Davies et. al, 2002; Garrity and Fronzagia, 2008; Leite and Boesten, 2011; Vlasblom et al., 2012; Lian et al., 2015a, 2015b). But in practical engineering, fiber ropes may be damaged or failure concerning the following reasons: (1) rope handling during installation; (2) wear experienced during service; (3) ingress of sand and marine growth; (4) material and manufacturing defects; (5) local sub-rope or element rupture during service (Williams et al., 2002; Denton, 2006; Ward et al., 2006a, 2006b; Ma et al., 2013; Gordon et al., 2014). This damage affects the working performance of the mooring lines. Therefore, lots of scholars have carried out researches on mooring lines with different damage levels. In theory, Karayaka (1999) first proposed the idea of adopting the continuum damage mechanics theory to explain the damage evolution of synthetic fiber rope, but a practical computational model is not presented. A significant progress was made by Beltran et al. (2003) and Beltran and Williamson (2004, 2005, 2009, 2010), who proposed a reliable computational damage model which was verified by small-scale breaking loading tests, but this model was not utilized to capture the evolution of dynamic stiffness of damaged fiber ropes due to lack of experimental data. In the aspect of experiments, Williams et al. (2003) and Ward et al. (2006a, 2006b) investigated the damage effect of fiber ropes on residual breaking strength of fiber ropes by adopting the method of cutting some sub-rope and components of fiber ropes. Similarly, the present industry standard only considers the residual strength and fatigue life to determine the mooring line service life. Cordage Institute (2001) proposed the method of evaluating the damage situation of ropes by considering fatigue life, wear and cross-sectional area reduction. Det Norshke Veritas (DNV, 2008) proposed the method of inspection of mooring lines by measuring the cross-sectional area of rope and evaluating the residual strength of ropes to determine the disposal methods of ropes. Liu et al. (2015) studied the evolution of dynamic stiffness of damaged fiber ropes and revealed the effect of damage level on the dynamic stiffness of fiber ropes.
Chen, Songgui (Tianjin Research Institute of Water Transport Engineering, bHohai University) | Chen, Hanbao (Tianjin Research Institute of Water Transport Engineering) | Zhang, Huaqin (Tianjin Research Institute of Water Transport Engineering) | Zheng, Jinhai (Hohai University)
In this work, wave propagation and deformation on the steep reef are studied based on large scale wave physical model. Due to shallow water depth on the reef and rather steep reef fore slope (larger than 1:1), strongly breaking occurs when waves reach the reef edge from offshore. Thus deviations brought by the scaling effect will become significant when small scale physical model is applied. The Large Wave Flume (LWF) of Tianjin Research Institute of Water Transport Engineering is employed in this work, which is 456m long, 12m deep and 5m wide. The simulated reef section with an 1:0.75 steep slope is 300m long, 50m wide and 50m deep. The modeling scale is chosen as 1:10, therefore a steel platform with 30m long, 5m wide and 6m deep (1m embedded under the ground) was built in the testing section of LWF. Regular with different heights and periods are generated under varies of water levels. Water levels varying with time on the reef platform and in the lagoon are recorded by the special video wave gauges. The relationships between transmission coefficient (Tc), wave setup (n) and dimensionless parameters H*=H/D, T*=gT2/H are analyzed. It can be found that Tc increases with T* and H*, and a power function between n and T* can be obtained. Wave spectrums at different locations on the platform are focused on in the experiment of irregular waves. It also can be found that the farther waves propagated the lower peak frequency becomes. The results in this work will be very useful to understand reef hydrodynamic.
Wave propagation and deformation on coral reefs are of considerable interest to a wide range of researchers. Expanding human activity on the reefs, such as foreshore reclamation for airport and harbour facilities, navigation aids located on reefs, is creating the need for more reliable engineering design data for coastal engineers. In particular, the wave parameters on the reef, including wave height and wave setup, are most concerned.
Xiong, Yan (Hohai University) | Liang, Qiuhua (Hohai University) | Amouzgar, Reza (Newcastle University) | Cox, Daniel T. (Newcastle University) | Mori, Nobuhito (Oregon State University) | Wang, Gang (Kyoto University) | Zheng, Jinhai (Hohai University)
This paper concerns tsunami modeling from wave propagation to inundation of dense urban area through reproduction of a laboratoryscale event. The adopted hydrodynamic model is based on the finite volume shock-capturing solution to the 2D nonlinear shallow water equations and is implemented on modern graphics processing units (GPUs) to achieve high-performance simulations. After being validated through reproduction of flow hydrodynamics, the model is further applied to quantify the tsunami impact on urban building structures by calculating pressure forces.
Tsunami represents a major type of natural hazards to the world’s coastlines. Once it happens, a tsunami may cause wide-spreading damage to both natural and social systems, and kill lots of lives. For example, the 2004 Boxing Day Tsunami, triggered by the M9.3 undersea earthquake offshore Sumatra, inundated a large number of coastal communities with waves up to 30m along the Indian Ocean coastline and killed 230,000 people in 14 countries. It was recorded as one of the deadliest natural disasters on book. On 11th March 2011, 20min after the M9.0 Tohoku earthquake, a mega tsunami struck East Japan coastline, travelled up to 10km inland with a maximum runup of over 40m, caused over 15,000 deaths and wide-spreading damage to buildings and infrastructure, including nuclear power stations. Tsunamis have long been perceived as extremely rare events. But worldwide statistics shows that, on average, one damaging tsunami event per year has been reported in the past two decades and tsunami is actually a common type of natural hazards of medium probability and potentially high risk to the world’s coastlines (NOAA Center for Tsunami Research 2016).
In order to protect coastal communities and save lives, attention has been given to better plan and design buildings and structures along the coastlines that are under threat of tsunamis, generally by following building guidelines/codes provided by relevant institutions in different countries, e.g. the Building Center of Japan’s Structural Design Method of Buildings for Tsunami Resistance (SMBTR) (Okada et al., 2004), Federal Emergency Management Agency (FEMA)’s Coastal Construction Manual (FEMA, 2011). Particularly, the Guidelines for Design of Structures for Vertical Evacuation (FEMA, 2012) suggests that the impact of tsunami forces may classify onto hydrostatic force, buoyant force, hydrodynamic force, impulsive force, debris impact force, debris damming force, uplift force and additional gravity load from the retained water on elevated floors.
Chen, Kaicui (Hohai University) | Liang, Qiuhua (Hohai University) | Xiong, Yan (Newcastle University) | Qiang, Juan (Hohai University) | Wang, Gang (Hohai University) | Zheng, Jinhai (Hohai University)
Storm surge and tsunami may induce extreme flow/wave conditions and cause tremendous damage to human lives, buildings and structures in the coastal areas. Bridges are among the most vulnerable structures to these extreme hazardous flows/waves. With a focus on sea-crossing bridges where the piers may be the only/main structure receiving flow/wave impact, this work presents a series of laboratory experiments to investigate the extreme flow/wave impact on a simplified bridge model. Subsequently, the experimental measurements are used to validate a hydrodynamic model for reliable prediction, with results further compared with those estimated using standard design formulae.
Storm surges and tsunamis may drive destructive flows and massive volumes of water onshore and cause tremendous damage to the coastal areas (Saatcioglu et al., 2005; Robertson et al., 2007). In addition to their direct threat to human lives, the resulting extreme waves and flooding may cause damages to and even destroy buildings and other structures. For example, in 2005, the extreme storm surge and floods following Hurricane Katrina caused the failure of man-made levees, rapidly inundated majority part of New Orleans, killed more than 1,833 people and left over one million people homeless (Robertson et al., 2007). On 11th March 2011, a mega tsunami struck East coast of Japan, travelled up to 10 km inland with a maximum run-up of over 40m, leading to over 15,000 deaths and wide-spreading damage to buildings and infrastructure, including nuclear power stations (Maruyama et al., 2012).
Field missions have been conducted following major tsunami and storm surge events to survey the wave and flood damages to buildings and infrastructure, trying to gain better understanding of extreme wave/flow-structure interaction and learn lessons (Iemura et al., 2005; Ghobarah et al., 2006; Akiyama et al., 2012). Particularly, a large number of bridges in coastal areas have been reported to be damaged and collapse during these extreme events. There are typically three types of failure mechanisms, i.e. 1) bridge superstructure (decks) failure, 2) bridge superstructure-substructure connection failure, and 3) bridge substructure (abutments, supporting piers and foundations) failure. Due to the relatively low deck height of the coastal and traditional bridges, they may be overtopped and submerged during the extreme disastrous events, most commonly leading to superstructure failures and connection failures. These failure modes have been often observed after tsunamis or extreme storm surges. For this reason, most of the current studies related to extreme wave/flow impact on bridges have been focused on vertical and horizontal loading on superstructures (Chen et al., 2009; Bradner et al., 2010; Azadbakht and Yim, 2014; Seiffert et al., 2014a; Hayatdavoodi et al., 2014b).
Storm surge is a major cause of coastal flooding. Robust models have provided useful tools for storm surge forecasting and flood risk management. In this work, a finite volume shock-capturing shallow water equation model originally developed for flood simulation is improved and tested for storm surge modeling. For storm surge modeling, additional source terms are included to represent the wind stresses and atmospheric pressure variation. The performance of the improved model is validated and demonstrated through application to benchmark test cases.
Storm surges and the resulting coastal floods caused by hurricanes, cyclones and typhoons are a major type of natural hazards threatening many coastal cities worldwide. Nowadays, numerical modeling has provided an indispensible tool for forecasting storm surge and managing the resulting flood risk. Numerous models that can be used to simulate storm surge and the resulting flooding processes have been reported in literature in the last few decades. Most of these models are based on the solutions to the governing depth-average fluid equations using finite element, finite difference or finite volume numerical schemes.
Ip et al. (1998) presented a finite element Galerkin model for simulating tidal flooding and drying in shallow estuaries with applications to hypothetical embayment and to the Great Bay, New Hampshire estuary system. The ADvanced CIRCulation (ADCIRC) is a two-dimensional, depth-integrated, barotropic time-dependent long wave, hydrodynamic circulation model based on an unstructured finite element numerical scheme (Leuttich et al., 1992; Blain et al., 1994); it has been widely used for predicting coastal circulations and storm surges. However, due to the adoption of unstructured grids, the ADCIRC model is computationally too demanding to provide high-resolution ensemble forecasts in an efficient way. Westerink et al. (1992) reported another unstructured grid-based finite element model to calculate tides and hurricane driven storm surges in the Gulf of Mexico, in a region ranging from the South Mississippi to the northwest coast of Florida.
In this study, a series of laboratory experiments on liquid sloshing in a rectangular tank are conducted in a shaking table excited by irregular wave-maker to estimate the pressure distribution on the tank walls and the free surface displacement by changing external excitation frequency of the shaking table and baffle location. The use of the upper mounted vertical baffle not only remarkably reduces the slosh wave amplitude and dynamic impact pressures acting on the tank wall but also shifts the natural frequency of liquid tank. The experimental results indicated that the upper mounted vertical baffle is an effective tool in reducing violent sloshing amplitude in the vicinity of the resonant frequency.
Free fluid surfaces in moving containers with related resonant sloshing phenomena pose a problem not only in marine applications but also in aircraft and rocket fuel tanks (Sprenger, 2012). Sloshing motions is extremely strong for excitations in the vicinity of the first natural frequency of the tank. The investigations on the sloshing dynamics are therefore very significant for designing liquid-tank. In general one of major issues caused by marine sloshing effects is structural problems due to high pressure on the tank walls. To install internal baffles inside the tank can change the nature frequency of the tank and reduce free surface fluctuation amplitude. The relative position, arrangement, configuration, size and modulus of elasticity of the baffles have direct effects on the hydrodynamic damping of the liquid motion in the liquid-tank.
To assess sloshing loads and the free-surface elevation of liquid sloshing in a baffled tank, theoretical analysis, numerical simulation and laboratory experiment (Pistani and Thiagarajan, 2012) have been employed by lots of researchers (Kishev, et al., 2006; Jeong, et al., 2014). Xue and Lin (2011) numerically simulated the effects of ring baffle inside the rectangular tank. It can be found from the results that the ring baffle is more effective in reducing violent liquid sloshing when it is placed near to free surface and increased in width. Jung, et al. (2012) numerically investigated the effect of the vertical baffle height on the liquid sloshing in a laterally moving three-dimensional (3D) rectangular tank. They point that the vortex generated by the flow separation from the baffle tip becomes weaker and smaller with increasing baffle height, leading to a diminished damping effect of the tip vortex on the liquid sloshing. Akyildiz (2012) investigated the liquid sloshing in a moving partially filled rectangular tank with a vertical baffle by changing the ratio of baffle height to the initial liquid depth. Koh, et al. (2013) investigated the effect of constrained floating baffle in sloshing mitigation by using improved Consistent Particle Method (CPM). Jin, et al. (2014) conducted experimental studies of liquid sloshing in a tank with an inner submerged horizontal perforated plate by varying excitation amplitude and frequency. The experimental results indicate that the horizontal perforated plate can significantly restrain violent resonant sloshing in the tank under horizontal excitation. Hosseinzadeh, et al. (2014) experimentally studied the effects of annular baffle in reducing fluid wave sloshing height in steel storage tanks typically used in oil and petrochemical complexes during an earthquake.
The trapping effect of oceanic ridge plays an important role on the farfield tsunami propagation pattern though the trapping mechanism is still unknown. Based on the linear shallow water equations, the trapped modes over a symmetric parabolic-profile submerged ridge are obtained analytically. The spatial energy distribution pattern for each mode is discussed. The wave amplitude gets the maximum at the top of ridge and decays gradually towards both sides, and the decaying rate is gentler with higher modes. A first-order approximation is derived to simplify the transcendental equation of dispersion relationship, which is useful to describe tsunami waves trapped over some shallowly submerged oceanic ridges in reality. The trapped wave length with the same period over the steep-profile and deep-submerged ridges is longer than that over the gentle and shallow ridges.
Aimed at the time effect, group effect and difference between negative skin friction (NSF) on pile in sand and that in clay, a series of model tests include single pile tests and pile group tests were carried out. The test results indicate that the NSF on pile increases with consolidation time of clay, which increases rapidly in early stage and slowly in later stage. The position of pile has significant influence on the group effect of NSF. Furthermore, the effective stress factor of NSF in clay is 85.73% of that in sand based on test results.
Sloshing flows can create significant global and local loads on the tank wall due to the impact of traveli ng hydraulic jump which is usually generated and observed in shallow water sloshing. This study therefore presents a numerical investigation of the shallow water sloshing characteristics in a rectangular tank. An in-house two-phase flows model is employed to simulate liquid sloshing phenomena with small fluid depth by changing the forcing frequency. The present numerical results are validated and in good agreements with the available experimental data from Bouscasse et al. (2013). The dominant response frequencies identified from the amplitude spectrum are presented for the different forcing frequency. Th e shallow water sloshing profile of the response amplitude operator is also obtained in a wide range of frequencies. The resonant frequency is shifted to a higher value than predicted by linear theory, which is like the response of hardening spring.