Display Mode： |
At present, more and more offshore wind farms have been built and numerous projects are on the drawing tables. Therefore, the study on the safety of collision between ships and offshore wind turbines (OWT) is of great practical significance. The present study takes the advantage of the famous LS-DYNA explicit code to simulate the dynamic process of the collision between a typical 3MW offshore wind turbine model with monopile foundation and a simplified 2000t-class ship model. In the simulation, the added mass effect of the ship, contact nonlinearity of collision, material nonlinearity of steel and aluminum foam and adaptive mesh technique for large structure deformation have been taken into consideration. Proposed is a crashworthy device for OWT of new conceptual steel sphere shell-circular ring aluminum foam pad, and the good performance of the device under the conditions of ship-OWT front impact and side impact has been verified from the views of theoretical analysis and numerical results. The new crashworthy device can effectively smooth the contact force and reduce the top structure dynamic response, using its own structure plastic deformation to absorb most of the ship collision energy. As a result, the main structure of the OWT and the inside key electric control equipments can be saved by scarifying the structural plastic deformation of new sphere crashworthy device. What is more, the sphere configuration design of the crashworthy device can effectively guide the ship to run away from the main OWT structure and reduce the damage of the ship and OWT to some degree during side impact.
This study deals with the nonlinear dynamic response of deep-sea risers subjected to parametric excitation at the top of a platform. As offshore oil and gas exploration is pushed into deep waters, difficulties encountered in deep-sea riser design may be attributed to the existence of parametric instability regarding platform heave motions. Parametric resonance in risers can cause serious damage which might bring disastrous accidents such as environment pollution, property losses and even fatalities. Therefore, the parametric instability analysis should attract more attention during the design process of deep-sea risers. In this work, an equation of motion for a deep-sea riser is derived firstly. The motion equation is analyzed by the Floquet theory which allows the determination of both system response and stability properties. The unstable regions in which parametric resonance easily occurs can be determined. The effects of damping on parametric instability are also investigated, and the stability maps are presented. The results demonstrate that the available damping is vital in suppressing the instability regions. The suggestions for reduction of instability regions are proposed in deep-sea riser design.
The main challenge in predicting global responses of floating vessels in deep and ultra-deepwater areas comes from the system’s coupled effect. In this paper, the coupled approach is used to analyze effects of riser on floating system in deepwater. The analysis results show that the coupling effects of risers will mainly affect the low frenquency (LF) motions of the system. That is because risers will provide the system with a significant low frenquency (LF) damping, which will vary with the sea- states. Under storm sea- state, the excitation term from drag force will govern the response. While under the moderate conditions, the damping effect from risers is more significant. In the practical engineering, the combination effect should be considered by other ways when the risers are omitted from the model.
In this paper, a numerical model for nonlinear wave propagation in currents is formulated by a set of enhanced fully nonlinear Boussinesq equations with ambient currents. This model is verified by comparison with the published results. Then the influence of currents on nonlinear focusing waves is studied by use of the numerical model. It is found that the effect of currents on the surface elevations at the focal location is negligible. Following currents can augment the maximum crest of focusing wave while decrease the focusing time, and vice versa for opposing currents. Furthermore, both opposing and following currents can shift the focal location forward relative to that in quiescent water.
Fourier transform (FT) is a commonly used method in spectral analysis of ocean wave and offshore structure responses, but it is not suitable for records of short length. In this paper another method, wavelet transform (WT), is applied to analyze the data of short length. The Morlet wavelet is employed to calculate the spectral density functions for wave records and simulated Floating Production Storage and Offloading (FPSO) vessels’ responses. Computed wave data include simulated wave data based on JONSWAP spectrum and the recorded data of Storm 149 from North Alwyn. Wavelet method is validated by comparing the statistical characteristics by WT method and those by fast Fourier trasform (FFT) method with those of target spectra. The spectral density functions’ shapes calculated by WT are less malformed and have less error of statistical characteristics compared with those by FT especially when the record lengths decrease.
The off-bottom tow is an important method to lay pipeline for offshore oil and gas transportation in shallow water area. During the off-bottom towing operation, the actions such as sea current resistance, friction force of seabed, pontoon buoyancy and tow force of tugboat can cause large deformation and fatigue damage to the pipeline. In order to keep the pipeline in safety, the lateral and vertical deformation of pipeline must be controlled within an appropriate range. Theoretical study is carried out in the paper on the environmental forces acting on pipeline, pontoons and chains, and the confirmation of tow parameters including tow forces of tugboat, number of pontoons and length of chains. Then the pipeline in the off-bottom towing process may be simplified into a continuous beam with elastic supports under the concentrated and distributed forces. A finite element method is applied to achieve numerical solutions describing the distribution of deformation and stress along the pipeline. The results show that the lateral shape of the pipeline is like an arc with the maximal deformation appearing at the middle of the pipeline. The distributions of stress are similar between two arbitrary pontoons. Moreover, both deformation and stress have the inverse relation with the tow forces of tugboat.
This study examines the performance of an air expansion oil fence system as deployed in two different configurations. Geometric and tension characteristics of the oil fence are investigated for (1) the system attached to two vessels and (2) the system attached to one vessel with spreading devices. Since full-scale testing can be time consuming and expensive, another component of the study was to examine how well models would perform. A scaling technique was developed and a set of experiments were conducted with a 1/20-th scale model of the oil fence in a towing tank facility. Performance results from the model tests were compared with those conducted at sea. The tension (N) in the full-scale results were represented by R=avb , where v is the current velocity in m/s and the values for a and b used for comparison purposes. Both sets of full-scale and model tests were conducted with an oil fence opening of 120 m and 160 m. In the full-scale experiment with spreading devices attached to the forward ends of the oil fence, the horizontal distance of the spreading devices has a tendency to linearly increase as the current velocity increases when the warp length is as short as 50 m, but to linearly decrease as the current velocity increases when the warp length is extended to 100 and 150 m. The spreading distance at uniform current velocitys in the model test increases as the warp length is extended. While there is no significant difference of spreading distance between the cases of 100 m and 150 m, the spreading distance becomes shorter when the warp length is 50 m.
Water jet thruster, which is a marine system that creates a jet of water for propulsion, has several advantages such as low noise, good anti-cavitation characteristics and maneuvering characteristics. The reaction thrust characteristics of water jet for conical nozzles directly determine the speed of autonomous underwater vehicles (AUV). Theoretical, numerical and experimental studies have been carried out to investigate the effects of the nozzle geometries as well as inlet conditions on the reaction thrust of water jet in this paper. The results show that: 1) the reaction thrust is proportional to inlet pressure, the square of flow rate and 2/3 power exponent of input power; 2) the diameter of cylinder column for conical nozzle has great influence on the reaction thrust characteristics; 3) the best values of the half cone angle and the cylinder column length exist to make the reaction thrust coefficient to reach maximum under the same inlet conditions．Those provide a basis for nozzles design and jave significant value, especially for developing high performance and efficiency water jet propulsion unit.
The national benchmarks on islands were mostly established by trigonometric leveling in Korea. This method results in inaccuracy, which is a serious problem in Geoga Grand Bridge construction work that tried to link the mainland and the islands. The Geoga Grand Bridge (Pusan-Geoje fixed link project) was selected as the study area, a huge construction work in Korea that will connect the mainland (Pusan) and an island (Geoje island). However, the orthometric heights issued at benchmarks (JINH and GOEJ) were not consistent, because they did not refer to the same zero point, which would make the linking of the sections problematic. This paper introduces the precise local geoid as a vertical datum for the construction area in order to establish a consistent height system. To determine the precise local geoid for the construction area, we firstly developed a precise gravimetric geoid for Korea and its adjoining seas as a whole. This gravimetric geoid was developed by use of all available gravity data, including surface and satellite data on land and on the ocean. The gravimetric geoid was computed by spherical fast fourier transform with modified Stokes’ kernels. The remove-restore technique was used to eliminate the terrain effects by use of the RTM reduction and to determine the residual geoid by combining the GGM02S/EGM96 geopotential model, free-air gravity anomalies and high-resolution DEM data. Finally, the gravimetric model was fitted to the geoid heights obtained from GPS and tide observations (NGPS/Tide) by least square collocation, to provide the final GPS-consistent local precise geoid model. The post-fit error (std. dev.) of the final geoid to the NGPS/Tide derived from GPS and tide observations was ±2.2 cm for the construction area. We solved the height inconsistency problem by calculating the orthometric height of the benchmarks and the control points using the final geoid model. Also, the highly accurate orthometric height was estimated through the GPS/leveling technique by applying the developed local precise geoid. Therefore, the precise local geoid is expected to improve the quality of the construction procedure of the Geoga grand bridge.
Suction caissons have been widely used as anchors and foundations for floating and fixed offshore platforms. The pull-out performance of conventional suction caissons (with upright walls) has been investigated by a number of researchers. However, no attention has been paid to tapered suction caissons. This paper deals with the performance of tapered suction caissons under vertical pull-out loads. A numerical approach is used for this purpose. The numerical model is first verified against test data available for common upright caissons. The verified model is then used to study the pull-out performance of tapered suction caissons. It is noticed that the pull-out capacities exhibited by tapered suction caissons are in general considerably higher than those from their corresponding traditional upright caissons. To obtain an insight into this superior behaviour, effects from certain soil/caisson/drainage parameters on the pull-out capacity of tapered suction caissons are studied. Soil cohesion is noticed to have a linear improving effect on the capacity of both upright and tapered suction caissons. The soil internal friction angle was noticed to have an exponential increasing effect on the pull-out capacity. With a constant caisson diameter, an increase in the aspect ratio is seen to particularly influence the pull-out capacity. With a constant caisson length, an increase in the aspect ratio is discovered to result in non-linear decrease in the pull-out capacity. Under undrained conditions, tapered models generally show less sensitivity to above mentioned soil/caisson parameters as compared with those under drained conditions.
Pile foundations of ports, mooring systems for ocean surface or submerged platforms are subjected to amount of uplift loading, and prediction of the uplift capacity is one of the most important subjects in structure designs. The paper pertains to the development of a simplified theoretical method on prediction of the uplift capacity of pile group embedded in clay assuming a composite failure surface (inverted and truncated circular pyramidal and cylindrical surface). Various pile and soil parameters such as the arrangement of pile group, pile spacing, length, diameter of the pile and the soil properties such as unit weight, angle of internal friction and the pile-soil interface friction angle, which have direct influence on the uplift capacity of the pile group, are incorporated in the analysis. A 3D numerical model is built by FLAC3D to analyze the pile group under uplift loading for comparison, and various effect factors, such as length to diameter ratio, pile spacing and pile numbers are considered. The predicted values of uplift capacity and failure surface of pile group with different length to diameter ratio, pile spacing and soil properties are then compared with numerical analysis results and field test results. The predictions are found to be in good agreement with numerical analysis and measured values, which validate the correctness of the developed method. It is also found that the uplift capacity is significantly influenced by the pile-soil friction coefficient, soil shear strength etc.
The capillary absorption of water by unsaturated cement-based material is the main reason of degradation of the structures subjected to an aggressive environment since water often acts as the transporting medium for damaging contaminants. It is well known that the capillarity coefficient and sorptivity are two important parameters to characterize the water absorption of porous materials. Generally, the former is used to describe the penetration depth or height of water transport, which must be measured by special and advanced equipment. In contrast, the sorptivity represents the relationship between cumulative volume of water uptake and the square root of the elapsed time, which can be easily measured by the gravimetric method in a normal laboratory condition. In the present study, an analytical method is developed to build up a bridge between these two parameters, with the purpose that the sorptivity or the gravimetric method can be used to predict the penetration depth of water absorption. Additionally, a new model to explain the dependence of sorptivity on initial water content of the material is developed in order to fit the in situ condition. The comparison of predicted results by the analytical method with experimental data or numerical calculation results, as well as some previous models, validates the feasibility of the methods presented in this paper.
This paper investigates the reflection of normal incident waves produced by absorbing-type breakwaters. The absorbing-type breakwaters in the present study consist of a vertical porous plate, a submerged permeable caisson, and an impermeable back wall. The flow field is divided into four regions: a porous caisson region, and three pure water regions. Under the assumptions of linear wave theory, Darcy’s law in the perforated wall, and the pore velocity potential theory of Sollitt and Cross (1972) in the porous caisson region, this study creates a 2-D BEM model to calculate the reflection coefficients of water waves using several breakwater properties. This numerical model is calibrated by previous numerical studies and limiting cases for a partially perforated-wall caisson breakwater and a vertical porous breakwater with an impermeable back wall. Generally speaking, the wave dissipation in absorbing-type breakwaters is bigger than that for a partially perforated-wall caisson breakwater. The reflection coefficient values imply the performance of wave absorbers in the present study. Therefore, we examine the major factors that affect the reflection coefficient.
On the basis of historical documents, this paper studies the evolutionary processes of the Jiangsu coast and re-establishes the positions of the shoreline in different periods. The East China Sea and the Yellow Sea mathematical models are applied to simulate and analyze the large-scale tidal waves changes under the influence of the coastal change in Jiangsu since 1855 when the Yellow River changed its lower course into the Bohai Sea. Results from this study can be summarized in the following aspects: (1) the coastline change strongly affects tidal waves in the region. Generally, the tidal amplitude decreases when the coastline changes in the north coast of the abandoned Yellow River Delta, whilst increases in the south of this region. The maximum variation of tidal amplitude takes place near the Radial Sandbank. (2) Following the erosion of the abandoned Yellow River Delta in the past century, the non-tidal points of M2 and K1 partial tides move to southwest gradually. (3) In the early 20th century, with the coastline changed, tidal range decreased 30~60 cm. From the abandoned Yellow River mouth to the Lvsi Port, the maximum increase of tidal range can exceed almost 150 cm. From the early 20th century to the early 21th century, the tidal range increases by 20~50 cm outside the radial sandbank. Whereas, the tidal range reduced near the abandoned Yellow River by 30~50 cm.
The control architectures of “SY-2” Remote Operated Vehicle (ROV) are introduced. Both hardware architecture and software architecture are discussed. PC/104 embedded computer is used to control equipment for collecting sensor data and sending control commands. PC/104 embedded computer is integrated with A/D, D/A, 8 serial ports card and power supply unit. The surface computer was a X86PC. They transfer data through a fiber line. For software, real-time OS VxWorks is embedded in PC/104. A/D, D/A and serial ports operation are based on VxWorks OS, which increase the real-time quality of control system. Surface computer is the center of motion control and data processing. It is communicated with underwater PC/104 by Socket. The whole system has been tested bothon land and in tank.
The vortex-induced vibrations (VIV) of barrel-shaped mast are calculated by three numerical methods, i.e. Newmark-b, HPD-L (High Precision Direct integration scheme-Linear form), and HPD-S (High Precision Direct integration scheme-Sinusoidal form). According to the measured value, the accuracy curves are given to show the advantages of HPD-S method over others. Based on the comparison above, HPD-S method is used to calculate the influence of ice covering on the mast to VIV responses. It has been proved that the vortex-induced responses of barrel-shaped mast are changed along with ice thicknesses and types.
Compared with the laboratory acoustic measurement of sediment samples, the in-situ acoustic measurement in marine sediment is considered more accurate and reliable, because it covers all of the surrounding environment factors and avoids the disturbance during the course of sampling and transporting of sediment samples. A new multi-frequency in-situ geo-acoustic measurement system (MFISGAMS) has been developed. The system can provide acoustic velocity (compressional wave) and attenuation profiles of the uppermost 4 ~ 8 m sediment in the seafloor. It consists of 8 channels with 12 frequencies (multi-frequencies) and 0.5~2 MHz sampling rates. The data collected can be transmitted in real-time. Associated with inclinometer and altimeter, it can provide the data for depth emendation. Acoustic velocity and attenuation data have been obtained from two in-situ experiments conducted in the Hangzhou Bay.
ScholarOne Manuscripts Log In
- Volume 33
- Issue 6
- December 2019
- Superintended by:
CHINA ASSOCIATION FOR SCIENCE AND TECHNOLOGY
- Sponsored by:
Chinese Ocean Engineering Society （COES）
- Edited by:
Nanjing Hydraulic Research Institute
Adaptive Predictive Inverse Control of Offshore Jacket Platform Based on Rough Neural Network
Numerical Simulation of Water Exchange Characteristics of the Jiaozhou Bay Based on A Three-Dimensional Lagrangian Model
A Global Reliability Assessment Method on Aging Offshore Platforms with Corrosion and Cracks