2022 Vol.36(1)
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2022, 36(1): 1-14.
doi: 10.1007/s13344-022-0008-9
Abstract:
A hybrid, porous breakwater−Oscillating Water Column (OWC) Wave Energy Converter (WEC) system is put forward and its hydrodynamic performance is investigated using the fully nonlinear, open-source computational fluid dynamics (CFD) model, OpenFOAM. The permeable structure is positioned at the weather side of the OWC device and adjoined to its front wall. A numerical modelling approach is employed in which the interstices within the porous structure are explicitly defined. This permits the flow field development within the porous structure and at the OWC front wall to be observed. The WEC device is defined as a land-fixed, semi-submerged OWC chamber. A range of regular incident waves are generated at the inlet within the numerical tank. The OWC efficiency and the forces on the structure are examined. Results are compared for the simulation cases in which the porous component is present or absent in front of the OWC chamber. It is found that the incorporation of the porous component has minimal effect on the hydrodynamic efficiency of the OWC, reducing the efficiency by less than 5%. Nevertheless, the forces on the front wall of the OWC can be reduced by up to 20% at the higher wave steepness investigated, through inclusion of the porous structure at the OWC front wall. These findings have considerable implications for the design of hybrid OWC−breakwater systems, most importantly in terms of enhancing the durability and survivability of OWC WECs without significant loss of operational efficiency.
A hybrid, porous breakwater−Oscillating Water Column (OWC) Wave Energy Converter (WEC) system is put forward and its hydrodynamic performance is investigated using the fully nonlinear, open-source computational fluid dynamics (CFD) model, OpenFOAM. The permeable structure is positioned at the weather side of the OWC device and adjoined to its front wall. A numerical modelling approach is employed in which the interstices within the porous structure are explicitly defined. This permits the flow field development within the porous structure and at the OWC front wall to be observed. The WEC device is defined as a land-fixed, semi-submerged OWC chamber. A range of regular incident waves are generated at the inlet within the numerical tank. The OWC efficiency and the forces on the structure are examined. Results are compared for the simulation cases in which the porous component is present or absent in front of the OWC chamber. It is found that the incorporation of the porous component has minimal effect on the hydrodynamic efficiency of the OWC, reducing the efficiency by less than 5%. Nevertheless, the forces on the front wall of the OWC can be reduced by up to 20% at the higher wave steepness investigated, through inclusion of the porous structure at the OWC front wall. These findings have considerable implications for the design of hybrid OWC−breakwater systems, most importantly in terms of enhancing the durability and survivability of OWC WECs without significant loss of operational efficiency.
2022, 36(1): 15-27.
doi: 10.1007/s13344-022-0001-3
Abstract:
This study proposed a wave power system with two coaxial floating cylinders of different diameters and drafts. Wavebob’s conceptual design has been adopted in the wave power system. In this study, a basic analysis of the wave energy extraction by the relative motion between two floats is presented. The maximum power absorption was studied theoretically under regular wave conditions, and the effects of both linear and constant damping forces on the power take-off (PTO) were investigated. A set of dynamic equations describing the floats’ displacement under regular waves and different PTOs are established. A time-domain numerical model is developed, considering the PTO parameter and viscous damping, and the optimal PTO damping and output power are obtained. With the analysis of estimating the maximum power absorption, a new estimation method called Power Capture Function (PCF) is proposed and constructed, which can be used to predict the power capture under both linear and constant PTO forces. Based on this, energy extraction is analyzed and optimized. Finally, the performance characteristics of the two-body power system are concluded.
This study proposed a wave power system with two coaxial floating cylinders of different diameters and drafts. Wavebob’s conceptual design has been adopted in the wave power system. In this study, a basic analysis of the wave energy extraction by the relative motion between two floats is presented. The maximum power absorption was studied theoretically under regular wave conditions, and the effects of both linear and constant damping forces on the power take-off (PTO) were investigated. A set of dynamic equations describing the floats’ displacement under regular waves and different PTOs are established. A time-domain numerical model is developed, considering the PTO parameter and viscous damping, and the optimal PTO damping and output power are obtained. With the analysis of estimating the maximum power absorption, a new estimation method called Power Capture Function (PCF) is proposed and constructed, which can be used to predict the power capture under both linear and constant PTO forces. Based on this, energy extraction is analyzed and optimized. Finally, the performance characteristics of the two-body power system are concluded.
2022, 36(1): 28-37.
doi: 10.1007/s13344-022-0002-2
Abstract:
In coastal sea areas with the bimodal Ochi−Hubble wave spectrum, such as parts of the China Sea and Indian Ocean, wave energy is the superposition of wind wave and swell. Traditional heaving buoy wave energy converters developed with narrowband wave spectrums suffer from big energy loss in these areas, leading to lower power absorption efficiency and higher generating costs. In contrast, multi-freedom buoy has different resonant frequencies and maximal power capture wave frequencies in different degrees of freedom (DOFs). Therefore, this study proposed using two DOFs to capture the energy of wind wave and swell correspondingly. A heave and pitch buoy model was established by potential flow theory and validated by experimental data. Coupling effect on the motion and power absorption, power capture frequency distribution and power absorption with different linear power take-off system damping coefficients were analyzed to reveal the hydrodynamic response and the power performance of the two DOFs. The results indicate that by using heave and pitch DOFs, the wave energy components of wind wave and swell were captured in a targeted manner. It demonstrates that the 2-DOF buoy is an effective tool to avoid the energy loss and realize the efficient power absorption in coastal sea areas with bimodal Ochi−Hubble waves.
In coastal sea areas with the bimodal Ochi−Hubble wave spectrum, such as parts of the China Sea and Indian Ocean, wave energy is the superposition of wind wave and swell. Traditional heaving buoy wave energy converters developed with narrowband wave spectrums suffer from big energy loss in these areas, leading to lower power absorption efficiency and higher generating costs. In contrast, multi-freedom buoy has different resonant frequencies and maximal power capture wave frequencies in different degrees of freedom (DOFs). Therefore, this study proposed using two DOFs to capture the energy of wind wave and swell correspondingly. A heave and pitch buoy model was established by potential flow theory and validated by experimental data. Coupling effect on the motion and power absorption, power capture frequency distribution and power absorption with different linear power take-off system damping coefficients were analyzed to reveal the hydrodynamic response and the power performance of the two DOFs. The results indicate that by using heave and pitch DOFs, the wave energy components of wind wave and swell were captured in a targeted manner. It demonstrates that the 2-DOF buoy is an effective tool to avoid the energy loss and realize the efficient power absorption in coastal sea areas with bimodal Ochi−Hubble waves.
2022, 36(1): 38-49.
doi: 10.1007/s13344-022-0003-1
Abstract:
Tidal current energy is a promising renewable energy source for future electricity supply. The flapping hydrofoil is regarded as a useful tool to extract the tidal current energy in shallow water. A concept of coupled tandem flapping hydrofoils under semi-activated mode was proposed in the present study. A two-dimensional numerical model, based on the computational fluid dynamics software ANSYS-Fluent, was established to investigate the power extraction performance of the coupled tandem flapping hydrofoils. The effects of the reduced frequency, pitching amplitude, moment of inertia, damping coefficient, and longitudinal distance between hydrofoils were studied. The vortices, pressure distribution, and kinetic characteristics of hydrofoils under various conditions were analyzed to reveal the interaction between the shedding vortex and hydrofoils. The energy extraction mechanism and hydrodynamic performance were analyzed. The positive interactions for energy harvesting were identified for improvements of the further performance. The peak values of efficiency and power coefficient were achieved at 0.69 and 2.13, respectively.
Tidal current energy is a promising renewable energy source for future electricity supply. The flapping hydrofoil is regarded as a useful tool to extract the tidal current energy in shallow water. A concept of coupled tandem flapping hydrofoils under semi-activated mode was proposed in the present study. A two-dimensional numerical model, based on the computational fluid dynamics software ANSYS-Fluent, was established to investigate the power extraction performance of the coupled tandem flapping hydrofoils. The effects of the reduced frequency, pitching amplitude, moment of inertia, damping coefficient, and longitudinal distance between hydrofoils were studied. The vortices, pressure distribution, and kinetic characteristics of hydrofoils under various conditions were analyzed to reveal the interaction between the shedding vortex and hydrofoils. The energy extraction mechanism and hydrodynamic performance were analyzed. The positive interactions for energy harvesting were identified for improvements of the further performance. The peak values of efficiency and power coefficient were achieved at 0.69 and 2.13, respectively.
2022, 36(1): 50-64.
doi: 10.1007/s13344-022-0004-0
Abstract:
For the offshore wind turbines installed in earthquake areas, their operation is affected by seismic loads in addition to wind and wave loads. Therefore, it is necessary to study the dynamic responses and vibration control of the wind turbines. In previous studies, the structural responses of offshore wind turbines are usually investigated in the parked case, while the blade rotation effect is usually not considered. The evaluation on the structural responses may be inaccurate under this condition, further affecting the evaluation on the vibration control performance of a control system. In view of it, this paper established a complete multi-body model of a fixed-bottom offshore wind turbine considering pile-soil interaction, and then performed simulations when the wind turbine was subjected to multiple external excitations. Continued, a single tuned mass damper (STMD) system and a multiple tuned mass dampers (MTMDs) system were applied to control structural vibrations of the wind turbine. Then, based on the construction of a simplified main structure-TMD system, TMD parameters were optimized. Finally, twelve load cases including operating and parked conditions were selected to perform simulations. Results show that the influence of the seismic excitation on blade responses is greater under the parked condition than that under the operating condition. Moreover, STMD/MTMDS exhibit better performance under the parked condition than that under the operating condition. Compared with STMD, MTMDS can better suppress the vibrations at both the fundamental and high-order modes, and exhibits significant robustness under the condition of changing soil parameters.
For the offshore wind turbines installed in earthquake areas, their operation is affected by seismic loads in addition to wind and wave loads. Therefore, it is necessary to study the dynamic responses and vibration control of the wind turbines. In previous studies, the structural responses of offshore wind turbines are usually investigated in the parked case, while the blade rotation effect is usually not considered. The evaluation on the structural responses may be inaccurate under this condition, further affecting the evaluation on the vibration control performance of a control system. In view of it, this paper established a complete multi-body model of a fixed-bottom offshore wind turbine considering pile-soil interaction, and then performed simulations when the wind turbine was subjected to multiple external excitations. Continued, a single tuned mass damper (STMD) system and a multiple tuned mass dampers (MTMDs) system were applied to control structural vibrations of the wind turbine. Then, based on the construction of a simplified main structure-TMD system, TMD parameters were optimized. Finally, twelve load cases including operating and parked conditions were selected to perform simulations. Results show that the influence of the seismic excitation on blade responses is greater under the parked condition than that under the operating condition. Moreover, STMD/MTMDS exhibit better performance under the parked condition than that under the operating condition. Compared with STMD, MTMDS can better suppress the vibrations at both the fundamental and high-order modes, and exhibits significant robustness under the condition of changing soil parameters.
2022, 36(1): 65-75.
doi: 10.1007/s13344-022-0005-z
Abstract:
With the increasing construction of artificial beach in coastal areas, it is of practical significance to study the beach surface deformation of artificial beach profile. Previous studies only focus on a single wave dynamic factor, and it is difficult to predict the beach deformation of artificial beach profile under the storm surge−wave co-action. To solve this problem, the cross-section physical model test method was used to study the beach surface deformation of a typical artificial beach profile in Shuangdao Bay, Weihai, Shandong Province, after continuous wave actions till they stabilize. The characteristics of beach surface deformation under the conditions of constant water levels, ladder-shaped water level combined with corresponding wave elements and storm surge−wave co-action are compared and analyzed. A beach profile model which satisfies the theory of Bruun model is proposed. The test results show that the maximum scour depth of beach under storm surge−wave co-action is smaller and the scour range is obviously larger than that under the condition of constant water levels or ladder-shaped water level. The evaluation of the maximum scour depth by traditional model test tends to be conservative while the evaluation of the scour range is insufficient. The research results can provide scientific reference for designing artificial beaches.
With the increasing construction of artificial beach in coastal areas, it is of practical significance to study the beach surface deformation of artificial beach profile. Previous studies only focus on a single wave dynamic factor, and it is difficult to predict the beach deformation of artificial beach profile under the storm surge−wave co-action. To solve this problem, the cross-section physical model test method was used to study the beach surface deformation of a typical artificial beach profile in Shuangdao Bay, Weihai, Shandong Province, after continuous wave actions till they stabilize. The characteristics of beach surface deformation under the conditions of constant water levels, ladder-shaped water level combined with corresponding wave elements and storm surge−wave co-action are compared and analyzed. A beach profile model which satisfies the theory of Bruun model is proposed. The test results show that the maximum scour depth of beach under storm surge−wave co-action is smaller and the scour range is obviously larger than that under the condition of constant water levels or ladder-shaped water level. The evaluation of the maximum scour depth by traditional model test tends to be conservative while the evaluation of the scour range is insufficient. The research results can provide scientific reference for designing artificial beaches.
2022, 36(1): 76-85.
doi: 10.1007/s13344-022-0006-y
Abstract:
For surface gravity waves propagating over a horizontal bottom that consists of a patch of sinusoidal ripples, strong wave reflection occurs under the Bragg resonance condition. The critical wave frequency, at which the peak reflection coefficient is obtained, has been observed in both physical experiments and direct numerical simulations to be downshifted from the well-known theoretical prediction. It has long been speculated that the downshift may be attributed to higher-order rippled bottom and free-surface boundary effects, but the intrinsic mechanism remains unclear. By a regular perturbation analysis, we derive the theoretical solution of frequency downshift due to third-order nonlinear effects of both bottom and free-surface boundaries. It is found that the bottom nonlinearity plays the dominant role in frequency downshift while the free-surface nonlinearity actually causes frequency upshift. The frequency downshift/upshift has a quadratic dependence in the bottom/free-surface steepness. Polychromatic bottom leads to a larger frequency downshift relative to the monochromatic bottom. In addition, direct numerical simulations based on the high-order spectral method are conducted to validate the present theory. The theoretical solution of frequency downshift compares well with the numerical simulations and available experimental data.
For surface gravity waves propagating over a horizontal bottom that consists of a patch of sinusoidal ripples, strong wave reflection occurs under the Bragg resonance condition. The critical wave frequency, at which the peak reflection coefficient is obtained, has been observed in both physical experiments and direct numerical simulations to be downshifted from the well-known theoretical prediction. It has long been speculated that the downshift may be attributed to higher-order rippled bottom and free-surface boundary effects, but the intrinsic mechanism remains unclear. By a regular perturbation analysis, we derive the theoretical solution of frequency downshift due to third-order nonlinear effects of both bottom and free-surface boundaries. It is found that the bottom nonlinearity plays the dominant role in frequency downshift while the free-surface nonlinearity actually causes frequency upshift. The frequency downshift/upshift has a quadratic dependence in the bottom/free-surface steepness. Polychromatic bottom leads to a larger frequency downshift relative to the monochromatic bottom. In addition, direct numerical simulations based on the high-order spectral method are conducted to validate the present theory. The theoretical solution of frequency downshift compares well with the numerical simulations and available experimental data.
2022, 36(1): 86-99.
doi: 10.1007/s13344-022-0007-x
Abstract:
Marine mammals could directly harvest energy from waves and obtain propulsive force through oscillating flapping fins or horizontal tail flukes, which in many cases have been observed and proved to be substantial. The propulsion generated by the flapping fin has been analyzed by many researchers from both the theoretical and experimental prospects; however, the structural and operational optimization of a flapping fin for the optimal propulsion performance has been less studied, such as the investigation of the effects of the phase difference between heave and pitch motion, maximum oscillation angle, fin shape, oscillation centre of the fin and the operating sea state on the generated propulsion. In this paper, the flapping fin is used as a self-propulsor to propel an autonomous underwater vehicle (AUV) for propulsion assistance. For the optimization design of the flapping fin, its propulsion effect is numerically investigated with different structural parameters and under various operation conditions using computational fluid dynamics (CFD) approaches. Verification and validation study have been implemented to quantify the numerical uncertainties and evaluate the accuracy of the proposed CFD method. Then, a series of case studies are thoroughly conducted to investigate the effects of different structural parameters and operational conditions on the generated propulsion of a flapping fin by CFD simulations. The simulation results demonstrate that different structural parameters and operation conditions would significantly impact the magnitude and distribution state of the fluid pressure around the flapping fin surface, thus, affect the propulsion performance of the fin. The findings in this study will provide guidelines for the structural and operational optimization design of a flapping fin for self-propulsion of mobile platforms.
Marine mammals could directly harvest energy from waves and obtain propulsive force through oscillating flapping fins or horizontal tail flukes, which in many cases have been observed and proved to be substantial. The propulsion generated by the flapping fin has been analyzed by many researchers from both the theoretical and experimental prospects; however, the structural and operational optimization of a flapping fin for the optimal propulsion performance has been less studied, such as the investigation of the effects of the phase difference between heave and pitch motion, maximum oscillation angle, fin shape, oscillation centre of the fin and the operating sea state on the generated propulsion. In this paper, the flapping fin is used as a self-propulsor to propel an autonomous underwater vehicle (AUV) for propulsion assistance. For the optimization design of the flapping fin, its propulsion effect is numerically investigated with different structural parameters and under various operation conditions using computational fluid dynamics (CFD) approaches. Verification and validation study have been implemented to quantify the numerical uncertainties and evaluate the accuracy of the proposed CFD method. Then, a series of case studies are thoroughly conducted to investigate the effects of different structural parameters and operational conditions on the generated propulsion of a flapping fin by CFD simulations. The simulation results demonstrate that different structural parameters and operation conditions would significantly impact the magnitude and distribution state of the fluid pressure around the flapping fin surface, thus, affect the propulsion performance of the fin. The findings in this study will provide guidelines for the structural and operational optimization design of a flapping fin for self-propulsion of mobile platforms.
2022, 36(1): 100-111.
doi: 10.1007/s13344-022-0009-8
Abstract:
This paper presents autonomous docking of an inhouse built resident Remotely Operated Vehicle (ROV), called Rover ROV, through acoustic guided techniques. A novel cage-type docking station has been developed. The docking station can be placed on a deep-sea lander, taking the Rover ROV to the seafloor. Instead of using vision-based pose estimation techniques and expensive navigation sensors, the Rover ROV docking adopts an ultra-short baseline (USBL) and low-cost inertial sensors to build an adaptive fault-tolerant integrated navigation system. To solve the problem of sonar-based failure positioning, the measurement residuals are exploited to detect measurement faults. Then, an adaptation scheme for estimating the statistical characteristics of noise in real-time is proposed, which can provide robust and smooth positioning results. It is more suitable for a compact and low-cost deep-sea resident ROV. Field experiments have been conducted successfully in the Qiandao Lake and the South China Sea area with a depth of 3000 m, respectively. The experimental results show that the functionality of autonomous docking has been achieved. Under the guidance of the navigation system, the Rover ROV can autonomously and efficiently return to the docking station within a range of 100 m even when the amounts of outliers exist in the acoustic positioning data. These achievements can be applied to current ROVs by an easy retrofit.
This paper presents autonomous docking of an inhouse built resident Remotely Operated Vehicle (ROV), called Rover ROV, through acoustic guided techniques. A novel cage-type docking station has been developed. The docking station can be placed on a deep-sea lander, taking the Rover ROV to the seafloor. Instead of using vision-based pose estimation techniques and expensive navigation sensors, the Rover ROV docking adopts an ultra-short baseline (USBL) and low-cost inertial sensors to build an adaptive fault-tolerant integrated navigation system. To solve the problem of sonar-based failure positioning, the measurement residuals are exploited to detect measurement faults. Then, an adaptation scheme for estimating the statistical characteristics of noise in real-time is proposed, which can provide robust and smooth positioning results. It is more suitable for a compact and low-cost deep-sea resident ROV. Field experiments have been conducted successfully in the Qiandao Lake and the South China Sea area with a depth of 3000 m, respectively. The experimental results show that the functionality of autonomous docking has been achieved. Under the guidance of the navigation system, the Rover ROV can autonomously and efficiently return to the docking station within a range of 100 m even when the amounts of outliers exist in the acoustic positioning data. These achievements can be applied to current ROVs by an easy retrofit.
2022, 36(1): 112-122.
doi: 10.1007/s13344-022-0010-2
Abstract:
Excessive displacement responses of monopiles affect the serviceability of offshore structures. Related to complicated pile−seabed−wave interactions, the actual behavior of monopiles in silty seabed under periodic wave action remains unclear, and relevant studies in the literature are limited. A series of experiments were conducted in a wave flume containing single piles in silty seabed with relative density of 0.77 subjected to regular waves. Two stages of wave loading were applied successively, accompanied by data recording which included pore water pressure, water surface elevation, pile head displacement, and pile strain. Development of pile-head displacement and pore pressure in silty seabed was the main focus, but the effects of pile diameter, pile type, and pile stiffness were also investigated. The experimental results indicate that, in silty seabed, piles of large diameter or with fins accelerate soil liquefaction, resulting in strengthened soil which allows a higher upper boundary of pore pressure. Using fins at deeper locations led to a quick failure of the piles, but the opposite result was observed with an increase in fin dimensions. Once pile-head displacement entered its rapid development period, the wave load calculated via the pile moment was an overestimation, especially for the piles of large diameter.
Excessive displacement responses of monopiles affect the serviceability of offshore structures. Related to complicated pile−seabed−wave interactions, the actual behavior of monopiles in silty seabed under periodic wave action remains unclear, and relevant studies in the literature are limited. A series of experiments were conducted in a wave flume containing single piles in silty seabed with relative density of 0.77 subjected to regular waves. Two stages of wave loading were applied successively, accompanied by data recording which included pore water pressure, water surface elevation, pile head displacement, and pile strain. Development of pile-head displacement and pore pressure in silty seabed was the main focus, but the effects of pile diameter, pile type, and pile stiffness were also investigated. The experimental results indicate that, in silty seabed, piles of large diameter or with fins accelerate soil liquefaction, resulting in strengthened soil which allows a higher upper boundary of pore pressure. Using fins at deeper locations led to a quick failure of the piles, but the opposite result was observed with an increase in fin dimensions. Once pile-head displacement entered its rapid development period, the wave load calculated via the pile moment was an overestimation, especially for the piles of large diameter.
2022, 36(1): 123-132.
doi: 10.1007/s13344-022-0011-1
Abstract:
As the anchoring foundation of the tension leg platform (TLP), suction caisson foundation is subjected to the long-term vertical pullout loads. But there are few studies on the mechanism of the unloading creep of soft clay and long-term uplift bearing capacity of suction caisson foundations. To address this problem, unloading creep tests of soft clay were carried out to analyze the strain development with time under different confining pressures. The test results show that the creep curve rapidly develops in the early stage and tends to stabilize in the later stage. The unloading deviator stress is higher, the unloading creep deformation is greater and the soft clay has typical nonlinear creep characteristics. Therefore, by introducing the creep model and considering the influence of the deviator stress, the stress-dependent Merchant model is proposed to describe the unloading creep of soft clay. Then, the stress-dependent Merchant model is extended to a three-dimension constitutive model, and a finite element subroutine is developed to establish a finite element analysis method for analyzing the long-term uplift capacity of suction caisson foundations and validated with the long-term uplift bearing capacity results of caisson model.
As the anchoring foundation of the tension leg platform (TLP), suction caisson foundation is subjected to the long-term vertical pullout loads. But there are few studies on the mechanism of the unloading creep of soft clay and long-term uplift bearing capacity of suction caisson foundations. To address this problem, unloading creep tests of soft clay were carried out to analyze the strain development with time under different confining pressures. The test results show that the creep curve rapidly develops in the early stage and tends to stabilize in the later stage. The unloading deviator stress is higher, the unloading creep deformation is greater and the soft clay has typical nonlinear creep characteristics. Therefore, by introducing the creep model and considering the influence of the deviator stress, the stress-dependent Merchant model is proposed to describe the unloading creep of soft clay. Then, the stress-dependent Merchant model is extended to a three-dimension constitutive model, and a finite element subroutine is developed to establish a finite element analysis method for analyzing the long-term uplift capacity of suction caisson foundations and validated with the long-term uplift bearing capacity results of caisson model.
2022, 36(1): 133-143.
doi: 10.1007/s13344-022-0012-0
Abstract:
Composite bucket foundation and one-step installation technology for offshore wind turbine are the integration of foundation construction, transportation and whole installation at sea. The cost of offshore wind turbine construction and installation has been largely reduced. Foundation stability is the key technology in the process of towing transportation. Field observation data can reflect the real state of the foundation. In this paper, the influence of water depth and towing speed on liquid level, the compartment pressure, and the pitch angles during towing of composite bucket foundation are studied. These data are analyzed based on the field measurements data from a 3.3 MW offshore wind power project in China. The results show that with varied water depths and towing speeds, the compartment pressure changes are small during the bucket foundation towing process. The offshore wind turbine composite bucket foundation is stable while being towed in the ocean.
Composite bucket foundation and one-step installation technology for offshore wind turbine are the integration of foundation construction, transportation and whole installation at sea. The cost of offshore wind turbine construction and installation has been largely reduced. Foundation stability is the key technology in the process of towing transportation. Field observation data can reflect the real state of the foundation. In this paper, the influence of water depth and towing speed on liquid level, the compartment pressure, and the pitch angles during towing of composite bucket foundation are studied. These data are analyzed based on the field measurements data from a 3.3 MW offshore wind power project in China. The results show that with varied water depths and towing speeds, the compartment pressure changes are small during the bucket foundation towing process. The offshore wind turbine composite bucket foundation is stable while being towed in the ocean.
2022, 36(1): 144-154.
doi: 10.1007/s13344-022-0013-z
Abstract:
A launch and recovery system for a seafloor drill was studied using a dynamic model that considered the influences of seawater resistance and the elastic deformation of the cable based on the lumped mass method. The influence of wave direction angle on heave, roll, and pitch motions of the ship was analyzed, and those motion characteristics were then used to assess the tension response of the armored umbilical cable at the lifting point under different wave direction angles. By analyzing the different wave direction angles we found that, when a ship experiences longitudinal waves it will express longitudinal movement. When a ship encounters transverse waves, it will have transverse movement. Under oblique waves from bow or stern, a ship will have both longitudinal and transverse movement, exhibiting obvious heave and pitch movements. Oblique waves, in this study, produced the most obvious impact on armored umbilical cable tension. However, the tension of the armored umbilical cable will change based on the weight of the armored umbilical cable and the seafloor drill in the water. This analysis has provided a useful reference for the study of heave compensation and the constant tension automatic control.
A launch and recovery system for a seafloor drill was studied using a dynamic model that considered the influences of seawater resistance and the elastic deformation of the cable based on the lumped mass method. The influence of wave direction angle on heave, roll, and pitch motions of the ship was analyzed, and those motion characteristics were then used to assess the tension response of the armored umbilical cable at the lifting point under different wave direction angles. By analyzing the different wave direction angles we found that, when a ship experiences longitudinal waves it will express longitudinal movement. When a ship encounters transverse waves, it will have transverse movement. Under oblique waves from bow or stern, a ship will have both longitudinal and transverse movement, exhibiting obvious heave and pitch movements. Oblique waves, in this study, produced the most obvious impact on armored umbilical cable tension. However, the tension of the armored umbilical cable will change based on the weight of the armored umbilical cable and the seafloor drill in the water. This analysis has provided a useful reference for the study of heave compensation and the constant tension automatic control.
2022, 36(1): 155-166.
doi: 10.1007/s13344-022-0014-y
Abstract:
Mooring system is a significant part of very large offshore floating structures (VLFS). In this paper, a single module pontoon type VLFS model considering four mooring types is studied through physical model tests to determine the effects of mooring conditions on the hydroelastic response, mooring force, incident coefficient, reflection coefficient and energy dissipation coefficient. Eight mooring cables are symmetrically arranged on both sides of the model. The floating body model satisfies the similarity of stiffness and gravity, while the cable satisfies the similarity of elasticity and gravity. The results show that the effect of mooring type on mooring force is greater than that on hydroelastic response. Increasing the initial tension of the mooring cable will reduce the amplitude of the leeward of the VLFS model. The mooring angle of the mooring cable will affect the maximum mooring force and the initial tension of the mooring line will affect the wave period in which the maximum mooring force occurs. The transmission coefficient and wave energy dissipation coefficient will change regularly with different mooring types. These results may provide a reference to facilitate the mooring design of VLFS.
Mooring system is a significant part of very large offshore floating structures (VLFS). In this paper, a single module pontoon type VLFS model considering four mooring types is studied through physical model tests to determine the effects of mooring conditions on the hydroelastic response, mooring force, incident coefficient, reflection coefficient and energy dissipation coefficient. Eight mooring cables are symmetrically arranged on both sides of the model. The floating body model satisfies the similarity of stiffness and gravity, while the cable satisfies the similarity of elasticity and gravity. The results show that the effect of mooring type on mooring force is greater than that on hydroelastic response. Increasing the initial tension of the mooring cable will reduce the amplitude of the leeward of the VLFS model. The mooring angle of the mooring cable will affect the maximum mooring force and the initial tension of the mooring line will affect the wave period in which the maximum mooring force occurs. The transmission coefficient and wave energy dissipation coefficient will change regularly with different mooring types. These results may provide a reference to facilitate the mooring design of VLFS.
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