Tension-leg-like mooring system suitable for shallow-water-depth floating structure

Abstract

The invention discloses a tension-leg-like mooring system suitable for a shallow-water-depth floating structure. The tension-leg-like mooring system comprises an anchor foundation, a mooring cable, a limiting spring and a suspension weight. The anchor foundation is fixed to the seabed and used for resisting horizontal and vertical mooring force. Three stand columns are arranged on the periphery of the floating body, balance weight cabins are arranged in the stand columns, overhanging weights are arranged in the balance weight cabins in the vertical direction in a sliding mode, limiting springs are installed at the top ends and the bottom ends in the balance weight cabins, one end of a mooring rope is connected with the anchor foundation, and the other end of the mooring rope is connected with the overhanging weights. The phenomenon that the mooring tension is sharply increased due to wave frequency motion of the floating body is avoided, and the problem that the catenary mooring platform with the low pretension design deviates too large is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of shallow water mooring, and in particular to a tension leg mooring system suitable for shallow water floating structures. Background Art

[0002] The challenge of shallow water mooring has attracted the attention of many scholars in recent years. Due to the shallow water depth, the length of the suspension section of the traditional catenary mooring is short and cannot provide effective restoring stiffness. To enhance the catenary effect, the most common method is to use counterweights. In addition, under the same offset, the mooring stiffness in shallow water increases faster, which can easily cause sudden tension increases and fracture damage. To reduce the mooring stiffness, the use of composite fiber cables is also a common method. However, the stiffness of synthetic fiber rope materials is path-dependent and lacks a unified model, which leads to design redundancy or risk, and lacks reliability verification in shallow water applications.

[0003] At the same time, for the tension-retreating platform, although it can provide good motion performance, due to the shallow water depth, the load changes caused by tide level changes, wave excitation, etc. are prominent, and the mooring tension fluctuates greatly.

[0004] The primary contradiction in shallow-water mooring design lies in the need to balance limiting platform excursion (rigidity requirement) with allowing for wave-frequency motion (flexibility requirement). Traditional mooring schemes, all based on a constant-length, variable-tension design, fail to fundamentally resolve this contradiction. Therefore, developing a new mooring system suitable for shallow-water floating structures that balances limiting platform excursion and allowing for wave-frequency motion has become a pressing technical challenge facing those skilled in the art.

[0005] To this end, the present invention adopts a new design concept, that is, adopting the idea of variable length and constant tension for mooring design, and proposes a specific implementation method of the mooring design and conducts an in-depth performance analysis. Summary of the Invention

[0006] The purpose of the present invention is to provide a tension leg mooring system suitable for shallow water deep floating structures to solve the above problems.

[0007] The present invention solves the technical problem by adopting the following technical solutions:

[0008] A tension-leg mooring system suitable for shallow-water deep-floating structures, comprising: an anchor foundation, a mooring cable, a limit spring and a suspended weight; the anchor foundation is fixed to the seabed to resist horizontal and vertical mooring forces; three columns are arranged around the floating body, and counterweight compartments are arranged inside the columns. Each counterweight compartment has a suspended weight arranged to slide in the vertical direction, and limit springs are installed at the top and bottom ends of the counterweight compartment. One end of the mooring cable is connected to the anchor foundation, and the other end is connected to the suspended weight.

[0009] Furthermore, the mooring cable is connected to the fixed anchor through a ring structure.

[0010] Furthermore, the weight of the suspension weight is 30%-50% of the breaking strength of the mooring line, and the stiffness of the limit spring satisfies that the maximum compression length under the action of the suspension weight does not exceed 2 meters.

[0011] Furthermore, it also includes a pulley group, which is installed in the floating body, and the mooring cable passes around the pulley group to change the arrangement path of the mooring cable.

[0012] Furthermore, a chock is provided at the bottom of the floating body, and a guide wheel is provided at the position of the chock for guiding the arrangement path of the mooring cable.

[0013] Beneficial effects:

[0014] The present invention avoids the phenomenon of rapid increase in mooring tension caused by wave-frequency motion, and avoids the problem of excessive offset of catenary mooring platforms with low pre-tension design; the tension-leg-like mooring design avoids the problem of excessive mooring tension fluctuations due to short tension legs in shallow water; the tension-leg-like mooring design only requires an extremely short mooring cable length, a small mooring radius, and small float motion offset, which is helpful for dynamic cable and wind turbine power generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of the system structure of the present invention;

[0016] Figure 2 A comparison chart of the static mooring tension and restoring force performance between the present invention and the traditional catenary scheme;

[0017] Figure 3 This is a diagram showing the simulation results of the free decay test of the present invention and the traditional catenary scheme;

[0018] Figure 4 A comparison diagram of the motion response of the floating body foundation of the present invention and the traditional catenary scheme;

[0019] Figure 5 A comparison diagram of the mooring tension response between the present invention and the traditional catenary scheme;

[0020] Figure 6 A comparison diagram of the dynamic response of the wind turbine of the present invention and the traditional catenary scheme;

[0021] Figure 7 A comparison chart of dynamic response statistics between the present invention and the traditional catenary scheme at different water depths;

[0022] Figure 8 This is a comparison chart of the dynamic response statistics of the present invention and the traditional catenary solution under different environmental conditions.

[0023] Among them, in the figure:

[0024] 1-anchor foundation; 2-mooring cable; 3-limit spring; 4-suspended weight; 5-seabed; 6-ballast compartment; 7-pulley block; 8-fairlead hole. DETAILED DESCRIPTION

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0026] refer to Figure 1 The present invention provides a tension leg mooring system suitable for shallow water deep floating structures, comprising: an anchor foundation 1, a mooring cable 2, a limit spring 3 and a suspended weight 4; the anchor foundation 1 is fixed to the seabed 5 to resist horizontal and vertical mooring forces; three columns are arranged around the floating body, and a ballast tank 6 is arranged inside the column. Each of the ballast tanks 6 is provided with a suspended weight 4 sliding in the vertical direction, and a limit spring 3 is installed at the top and bottom of the ballast tank 6. One end of the mooring cable 2 is connected to the anchor foundation 1, and the other end is connected to the suspended weight 4.

[0027] The present invention is connected to the suspended weight 4 by a mooring cable 2, and the suspended weight 4 can slide in the counterweight compartment 6, thereby achieving basic fixation of the mooring tension within a large range of horizontal displacement of the floating foundation. Under actual sea conditions, the semi-submersible platform will move back and forth and up and down with the waves, and the mooring cable will be released and retracted from time to time. Energy storage is achieved through the change of the gravitational potential energy of the suspended weight, and the mooring tension will remain relatively stable. In a mild ocean environment, the lifting and lowering movement of the suspended weight relative to the platform is relatively small, and the mooring tension is approximately equal to the weight of the weight; in extreme sea conditions, the lifting and lowering movement of the suspended weight is more intense, and the fluctuation of the mooring tension is closely related to the acceleration of the suspended weight. The setting of the limit spring can avoid the impact of the suspended weight on the counterweight compartment, and can play a further buffering role. At the same time, it prevents the suspended weight from escaping from the constraint of the counterweight compartment, ensuring that the lifting and lowering range of the weight in the worst environment does not exceed the limit range when the spring buffer is not under load.

[0028] To further optimize the technical solution, the mooring cable is connected to the fixed anchor through a ring structure.

[0029] To further optimize the technical solution, the weight of the suspended weight is 30%-50% of the breaking strength of the mooring cable, and the stiffness of the limit spring is such that the maximum compression length under the action of the suspended weight does not exceed 2 meters.

[0030] To further optimize the technical solution, the present invention also includes a pulley block 7 installed within the buoy. The mooring cable passes around the pulley block 7, changing the mooring cable routing. The combination of the pulley block and the overhanging weight achieves a variable-length mooring cable design, maintaining a stable mooring tension and ensuring the safety of the floating wind turbine in extreme sea conditions such as typhoons.

[0031] To further optimize the technical solution, a fairlead hole 8 is provided at the bottom of the floating body, and a guide wheel is provided at the position of the fairlead hole 8 for guiding the arrangement path of the mooring cable.

[0032] This invention avoids the dramatic increase in mooring tension caused by wave-frequency motion and the excessive deflection of catenary mooring platforms with low pretension designs. A tension-leg-like mooring design also avoids the problem of excessive mooring tension fluctuations caused by short tension legs in shallow water. This tension-leg-like mooring design requires only a very short mooring cable length, a small mooring radius, and minimal buoyant deflection, which is beneficial for dynamic cable and wind turbine power generation. The present invention utilizes a tension-leg-like mooring design, which keeps the mooring cable taut at all times, eliminating the need for manual release of weights based on sea conditions. Furthermore, the mooring cable is short and deflection is minimal. Furthermore, the weights are positioned within the buoyant columns, avoiding fatigue, corrosion, and kinematic coupling issues associated with their pendulum motion in the water. This invention provides a high restoring force, minimizes the platform's motion response, and maintains the platform's translational displacement within an acceptable range, making it suitable for floating wind turbine mooring systems in shallow water.

[0033] To verify the performance advantages of the present invention over traditional mooring solutions, based on the design load conditions required for normal power generation conditions in the DNVGL-ST-0437 specification, typical DLC1.1 and DLC1.6 conditions were selected as environmental conditions for subsequent model verification and mooring response comparative analysis. DLC1.1 represents the most common power generation operating condition, with wave parameters selected corresponding to wind speed, while DLC1.6 represents the worst sea conditions during normal power generation. To demonstrate the performance of the simulation model and mooring response under typical, harsh environments, we prepared three calculation conditions. The wind speed was 12 m / s, slightly higher than the rated wind speed, with the aerodynamic load close to the maximum value and within the range of frequent pitch and speed changes. In terms of wave parameter selection, LC1 selected wave height and period parameters corresponding to the wind speed, while LC2 and LC3 selected wave parameters for 1-year and 50-year return periods, respectively, based on the environmental conditions in the northern part of the South China Sea. The effect of flow loads was not considered in the calculations of the present invention. The following will compare the dynamic response of the present invention and traditional mooring solutions under different environmental conditions from the aspects of still water characteristics and dynamic response through coupled simulation analysis. The environmental conditions of the load conditions are shown in Table 1.

[0034]

[0035]

[0036] Table 1

[0037] First, horizontal stiffness tests and hydrostatic attenuation test simulations will be conducted on two mooring systems (represented by W-CMS and SC-TLMS, respectively) by simulating water tank model tests, so as to obtain the static characteristics of the two mooring systems and provide a reference for subsequent dynamic response analysis.

[0038] Figure 2 The mooring tension of mooring line #1 and the overall mooring horizontal restoring force are given under different horizontal displacements along the X-axis. Figure 2 (a) It can be seen that under static conditions, the mooring tension of the SC-TLMS remains unchanged, while the catenary mooring tension changes with the platform position offset and the suspended length of the mooring cable. The mooring tension increases rapidly after the horizontal positive offset exceeds 4 meters. Figure 2(b) It can be seen that when the horizontal displacement direction is positive, the overall restoring force of the mooring is positive, and vice versa. Therefore, under static conditions, the mooring restoring force always prevents the floating body from deviating to a larger displacement. Specifically, the catenary mooring system is relatively smooth at small displacements, while when the positive displacement is large, the horizontal restoring force increases sharply, and the horizontal restoring force value is equivalent to the mooring tension of the #1 cable, indicating that under shallow water conditions, when the floating body is displaced larger, the horizontal angle of the mooring cable is small and the mooring stiffness is large. When displacing in the negative X direction, the mooring stiffness is milder than that in the positive direction, indicating that when the environmental load is 0 degrees, the mooring tension response is more critical than that in the 180-degree wind and wave direction. For the SC-TLMS mooring, its horizontal restoring force shows a nearly linear change phenomenon with the displacement, which can be seen from the following derivation.

[0039]

[0040] Where N is the number of mooring cables, T lin is the mooring tension, which can be considered equal to the mass W of the weight under static conditions clump , θ is the horizontal angle of the mooring cable at the fairlead, X is the horizontal displacement, and h is the initial height of the fairlead from the anchor point. This simple horizontal stiffness characteristic allows for rapid design and optimization of steady-state tension leg moorings, and its static performance is easily evaluated.

[0041] Figure 3 Simulation results for the hydrostatic attenuation of the float's surge, heave, pitch, and yaw for two mooring system configurations are presented. The corresponding natural periods and damping levels are clearly shown in the blank spaces in the figure, with the preceding values representing the results for catenary mooring. Figure 3 (a) It can be seen that the natural period of the catenary mooring is almost twice that of the tension-leg mooring, which are 123.1 s and 65.8 s respectively, and the damping levels of the two are similar. Figure 3 (b) and Figure 3 (c) It can be seen that the influence of the mooring system on heave and pitch is relatively small, and the natural periods are relatively close, with some natural periods exceeding 20s and some pitching natural periods exceeding 30s. Figure 3 (d) It can be seen that the tension-leg mooring system has a large restoring stiffness in the yaw direction, and the yaw natural period is 47.5 s, which is much smaller than the natural period of the catenary mooring system, but still significantly deviates from the period range of the main wave energy distribution.

[0042] Without loss of generality, the dynamic responses of the two mooring systems under the 50-year return sea condition (LC3) will be compared and analyzed below. Figure 4 (a)~ Figure 4(h) shows the time history curves and power spectrum density comparison of the floating body motion response under the two mooring system configurations. The power spectrum density on the right corresponds to the motion response on the left. As can be seen from the figure, the mooring system has a relatively small impact on the floating body's wave-frequency motion, but a significant impact on low-frequency motion. Specifically, the mean longitudinal surge motion of the floating body under the tension leg system configuration is greater than that of the catenary mooring scheme, and the wave frequency fluctuations of the two have similar fluctuation trends. See Figure 2 The mooring restoring force information in (b) shows that the relatively large mean surge value of the TLR mooring is primarily due to its smaller restoring stiffness. The power spectral density (PSD) plot shows that the wave-frequency motion responses of the two mooring system schemes are essentially the same, while the TLR mooring exhibits more prominent low-frequency surge motion, reflecting the design concept of the TLR mooring system, which emphasizes "softness overcoming rigidity." In terms of heave, the heave responses of the two mooring systems are similar, dominated by wave-frequency motion. Overall, the TLR mooring exhibits slightly smaller heave fluctuations, primarily due to the larger vertical component of tension in the TLR mooring system, which has a certain degree of suppressive effect on heave motion. In terms of pitch, the TLR mooring scheme exhibits a slightly greater overall response than the catenary mooring system, with similar wave-frequency fluctuations between the two systems. The frequency spectrum shows that the TLR mooring scheme exhibits slightly less wave-frequency pitch motion than the catenary mooring scheme, but is more pronounced in low-frequency pitch. In terms of yaw motion, the yaw motion mainly manifests as low-frequency motion. The yaw motion range of the tension leg mooring scheme is much smaller than that of the catenary mooring system scheme, which has good advantages for the yaw control and power generation of floating wind turbines.

[0043] Figure 5 (a)~ Figure 5(f) Comparison of the time-history curves and power spectrum densities of the tension responses of the three mooring cables under the two mooring system configurations is shown, where the power spectrum density on the right corresponds to the tension response on the left. As can be seen from the figure, the tension response characteristics of the quasi-TLE mooring and catenary mooring systems differ significantly. For the catenary mooring system, the mean and extreme values of the tension of the mooring cable on the wave-facing side are much greater than those of the mooring cable on the leeward side. In contrast, the mean tension of the different mooring cables in the quasi-TLE mooring system is close to the weight of the weight block, exhibiting small fluctuations around the mean, with the amplitude and phase of the fluctuations being essentially consistent. In terms of maximum value, the maximum tension response of the catenary mooring system is much greater than that of the quasi-TLE mooring system, requiring a thicker and longer mooring design. In terms of fluctuation amplitude, the tension fluctuation amplitude of the quasi-TLE mooring system is smaller, which is advantageous for fatigue damage. From the spectrum, the tension response of the catenary mooring system includes both low-frequency and wave-frequency components. Wave-frequency motion is the dominant factor in mooring tension on the wave-facing side, while low-frequency motion is the dominant factor on the leeward side. This further illustrates the challenges that wave-frequency motion poses to mooring design in shallow water. For tension-leg-like moorings, the mooring tension response is dominated by wave-frequency motion, with the influence of low-frequency motion being almost negligible. However, due to the pulley-weight design of tension-leg-like moorings, fluctuations in mooring tension are primarily due to the inertial force of the weights caused by wave-frequency motion. There is no situation where the mooring system rigidly resists the wave-frequency motion of the floating body, thus avoiding excessive mooring tension response.

[0044] Figure 6 (a)~ Figure 6 Figure (f) compares the dynamic responses of the wind turbine under the two mooring system configurations, including rotor speed, pitch angle variation, generator power, and rotor aerodynamic thrust, torque, and bending moment. It can be seen that under the same incoming wind and wave conditions, the corresponding curves for the W-CMS and SC-TLMS exhibit roughly the same trend, indicating that the different mooring systems have relatively little impact on the overall dynamic behavior of the wind turbine. Overall, the rotor speed, pitch angle, and generator power fluctuations for the SC-TLMS mooring configuration are slightly smaller than those for the W-CMS configuration, which is advantageous for addressing wind turbine structural fatigue loads and grid surges. Similarly, the peaks and valleys of rotor aerodynamic thrust and aerodynamic torque for the SC-TLMS mooring configuration are slightly smaller than those for the W-CMS configuration. This indicates that the dynamic response of floating wind turbines is significantly affected by the wave-frequency motion of the float and less so by its low-frequency motion. The relatively small wave-frequency motion response of the float in the tension-leg-like mooring scheme is beneficial for the wind turbine control system, structural fatigue, and power generation efficiency.

[0045] For shallow-water, deep-floating wind turbines, particularly those designed with tension-leg platforms (TLPs), tidal fluctuations significantly impact the static and dynamic responses of the mooring system. To analyze the mooring system's adaptability to tidal fluctuations, coupled numerical simulations were performed with water depths increased and decreased by 2 meters, while maintaining the mooring design (e.g., mooring line length and anchor point locations) unchanged. Figure 8 Statistical comparisons of the float motion response and mooring tension response for two mooring schemes under three water level conditions are presented. MSWL represents the medium water level condition, i.e., the original water depth of 50 meters, while LSWL and HSWL represent the low water level (48 meters) and high water level (52 meters), respectively. The figure shows that, unlike conventional TLP mooring, water level changes have relatively little impact on catenary mooring systems and tension-leg-like mooring systems. Specifically, as water level increases, the mean and maximum values of the catenary mooring float's surge motion decrease, while those of the tension-leg-like mooring scheme increase. This is because, while the mooring length remains constant in the catenary mooring scheme, the mooring pretension increases, while the length from the fairlead to the anchor point of the tension-leg-like mooring scheme varies with water depth. As shown in the restoring force formula, for the same mooring restoring force, the horizontal displacement is proportional to the length from the fairlead to the anchor point. Water depth changes have little impact on the heave and pitch motion responses. The change of water depth has little effect on the mean of yaw motion, but has a certain impact on the yaw response range, and its influence pattern is similar to that of longitudinal surge.

[0046] For the mooring tension response, Figure 7 (a) with Figure 7 (b) It can be seen that for the catenary mooring system, as the water level increases, the mean mooring value on the wave-facing side increases slightly. The reason is that the pre-tension increases due to the unchanged mooring length, but the maximum mooring tension decreases significantly. The reason is that the greater the water depth, the more significant the catenary effect, which reflects the limitations of the traditional catenary mooring system in shallow water conditions. For the tension-leg mooring system, although the water level changes, the displacement range of the suspended weight remains within the upper and lower limit springs, and the effect of water level changes on the mooring tension is almost negligible. This shows that the tension-leg mooring system design can better adapt to tidal changes in shallow water conditions compared to the traditional tension-leg mooring system.

[0047] In order to further analyze the influence of different wind and wave environmental conditions on the dynamic response of anchored floating wind turbines, the three environmental conditions listed in Table 1 are used to perform numerical simulation analysis on the two mooring schemes. Figure 8 (a) with Figure 8(b) Comparison of the statistical results of the floating body motion response and mooring tension response for catenary and tension-leg-like mooring schemes under different environmental conditions is presented. Table 2 shows that the parameters of the incoming turbulent wind are the same in conditions LC1 to LC3, and the average wind speed is slightly greater than the rated wind speed. Under these conditions, the floating wind turbine experiences a higher aerodynamic thrust, while the wave period and wave height increase sequentially. The figure shows that the mean value of the surge motion response varies little from LC1 to LC3, indicating that the load causing the average drift of the floating wind turbine primarily comes from the rotor wind load. The maximum surge response occurs in the 50-year condition LC3 and is significantly greater than the maximum surge response of LC1 and LC2, indicating that the amplitude of the surge reciprocating motion is closely related to the wave parameters. For heave motion, its mean level depends primarily on the difference between gravity and buoyancy, with little correlation to wave and wind parameters. Its motion amplitude is closely related to wave height. For pitch motion, the mean variation in pitch is essentially negligible, indicating that the overturning moment causing the floating wind turbine to tilt primarily comes from the rotor wind load. Similar to surge motion, the maximum pitch motion response occurs in the LC3 condition but remains within 10 degrees, meeting the design requirements for pitch angles in most wind turbines under extreme sea conditions. The mean and maximum yaw motion values are less affected by wave parameters, and the yaw motion load primarily comes from the rotor deflection moment.

[0048] Regarding the mooring tension response, the maximum mooring tension response on the wave-facing side (cable #1) of the catenary mooring increases rapidly from LC1 to LC3, indicating that the influence of wave-frequency motion on the catenary mooring tension exhibits nonlinear characteristics. Although the maximum tension of the tension-leg-like mooring is also closely related to the wave-frequency motion response of the floating body, the change in its tension response is mainly due to the inertia force of the weight. Therefore, the relationship between the maximum tension response and wave-frequency motion is more linear. Therefore, compared with the catenary mooring system, the tension response of the tension-leg-like mooring system has better adaptability to extreme environmental conditions.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A tension leg mooring system suitable for shallow water deep floating structures, characterized in that: include: Anchor foundation, mooring cable, limit spring and suspended weight; the anchor foundation is fixed to the seabed to resist horizontal and vertical mooring forces; three columns are arranged around the floating body, and ballast compartments are arranged inside the columns. Each ballast compartment is provided with a suspended weight that slides in the vertical direction, and limit springs are installed at the top and bottom of the ballast compartment. One end of the mooring cable is connected to the anchor foundation, and the other end is connected to the suspended weight.

2. A tension leg mooring system suitable for shallow water deep floating structures according to claim 1, characterized in that: The mooring line is connected to the fixed anchor through a ring structure.

3. The tension leg mooring system suitable for shallow water deep floating structures according to claim 2, characterized in that: The weight of the suspended weight is 30%-50% of the breaking strength of the mooring line, and the stiffness of the limit spring satisfies that the maximum compression length under the action of the suspended weight does not exceed 2 meters.

4. A tension leg mooring system suitable for shallow water deep floating structures according to claim 1 or 3, characterized in that: It also includes a pulley block installed in the buoy, and the mooring cable passes around the pulley block to change the arrangement path of the mooring cable.

5. The tension leg mooring system suitable for shallow water deep floating structures according to claim 4, characterized in that: A chock is provided at the bottom of the floating body, and a guide wheel is provided at the position of the chock for guiding the arrangement path of the mooring cable.

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