Research on the safety and practicability of the combination of offshore wind and solar energy

Wind power is a renewable way of power generation. It features simple scale construction, considerable efficiency and clean and pollution-free. The current situation of offshore wind power generation is also facing many challenges, safety, economy, feasibility. In this paper, the design method is adopted and the structure is checked and analyzed, and the current problems and solutions of offshore wind power generation are analyzed from the perspectives of structural safety, cost estimation and design feasibility. Through practical design and calculation, this study makes a breakthrough in the structure of offshore floating wind power generation, and provides a solution for the spar structure to limit the sea state.


Introduction
The ocean takes about 66% of the earth surface area, there are many clean energies, including wave energy, tidal energy, temperature difference energy, salt difference energy and offshore wind energy.To mitigate the gradual depletion of non-renewable energy sources, new distributed clean energy is integrated into the distribution network are gradually becoming research priorities in countries around the world.In recent years, wind power generation has become an important part of China's energy structure, with an influence that cannot be ignored [1].In the all kinds of clean energy, wind energy is the most common, which has great potential of development [2].The wind is much stronger on the sea than land, and the power of offshore wind generator is more efficient.But at the same time, compared with onshore wind power, there are lot of retractions of offshore wind power.Wind power plant is requested for higher level for the stability of structure because there is higher wind speed on the sea level.In addition, wave load also is an important limitation.Compared with onshore wind, offshore wind is operated in a more extreme environment.Tempering must always be done in wet, cold, offshore, strong winds and corrosion resistant conditions, with higher safety and stability requirements [3].Among many marine green energy sources, some of them have low energy and high cost, which cannot supply a lot of household and factories electricity.However, there are some energies such as wave energy, tidal energy and wind energy with relatively high power and feasibility.In terms of power investment, recently, the completed investment in clean energy such as hydropower, nuclear power and wind power accounted for 91.7% of the completed investment in power supply [4].Tidal power generation is an important form of using tidal energy, which harness the energy of the periodic ebb and flow of ocean water.At high tide, the kinetic energy in seawater is converted into potential energy, and at ebb tide, the potential energy is converted into kinetic energy.Tidal power generation requires a strict geographical location, in usual, people will set up the dam beside the bay as the reservoir to stockpile plenty of seawater.therefore, tidal power can be harnesses to its full potential only when the energy is concentrated and in the right geographical location.Wave energy generation is an effective way to use wave energy.Because wave energy is a type of potential energy, the main method is to harvest energy and convert energy.In China, there are about 2000 kW available tidal energy, and the wave energy is about 2300 kW to 3500 kW.However, the technical development of wind energy resources within 5-50 m offshore water depth is 500 million kW [5].In 2021, 84 new offshore wind farms with a total of 18.5 GW will be put into operation, including more than 3,400 offshore wind turbines, with a cumulative production scale of 58% higher than that in 2020 [6].Thus, offshore wind energy has huge sustainability.Meanwhile, since the sea surface is wide and having no few buildings in the way, developing photovoltaic power is also an efficient and feasible choice.The integrated development and utilization of offshore wind power, photovoltaic and other energy sources will also be an important development direction in the future.In an offshore wind farm, photovoltaic power stations and energy storage power stations are connected to the urban power grid, and it is bound to have a positive impact on the power supply reliability of the grid [7].In this study, a structure of offshore wind and photovoltaic power generation is designed, and the total power is not less than 12 MW.The feasibility and economic issues are analysed.

Structure and shape design
This design mainly involves several parts including independent offshore wind motor, floating platform and solar photovoltaic power generation panel.The floating platform includes a tower and a buoy.The size selection of the tower shall be based on the local sea conditions.It is known that the local water depth is 100 m, the maximum wave height is 9 m, the period is 12 s, and the wavelength is 120 m according to the dispersion equation.For security considerations, the tower size should avoid its wavelength.Secondly, due to the wave height of 9 m, the distance between the lower surface of the corresponding floating platform and the sea surface should be greater than 9 m to avoid the impact of seawater rushing onto the platform on the power generation efficiency of photovoltaic panels.For the selection of photovoltaic panels, considering the limited area of floating platform, in order to maximize the power, it is necessary to improve the power generation efficiency.After investigation, it was found that the common photovoltaic panels on the market are in 100 W/m 2 , and the design is 200 W/m 2 photovoltaic panels, and the best tilt Angle of 40°.According to the illumination conditions in our country, the best orientation of photovoltaic panel is due south, but considering the interference of external factors such as wind and wave on the sea, the rotary support is equipped for photovoltaic panel, which can take the most appropriate measures according to the local illumination conditions.

Economic
The construction cost of offshore wind power is mainly composed of construction investment and operation and maintenance costs, among which the construction vessel and machinery equipment cost accounts for more than 60% [8].For floating independent wind turbine, its construction cost is much higher than that of small wind turbine with the same power.In addition, considering the difficulty of ocean engineering operations, the transportation and labour costs are higher than those on land, so the proportion of labour costs should be increased in cost accounting.

Stability checking calculation
This design uses the spar structure, which is characterized by the center of gravity is lower than the center of buoyancy, but because the offshore fan belongs to the towering structure, when it floats on the sea, the center of gravity is much higher than the center of buoyancy, so the need for pressure load to make its center of gravity lower than the center of buoyancy.The concrete method of pressurizing load is to inject C50 concrete into the bottom of the buoy so as to meet the requirements of stability checking calculation.The following is the calculation process of stability check.
In formula (1),   is the moment of inertia of the profile with respect to the central axis, B is the width of the structure, and L is the length.Since the buoy designed in this paper is a cylinder, both length and width are taken as diameters D.
In formula (4), BG is the height difference between the design center of buoyancy and the center of gravity, and GM is the initial stability height.GM>0, based on the stability requirements of spar structure, GM>0 should be made, the stability check is qualified.

Load calculation 2.4.1. Calculation of wind load.
The calculation of wind load refers to the calculation specification of ABS (2014) edition, which refers to the following formula.
=        0 (5) In formula (5),   is the wind vibration coefficient at the height z;   is the shape coefficient of wind load of each tower segment.  is the change coefficient of wind pressure height at the corresponding height. 0 is the basic wind pressure, / 2 [9].As the fan adopted in this design is as high as 180 m and the blade tip is as high as 274 m, the influence of height on wind speed should be considered in the calculation of wind pressure.This design adopts the maximum wind speed of 15 m/s at 10 m water surface.SACS is used to automatically calculate wind load on components above the water surface and user-defined wind area.The wind pressure p on each component or wind area is calculated according to the following formula [9].
In formula (6), V is the wind speed at the component, which is obtained from the wind shear function at the corresponding height;  ℎ is the height coefficient and exists only when the ABS specification is used,   is the shape coefficient, which is 0.5 for the round tube and 1.5 for other components and planes.Coefficient 0.003 38 takes into account the system of parameter units and air density [9].It should be noted that for low buildings and structures do not need to consider the wind vibration coefficient and natural vibration frequency, the range is below 30m.For tall buildings and structures, the wind vibration coefficient and natural vibration frequency should be considered.For low buildings, natural vibration period is 0.25 s, damping ratio is 0.01, and wind vibration coefficient is 1.The photovoltaic panel and floating platform in this design are within this standard, so the above values of these two structures are consistent with the above principles.

Calculation of wave load.
The design of the wave load calculation adopts the following standard calculation.
In formula (7),   is the maximum wave height,   is the maximum period, d is the wave height.Among them, as the maximum wave height of the design area is 9 m, η is taken as 1/2 wave height.According to the standard of Norwegian Register of Shipping, the wave type of the above sea state is judged as linear wave.The stable flow towing force coefficient   is the premise for calculating CD and CM, which can be calculated by the following formula [10].
In formula (8), k is the surface roughness, and the newly sprayed steel structure can be assumed to be smooth.When Marine organisms grow, k=0.005 ~ 0.05 m is assumed [10].

Structure and shape
This design chose a free-standing offshore wind generator which power is 10 MW, the fan blade length is 94 m, a single blade weight 35 t, rotor diameter of 193 m, sweep area of 29,300 square meters.The tower barrel is 180 m high, 8 m in diameter, 274 m in blade tip height, and the weight of the whole fan is about 695 t.The tower is a triangular truss structure with three pontoons at three vertices.The height of each buoy is 45 m, the upper part is 35.5 m and the diameter is 15 m.The lower part is 7.5 m high and 20 m in diameter, and the weight of the whole tower is about 863 t.The upper surface of the triangular tower is laid with photovoltaic panels, photovoltaic panel power is 200 W/m 2 , each photovoltaic panel area is about 208m 2 , a total of 69 photovoltaic panels, and the Angle of the sea level is 40°, the total power generation is 2.88 MW, each photovoltaic panel has a support below the rotation Angle can be adjusted, the total weight is about 14.4 t.Therefore, the total weight of the whole floating structure, including the fan, tower and photovoltaic panel, is approximately 1,572.3 t.The outline design of the floating wind motor can be seen in figure 1.

Figure 1. Shape design.
The design parameters and dimensions of the floating generator are summarized in Table 1 to 4 Due to the stipulated water depth of 100 m, the sea condition once in 50 years and the maximum wave height is 9 m, the photovoltaic panel is finally 20.5 m higher than the sea level after ballast application and stability checking calculation in this design.The photovoltaic panel is 16 m away from the wave whose maximum wave height is 9 m.Under the influence of extreme weather, waves will not wash up on the photovoltaic panels, causing safety risks.In terms of safety, the design of the floating platform adopts spar structure, that is, the centre of gravity is lower than the centre of buoyancy structure, so the structure has good stability, can withstand extreme weather wind load and wave load.For a 10 MW single wind motor, the installation and construction are difficult, but the reason why do not choose a number of small wind motors to form wind turbines, is that considering the combination of scenery, floating platform should not be too large, in the limited space to arrange a small power generator, between the blades of a single generator will cause interference, thus reducing the efficiency of power generation, therefore, this design uses 10 MW independent wind motor.The side length of the floating platform is 152 m, and the fan is located in the middle of the platform.The distance between the fan and each side is 43 m, which greatly increases the difficulty and risk factor of installation.From the perspective of construction, for the prefabricated structure, the most important thing is installation and transportation, and this design for installation and transportation test are great, which greatly increases the transportation cost and labour cost.For the installation of offshore wind power generation combined with solar power generation, it cannot be completely referred to the installation of only offshore wind power generation, because the latter does not need to consider the installation location of photovoltaic panels, while the former needs to consider the installation location of photovoltaic panels and power generation efficiency, such a design is less effective.

Economic
The economic analysis results can be seen in Table 5.The labour cost is calculated as 25% of the cost of each kW, which is about 4,899 yuan /kW.The final estimate is 24,496.6yuan /kW, and the total cost is about 315 million yuan.The existing domestic offshore wind power cost about 15000~17000 yuan /kW, so in general, the design cost is on the high side.From the point of view of feasibility, the biggest defect of this design lies in the difficulty of installation and construction.First of all, the construction of floating structures at sea is generally in the dock, and the long-span steel structure adopted by this floating platform is up to 170 m in length, so it is difficult to find a matching dock.

Stability check
This design adopts the floating platform spar structure, which requires that the centre of gravity of the platform is lower than the centre of buoyancy after loading.Such a structure has the advantages of good stability, strong wind and wave resistance, but the disadvantage is large consumables and high requirements for local sea conditions.Spar type platform is one of the most popular floating fan platforms at present.The centre of gravity of the platform is below the centre of buoyancy through ballast, and the waterplane is relatively small, so it has good stability and hydrodynamic performance [11].As shown in Table 6, the loading weight is 47,860.20 t, the total mass after loading is 50291t, the drainage volume becomes 47896.20 m³ , the centre of gravity becomes 7.21 m, and the centre of buoyancy is 12.25 m.The formula of initial stability height is as follows.It is calculated that the height of the centre of gravity is 48.7 m before the pressurized load, while the centre of buoyancy is 1.86 m underwater, the centre of gravity is much higher than the centre of buoyancy, the centre of gravity of the structure is too high, the draft depth is shallow, and it cannot bear the wind load and wave load of the current sea condition, so the structure is applied ballast.The loading method is to use C50 concrete for ballast and fill the bottom of the three pontoons with concrete so that the centre of gravity is lower than the centre of buoyancy to meet the stability checking calculation requirements of the spar structure.The height of centre of gravity and centre of buoyancy after loading are summarized in Table 7.
The loading weight is 47,860.20 t, the total mass after loading is 50,291 t, the drainage volume becomes 47,896.20 m³ , the centre of gravity becomes 7.21 m, and the centre of buoyancy is 12.25 m, so it can be calculated that GM is 4.57, and the stability checking calculation is qualified.

Wind load
The following is the calculated value of wind speed and final wind load at different altitudes based on the above formula, and the results can be seen in Table 8.The maximum wind speed chosen in this study is 15 m/s.It is calculated that the overall wind load of the structure is 2,228.23 kN, and the structural unit wind load is 152.39N/m 2 .

Wave load
The following is the value of wave load calculated according to the above formula, and the results can be seen in Table 9.
The results of the wave load is 41,116.38kN , and the wave load per unit area is 33,344 N/m 2 In this research method, the wave shape in the calculation of wave load is approximated as a cosine trigonometric function, and the period is 12 s.The wavelength is about 120 m according to the dispersion equation.In this design, the total length of the floating platform tripod and buoy is about 180m, which avoids the range of wavelength and makes the structure more secure.

Discussion
Through this design and research, reflected a lot of difficulties and problems of sea scenery combined project.The first is safety, and safety is divided into construction safety and structural safety.Among them, the structural safety includes the stability of the structure, whether the structure can withstand the wind load and wave load at sea, as well as the local sea conditions which is suitable for the construction of wind motors.For stability calculation, the spar structure is adopted in this study, but the draft depth of spar structure is generally above 30 m, so the requirements for local sea conditions are higher.The ocean depth described in this study is 100 m, which conforms to the construction conditions of spar structure.However, if the water depth is relatively shallow, we should carefully consider whether to choose spar structure to increase its stability.Safety has been a bottleneck in the development of offshore wind power.If safety is not guaranteed, the project is not feasible.There are the following problems in the construction of offshore wind power projects: the construction scope is large, the water area is wide, the construction conditions are complex, the operation risk is large, and the safety accidents are prone to occur frequently; The construction period is long, spanning the four seasons, and the process links are many; The construction of the use of complex ships, offshore construction personnel are more and more complex, the field operation personnel mobility, uneven quality.Under the new situation, the safety supervision and management of offshore wind power construction are put forward higher requirements.In the construction process, safety supervision and management should be focused on three aspects: (human) personnel, (aircraft) ship and aircraft equipment, and (environment) weather operation environment, so as to ensure the safety and stability of offshore wind power project construction and controllable risks [12].Therefore, the feasibility and safety of construction should be considered in the design.Unlike land construction, offshore construction has a higher construction and transportation budget, so the economy of the structure should be combined with the market and local conditions.

Conclusion
Through this design and research, some problems in the safety of offshore wind power generation at the present stage are reflected.For the sea area with a depth of 100m, it is feasible to use spar structure to increase its stability.However, for the spar structure, to make it feasible, it is necessary to reduce the dead weight of the wind motor as much as possible, and for a single 10 MW floating generator, there is a problem of too much dead weight.Therefore, in order to reduce the limitation of spar structure to the sea area, it is necessary to use the wind motor with less power and less dead weight.However, the feasibility of the design of the structure itself remains to be discussed.This shows that in the structural design should not only consider the structure itself, but should pay more attention to the feasibility of construction.

Table 1 .
. The parameters of the fan.

Table 2 .
The parameters of the photovoltaic panel.

Table 3 .
The parameter s of the floating platform.

Table 4 .
The parameters of the steel tube.

Table 5 .
The price of each part of motor.