Floating solar has been rising in popularity in the last few years with over 2.1 GW of installations by the end of 2020. We have seen many innovative floating systems developed to solve the challenge of supporting the PV modules on the surface of the water. This article is about the part we do not see, the anchoring and mooring system under the surface of the water.
Floating solar islands are affected by environmental loads, wind, waves, and current. The function of the anchoring and mooring system is to prevent the islands from drifting or changing their orientation under these loads.
The problem of mooring a floating structure is thousands of years old with many sound engineering practices established. The floating solar industry is adapting existing standards and guidelines on mooring from the maritime industry.
Floating solar is mainly installed in inland water bodies. The environmental loads affecting a floating structure in inland water bodies are significantly lower than those it may experience offshore. However, this type of mooring comes with its challenges:
Offshore mooring systems are generally designed for large deployed lengths of mooring wire or chain whereas inshore/inland moorings will generally have short taut lines which can lead to very high tensions and can result in uplift on anchors
High consequence of failure given the proximity to shore, other assets and limited response time
Uncertainty in the calculation of environmental forces due to wind shear effects and shallow water blockage effects
Potential difficulty in knowing the actual tensions in the lines, in other words, a lack of instrumentation
The basic design philosophy for inland/inshore mooring systems is the same as offshore applications although it should be modified to account for the main features of inland applications. Therefore, the standards and guidelines on the design of inshore mooring and floating structures in specific have proven very helpful for floating solar applications.
The process of designing a sound mooring system for floating solar is divided into six steps.
1- Establishing Operation Conditions:
In the case of floating solar, this is usually a passive, unmanned, permanent mooring system for 25 years. The term passive mooring system refers to a system where the tension/length of mooring lines cannot be adjusted.
2- Selecting the Design Criteria:
The design criteria should be established. In other words, the code or standard to which the design has been carried out.
Usually, it is necessary to draw upon more than one code to cover all of the design and practice recommendations. However, coherent input data, analysis methods, and safety factors must be used; in general, this means that these should be taken together from a single source. Combining the least conservative options from different sources is not acceptable.
3- Collecting Site Information:
This includes all the information needed for a complete mooring system design:
Location
Bathymetric report (shape of reservoir bed)
Water level variation profile
Geotechnical report (reservoir bed soil type and properties)
Surrounding topography (needed to determine wind sheltering effects and wind shear profiles)
Details of any existing installations/structures on the surface and underwater.
4- Establishing Design Environmental Conditions:
Environmental information should include:
Maximum and significant wave height
Maximum wind speed and gust in the last 100 years
Current
Minimum temperature
5- Determining Floating System Characteristics:
The characteristics of the floating structure should be established in this step. This includes:
Draught (the part of the floating system under water)
Wind load coefficients (calculated according to the geometry of the floats and PV module tilt angle using Computational Fluid Dynamics analysis)
Current load coefficients
Wave and wave-frequency response characteristics (numerical models to calculate the load of waves).
Strength and pitch of mooring points on the floating structure.
6- Designing the Mooring System:
The different mooring system components are designed and specified according to the previous steps. These components include:
Anchors: The number, type and weight, and position of the anchors is selected. Different types of anchors such as deadweight, screw anchors, or percussive anchors are available. The anchors can be located at the bottom of the reservoir, reservoir wall, or the reservoir bank.
Mooring Lines: The length, type, diameter, breaking load, and pre-tension of each mooring line are specified. There are two main types of mooring lines, static and elastic. Static mooring lines are composed of one or a combination of polyester ropes, metal wires, and metal chains. Elastic mooring lines are made from elastic mooring units based on natural or synthetic rubber and polyester ropes or metal wires. The difference between elastic and static mooring lines will be discussed in more detail in future articles.
Connecting Equipment: This includes the equipment used to connect the anchors to mooring lines, different parts of the mooring lines to each other, and the mooring lines to the anchor points on the floating platform. These have to be selected to sustain the maximum breaking load of the lines and to withstand corrosion during the 25 years lifetime of the mooring system.
This concludes the first part of our series of articles on floating solar anchoring and mooring systems. We will discuss each of the steps above in detail in the upcoming parts of this article.
HydroSolar specializes in providing floating and mooring system solutions in the field of floating solar. For more information and please contact us at info@tyt.com.tr
This article uses information and text from GL’s GUIDELINES FOR MOORINGS 0032/ND document.