Floating Solar

When engineers come up with a floating solar photovoltaic (FSPV) system, the majority of the obvious complexity is above the waterline.

Design teams are usually concerned with:

  • Solar panel arrays
  • Floating pontoons
  • Electrical cabling
  • Inverters and transformers

But the stability of the whole installation in the long run is determined by something that cannot be seen, the state of the bottom of the reservoir beneath the system.

The nature of the reservoir beds directly affects the performance of anchoring systems with time. It is these conditions that dictate that anchors will stand steady after decades of environmental loading or they will slowly start to:

  • Sink
  • Tilt
  • Migrate
  • Lose holding capacity

In a floating solar project or application where the lifespan is projected at 25 years or more, knowledge of underwater geology is not an option. It is a structural necessity.

The article discusses the impact of three important factors on anchoring stability of floating solar reservoirs:

  • Sediment composition
  • Sedimentation and erosion
  • Reservoir bathymetry

It also describes the way engineers consider these variables in creating effective anchoring systems.

A key fact can be discerned in most FSPV projects:

The most common — and avoidable — causal factors of anchoring system failure is a lack of geotechnical investigation.

The importance of Reservoir Bed Conditions

A system of anchoring can only be as good as the ground on which it is embedded.

In offshore sectors like oil and gas, large scale geotechnical exploration is the norm. Floating solar is, however, a relatively new industry.

The reservoir beds are hardly homogenous. They are usually made of layered materials e.g.

  • Organic silt deposits
  • Clay sediments
  • Sand layers
  • Compacted gravel
  • Exposed bedrock

All these materials respond differently in the event of structural loading.

With time, these materials too evolve as a result of sediment deposition, erosion and consolidation.

Owing to this, the performance of the anchoring systems is subject to three key environmental factors.

The Three Reservoir Bed Variables

  1. Bed Material Strength

Establishes the capability of anchors to resist loads.

  1. Sediment Dynamics

Continuous deposition and erosion influence anchor embedment depth.

  1. Bathymetry

Anchor layout and mooring geometry are affected by the shape of the underwater terrain.

An efficient anchoring system design should consider the three variables prior to installation.

Reservoir Bed Materials: The Ground Beneath the Water

The reservoir floors reflect natural geology and historical sediment.

The materials under floating solar projects are relative to:

  • The upstream watershed geology.
  • The age of the reservoir
  • Inflowing river sediment loads.
  • Reservoir working patterns.

Due to it, engineers often come across the layered sediment structures under a floating solar site. The following is a simplified description of typical reservoir bed materials and their effect on anchoring design.

To choose the proper anchoring technology, it is important to understand which material predominates the reservoir bed.

The Puzzle of Soft Sediments

Silt and clay are some of the most difficult environments in which anchoring systems have to work.

These materials usually contain:

  • Low shear strength
  • Low bearing capacity
  • High water content

Anchors in soft sediment may slowly:

  • Plunge deeper in the reservoir bed.
  • Tilt under uneven loading
  • Migrate gradually due to recurrent environmental forces.

These motions usually arise because of cyclic loading, caused by waves of wind and water currents acting on the floating structure.

Engineering Response

In the case of soft sediments, engineers often choose helical screw anchors.

These anchors create holding capacity by:

  • Rotational embedment
  • Helical plates which resist uplift and lateral movement.

Screw anchors can work effectively even in soft ground when deepened to reach firmer layers of sediment.

Sedimentation: An Ongoing Process

Sedimentation refers to the process of accumulation of particles that are carried by rivers and run-offs over time.

This process is continuous in most reservoirs.

The tiny particles like silt and clay gradually accumulate on the bottom of the reservoir creating new layers of sediment.

The rate of sedimentation depends on a wide range of factors including:

  • Catchment erosion rates
  • River sediment loads
  • Rainfall patterns
  • Reservoir management techniques.

At some reservoirs, the annual sedimentation is just a few millimetres. Some other ones can gain a few centimetres annually.

Even minor annual deposition rates will cause substantial changes to bed conditions over a 25 year floating solar project lifetime.

Effects of Sedimentation on Anchoring Systems

Anchors are most commonly affected by sedimentation via a gradual burial.

When the sediment builds up around the anchor hardware, a number of changes take place.

Mooring Geometry Changes

Buried anchors can change the mooring line connection angle to the floating array.

This shift has the potential of redistributing loads in the whole mooring system.

Increased Tension Forces

In the event that anchor positions are eventually changed as a result of sediment deposition, mooring lines can develop increased loads than initially anticipated.

Maintenance Challenges

The buried anchors in question are much more difficult to check and maintain.

Accessing them may require:

  • Professional diving teams
  • Remotely operated vehicles (ROVs).
  • Underwater specialised equipment.
  • These complicate the maintenance.

Sediment Erosion: The Opposite Problem

Whereas anchors can be buried during the process of sedimentation, they can be revealed through erosion.

Sediment erosion happens when there is powerful currents sweeping away materials around anchor foundations.

If erosion continues:

  • The depth of anchor embedment reduces.
  • The stability of the structure becomes weak.
  • Failure risk increases

This is commonly related with scouring.

Scouring Around Anchor Foundations

Scouring is a phenomenon taking place when water flow strips the sediment around the structural elements.

Mooring chains and anchors may change the flow field around them.

Examples include:

  • Chains that divert water flows.
  • Cables that form turbulence around the reservoir bed.
  • Localised flow velocity increases.

When the process of sediment erosion starts, it may gain momentum.

A bare anchor will lose the stabilising pressure of the surrounding sediment, and its holding capacity would decrease.

Scouring can also increase due to reservoir operations.

Drawdown Events

When the water level in the reservoir falls at a high rate, there can be intense currents as water is directed out to outlet structures.

These currents are capable of enhancing sediment erosion along anchors.

Frequent underwater inspection is necessary to prevent early scouring before severe damages are experienced.

Bathymetry: The Underground Landscape

Bathymetry is three-dimensional mapping of the underwater topography.

Reservoir bathymetry can be a reflection of the pre-reservoir topography.

The common underwater characteristics are:

  • Shallow shelves by the waterline.
  • Sloping valley sides
  • Deep central channels
  • Former riverbeds

These characteristics play a big role in the design of anchoring systems.

Bathymetric Factors Which Influence Mooring Design

There are a few bathymetric features that have a direct impact on anchoring layout.

Underwater Slopes

The anchors on sloping soil are subject to unequal loading.

This imbalance can lead anchors to:

  • Rotate
  • Slide downslope
  • Gradually shift position

Depth Variation

Reservoirs commonly exhibit high depth variations over a floating solar field.

This will result in disproportionality of mooring lines and tension distribution.

Engineers solve this issue by making variable-geometry mooring systems.

Sediment Distribution

Bathymetric survey with sediment sampling shows the locations of soft sediments and hard substrates.

This data aids engineers to lay anchors in areas that have maximum holding capacity.

Bathymetric Survey Methods

The new floating solar developments are based on the high-resolution bathymetric survey.

Common techniques include:

  • Single-beam echo sounding
  • Multibeam sonar mapping
  • GPS-referenced survey boats.

These surveys produce step by step maps of the bottom of the reservoir.

This information is utilized by engineers in the design of mooring layouts and in setting baseline conditions to be used in long-term monitoring.

Reservoir Beds Are Not Static

The fact that reservoir beds keep changing is one of the most significant factors to be considered when designing anchors.

A number of processes change bed conditions with time:

  • Sediment deposition
  • Sediment erosion
  • Consolidation
  • Redistribution associated with water movement.

Sediment Consolidation

New sediments are usually rich in water.

With time, the overlaying sediment load puts pressure on the material.

The expulsion of water in pore spaces increases the strength and density of sediment.

The result of this consolidation is a change in the interaction between the anchors and the reservoir bed.

It enhances holding capacity in certain situations. In other ones it changes load geometry and loads mooring components with higher stress.

Due to these shifts, engineers are forced to come up with anchoring systems that will be stable decades after they are put into use.

Engineering Strategies for Managing Reservoir Bed Risks

The problems related to the reservoir bed conditions are well comprehended.

Effective floating solar projects can address these risks in 3 phases.

Phase 1: Pre-Construction Research

Design of anchoring ought to be preceded by a thorough geotechnical study.

Common investigations comprise:

  • Bathymetric mapping
  • Sediment core sampling
  • Sediment property testing at the laboratory.
  • Lab results measure:
  • Shear strength
  • Compressibility
  • Consolidation behaviour

Hydrodynamic modelling is also carried out by engineers in certain scenarios to determine the patterns of sediment transport.

Stage 2: Anchoring System Design

Engineering design directly takes into account the outcomes of site investigations.

Engineers determine:

  • Appropriate anchor types
  • Anchor embedment depth
  • Mooring line geometry
  • Anti-scour protection measures.

In uncertain reservoir conditions, the engineers can include design redundancy as follows:

  • Additional anchors
  • Higher safety factors
  • Enhanced scour protection

Stage 3: Operational Monitoring

Even properly designed anchoring systems need continuous monitoring.

Best practices include:

  • Periodic bathymetry surveys.
  • Inspecting anchors under the water.
  • Measurement of mooring line tension.

These checks assist in establishing early warning signals, including:

  • Anchor burial
  • Sediment erosion
  • Mooring line fatigue

Early intervention helps avoid minor geotechnical alterations that can turn into the large-scale structural failures.

Conclusion: Understanding the Invisible Foundation

Floating solar projects rely on the conditions that are usually concealed beneath the water surface.

The anchoring stability depends on the reservoir bed conditions by:

Substrate strength

Sediment accumulation

Scouring processes

Changing mooring geometry

All those mechanisms are foreseeable and controllable, however, only when they are examined properly by engineers prior to the start of installation.

Those floating solar projects that are reliable in their 25-years or longer service are those that are willing to invest in the early knowledge of the geology of the reservoirs.

In floating solar engineering, the stability of the whole system depends on the quality of the underwater investigation.

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