Monthly Archives: January 2020

Slingers & trenching machines in wind farms: a guest post by Christopher James

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After posting this article on Slingers I have been contacted by Christopher James, an expert on the topic with an exceptional amount of real world experience.

He has been so kind to share is knowledge on the theme and I am thankful for that. I am sharing it with you in the very comprehensive post below with almost with no edits.

You can reach him via LinkedIn or following one of these links:

https://slingers.com

http://buckeyetrenchers.com

(Beginning of guest post)

This video link shows some of the applications of the Slingers working on wind farms.

Slingers are high speed material placing machines.  By introducing them to backfill cable trenches most companies can remove one piece of earthmoving equipment, one operator and two or more labourers.

While doing all of this they are able to increase production of over 10 times faster than some operations and while almost reducing waste of the imported material.  In the  video below these speeds of backfilling are real time speeds.  The machine was backfilling cable trenches on a wind farm in NSW, Australia.

 

Speed is obviously an advantage but it is the ancillary savings like reduced labour and equipment.  Also the waste of the imported material is huge with traditional methods.  In some cases on large scale wind farms we are able to off-set the cost of a Slinger to just the savings alone…all on one job.

These machines can move up to 3.5 cubic meters per minute.  This is in a perfect world and perfect conditions.  Generally speaking a good average to work on is 8 to 10 meters per minute on most 400mm wide trenches.  As you can probably tell this is huge production compared with some of the old methods.

This wind farm in NSW where we brought this TR-30 Slinger (rubber tracked with cab) they were using a 25 tonne articulated dump truck and a 20 tonne excavator.  The excavator would scoop material out of the truck and place into the trench.  We were around 15 times faster than this operation with the obvious one less machine and operator.

I have also added the below the link to a video by Buckeye Trenchers working on the cable trenches on wind farms in the USA.  We are agents for Buckeye Trenchers and these really are high speed trenching machines.  A Bucketwheel trencher is suited to soils or small stoney ground.  They are not suited to rock at all.  Once you hit rock you have 2 options, either a Chain Trencher or a Rock Wheel trencher.  Either way there are options.

A chain trencher is one of the most common trenchers around the world, as seen in the Vermeer video link below.  These units can range in trenching widths from 300mm up to over 2 meters wide.  Ranging in weight from 20 tonne up to over 200 tonne.

A rock wheel trencher as shown in the video below by Vermeer will generally only go up to 350mm wide trench.  Now I have seen up to 450mm wide but not a very common machine.

Next you step up into the “dig, lay, bury” machines.

Rivard make a unit in the attached video. These are great units for single pass operation.  From experience though if one link in the chain stops, like the trencher or the cable spooling and so on, everything stops.  This can be quite costly.

From my experience having crews working on multiple fronts at once limit your risk and usually allows for a more productive environment.  For example a bucketwheel trencher should get around 3 kilometres of trench done in a shift, the same goes for backfilling with a Slinger (depending on many factors).

If you were doing a single pass operation you would not achieve numbers like this.

It really comes down to how much production you really want, or you can achieve.

Each operation has its pluses and minuses, no doubt about it.  As I was always taught in pipelines, get the basics right.  Get the equipment right.  Lower your risk as much as possible and then find minutes to shave off each process.  Laying cables is as repetitive as it comes, just like pipelines and it is all about streamlining processes as much as possible to get as efficient as possible.

This is why we have always used trenching machines for pipeline works.  They replace many excavators (bucketwheel trencher can replace up to 10 x 25 tonne excavators).  Trenchers also make exceptional backfilling material, excavators just cannot do this.

As a general rule (very general as all ground conditions change dramatically) please see the below for trenching equipment production:

  • Bucketwheel in good, dry soil ground conditions: 3 kilometers up day digging 400mm wide trench at 1.5 meters deep
  • Chain trencher in firm, dry, small stone ground conditions: 800 meters to 1000 meters per day digging 460mm wide and 1.5 meters deep with a 45 tonne class machine

I cannot give production in rock as it is too much of an unknown with the hardness and so on.

I have had 45 tonne class chain trenchers take 3 days to cut 4.5 meters of pink quartz and then dig 600 meters of hard limestone. It is too much of a variant to give some solid figures.

Additional links that you might find useful:

https://www.linkedin.com/pulse/where-how-use-my-slinger-episode-10-wind-farm-christopher-james/

https://www.linkedin.com/pulse/7-reasons-why-bucketwheel-trenchers-bring-efficiency-your-james/

https://www.linkedin.com/pulse/where-how-use-my-slinger-episode-2-trench-backfill-imported-james/

https://www.linkedin.com/pulse/stop-wasting-money-when-backfilling-your-trenches-imported-james/

Can a rotor be smart? Passive optimization systems

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The spectacular growth of the dimension of the wind turbine has led to the introduction of several interesting technical solutions. Different type of towers (concrete, hybrid, lattice, self-erecting, etc.) and new technical solutions for the foundations appeared in the last decade, trying to have the lowest possible cost of energy.

The rotor of the turbine has followed the same trend. With the impressive size of the blades currently in the market (today we are around 70 meters, the width of a football field) it is not surprising to see a variety of new concepts already in the market or under development.

One of the key issues of very long blades is that it is difficult to optimize them: finding the “sweet spot” for a component exposed to a variety of wind flow characteristics during its operational life is not easy, above all if such characteristics are not uniform along the blade.

The engineers designing the blades are trying to achieve several goals, such as:

  • Increase the energy production (maximizing the aerodynamic efficiency and the power extracted from the wind)
  • Reduce the loads on the structure
  • Create a solution that can easily be transported on public roads and installed with cranes already in the market
  • Extend the life of the blades (we are moving to the 25 or even 30 years mark)

All these objectives should be reached with the lowest possible cost.

The term “smart rotor” refer to a variety of technical solutions whose purpose is to increase the production or reduce the loads.

They belong to two categories, passive systems (not controlled by a software or an operator) and active systems.

Among the passive systems the most interesting are:

Vortex generators. Believe it or not, you can buy the elements generating the vortex from 3M (the same company that invented the Post It all around my desk). You can use them to retrofit pitch-regulated turbines sticking them near the root of the blade, where the air flow is separated by the blade (that is, it is “stalled”).

They basically work reducing the separation of the flow, increasing the production 1 or 2 percentage points (they can be a lot of money).

A second passive technology is the bend-twist coupling.

As you will guess from the name it creates a link between the bend and the twist of the blade, with the object to reduce fatigue loads created by sudden inflow changes during turbulent wind conditions.  

The wind blowing on the blade is creating 2 forces – “lift” (the one pulling up the blade and making the rotor turn) and “drag” (the one bending the blade backwards). As a general rule the engineers try to minimize the drag and maximize the lift, achieving a high “lift to drag” ratio.

With the bend-twist coupling the loads are reduced because the blades “adapt” its shape changing its shape and the angle of attach when deflected.

This coupling can be achieved changing the geometry of the blade (geometric coupling) or by changing the direction of the fibre inside the composite material (resin + fiber) that constitute the blade.

This interesting technology is currently being investigating by several entities, including a heavy weight such as the Fraunhofer Institute for Wind Energy Systems.

Image Copyright Mark Capellaro, 2012 Sandia Wind Turbine Blades Workshop

I also described some months ago the serrations. They have a different scope (reduce the noise) but I believe they can be consider a type of passive optimization system.

Second life: the destiny of turbine foundations after decommissioning

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I have discussed in another article the challenges associated with the disposal of the blades when a wind farm is decommissioned or repowered.

But what happen to the foundations?

The destiny of a foundation will depends on local regulations, on the environmental requirements that are normally given with building permits and on the wish of the owner of the wind farm and of the land.

As a general rule, foundations are at least partially dismantled. The first centimetres (20, 50 or even one meter) are removed and the rest of the foundation is left in place and buried below a layer of organic soil.

Sometime the entire foundation is removed. This is a complex activity, and blasting or at least many hours of hydraulic hammer are needed.

The third option is to bury the foundation below a small hillock.

In case a repowering is planned there is also a fourth and more interesting possibility: giving a second life to the old foundation integrating it in the bigger, new foundation. There is a group of company that is studying this possibility under the name “FEDRE” (Fondations d’Eoliennes Durables et Repowering – French for “Long lasting wind turbine foundations”).

The concept that is being developed is how to reuse part of the existing footing for the new foundation - adaptation the existing one on the short term and working with reusable foundations designed ad hoc on the long term.

In case the foundation has to be dismantled some difficulties may be experienced.

They are very “dense” in steel (on average a foundation can easily have more than 100 Kg of steel for each cubic meter of concrete). Due to the concentration of rebars in some areas of the foundation (above all, in the centre) it can be more difficult and time consuming to separate the steel from the concrete.

Usually steel is separated from concrete and melted again. In some countries the reinforcement bar are even used “as they are” without being melted and reformed (i.e. they are straightened and used again in another structure).

The presence of steel makes more complicate grinding the foundation in smaller elements to use it again as a construction material, for instance to build roads (in the nucleus of the embankment) or for earthworks as a filling material.

It is worth mentioning that some with turbine have concrete tower or hybrid (concrete + steel) solution. They could be equally difficult to recycle.

Repowering and decommissioning will probably gain momentum in the near future.

2018 and 2019 saw only a few hundreds of MW decommissioned (unsurprisingly mostly in Germany, where the installed capacity is huge). However the numbers should increase steeply in the next years when more and more wind farms will end their 20 years of supported tariff.

Unless they are able to close some king of PPA (power purchase agreement with a counterpart willing to buy the electricity at a certain price) it could prove to be not economically viable to sell the power at spot prices.

 

Good vibrations: wind induced resonance and turbines oscillations

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Have you ever wondered why sometimes the wind turbines (and other similar tall structure) sometime vibrate?

Under some conditions the wind blowing on the tower create vortices.

These vortices appears regularly on both sides of the tower, creating low pressure zones first on one side and then in the other.

This beautiful sequence of vortices is called von Karman vortex street. von Karman has been a pioneer of aeronautics

For these reason the tower will start moving perpendicularly to the wind, first toward one side and after toward the opposite.

The tower has a very low structural damping – when the oscillation start its reduction is very slow because the steel tower has a limited capacity to absorb the kinetic energy.

It also has a very low natural frequency (the frequency at which the tower will tend to vibrate when subjected to external forces).

The vortices created by the wind will appear at a frequency that depends on the speed of the wind and the diameter of the tower.

The formula to calculate this frequency is very simple:

f = St · U / D

where

f is the vortex shedding frequency

St is a value called Strouhal number (in our case it is around 0,2)

U is the wind speed

D is the diameter of the tower

At a certain wind speed the vortices will appear and shed at a frequency equal to the natural frequency and the tower will resonate. The wind speed that start the resonance is called the critical speed.

If the wind blow at the critical speed for enough time the amplitude of the vibration will increase and you will see the tower oscillating.

This is a simplistic summary of a very complex phenomenon. It is however a limiting factor for the use of higher steel tower. In additional to the risk of catastrophic failure the wind induce vibrations will also generate additional fatigue loads, shortening the life of the tower.

It is also interesting to observe that a structure has more than a natural frequency.

Vibrations in the lowest natural frequency (first mode) will have this shape:

First wind turbine tower vibration mode

However, sometime a turbine can vibrate in what is called “the second mode” the second lowest natural frequency):

Second wind turbine tower vibration mode

Following this link you can see a real world example of how is a tower vibrating in the second mode.

What can we do to avoid these dangerous vibrations?

Some solutions are structural - basically aimed at increasing the damping of the tower or changing the way the mass is distributed.

It is not easy to change the mass distribution in a wind turbine tower (basically its an inverted pendulum).

Nevertheless it is possible (and it is becoming increasingly common) to install dampers in the wind turbines, either only temporarily during the installation or as a permanent feature.

Other solutions are aerodynamic - the idea is to change the shape of the tower adding elements that disrupt (or "spoil") the vortices. Conceptually they are similar to the spoilers used in cars or planes, in the sense that they are intended to mitigate an unwanted aerodynamic effect.

An example are helical strakes, sometimes called "coils" "ropes". This is a concept developed by Christopher Scruton and other scientist such as William Weaver in the fifties and sixities and used often in chimney and similar structures.

However sometimes they do not work as you can see in the video below.

In general, the effectiveness of this solution is driven by two parameters:

  1. Diameter of the strake (usually defined as a ratio of the diameter of the tower)
  2. Pitch (the distance along the cylinder axis that is needed to complete one full turn of the strake)

Definition of pitch and height

From experimental tests in wind tunnels it has been found that the optimum height of the strake is approximately 10% of the diameter of the tower (that is, around 40 cm for a standard steel tower with a diameter of around 4 meters).

For the pitch the results of the test shown an optimum value in the region between 5 and 15 diameters (that is, one full turn of the rope should be done between 20 and 60 meters).

Obviously the smaller the pitch, the more the strake is parallel to the tower.

There are however other secondary factors that influence the efficiency of this solution, such as the area of the cylinder covered by protrusions (usually called “strake coverage”) and the pattern of the ropes (usually three or four independent ropes are used to create the helix).

One of the biggest problems of the strakes is that they greatly increase the drag coefficient (the resistance opposed by the wind turbine tower to the flow of the wind).

The implication is that the loads at the bottom of the tower will increase as well, so that a bigger foundation could be needed.

For this reason strakes are normally used as a temporary solution only during the installation – above all in the most critical part of the procedure, when the tower is installed but there is still no nacelle on top.

Removable strakes are wrapped around the tower, either before the lift while the tower segment is on the ground or after installation.

Bonus: poetry & technology (from a LinkedIn post of Hervé Grillot)