Unconventional Offshore Structures-Wind Farms

Offshore Structures have been one of the most indigenous creations of man as it gave him access to the nature’s treasury of fuel and mineral reserves present in ample amounts underneath the oceans. A very important objective behind offshore structures is that man still has his limitations at land. Venturing into the oceans for the quest of abundant resources which were by far, unavailable or scanty in land can meet most of his needs.

Figure 1. Illustration of offshore extraction ( Courtesy: www.googleimages.com)

With various types, their research and development needs a lot of work force and statistical data. Different parts of ocean have different kind of weather and wave pattern to which an offshore structure is subjected. The external forces includes waves, wind, currents and corrosion along with other environmental vagaries.We know these structures are built at higher stakes and thus must reciprocate greater revenues which needs the efficiency to be maximum.

So far we had known of conventional ones like Jacket Platforms, Rigs, Drill ships, Spars, FPSOs, TLPs, Fixed or gravity based structures. These are segregated as: movable and immovable. Most of them are dedicated to more or less the same types of purpose: exploration, extraction, processing and/or transportation of petroleum and oil products derived from the deep sea reserves. A lot of analysis is done on them including strength calculations, advanced extraction techniques, software interpretations or safer installation-cum-maintenance goals.

Figure 2.: Conventional Oil Rig (Courtesy: www.offshoretechnology.com)

 

But have we ever wondered, can offshore structures be solely employed for the sake of exploiting resources that were limited or absent on land? Can they be utilized as improvements over infrastructure that are already present on land for better outcomes?

The answer lies in certain unconventional offshore structures such as wind farms, subsea pipelines, unconventional offshore gas stations or hydel-power installations which are focused mainly on tapping abundant 'oceanic' natural resources ( plentiful, renewable) for the purpose of generation of power on a widespread scale. As we are well aware of, renewable resources such as coal, natural gas and other earth derivatives are dwindling drastically. Thus to meet the insatiable needs of the ever-inflating population, Alternative forms of natural resources should be ingenuously exploited. Some of them are already materialized, but more needs to be done for achieving a global trend. 

Here we limit our discussion only to Offshore Wind Farms- a big trend setter . Later we take some of the others.

What are Offshore Wind Farms?

 Offshore Wind Farms are exemplary modern developments of tapping wind energy at seas for widespread harnessing of power as compared to landmass.

Figure 3: Offshore Wind Farm ( Copyright: www.tokyotimes.com)

Despite wind farms being relatively old technology on land, they have gained more credibility on seas in the recent years. Have you thought why?

The wind pattern on seas or vicinal to the shoreline is much more uniform, stronger and consistent as compared to that on land surface. Thus, this can be opportune in tapping wind energy on a much wider scale wielding more influence in generation of electrical power. According to a recent study by the United States Department of Energy, up to 50% of more power can be generated by an offshore wind installation than a land installation given the same turbine capacity. This is a boon actually for us! Given our vast ocean areas, if judiciously tapped, offshore wind farms can supplement a huge stake of our global energy demand.


The first offshore wind farm technology came up in Denmark in 1991. Since then, it is having a dramatic surge, especially in the Western frontiers. Till now about 4.5 GW of offshore wind installations are present worldwide, European countries being the maximum stakeholders and the Americas following close by. Around 30 GW is coming up and by 2020, an optimistic target of 75 GW is sighted. Major exploiters of them are Denmark, UK, Germany, US, Japan, China, South Korea, Belgium, Sweden, Portugal, Norway and Netherlands. India is also on the move with 100 MW planned along the Gujarat coast.

Requirements and Operation

Figure 4: Wind Farm (Copyright: www.theguardian.com)

Wind turbines convert the kinetic energy in the wind into mechanical power, which in turn generates electric power by induction. A generator can convert this mechanical power into electricity for commercial use. They are mostly designed for shallow waters, mostly continental shelf (preferably depths of 9-14 metres). Why?

• Installation procedure is cheaper and easier  
•    Moreover, building them on the continental shelf makes it relatively easier for drawing             transmission lines to the shore than from deep sea. 
•   Maintenance can be done frequently and as and when required.
•    Safeguard from adverse, unpredictable weather conditions deep sea.
•   To avoid interference with the mid-ocean traffic. 

f     Now most of the offshore wind farms have a strong pile that is driven into the seabed. This  supports the nacelle and the tower. 

Figure 5: Nacelle of a Wind Farm installation (Courtesy :Wikipedia)

      The Nacelle is a shell adjoining the blades which houses the blade hub, rotor, generator, gearbox and the remaining electronic components.The Tower is the extension of the monopile and rises vertically above the waterline supporting the entire structure. The entire setup has a height of around 200 feet with additional 80-100 feet of piling foundations below the mud line. They are mostly three-bladed with the height of the farthest tip being up to 500 feet above seabed. Following image illustrates better.

Figure 6: (Courtesy: www.offshorewind.blz)

      The blades are similar to aircraft propeller blades (aerofoil sections) which are designed to turn in response to blowing air at a relevant speed and direction, which spin a drive shaft connected to a high-capacity generator producing electricity. Operational nameplate ratings of wind farms can vary from 2 to 5 MW (Find out about nameplate ratings).  

     After Production

Once the power is harnessed, it has to be distributed to the shore substations for further use. This is the most cumbersome thing in case of wind farms as contrary to the oil platforms (where fuel could be stored or transported through ships or subsea pipelines). One essential feature is the use of Electric Service Platforms (ESPs), which act like intermediate ‘offshore substations’! They are placed in a region within the radius of vicinal substations, or reckoned as the turbine array.  The power output from the turbines is transmitted through high tension undersea cables to the ESPs. These either store or transmit the power to the grid at shore, from where it is distributed elsewhere. Underlaying of subsea pipelines is a critical task and has to be done with precision, high design reliability and safety factors.That’s the prime reason for keeping the wind farms as much closer to the shore as possible (as we had discussed before). 

Figure 7: Distribution layout of an offshore wind farm(Courtesy: googleimages)

Figure 8: An Electric Service Platform (Courtesy: www.boem.gov)

 A big problem with these offshore wind farms is that they are constantly exposed to the ocean environment. Thus their design criterion and basis for modification is different from that of onshore or land-based structures. Let’s see some of them:

• Assessment of the structural design criterion is a very difficult task as the loads on the sea are highly large and unpredictable. Prediction of the dynamic loads such as wave, wind, current, pressure loads are analysed in details. The climatic patterns or the predominant wave actions under variant sea states are studied in detail before installation.     
•  A site-based design is done for the particular installation as unlike land, the environmental condition in the waters are highly non-uniform. Thus design should always have a ‘fit’ to the location of operation. Water depth, topography of seabed, salinity, geology of the type of underwater soil and other miscellaneous geotechnical details are studied before laying out the design. 
• The pile foundation has to be penetrating deep into the seafloor and strong enough to withstand the uncertain loads encountered in the sea. The effect of all the six degrees of motion in water (surge, sway, heave, yaw, roll, pitch) are taken into account as for any marine structure.
•  As the weather in oceans is highly varied, specialized internal climate control systems are employed to maintain proper conditions congenial to the operation of the components.
• Measures for stability of the towers and its resistance to waves, wind or icing accretions is done.
• Corrosion is a pressing issue in oceans. Thus higher grade paints along with other modern anti-corrosion measures are applied on these.
• As maintenance is not that easy every time, automated greasing and drainage system is inbuilt into them. Self-heating and cooling systems are also employed.
• Problems of navigation are also sometimes grave. Thus, apart from selection of leaner marine traffic sites for installation, they are also specially equipment with emitters for navigational safety. 
• A high degree of precision is used for supply of power through lines post-production to the land-shore.

Figure 9: Often wind farms are a big problem to ship traffic

 Advantages and Disadvantages

Like it is said, everything has its pros and cons. Though wind farms have emerged to be one of the most ideal applications of oceanic exploitation for mass productions, it is facing challenges. However, on judicious planning advantages outweigh them.

Advantages:

ü   Primary advantages of wind power are that it is free, renewable, clean, non-polluting source of energy. 

ü  Land acquisition problems are eliminated.

ü  Constant wind to enable greater productivity.

ü  Facilitates port and harbour infrastructure. Also acts as the primary source of electricity in remote coastal villages.

     Disadvantages:

•   Ecological problems to marine underwater life.
•  Proximity of seabirds to get struck by.
•  Very high initial investment
•  Navigation  problems
•  Tedious job of laying high-voltage underwater wires over long distances to the shore.
•   Problems of fishing and trawling
•   High noise and vibration created by installation and operations.
•   Prone to damage by lightning or very adverse sea states 

     

      The potential of offshore wind is enormous. It could meet Europe's energy demand seven times over and United States energy demand four times over. In India too, it can supplement the shortfall power over a large number of regions, especially rural areas and large cities at the same time, where excess consumption often leads to crisis. Offshore wind power generating stations have a few advantages over the onshore ones like wind blows at a higher speed as we move far away from the shore so more power can be generated from few turbines. Most of the world's largest cities are located near a coastline avoiding the need of long transmission lines and constructing an offshore wind farm makes sense in a densely populated coastal region with high property value.

Figure 10: Some of the global statistics ( Courtesy: www.gwec.net)

            

      Presently only 3% of global installed capacity is offshore. Relatively high costs remains the biggest challenge for offshore wind development. But forgoing its problems and making it instrumental to meet the quantum of our short falling energy reserves. Also as we know, time is money. Thus, improper and slack utilization from the early stages can make any asset a liability.LSD

        

     Article by:   Kartik Garg