Offshore Wind Turbines Support Structures Types

Offshore Wind Turbines Support Structures Types

 

As the OWTs are located on the sea, therefore, support structure and its foundation are important for design considerations.

There are several conventional bottom-mounted support structures, which can be categorized into five basic types:

  • monopile structures (using pile foundation),
  • tripod structures (using pile foundation),
  • lattice structures (using pile foundation),
  • jacket foundations,
  • gravity structures,
  • floating structures.

There are also hybrid support structures which use the combined features of the above categorized structures. For the first three type of supporting structures (monopole, tripod, lattice structures) pile as the foundation is usually used. Pile foundations are one of the most common forms of offshore structures; they are widely used for both offshore oil platform and OWT.

The standard method of piling method is to lift or float the structure into position and then drive the piles into the seabed using either steam or hydraulic powered hammers.

1. Monopile Structures

As shown Fig.1, monopole has the simple fabrication and installation.The tower of the turbine directly sits on one pile. Monopile foundations are one of the most frequently used support structure to date. Most of the offshore wind farms in shallow waters are monopole structures, which have the advantage of simple design for manufacturing.

However, failure of the grouted connections between the monopile andthe transition piece is one of its disadvantages. This transition piece is responsible for connecting the monopile to the turbine tower. In addition, there is no proven solution using monopiles for larger turbines with 5 MW or more powerful turbines.

Fig. 1. Wind turbine on the monopile foundation

2. Tripod and Lattice

As shown in Fig.2, the turbines directly sit on a tripod or a lattice, which are supported on the pile foundations. The tower can be further stabilized by the tripod.

Fig.2. Wind turbine on tripod

3. Gravity Foundations

As shown in Fig.3, this type of foundation achieves its stability solely by providing sufficient dead loads by means of its own gravity. Ballast can be pumped-in sand, concrete, rock, or iron ore to add extra weight. Gravity structures are suitable for modest environmental loads such as wave load that are relatively small and dead load is significant or when additional ballast can easily be provided at a modest cost.

The gravity base structure is especially suited where the installation of the support structure cannot be performed by a heavy lift vessel or other special offshore installation vessels, either because of non availability or prohibitive costs of mobilizing the vessels to the site.

Gravity-base foundations are the second most popular sort of support structure to date. They have been used mostly to support smaller turbines in shallow waters near shore locations with a rocky seabed where the operation of piling is extremely complicated and expensive. However, for waterdepth beyond 35 m, the new generation of wind farms are needed.

Fig.3. Wind turbine on the gravity-base foundation

 

4. Floating Structures

Floating structures are especially competitive at large water depths where the depth makes the conventional bottom-supported structures non-competitive. Detailed design guideline can refer to DNV-OS-J103,” Design of Floating Wind Turbine Structures”.Fig.4 shows a floating form using space frame style floater.Fig.5 gives the other two examples.

 

Fig.4. Floating wind turbines

Fig.5 Floating wind turbines examples

 

5. Jacket Foundations for Offshore Wind Structure

As shown in Fig.6, jacket foundation uses four-legged jackets to supportthe OWTs, which can support larger OWTs such as 6 MW turbines. Jacket foundations provide a solution for foundations in offshore wind farms in water depths of 35 m and beyond which is less risky, less expensive, and more reliable than monopiles and gravity-base foundations.

Fig.6. Steel jackets on a barge

Movable and Fixed Oil Platforms Types

Movable and Fixed Oil Platforms Types

 

While choosing the correct type of platform, several factors need to be considered: such as its function and the water depth. Based on these two factors the oil platforms are designed into two major categories: one is movable oil platform, the other kind is fixed platform.

1. Movable Oil Platform

 

1.1 Drilling Barges

Drilling barges are shown in Fig.1. They are mostly used for shallow,in land waters; this would typically be lakes, rivers, and canals. When the drilling barge tends to be large floating platforms must be moved using a tugboat to the location of drilling. They are not suitable for large open waters as they are not able to with stand water movement.

Fig.1. Drilling Barge

 

1.2 Drillship

As shown in Fig.2, drill ships are ships with the drilling platform installed inthe middle of the ship deck to allow the drilling string to reach the sea bed through ship’s. Drill ships can drill even in deep waters. Drill ships maintains its position using dynamic positioning systems and other sensors. The ships also use satellite-positioning technology with motors integrated below the hull to position the ship directly above the drill site.

Fig.2. DrillShip

1.3 Jack-Up Platforms

Jack-up platforms (Fig.3) are mobile drilling platforms made of floatable deck with three or four legs lowered on to the seabed upon being towed to the drill site. The platform is then towed to the location with raised legs. It can raise its hull over the surface of the sea. Jack-up rigs are suitable for shallow waters with a maximum depth of water of up to 130 m in gentle water and up to 70 m in harsh waters. Jack-up platforms tend to be a safe alternative than drilling barge since during operation they operate similarly to a fixed platform.

Fig.3. Jack-up rig

1.4 Submersible Platforms

As shown in Fig.4, submersible platforms are mobile structures which are designed that can be floated to location and lowered onto the sea floor for offshore drilling activities. However, due to its own feature, it is only limited to shallow waters.

It normally consists of two platforms one is on top of another. The upper platform is the actual drilling platform, the lower platform provides buoyancy during towing of the platform between drilling sites.Once the platform has been positioned, air is let out of the hull and then it submerges to the sea floor. Due to the design of the platform, it is limited to shallow waters.

Fig.4. Submersible Platform

1.5 Semi-Submersible Platforms

As shown in Fig.5, semi-submersible platforms are similar in design to submersible platforms with same principles. As the lower hull can be inflated and deflated this type of platform is only partially submerged and uses water to fill the bottom hull for buoyancy. A semi-submersible platform can be used to drill in deep water depths of 1800 m.

The depth of the lower hullthat submerges into the water is predetermined, and the platform is held in position by anchors weighing upwards of 10 tones. The dynamic positioning can also be used for the same purpose, and this can ensure safe and stable conditions of the platforms.

Fig.5 Semi-submersible platform

 

2. Fixed Type Oil Platform

The fixed platforms are normally used in shallow waters. They have the legs which are made of concrete or steel tubular structures anchored to the seabed to ensure stability of the platforms. However, the cost of these types of permanent platform would be too much to use in deep water conditions, therefore, if most of them are used in the shallow sea water, except for compliant tower (CT), which can be used for>600 m. There are several types of fixed platforms such as, Jacket platforms, tension leg platform(TLP), gravity platform, or even an artificial island.

2.1 Jacket Offshore Platforms

As shown in Fig.6, jacket platforms are one of the most commonly used fixed type of platform, about 95% of the offshore platforms in the world are using jacket platform. The platform is supported by steel space frame which consists of plate girder or deck truss structure supported by welded tubular that is piled to the sea floor. The steel frame is called jacket. It is used in water when the depth does not exceed 500 m.

Fig.6. Jacket offshore platforms

2.2 Tension Leg Platforms

Fig.7 shows the TLPs. TLPs is evolved from semi-submersible platforms, which consist of a buoyant floating platform kept in position by pretensioned anchors fixed to the seabed using vertical mooring lines called tension leg. Although vertical movement is restricted, they do allow for significant sideway movements. The pretensioning of the tethers is huge in size to avoid compression due to waves.

Typically, thetethers consist of a 12- to 15-m-long section connected by welds or threaded joints, with a pile or gravity skirt foundation into the seabed.To further increase the stability of the platform the lower hull is filled with water during drilling to resist movement of ocean and wind forces.The TLPs can operate in waters up to 2.13-km-deep waters. Sea star platforms are a miniature version of a TLP made for waters of depths up to 1 km.

Fig.7. Tension leg platform

2.3 Gravity Platforms

Gravity platforms consist of a steel deck and concrete framework as well as as teel skirt foundation where petroleum is normally stored in the skirts. Gravity platforms have been made for water depths of up to 300 m in harsh waters.

They are built in an upright position and towed out to the off shore drilling site and installed by ballasting, where a heavy substance is used to provide floating stability.

Gravity platforms are cost-efficient option dueto reusability of a platform as the structure can be towed to another location.A gravity platform is shown in Fig.8

Fig.8 A gravity platform

2.4 Spar Platform

This is one of the largest fixed type offshore platforms. The spar is a lowmotion floater that can support full drilling assistance of flexible or steel cat-enary risers.

A large platform consists of a large cylinder that does not extendall the way to the seafloor but is tethered to the seabed by cables and lines.

A cylinder is used to stabilize the platform in the water and allows the move-ment to absorb the force of the hurricanes.

There are different types of spars: original cylindrical classic spar, truss spar, the cell spar, the Arctic Spar, and spar with storage.

Three main types of configurations of Spar platform cylinders are mentioned below:

  • Conventional spar: One-piece cylindrical hull, as shown in Fig.9.
  • Truss spar: Midsection is made of truss elements as shown in Fig.10.
  • Cell spar: Midsection made of multiple vertical cylinders, as shown in Fig.11.

Fig.9 Conventional spar

Fig.10 Truss spar

Fig.11 Cell spar (Skaug, 1998)

2.5 Compliant Towers

A CT is a fixed rig which is similar to the fixed platform, but consists of anarrow tower attached to piled foundations on the seafloor and extend up to the platform shown in Fig.12.

In deep water, in order to prevent excessive amplification of wind, waves, and current, the natural period of the bottom founded structure should be substantially different to the dominant period of hurricane. Therefore, one of the methods to achieve this is to be able to control the mass and stiffness of the rigs.

The advantage of CTs use the flex elements such as flex legs and axial tube, which can control its natural periods therefore resonance is reduced and wave forces are de-amplified. Therefore, they are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 450 to 900 m.

Fig.12. Compliant tower

Outrigger Structures Types

Outrigger Structures Types

 

1. Introduction

The concept of outrigger dates back to 50 years, it origined in deepbeams. It has been derived from deep beam into concrete walls, and now in the form of one or several story outrigger trusses.

Outriggers are one of the most widely used systems for relative regular floor plan. It is constructed using steel trusses, girders, concrete walls, or deep beams to connect the core and the columns at the perimeter.

The outrigger trusses are normally one-story high, some even occupy several storys. The cores are normally located at the center of the building, whereas the outriggers extend out to the outer columns (as it is shown in Fig.1).

Therefore, the outriggers and the outer columns work together as a further restrain to the core wall. Under lateral load, the belt trusses act as lever arms that directly transfer axial stresses to the perimeter columns. The bending, axial tension, and compression of the outer columns connected to the outriggers help resist the external moments of the structure. This resistance enhances the overall stiffness of the core, helps in reducing the lateral deflections, and overturning moments.

The outrigger columns work together particularly helping to restrain the rotation of the core. Overall, major advantage of using the outrigger is to resist the rotation of the core and significantly reduce the lateral deflection and overturning moment.

One of the famous examples of this system is Shard, London BridgeTower (Fig.2). It has core wall at the center and outrigger truss at highlevels, inside the plant room, to connect the central core and outer raking columns.

Fig.1. Outrigger structures

Fig.2. The Shard.(Adapted and reuse with the permission of Asset bank, City, Universityof London)

If the outrigger is used together with external tube systems, it can more evenly distribute the large vertical forces applied by outriggers across the multiple columns. Analysis and design of a core-and-outrigger system requires the use of computer program. This is because the distribution of forces between the core and the outrigger system are determined by the relative stiffness of eachelement: the core, the outrigger, and the columns. Therefore, it is difficult to calculate manually.

2. Types of Outriggers

There are several different types of outrigger system, such as steel outriggers, concrete outriggers, and hybrid outrigger (using both concrete and steel material). Among them, steel outriggers are most conventional type outriggers.

The famous examples are: Twin Tower (collapsed in the 9/11 attack) and the Shard in London. Concrete outriggers are used in some tall buildings. One of the famous examples is 432 Park Avenue building in NewYork.

With the development of the construction technology, new types of outriggers such as hybrid outriggers and damped outriggers have emerged in the construction projects.

2.1 Steel Outriggers

Steel outrigger systems are extensively used in a lot of tall buildings as most of tall buildings are either steel or composite structural system. In the conventional design, the outrigger is designed to be a story height truss.

2.2 Concrete Outriggers

The benefit of concrete outrigger system verses steel is high stiffness and lowcost. Under wind load cases, the outrigger system needs to be of stiff concrete deep beam or of concrete wall which can be easily achieved by this.

Fig. 3 shows a typical outrigger using concrete wall. This type of system is more common in a concrete structure rather than in a steel frame structure.

Fig.3. A typical outrigger using concrete wall modeled using ETABS

2.3 Hybrid Outriggers

The steel outrigger is not as stiff as concrete outrigger. However, a pure concrete outrigger system is very brittle.

An innovative type of steel-concrete hybrid outrigger truss was developed in two 370-m tall mega-high-rise towers in Raffles City Chongqing, in which the steel truss is embedded into the reinforced concrete outrigger wall as shown in Fig.4. Both the steel truss and the concrete outrigger wall work compositely t oenhance the overall structural performance of the tower structures under extreme loads.

Fig.4. Fused outriggers (a concept originally developed by Arup)

2.4 Damped Outrigger

In the event of severe earthquake, the overall structural system should be able to dissipate energy and maintain its robustness against the collapse. Additional viscous dampers can be installed in the outrigger for a nonlinear response and tuned to meet multilevel performance objectives.

In case the dampers fail, the outriggers which is designed to yield in a ductile manner will remain intact. Thus, it can reduce wind-induced vibration and can also be used as fuse to protect the building under a severe earthquake condition.

Fig.5. Arup Damped Outrigger system adopted in St Francis Towers

Shear Wall and Core System in Tall Buildings

Shear Wall and Core System in Tall Buildings

 

Shear walls are widely used for both tall buildings and low-rise buildings. They are important structural members used in the lateral resisting system. They work as a deep vertical cantilevered beam supported at the ground. They also carry vertical load together with columns.

Some structures may require coupled shear walls, where girders and the floor system join the two or more walls together as a coupled system to provide morestiffness.

In tall buildings, shear walls are generally located at the center of the building, normally in the form of core wall system to accommodate the vertical translation system such as lifts for the tall building. It is a very common form of lateral load support system in tall buildings.

1. Type of Cores

There are two major types of cores: concrete core and steel framed cores. Concrete walls are used widely in the tall building design; on certain occasions, steel core can be found in buildings built before 9/11, they being much lighter, can save the cost of the foundation. However, they are gradually abandoned after 9/11 attack.

1.2 Concrete Core

Fig.1 shows a concrete core for a tall building under construction. Reinforced concrete cores are a more standard option for tall buildings in general, as seen from the history, concrete structure is dominant in the market because they provide more stiffness than steel cores, and it is relatively cheaper to use a concrete core in certain countries such as China.

In certain countries such as China, the steel production was not sufficient in the past; therefore, most of the tall buildings were built in concrete. In addition, some codes require that the core of the building be constructed using reinforced concrete in case of fire and for emergency safety.

When designing a concrete core, there are several issues that must be considered:

  • Constructability and construction sequencing: The erection sequence for building with a concrete core will see the casting of the core proceed ahead of the steel framing. The core and elevator shafts can therefore allow for the use of a climbing crane rather than having to use separate tower cranes.
  • is also quite common to install the steel embedment such as steel plate in the core to further strengthen the core. One of the project example is the China Zun Tower in Beijing.
  • Due to the shortening effect, differential movement may incur especially in the structural system of the building, which is a combination of steel and concrete, the tendency of concrete to creep or shrink over time and thermal expansion of steel need to be considered.

Fig.1. Concrete core of a tall building under construction

 

1.2 Steel-Framed Cores

Steel core was quite common for tall building design before 9/11 attack. Most tall buildings in the United States at that time were predominantly using the structural steel for the cores. The twin towers in World Trade Center is one of the examples. It used the steel core at the center of the building. Another example is Swiss Re Tower in London, also called as 30 St Mary Axe or Gherkin.

The main reason for using steel core is that it provides a light weight structure solution. The total weight of the structure is quite important for tall building design, as it will directly affect the foundation design. Therefore, a light weight solution will make a cost-effective foundation design possible.

However, the investigation of NIST NCSTAR  “Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers,” shows that fire was the major cause of the collapse of the World Trade Center as majority of its structural members were steel. So for supertall buildings designed after 9/11, steel core is rarely used.

When designing the steel-framed cores, one need to check the lateral loads, fire protection, constructability, and erection sequencing. Among them, the most important issue is the fire protections, due to the lessons learnt from the twin tower. Different fire protection strategies can be considered, such as intumescent paint, board, spray, etc.

2. The Importance of Core Design

The structural system of Twin Towers is theso-called framed tube system. An internal steel core was used for TwinTowers, the floors were made of the steel composite truss floor system, under the fire, the truss starts sagging, which pulled inward on the perimeter columns: “This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers”.

As a result of 9/11 attack, more and more design engineers began to focus on how to design a tall building to be able to resist a similar attack. Therefore, a concrete core became one of the major choices for consideration. As discussed, core is also part of evacuation route when hazards happen,therefore, concrete core would also be a good option.

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