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LORC tests and demonstrates technology for harvesting renewable energy offshore

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Meeting industry’s needs for environmental testing of large components for offshore use is now one step closer with the ordering of a climatic chamber for a new LORC test centre.

Read more about the environmental testing of structures here




Inward Battered Guide Structure

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onsdag 20. april 2011

Faster installation and fewer installation manoeuvres, using less steel and fewer components, all add up to an estimated 10% decrease in costs: this foundation concept has a lot in its favour compared to a four legged reference jacket, explains engineer Rudy Hall

By Anne Korsgaard
The concept has been proven by independent push-over analyses to have a reserved strength ratio (RSR) slightly greater than a traditional four-pile jacket-type structure. Therefore, the concept has significantly more redundancy than a tripod. Rudy Hall, one of the conceivers of the concept, describes it in more detail:

- Our concept is optimal for Round 3 offshore wind farms in the UK, because it is good for water depths greater than 30 meters. The typical jacket foundations for water depths greater than 30 meters have more than 60 components - transition, legs, and braces - to assemble. Our structure only has 20 components to assemble; requiring fewer welded connections, less work above ground level, less space to assemble, less space to store, etc.

Inward Battered Guide Structure technology (IBGS). Illustration: Keystone Engineering Inc

Keystone has performed ship impact analyses. Initial independent fabrication and load-out cost estimates indicated that the IBGS concept would be less expensive based on fabrication along the US Gulf Coast. And because the IBGS concept can be installed with smaller, less expensive marine assets available from the Gulf, the concept also provided a potential savings with respect to installation for the US East Coast site.

The advantages regarding fabrication are that the concept is less complex to fabricate than a tripod or four-leg jacket, easier to transport in larger numbers with existing or planned multi-purpose transportation and installation vessels currently used by the offshore wind industry, and requiring less time to install than pre- or post-piled jackets, and with no underwater work required.

When asked about his hopes for the near future of IBGS technology the engineer says:

- Our number one goal is a photo of our foundation supporting a wind turbine generator in an offshore wind farm. Up until now, the IBGS has only been in use in the oil and gas industry. An installation in the Gulf of Mexico proved to be very sturdy since it suffered no structural damage when hit by Hurricane Katrina. Now funding is pending and Keystone can begin with prototypes and the proofing of the concept. Designing the prototype demonstrator will hopefully occur during the next half year.

The installation process

Multiple foundations may be loaded onto a single vessel

Guide Structure Transportation
On site, the installation vessel is positioned and the self-standing caisson is lifted into position, allowing it to “free run” onto the sea floor and then driven to the design penetration using a hydraulic hammer.

A guidance tool mounted on the lift vessel is used to locate and support the caisson during the setting and driving operations.

Illustration: Keystone Engineering Inc

Guide Structure Installation
After the caisson is installed, the multipurpose vessel transporting the IBGS guide structure is brought into position. The guide is set over the caisson and lowered until the internal bearing points located in the guide structure caisson sleeve rest on the caisson.

The guide structure is levelled and centralized with a series of levelling jacks located on the internal bearing points.

Illustration: Keystone Engineering Inc

Pile installation
Piles are driven to the design depth using a hydraulic hammer and follower that can drive the top of the piles below the top of the steel elevation of the upper access landing. The structure is grouted using a smaller specialty vessel outfitted for this purpose.

The grouted pile splices, pile to guide structure annuli, and the caisson to guide structure annulus are all grouted. After grouting, the wind turbine is installed.

Illustration: Keystone Engineering Inc

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