Reducing Costs of Gravity Foundations

Since 2011 STX Finland has been leading the development of systematic ‘design-to-cost’ turnkey deliveries of foundations for offshore wind turbines, collaborating with offshore wind farm developers. The aim has been to develop new cost-effective foundations for offshore wind energy projects in the Northern Baltic Sea, where due to icy conditions monopiles are not very suitable except if blown into rock. To create a versatile structure for a variety of seabeds, STX has employed a gravity-based design that reduces manufacturing, logistics and installation costs and time. In August 2013, the first two of these innovative offshore foundations were produced and installed successfully in cooperation with Terramare Oy to serve as fairway marker substructures in Gävle, Sweden. This article describes the design approach used and the experiences gained in the Gävle project.

 

The Baltic Sea region has excellent wind conditions (see Figure 1). However, although offshore wind power is needed to reach renewable energy targets, existing tariffs are too low for wide offshore deployment unless costs are significantly reduced. A key driver is the cost of manufacturing, deploying and installing offshore foundations.

Applying Design-to-Cost to Offshore Foundations
STX Finland, together with leading wind farm developers and wind turbine manufacturers, has been developing a novel type of highly cost-effective foundation for the icy conditions in the Northern Baltic Sea [ref. 2] since 2011. The goals were reduced manufacturing costs and lead time (minimising working capital), leveraging existing equipment and infrastructure in shipyards for production (eliminating the need for investment into manufacturing infrastructure) and minimised logistics and installation costs. The result is a new foundation type, which is now being patented. Between May and August 2013 first demonstration examples of this design were manufactured at the STX Finland Turku shipyard, and installed on the Swedish coastline near Gävle. Here the concept was applied to fairway markers with a design lifetime of 100 years. This type of structure is useful beyond wind turbine and OSS foundations, for a range of offshore substructures required in the Baltic Sea and similar waters.

Novel Gravity Foundation Design Applied to Fairway Markers
Having done the product development work for wind turbine gravity foundations, the concept that had been developed was applied to fairway markers destined for the Gävle shipping lane in Sweden (see Figure 2). Due to the modularity of the original concept, adaptation and design reviews could be quickly completed. The design was completed in March 2013, the construction contract was signed in April, the fabrication was started in May and completed at the beginning of August. Installation followed within a couple weeks and was swiftly completed. The Gävle substructures, similar to the offshore wind turbine foundations they are based on, were specifically developed for serial production (see Figure 3). This made it possible to make maximum use of the existing automated shipbuilding manufacturing capability and equipment of the STX Turku shipyard in order to reduce fabrication costs and lead time.

Lightweight Steel Structure
A key feature is that the foundation steel structure is very light compared to a conventional concrete caisson foundation. Using a shape-stiffened structure reduced the steel amount by 30% compared to traditional steel structure designs. In addition to easier handling, transportation and installation, this provided advantages in fabrication lead time and quality, because low steel plate thicknesses make mechanised welding more effective. In the case of the Gävle fairway markers, in order to achieve as thin steel plates as possible the ballast pockets of the steel structure were developed to serve as a mould for the dense ballast material infill. This was also a key step in adapting the design to the long lifetime required (100 years). Using a lightweight steel structure enabled a short installation time and low-cost installation equipment with better availability of suppliers. In addition, there were fewer restrictions on the allowable offshore conditions, thus widening the installation window.

Open Bottom Structure and Modularity
Opting for an open bottom structure helped to reduce manufacturing complexity while speeding up the installation process, as the work required for bottom infill grouting was eliminated. Here, as well as in other key features, the Gävle fairway marker followed the modular design of the gravity foundations for offshore wind turbines developed by STX Finland. Thus it is an adaptation of the same design concept. The design foundation is easily adjusted for different water depths by changing the ballast bucket height and the centre tube length. Features like the access platform and boat landing are separate generic modules. This drives maximal manufacturing flexibility, and the ability to mass-produce these structures with a short lead time. The base design is suitable for other marine foundation applications with only minor modifications. The same concept can be adapted to transformer stations, bridge pillar constructions, lighthouse applications and mooring structures.

Using a Smaller Structure to Verify the General Concept
While STX Finland’s work seeks to bring a novel cost-effective gravity foundation to the offshore wind market, the Gävle fairway markers provided a good opportunity for a practical test of the design. The concept aims to be suitable for varying seabed conditions, depths from 5 to 40 metres, and needs to handle the harshest arctic offshore conditions. A fairway marker, while approximately one-third the size of a full wind turbine foundation, has similar requirements and is well suited for concept verification. Initial fabrication cost estimations were verified with actual production statistics. The logistics and installation process were confirmed (Figures 4 and 5). The performance of the full delivery chain from steel plate cutting to final installation was shown to work well. Since two fairway markers were made, some tests on further improving productivity were also possible.

Leveraging Existing Shipyard Infrastructure and Creating Synergies
Throughout the process the standard shipyard manufacturing process was used. However, careful preplanning (similar to the approach used in large-volume series production), designing for a very high degree of parts commonality, and maximised automation (panel lines with robot welding) and mechanisation (final assembly) were emphasised. Past lessons learned from shipbuilding were obviously taken into account for best productivity and synergies. For example, the design of the foundation radial bulkhead was applied from a ship bulkhead. As a result, the bulkhead could be robot welded on the automated panel line, taking only 40 minutes per bulkhead. Throughout this effort, a key principle has been to leverage these types of synergies between shipbuilding and offshore structure production, delivering benefits for both.

Simple Design and Reduced Weight Improved Execution
The effort confirmed that the design concept indeed has large benefits at all stages of design, manufacturing, logistics and installation. Applying design-to-cost and modular design, with a high degree of parts commonality, simplified the solution and made it also more robust against quality failures. Using design-to-cost and design-to-manufacture iteratively in collaboration with a cross-functional team involving manufacturing, logistics, installation and end users, resulted in a simple and cost-effective concept. A conventional reinforced concrete fairway marker with infill would weigh 1,000 tonnes. The new design reduced the weight to about one-tenth, simplifying handling, logistics and installation. Overall cost was also reduced, and the project was executed on-time, on-quality and on-budget. During the production of the two fairway markers the shipyard was simultaneously in the middle of constructing a large-sized advanced cruise ship, but efforts ran smoothly in parallel.

Simple Installation Process with Easily Available Equipment
The logistics and installation process was done with equipment similar to that used in actual wind farm installation. Obviously the scale was somewhat smaller, but the principal process was the same. The project confirmed that the benefits of using a lightweight structure that is ballasted once it has been installed on the seabed are significant. Installation proceeded smoothly and on-schedule, and without interruptions. The robustness of the installation process for this type of solution will be a considerable advantage to offshore wind farms, where on-time project completion and the ability to complete work despite weather changes is critical. Reducing installation equipment complexity is also a key benefit. In offshore conditions, a heavier weight directly drives the need for more advanced equipment and a smaller installation weather window.

Summary and Next Steps
The foundation makes full use of existing shipyard equipment, making large-volume production possible without large investments. The design is manufacturing friendly, allowing the use of automated lines and mechanised welding, while meeting all quality requirements. High parts commonality optimises manufacturing, achieving high-volume and variant flexibility. The throughput time is short, providing a base for minimisation of working capital. The structure blends well into standard shipyard production, and can be cost-effectively produced side by side with advanced large ships without causing disruption. The prototype demonstrated considerable benefits in handling, logistics and installation of this type of structure, paving the way for more reliable project execution of large-scale offshore wind farms.

For STX Finland, the next step is delivering and installing a full-scale arctic offshore wind turbine foundation. This will serve to optimise the end-to-end process from design to delivery, and is a precursor to delivering a full arctic offshore wind farm in the Baltic Sea.

Biographies of the Authors
Dr Per Stenius, is SVP Corporate Development and Head of STX Finland Windenergy. He has been leading the Windenergy business unit at STX Finland since its inception in 2011. He holds a PhD (Electrical Engineering) and MA (Economics) from the University of California, and an MSc (Electrical Engineering.) from Aalto University (formerly Helsinki University of Technology).

Jukka Mäkiranta is a Project Manager at STX Finland. He holds an MSc (Naval Architecture) from Aalto University (formerly Helsinki University of Technology) and an MSc (Economics) from Turku School of Economics, Finland.

Juha Papinoja is a Classification Engineer at STX Finland, and is responsible for technical design and engineering within STX Finland Windenergy. He holds a BSc (Naval Architecture and Marine Engineering), and has been involved in multiple design and development projects working both in shipbuilding and within classification.

Ilkka Rantanen (MSc) is working as a Project Manager at STX Finland. Previous to that he held various positions in production management, corporate strategy, sourcing, business development and R&D. He graduated in 2001 as an MSc in Industrial Engineering and Management from the University of Technology in Tampere, Finland.