Heerema Marine Contractors has tested its innovative rotor nacelle assembly (RNA) installation technique for the first time in the offshore environment
The company recently undertook a demonstration project in collaboration with Delft Offshore Turbine (DOT), a research project that focuses on reducing the cost of offshore wind energy, and Delft University of Technology (TU Delft) to collect operational data and test installation techniques.
The demonstration project, known as floating offshore installation of XXL wind turbines (FOX), was a collaborative effort between Heerema, DOT, and TU Delft, supported by the Dutch Ministry of Economic Affairs and Climate Policy and Eneco. The project partners worked together with a number of subcontractors, including Heerema Engineering Solutions, F&B Group, Harco Heavy Lifting, Ampelmann, Sif, and CAPE Holland.
The RNA concept is designed to enable offshore wind turbines to be installed from a floating vessel, rather than from a jack-up.
The company said the greatest technical challenge when using a floating installation vessel is relative motion between the vessel’s crane and the foundation of the offshore wind turbine. Blade installation, which has been identified as the most critical part of the turbine installation process.
To combat these challenges, Heerema developed a ‘guided root end positioning tool’ known as the GREPT. This blade assembly tool enables offshore handling and installation of blades safely and efficiently.
Development of the RNA technique has been ongoing over the last two years and has been tested extensively at Heerema’s Leiden-based simulation centre. The offshore technique was put into practice for the first time on Eneco’s Prinses Amalia offshore windfarm in The Netherlands, using the vessel Sleipnir. The test results will be used to continue improving the RNA at the simulation centre.
Heerema chief executive Koos-Jan van Brouwershaven said, “Testing the RNA installation technique offshore is a significant step forward in our ambition to deliver the solutions clients need for next-generation offshore wind turbines.”
In addition to testing the RNA installation concept, the offshore tests were also used to test DOT’s slip joint connection. Wind turbines usually use bolted flange-to-flange connections and multiple lifts. The slip joint provides an alternative connection between an offshore wind turbine and its foundation. Heerema says it works “like two paper cups stacked upside down on top of each other.”
The connection is based on friction, and the weight of the component ensures a firm, stable connection. Installation is done by simply sliding the two-part over each other without the use of grout or bolts. This simple mechanism allows for cost reduction in materials, equipment, and personnel and it reduces the time needed to install a turbine.
Two separate slip joint connections were used during the ‘FOX project,’ connecting the wind turbine’s lower tower to a monopile foundation and connecting the nacelle to the upper tower. A slip joint sea-fastening was used to transport the complete tower section.
During the tests, Heerema assembled the complete wind turbine generator onboard one of its vessels, making use of the GREPT concept to install the blades. After that, the tower was installed on a pre-installed monopile using the DOT slip joint connection. The single-lift RNA was installed using a slip joint connection and a flange-to-flange connection. After the test project was finalized, the wind turbine generator was disassembled and the monopile removed using a vibro lifting tool.
The monopile used was installed in 2018 as part of an earlier project known as SJOR. This was the first time a vibro lifting tool had been used with a dynamically positioned (DP) vessel to installation a monopile without the use of a gripper frame or similar concept. This time, the process was reversed to execute the removal of the monopile.
The removal process tested research undertaken by Heerema Engineering Solutions and CAPE Holland and included a series of tests executed whilst reinstalling the monopile and safely removing the structure before transferring it to the deck of Sleipnir.
Heerema said the tests provided valuable data that can be used to demonstrate the feasibility of monopile installation without the need for gripper frames.
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