Jan de Nul is contacted by Nobelwind for the installation of 51 foundations for the Bligh Bank 2 Offshore Wind Farm. TWD has provided Jan de Nul with designs and know-how for seafastening and installation of these foundations. Jan de Nul will use their HLJV Vidar installation vessel for the installation. Due to a short time schedule, they will reuse various seafastening and installation equipment from a previous installation project.

Bligh Bank 2 seafastening structures and Fabrication Assistance

block white

The Vidar will be equipped with eight monopile cradles of which six were reused. Besides these monopile cradles an upending hinge and four transition piece grillages were reused as well.
block white
In addition to the modified reused designs TWD also designed new seafastening structures for various items on board of the Vidar, including crawler cranes, grout spread to monopile cradles and containers. By making use of our experience in wind foundation installation projects, we assisted Jan de Nul with the installation preparation and concluded that additional access structures were required.

PROJECT HIGHLIGHTS

  • Reuse of existing monopile cradles and transition piece grillages on a different vessel, with minimum extra steel
  • Providing designs in steps to the fabricator, work in parallel and finish the project in a tight time schedule
  • Upending above deck in the top monopile line
  • Fabrication assistance

block white
MONOPILE SEAFASTENING

block white
The reused monopile cradles were designed for transverse placement of monopiles on deck. However, for the Bligh Bank II project the monopiles were oriented longitudinally. This created two challenges. The first challenge was to fit the original seafastening structures in between the two spud houses. Secondly, to provide sufficient resistance to transverse accelerations. This was achieved by replacing internal braces and shifting the seafastening structures on the deck to find a position in which all monopiles can be transported and the loads are spread over the deck.

block white
TRANSITION PIECE SEAFASTENING

block white
The original transition piece grillages were designed for transition pieces of larger diameters than used in the Bligh Bank 2 project. To properly support the smaller transition pieces, the grillages were cut in the middle and a new middle piece was designed to make the diameter smaller. The clamps can be repositioned over the transition pieces flanges and spread the loads over the deck of the Vidar. This solution minimized the required steel and modifications on the transition piece grillages.

block white
UPENDING HINGE

block white
To allow rotation of the hinge above the deck of the Vidar, the entire upending tool is elevated by a support frame. Due to the increased pile weight, the bucket and the bottom beam required strengthening. Because of the bucket strengthening modifications, further adjustments were made to the original diagonal bracing in the upending hinge support frame to prevent clashes.

block white

Fabrication Services

block white
On behalf of Jan de Nul, TWD’s fabrication services department assisted during the fabrication phase by performing site visits, quality assurance and quality control on the Bligh Bank II equipment. Furthermore, we monitored the progress, to ensure the production is according to design drawings and design objectives.
block white
During the fabrication, TWD assisted in technical queries from the fabricator and field engineer. In particular for solving deck interfaces due to unforeseen differences in the deck. We provided updated construction drawings to keep the fabrication process in motion.
block white
Finally manuals were written to perform various site acceptance tests after the completion of all welding, mobilization and connection of the hydraulic systems. After evaluation of the tests the operation manuals were finalized and used during safety reviews and informing all involved parties.

block white

Naval Engineering

block white
To determine the dynamic loads on the crane and pile gripping frame, a dynamic analysis of the monopile and J-tube cage lowering operation was necessary.
block white
TWD was requested to perform the dynamic analysis of the lowering operation for a range of monopiles and a single J-tube cage. The findings, including recommendations on limitations of environmental parameters such as significant wave heights, wave peak periods, current and wind speed, to ensure a safe installation operation were communicated to the client.
block white
Based on close to a thousand simulations performed using the dynamic analysis software OrcaFlex, TWD determined the workability of the monopile lowering and monopile piling operation for a range of monopile diameters and lengths based on Dynamic amplification factor (DAF) on the crane and dynamic forces on the Pile Gripper Tool.
block white
Additionally, TWD performed simulations for the J-tube cage to determine the behavior of the J-tube cage in the splash zone. Based on the simulation results, TWD made recommendations relating to the following aspects of the operation:

  • Optimum lifting radius of the crane
  • Limiting value of significant wave heighs (Hs), wave peak periods (Tp) and headings
  • Natural period of lowering system and critical lowering depth
  • Sequence of lowering operation

block white
Watch the movie of the simulations below.

Photo’s courtesy JDN/NBW
Video

Dynamic lowering analysis of monopiles and J-tube cage using jack-up vessel

Top