Credit: Mammoet

The WTA assembles wind turbine generators by attaching directly to the tower itself, using a series of clamps to self-assemble and then climb to each lift location. It assembles tower sections, hubs and nacelles and has a capacity of 150 tons.

It operates in wind speeds up to 20 meters a second, reducing downtime during construction and extending the build season.

As the WTA has a reduced footprint compared to other crawler cranes and actively lowers the need for groundwork on site. Pads can be smaller, and ground pressure requirements are lessened — maxing out at the 15 tons per-meter-squared typically needed for assist cranes.

The system’s small size means quicker and more cost-effective mobilization. While a conventional crawler crane can require up to 50 truckloads to reach the site, the WTA gets there with just nine.

With no boom laydown requirement, fewer components and a lower total weight, the WTA is also faster from pad to pad, Mammoet stated in a press release. The system is designed to reduce relocation time compared to using crawler cranes and can shave weeks off wind farm construction schedules.

Powered entirely by electricity, it also opens the door for a 100% emissions-free journey from factory to first megawatt — with transport to site via electric or hydrogen-powered truck, on-site maneuvers via ePPU-enhanced SPMT and carbon-free WTA lifting.

The WTA system is now design-ready and can be ready to enter the market during Q2 2023.

News item from Mammoet

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This offshore wind manufacturing facility will be the largest in the U.S. and will be able to build 400-foot long, 2,500-ton monopiles to supply the 1,100 MW Ocean Wind farm off the coast of southern New Jersey.

The Harman Group provided structural engineering for concrete foundation systems for Phase 1 of construction.

The 70-acre site requires a concrete mat foundation to support the manufacturing of monopiles, which are 40 ft in diameter and weigh the same amount as 1,572 average cars. With this weight upwards of 5.5 million pounds, it is an enormous load to be put on a foundation system. These mammoth monopiles must be made on the waterfront, for direct loading onto barges as roads are unable to support this kind of load.

To ensure the manufacturing facility does not settle excessively, 9,250 cubic yards of concrete must be poured to create a supportive foundation system.

“The Harman Group has substantial knowledge of constructing and designing mat foundations, especially on demanding sites, which allows us to take on this engineering feat,” said Jan Vacca, principal and VP of The Harman Group. “We are proud to be a part of the team that is building offshore wind and clean energy, making New Jersey a leader in this category.”

Once completed, the foundation system will support multiple buildings on this 70-acre site. The project construction is ongoing.

News item from The Harman Group

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The offshore wind market is maturing rapidly as the world transitions to cleaner energy. Indeed, the BP Energy Outlook 2019 anticipates significant growth of the sector, suggesting that the percentage of generated wind energy in the renewable market will be more than double by 2040. Meeting this rapid growth in demand presents huge challenges in design, technology and engineering. To make the transition economical, wind farm developers and operators are prioritizing efficiencies in cost and performance across all aspects of the operation.

Sea fastening is the routine practice of fastening cargos to the ship for transport, either to the site of installation or transit from port to port. As component sizes increase, the challenge of transportation becomes more acute. Executed properly, sea fastening enables the safe and efficient transportation of project equipment, minimizing the number of trips required to install the wind farm equipment. This critical stage of the installation process must not be underestimated — not least in terms of the value it has the potential to deliver.

Sea fastening, but not as you know it

Over the last 30 years, the blade diameters of offshore wind turbines have more than quadrupled. Indeed some of the newest blade designs are double the length of a Boeing 747, and it is anticipated that this growth will continue. As experience is gained, and the technology develops, the turbines get larger to generate more power.

As the components increase in size and at a significant rate, the industry is pondering the optimum design for the next generation of installation vessel. In the meantime, however, the existing fleet is being pushed to the limit. Particularly as there is commercial pressure to maximize the amount of equipment on the vessel per trip to help decrease the overall cost of offshore wind farm installation. This significantly increases the project risks associated with transportation and installation.

The importance of sea fastening in this context is often undervalued — this is not as simple as a standard offshore container sitting on a deck well within the allowable variable deck load. For today’s wind farm installations, the components do not only differentiate in size and weight, but also shape. The blades need to be transported in racks, the monopiles in cradles, and the towers on grillages; all of which must be designed to fit. This is sea fastening, but not as we knew it.

Stability and motion analysis informs effective design

When particularly large equipment is secured to a vessel, the loading conditions must be checked to ensure the vessel remains stable and within the operationally compliant restraints of draught and trim.

Additionally, the weight and height of these components adjusts the vessel’s motion characteristics. Therefore, bespoke vessel motion must be derived to determine the forces the equipment will impart into the hull as the vessel rolls and pitches while at sea. The length of the blades and sometimes monopiles means that they can overhang the edges of the hull, meaning additional green water analysis may be required.

The structural analysis, design and engineering work follows confirmation of stability and the determination of vessel motions. It is essential that the structural interface for each piece of equipment is designed to transmit the loads into the vessel structure without overstressing and damaging the hull or the connecting interface.

For jack up vessels, it is also important to check hull strength in the jacked-up condition, and that the forces pushing down on each of the legs does not exceed the allowable seabed limitations. The leg forces can vary significantly when the vessel crane lifts the wind farm components, and so several scenarios must be considered.

Integrated thinking across structural analysis, design and engineering

Experienced, practical analysis is essential to ensure proper securing and sea fastening of high-value cargoes to guarantee a project’s success. Developing an offshore wind farm involves specific and expert engineering, from concept design to installation, into operation and finally decommissioning. Every element is closely interlinked and therefore decisions must not be taken in isolation — the wider picture must always be considered.

This is why analytical capability alone is insufficient; structural analysis is just one piece of the puzzle. There is a seamless chronology between understanding stability and vessel motions before then delivering on design. For example, what are the practicable options when an allowable vessel limit is exceeded or is so-close to exceedance that further calculation is required to prove it is acceptable? Every decision has a knock on effect, which is why — throughout the entire process — considerable experience is needed across each and every element to engineer reliable solutions which facilitate safe, timely, and cost-efficient delivery.

Progress necessitates change in the swiftly advancing offshore wind space, particularly as global societal pressure increases the move to cleaner energy sources, while development costs continue to be driven down. To safeguard investment, protect assets and maximize efficiencies, integrated design and engineering remains critical in navigating the evolving challenges of this swiftly emerging sector.

Houlder has completed multiple fixed bottom offshore wind sea fastening projects for various clients. During the course of these projects it has designed the structural interfaces between all of the primary offshore wind components and the vessel — tower grillages, blade racks, substructures, TP grillages, monopile cradles — as well as various ancillary equipment that goes along with the mobilization. Each project is different and presents its own unique challenges.  

When designing the primary support structure for any of the main components, the main interface is usually a straight forward process. The challenge is transferring the loads into the vessel to avoid underdecks strengthening or fatigue issues while working within fixed vessel parameters such as crane reach and height, loading condition ballast limits, and avoiding clashes with areas that require access or walkways for safe operations, etc. Careful consideration to each limitation, and a multidisciplinary team that can work effectively together and in parallel is the key to a successful outcome within the planned timeframe.

There are always additional requirements that present themselves where an efficient engineering team can add real value to the project. Houlder has designed lifting arrangements (including spreader beam) to improve mobilization times of blade rack substructures that were not originally designed to be lifted in one unit. It has also added and repositioned boat landing platforms, and relocated crane boom rests, for example. In addition to back deck equipment, Houlder has relocated life rafts and FRCs to avoid project equipment and is familiar with the requirements for class approval of safety-related equipment.

Houlder has also provided quick responses to queries during mobilization. Wrong bolt grades being delivered, slings that are shorter, and a lower SWL than requested, uneven decks, are all things that have arisen and been resolved quickly without holding up the mobilization.

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