Lifting instead of RoRo safeguarded the project against changing tides and additional charter costs.

RoRo is often the preferred method for moving large and heavy offshore wind foundations onto and from vessels at port. Under ideal conditions, driving foundations directly to a set down point – rather than lifting and then driving them – takes less time.

However, it is not always the most efficient or safest option under real-world conditions. In this instance, external factors mean a ring crane approach provides significant advantages.

In busy ports with heavy marine traffic, the swells created by moving vessels can have a significant impact on RoRo maneuvers. Tidal windows and poor weather conditions can also delay and even halt operations, as the bridge between quay and vessel is too steep – resulting in significant costs.

Mammoet-Giant was chosen by Ørsted Taiwan to marshal 66 wind turbine jackets for the 920 MW Greater Changhua 2b and 4a wind farms in Taiwan. The project is notable for using suction bucket jacket (SBJ) foundations and being a debut for SBJs in the Asia-Pacific region on a large offshore wind farm.

Mammoet-Giant's engineers played a key role in coming up with a method to offload the jackets that ensured the changing tides had no impact on the timescale of the project. The approach further reinforced strong operational safety and reduced the number of vessel-charter days as the load-in and load-out schedules could be optimized.

To lift or not to lift?

When the SBJs were being introduced to this project, Ørsted sought solutions for their marshaling in the port of Taichung and approached Mammoet.

“We considered both RoRo and ring crane options but eventually came to them with a very clean solution of using our 5,000t capacity SK350 ring crane,” explains Joey Yu, Manager of Operations at Mammoet Taiwan.

“This was because the port is quite famous for big tides – sometimes up to six meters in difference.

For a RoRo operation, if the tide is not up to a certain level and the weather conditions are poor, it would take much longer to offload the jackets. What we proposed was a solution that wasn’t dependent on those factors”.

The 80-meter tall, 2,400t jackets arrived on a deck carrier in batches of four. They were hoisted from the deck using the SK350 and placed onto concrete supports on the quay.

Thanks to the SK350’s centralized ballast weight, the entire ring track didn’t need to be constructed on the quay – only the section needed to make the 130-degree slew from vessel to landing support – saving a lot of mobilization time and working space.

Next, with the three legs of the jacket elevated on supports, 32 axle lines of SPMT drove underneath each foundation and lifted it in tandem, using their on-board stroke. 96 axle lines of SPMT were used in total across the project.

Next, the jackets were driven to a temporary storage area in the port and placed on steel foundations resting on load-bearing mats made from sustainable bamboo.

The steps were reversed for the final load-out phase – the jackets were driven back to the quayside and then lifted by the SK350 onto a deck carrier, which ferried them to the offshore installation vessel in batches of four. The efficiency of this approach helped to minimize the uptime of high-value assets at sea.

De-risking projects; optimizing schedules

Tides and wind speeds were the biggest considerations for this project. These could have caused significant delays, incurring costs, had chartered vessels been left waiting in the port.

Because the team at Mammoet-Giant had considered this at the start, the methodology and equipment used helped to de-risk the project and optimize delivery schedules.

“Initially, Ørsted had some doubts as they had seen how capable and efficient our team is at doing RoRo operations – but this is only possible in perfect tidal conditions,” explains Yu.

“Not every tide would be sufficient for a RoRo operation, meaning days could have been lost with vessels waiting there for the right weather windows. With our method the vessels could be offloaded without any such constraints”.

For more information, please visit Mammoet's website.

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