Achieving new capacities

Huisman’s 900-tonne RL-KBC with long outreach. Photos from Huisman.

Subsea 7’s Seven Arctic is due for delivery in 2016. Its 900-tonne capacity crane was developed in close cooperation with Huisman.

Earlier this year, Subsea 7’s latest vessel design was unveiled, the new offshore construction vessel Seven Arctic. Currently under construction at Hyundai Heavy Industry’s yard in South Korea, it is due for delivery in 2016 (OE: March 2014).

It’s not just a new design for Subsea 7; the vessel also sports a new crane design, developed in partnership with Huisman—a 900-tonne safe working load (SWL) capacity rope-luffing knuckle boom crane.

For Huisman Equipment, based in Schiedam, the new crane is the start of a new product line in rope-luffing knuckle boom cranes, with capacities reaching 1200-tonne SWL and 5000m water depth.

The investment in the new design is already paying off. Since launching the new crane at May’s Offshore Technology Conference in Houston, Huisman has won four orders, two for 900-tonne cranes and two for the 250-tonne versions of the crane, in addition to the Seven Arctic’s crane. Discussions with several clients for 400 and 600-tonne versions were ongoing at press time.

Huisman Chief Executive Officer Joop Roodenburg and Mechanical Engineering Manager Henk Weterings hatched the design about two years ago, based on indications from the offshore industry about future needs, as projects are venturing into deeper waters with heavier loads in more remote and harsh environments.

Huisman’s 900-tonne RL-KBC’s two-fall anti-twist device.

The new rope-luffing knuckle boom crane is Huisman’s answer to those challenges, says Gerben Roks, product and sales manager, Cranes. The crane combines a traditional rope-luffing crane design (usually based on a pedestal or mast) with a traditional knuckle boom crane, which uses hydraulic cylinders to lift or extend the knuckle of the crane.

“The principle has been used on land cranes, specifically for offloading cargo. Offshore, it hasn’t been used before,” Roks says. He thinks this could be because people, until now, have simply increased the size on established crane designs. But, with projects increasing in size, crane capacities based on existing designs were reaching their limit, he says.

“Knuckle boom cranes have increased in size,” he says. “Traditionally, they went up to 250-tonne capacity. A couple of years ago, they stepped up to 400-tonne, and then 600-tonne and even 900-tonne, due to the demand in the market. With the additional capacity comes disadvantages, due to having a higher boom weight, which can cause rolling motion problems when slewing the crane, and potentially a need to ballast.“

By using rope-luffing, the boom does not have to be as big, and therefore heavy. The hydraulic cylinders, which put bending moment on the boom and limit working angles, can be removed, increasing the operating radius/outreach, Roks says. The new design also has larger capacity in high-lifting mode. Unlike traditional knuckle boom cranes, Seven Arctic’s crane can lift up to 600-tonne at 50m height from the main deck. With a 250-tonne load, it can also reach out to 58m radius, or 300-tonne at 53m radius. Retaining the knuckle means a shorter pendulum, reducing hook swing. Up to 700-tonne can be lifted down to 3000m water depth using double fall mode, which uses an anti-twist device (a type of rope spreader) incorporated into the crane’s base.

Huisman’s 900-tonne RL-KBC in high lift mode.

A degree of limitation on the range of the cranes has been introduced by the amount of wire currently available— the maximum amount of 156mm wire available on the market is 600-tonne piece weight. For the wire used on Huisman’s RL-KBC, this means that on a 1200-tonne maximum capacity crane, the deepest it can go in single fall is 5000m carrying 600-tonne. In double fall, it could go 2500m carrying 1200-tonne. The lifting capacity in high-lift mode is 1200-tonne.

The cranes are completely electric and use a traction and storage winch configuration, which stops drum crushing and rope cut-ins. The system also uses cylinder-type active (AHC) and passive heave compensation. In addition to being a backup to the AHC system, the passive system acts as a shock absorber for operations in the splash zone.

The latest challenge for the design team at Huisman has been incorporating all the available configurations into one system. “There are quite a lot of configurations—single fall, double fall, even triple fall, and high-lift mode—and they all have to be incorporated in to one system,” Roks says.

For Weterings and Roodenburg, it is rewarding to see the design being taken on by the market.

Huisman’s 900-tonne RL-KBC in storage.

“Two to three years ago, we noticed the market was changing and requiring larger capacity deepwater cranes (>300t SWL) with the advantage of the knuckle boom concept having a low swing point,” Weterings says.“The traditional knuckle boom crane concept has, in our opinion, reached the limit, resulting in extreme own-weight—SWL ratio.

“Therefore the idea was born to development a new crane type, with low own-weight, a center of gravity close to the crane center line, and all main drive components mounted inside the cranehouse or below the vessel deck. Our concept also had the possibility to increase the radius capacity, allowing greater use of deck space.”