Making room for a bigger boom

9 September 2011

Newer and bigger offshore wind turbine construction sites, such as the London Array that A2SEA is building in the Thames estuary, will place unheard-of demands on turbine construction. Cristina Brooks talks to Liebherr, GustoMSC, and Neuenfelder Maschinenfabrik (NMF) who are all racing to build the innovative crane vessel for the job, the crane-around-the-leg (CAL).

The new generation of cranearound-the leg (CAL) is targeting a broad mix of offshore and onshore construction firms, shipyards and shipping firms—businesses from differing arenas going into offshore wind turbine installation.

Contractors will need specialized turbine installation cranes with lifting capacities over 300t, to install the turbines’ monopile foundations, and long slender booms for lifting 200t nacelles 100m above sea level, which is twice the height of average offshore crane booms designed for oil drilling.

There are only a handful of dedicated turbine installation vessels. Many of these, like the MPI Resolution built by US engineering firm Mayflower Energy, are jackup vessels.

Jackup vessels originate in offshore drilling. They motor to the site, lower legs to the seabed to enhance platform stability, and then raise the long boom stowed on deck. Because the jackup’s legs touch the ground construction on a jackup is more comparable to being on land, as it makes the ship independent of the waves so the crane operators only have to worry about the wind.

Despite these advantages, manufacturers have sought to eliminate the need to stow the crane boom on the deck instead of cargo, by moving the crane from the deck to around the leg of the vessel, creating the CAL.

Positioning the crane on the leg frees up valuable deck space. “Its advantage over a stand-alone-crane is that it rotates around one leg of a jackup vessel, and that due to this position only relatively little deck space is covered by the crane,” says Tanja Strele, sales assistant at Liebherr offshore.

Shipbuilders are looking to place a maximum number of turbines on the deck to minimise contractor spending on supporting ships for turbine transport to the site.

Ralf Ressel, CEO of NMF Kran, and previously director of research and development at Terex, says, “They have found it’s the most efficient to have one ship where you can take the parts from the shore and install them right away instead of having separate ships, one which is doing the transport and one doing the installation. The calculation in the end for fewer ships is more economical.”

Contractors also want to use CAL vessels to increase the speed of construction, seeing that downtime due to adverse weather brings heavy costs. Ressel says: “This jackup system gives them the possibility to work with a broader weather window, because if you’re in a jacked up condition, it’s like land and it doesn’t matter how high the waves are. In the end, if you can work half the time you are making half the money.”

As wind turbine construction has taken off, major companies like Liebherr, GustoMSC and NMF Kran have been rolling out new CAL designs for clients.

Dutch engineering firm GustoMSC built the first CAL, the Cowrie One, in 1968. It was a smaller forerunner of this new generation of CAL, designed for oil installation. The firm started the turbinespecific CAL race in December 2009 with the announcement it had a contract with Drydocks World in Dubai for a CAL vessel, the NG-9000C; four more orders followed.

The NG-9000C has a capacity of 800t and can install all turbines currently on the market, says Gerrit-Jan Schepman, GustoMSC’s manager for marketing and business development. “The majority of offshore wind turbines will be in the range of 3.6 MW (with gearboxes) or 5 MW direct drives. These weights of the nacelle are not differing significantly, thus the 800t crane can perform the job,” says Schepman.

German crane manufacturer NMF, announced about a year later that it would develop a CAL vessel for client Van Oord, and Liebherr announced its order from Beluga Hochtief the following February.

Despite being the last to secure a CAL contract, Liebherr is likely to deliver the first dedicated wind installation CAL to a client, as the order will be totally delivered by January 2012; GustoMSC’s five cranes are scheduled for delivery starting the second quarter of 2012 and NMF’s will be delivered in the third quarter.

As the firm first linked to historic CAL cranes, engineering firm GustoMSC’s strength lies in its design expertise, and in its reputation as the largest supplier of CAL cranes for CAL vessels.

GustoMSC belongs to a group selling systems and services for oil exploration. It designs both cranes and jackups, which are then manufactured by partner shipyards. It can offer the advantage of an integrated crane and vessel combination, designed to ensure uneven loading doesn’t cause a leg to ‘punch through’ the seabed, capsizing the vessel.

Schepman says: “The point of overturning stability of the jackup during crane operations is one of the design criteria we are considering seriously. If the leg(s) are overloaded this could result in a ‘punch-through’ and severe listing of the platform, because the support of one leg is failing.”

Like GustoMSC, NMF designs jackups. Having a combined ship and crane supplier can save the client servicing time. Ressel says: “Our people are involved in the design. They’re trained from the beginning. It’s good that we design and build, it’s also an advantage with the customer if they have a complaint.”

Saving the client additional transport hassle, NMF’s ships may be assembled on the water in Europe by its parent company Sietas, a shipbuilding company and deck crane specialist, as opposed to being assembled at international shipyards. “Because our shipyard is located right on the water we can load everything on the ship,” says Ressel.

Involving itself much less in the process of vessel creation, Liebherr is a manufacturer with 35 years experience supplying equipment ‘a la carte’ for the oil and gas industry. Supplying a modular crane does require detailed knowledge of the ship, Liebherr’s Strele says. “Due to the around-the-leg design, there are various interfaces with the vessel and the crane carrying structure. We are therefore in permanent contact with the vessel designers and the customer to ensure the correct implementation of the crane into the vessel design,” says Strele.

Neck-to-neck competition has pushed firms to come up with innovative features to simplify what is currently a hazardous operation in the budding industry of wind turbine installation. The features designed have included split booms and boom parking systems, intended to capitalise on the deck-space saving format and allow vessels to stow more turbines.

Liebherr is introducing a CAL crane, CAL 64000 – 1500 Litronic, with the largest tonne metre rating yet. The crane’s great capacity means that current models will be capable of handling the larger 10mW turbines proposed by firms, such as Clipper, for the future.

Liebherr’s contribution with the CAL 64000 – 1500 Litronic is the optional twin boom which may provide a more space efficient way of parking the boom. Typical offshore cranes usually take up nearly half the deck, thereby preventing large turbine loads. “The twin boom design had been especially developed for cranes on jackupvessels where users often claimed the ‘problem’ of having to park the boom laterally over the deck,” says Strele.

Instead of a single lattice boom of a truss work that can only be stowed by folding diagonally to one or the other side of the leg longitudinally opposite of it, the twin boom is divided into two smaller booms, linked by massive steel ropes, that sit on either side of the lateral leg.

Rivals question, however, whether the deck space saved this way is worth it. “A split boom could be fitted over the forward leg but this operation can’t be done with the legs fully retracted in floating condition. Thus the crane can’t be used during harbour conditions,” says GustoMSC’s Schepman.

By contrast, the GustoMSC crane can be folded for storage either across the deck or along the leg at the outside of the unit.

These bulky booms are not only difficult to stow, but difficult to manoeuvre during parking. With NMF’s OC900030 crane, it has innovated a unique boom parking system that compensates for human error by automatically folding the crane the last metre. “Imagine if you are parking the crane and you see the tip of the boom 100m from a little hold in the boom, you’ll have problems hitting it. You can cause damage if you do something wrong, or you may need someone to assist you. If you do it automatically you don’t have to worry about it,” Ressel says.

NMF claims that its ship and crane is one of the greenest, since, it says, its use of efficient power systems and biodegradable oil promote low energy consumption, only 2,000kW, which is half that of some competitor companies.

In the past some contractors were wary of buying dedicated turbine installation vessels, forseeing stagnation in wind turbine construction, but these CAL cranes are designed to be multi-purpose. “As a back-up use of these vessels, in the offshore oil and gas market there will be a need for installation and decommissioning work. Especially in the Southern part of the North Sea, a large number of jacket type production platforms have been installed, that will need to be decommissioned at some time,” says Schepman.

NMF notes that the diverse uses for CAL cranes is typified by the variety of potential clients. “Van Oord is coming from the dredging business and used to building things in the water,” he says. “Then we have requests usually from people who do heavy transportation, so regular ship owners. We have people who own ships.”

NMF, scheduled to deliver its first ship to Van Oord, has seen interest from international clients. “At the moment most of the clients are in Europe. But we already had a request from a Taiwanese owner and a Chinese owner. It’s starting also on the other side of the globe. And there is also some discussion from the US side. I think UK will build most of the wind parks so it’s concentrated right now around Europe, but its spreading.”

While GustoMSC is far ahead of the competition in the number of orders, other crane builders are introducing features to improve what is currently a challenging sector of offshore construction. All are looking to provide the industry with bigger and better booms.

NMF's OC900030 Crane Around the Leg NMF's OC900030 Crane Around the Leg
The CAL 64000's unique twin boom divides in two The CAL 64000's unique twin boom divides in two
Liebherr's Crane Around the Leg:CAL 64000 - 1500 Litronic Liebherr's Crane Around the Leg:CAL 64000 - 1500 Litronic
GustoMSC developed the NG-9000C for erecting offshore wind turbines GustoMSC developed the NG-9000C for erecting offshore wind turbines
Liebherr's Crane Around the Leg:CAL 64000 - 1500 Litronic Liebherr's Crane Around the Leg:CAL 64000 - 1500 Litronic