Strand jacks, gripper jacks, towerlift systems and telescopic gantries are today all well established as alternatives to cranes. Often two or even three of these elements are used in combination. But what are they, and what advantages can they offer over cranes?
Strand jacks have been around since the 1940s when the French engineer Eugene Freyssinet developed pre-stressed concrete. Strand jacks were used to post-tension the reinforcing steel within concrete beams.
Strand jacks are hydraulically powered jacks that move along a steel strand. They grip the strand and then a hydraulic stroke moves the jack along to grip further along and release the first hold, just like a gymnast climbing a rope, hand over hand, except usually they are fixed to a high point and pull the rope upwards rather than pull themselves up the rope. Several strands are used simultaneously. The more strands that are used, the greater the power that can be achieved. While the strokes of the jacks are hydraulically powered, the loads are always held mechanically. Jacks may be used singularly, in pairs, or in groups to give any lifting capacity required. They can be precisely controlled and synchronised.
At the end of the 1970s PSC Freyssinet (PSC stood for pre-stressed concrete) remained a world leader in post-tensioning. “We were challenged to develop a new market for strand jacks. The offshore industry was in full flow, so we aimed there and at lifting,” recalls Les Brown, managing director of successor company Fagioli PSC. (Italian heavy transport specialist Fagioli took a 25% stake in PSC Heavy Lift in the 1990s to formalise an alliance forged when working together, and took full ownership in 1999.)
As the 1970s came to an end, rival post-tensioning specialist VSL (Vorspann System Losinger) was also introducing strand jacks to lifting applications and, between them, PSC and VSL pioneered the use of strand jacks for heavy lifting in the early 1980s.
PSC’s first application was in Kishorn, Scotland, using a 12-strand jack, called the K350L, working for an offshore contractor that did not have room to get cranes in. “That got us into the market,” Brown says.
In 1980 McDermott needed to put huge pile clusters onto an offshore platform jacket. The engineering contractor had built a huge goliath crane for the job but it collapsed under testing. “They came to us for four 600t strand jacks, which we had to design and build in eight weeks.” After much overtime, PSC delivered, sold both the jacks and its services as a subcontractor on the job and impressed the client. An order for four more jacks swiftly followed.
Another early use of strand jacks was to lower the seals into place on the Thames Barrier in London. Each seal weighed about 3,000t. Eight jacks, each with 27 strands, were used. It was the beginning of a steep learning curve. Thirty people worked on each operation. Today it would take just one, Brown says.
Strand jacks versus grippers
In 1981 PSC Heavy Lift introduced its towerlift system – towers like the masts of tower cranes with a cross-beam on which to mount the strand-jacks – to compete against the then better established gripper jack system. Kramo Montage of Norway and RMS of Sweden both were using gripper jack systems for heavy lifting.
Gripper jacks, also known as climbing jacks or even ‘koala bear jacks’ work by climbing a rigid square bar of solid steel in compression instead of strands in tension. They push loads upwards rather than pull them. On gripper jack systems, the cross-beam supporting the load is itself raised between two towers. On a strand jack tower lift system, the beam remains on top of the towers and the jacks pull up just the load.
According to Brown, this gives strand jack towerlift systems the advantage that the whole system can be tested before starting. But he readily acknowledges that “each application has its niche”. Another advantage is that, with pressure transducers in the jacks, strand jacks can weigh loads. This is not possible with gripper jacks.
Strand jacks also offer more flexibility, Brown says, because they can be used through 360 degrees. This means they can be used as a guying system, as they are at London’s Wembley Stadium, where they have been used to hold up a giant steel arch temporarily.
“A gripper bar is a rigid system. It is difficult to make adjustments. Strand jacks are more flexible,” Brown says.
Dorman Long Technology has both systems in its equipment fleet and says that climbing jacks offer an attractive alternative to strand jacks where it is desirable to keep the load path beneath the item being lifted.
Brown says: “We have looked at whether we need gripper jack systems in our hire pool and I wouldn’t say definitely that we don’t. But we have gone down the road of strand jacks and we have now got 500 strand jacks in our hire pool.”
Strand jacks can also be fixed just above the load instead of on top of the towerlift system, and used upside down, climbing up the strands rather than pulling the strands up. This option might be used, Brown explains, if there is a lack of headroom or if it is not possible to get access to the tope of the tower after the job to get the equipment back down again. (This latter point is not an issue with gripper jacks, since they can also be lowered afterwards.)
PSC’s first towerlift system comprised two towers with a lifting capacity of 1500t, standing 100m high. The towers could be freestanding up to a height of 48m but higher than that required guy ropes to support the structure. (Gripper jack tower systems also need multiple guys.)
Jacks versus winches and cranes
The disadvantage of both gripper and strand jacks, compared with winches, is the comparatively slow lifting speed. Winches can lift much more quickly. They can also be reeved to go around corners. A winch could be mounted on a tower lift system, like an old-style gin pole, but the size and weight to capacity ratio of strand jacks is far superior. “With the cost of moving them and setting them up, there is no competition,” says Brown. It is also much harder to synchronise the control of multiple winches.
Proponents of strand jacks are also able to claim safety advantages for their system. A crane ultimately depends on a single rope to the winch, whereas a strand jacking system has multiple ropes in each jack. Strand jack systems are also less dependent on calm weather than cranes.
Cranes will always be the preferred option where there are numerous loads to be lifted as they can move a round a site from lift to lift. But where there is no room for a crane, or the lifts are super-heavy, an alternative solution is required. The world’s largest crane, Mammoet’s PTC, has a maximum rated capacity of 1600t. Its MSG sliding gantry machine, which looks like a crane but uses strand jacks instead of winches, has a maximum rated capacity of 3,070t. These machines also have outreach capability that a towerlift system does not have. However, strand jacks mounted on vertical towers – where all the forces on the structure are carried straight down to the ground – can lift extraordinary weights. In August, Fagioli PSC lifted a staggering 26,000t. Working for Samsung Heavy Industries, it was contracted to lift the LUN/A and PA/B topside decks for the Sakhalin Energy Development. Each deck had to be taken from a multi point support (on which they were fabricated) to a four point support in preparation for lifting onto load-out support frames next year. The LUN/A deck weighed 16,900t and was lifted using fifty L600 (600t capacity) jacks. The PA/B deck weighed 26,000t and was lifted with 66 of the L600 jacks. For these lifts, they were raised just a metre, but for the second operations they will be raised to a height of 24m and will weigh 23,500t and 31,000t respectively. Brown believes that this will be an all time record for heavy lifting.
There is not a crane in the world that could lift, for example, that can lift these kinds of loads.
A telescoping hydraulic lifting gantry is a much simpler solution than a towerlift system for many applications. As a ready-made piece of kit, they are much quicker to set up and, with a runway or skid track, they can travel horizontally. In general, however, they do not offer the same sort of heights as a towerlift system, and, unless there is some kind of winch or jacking system mounted to it, they cannot lower loads into a hole. Their lifting range is limited to the scope of the telescoping action.
Increasingly common is rigging up a hydraulic gantry on top of a four-tower towerlift system, to get horizontal travel capability on top of the towers. It is the kind of arrangement often seen in Specialized Carriers & Rigging Association’s rigging job of the year competition entries by the likes of Barnhart and Emmert.
This year Fagioli PSC has added 750t capacity strand jacks, the L750, to its equipment range. Its previous biggest was the 600t capacity L600. The L750 jacks operate on 50 strands, each with a nominal capacity of 15t. It is designed to be used with a new second generation towerlift system.
Demand for its towerlift system on projects around the world meant that PSC needed to fabricate more equipment but has made some significant design changes. There were three reasons for this.
* Firstly, the size of petrochemical vessels keeps increasing – 2,000t and 100m high is not uncommon these days, particularly on gas to liquid projects. The new system needed more lift capacity.
* Secondly, getting the cross-head beams on top of tower systems requires very large cranes for erection and dismantling. As Fagioli PSC does not own cranes, this means calling on the services of competitors. Some kind of self-erecting system was required.
* Thirdly, the stability guys that support towers on the original system often interfere with other parts of the site, which on something like a live refinery can be a particular nuisance. An unguyed, freestanding system was required.
“We decided the market now needs a 3,000t capacity, 100m high system with no stability guys,” Brown says.
The second generation system has been built and used but is still in the process of further refinements. Parent company Fagioli SpA is using it to lift a 635t flare tower on the Hammerfest project in Norway. The towers are 4m x 4m in section rather than 3m x 3m, which is enough to make them securely freestanding in either operational or storm conditions.
A variation on the second generation two-tower towerlift system has been sold to a customer in China. The customer’s consultant specified luffing capability, so stability guys were still required. This system is self-erecting, using strand jacks to raise the cross-beam. Even in self-erect mode, the system still requires a ‘small’ 300t capacity crane, but everything is built at ground level rather than in the air.
A self-erect system for unguyed towers is in development, Brown says.
In the past 10 years, says Brown, project designers have embraced the capabilities of strand jacks and other alternative lift systems and design projects around using them. He says: “As vessels grow on petrochemical projects, so the towerlift market grows. Vessels have outgrown the ability of cranes to lift them. A 500t vessel 10 years ago was a big vessel, but it’s not anymore. There are not many industries that don’t use strand jacks these days. Everyone has strand jacks in their fleet. They are becoming a common tool.”
