Offshore Highway Construction - Lifting Challenges
Building an offshore highway has many challenges, not least of which is the requirement to lift and accurately place concrete foundation blocks on the seabed, position the supporting pillars and finally assist in the connections with the bridge deck. Enerpac Heavy Lifting Technology, reviews the lifting and handling requirements and explains how they shaped Enerpac’s development of the world’s largest offshore crane.
French consortium Bouygues Travaux Publics, VINCI Construction and Demathieu Bard Construction is constructing an offshore highway for the Reunion Island, East of Madagascar in the Indian Ocean. The 5,400m coastal highway (Route du Littoral) will connect Saint Denis - the administrative capital of La Réunion - with La Possession. It replaces the existing coastal road, which is exposed to falling rocks and flooding from swells and tropical storms.
The Bouygues Travaux Publics, VINCI Construction and Demathieu Bard Construction consortium is building a jack-up barge to construct the offshore highway. Enerpac is supplying an overhead gantry crane with an overall width of 30m and a lifting capacity of 4,800 tonne capable of lifting, moving and lowering the concrete gravity base, pile head and segments was selected for the jack-up barge. The crane is classified as A5 for the structure classification and M5 for the mechanism classification and designed to operate in tropical marine conditions (IP66).
The offshore highway is of a multi span bridge structure located in the open sea 100m to 200m from the shoreline. Each bridge pier consists of various concrete components, one of which will be a reinforced concrete, gravity base, which is placed on the seabed. After the base is secured and grouted an intermediate pier element will be placed on the base (protruding from the sea). Then, once the intermediate element is secured to the base, a pier deck segment will be placed on top. The bridge piers are spaced between 85m and 120m with 48 units in total. The bridge deck segments, which will span between the piers, will be fabricated onshore and placed using classic bridge launching techniques.
During the offshore highway construction, the Enerpac crane will make 288 heavy lifts made up of 144 cycles. Each cycle consisting of:
- at the quay: take the load from the jetty fabrication area and transfer it to its travelling position on barge and lower it down on the barge deck,
- at the designated offshore location: take the load from the barge deck and transfer the load to its deployment position, lower the load and retrieve the unloaded hooks.
The key technical requirements for the gantry crane are the ability to lift, move and accurately place the concrete blocks in a tidal environment. Of course there are also project and commercial considerations including the weight of the crane: speed of handling, vessel time / cost, and lifting systems costs and cost-of-ownership.
Selecting Crane Technologies
The kinds of crane technologies best suited to this particular project are strand jacks or winches to lift and lower, and skidding or trolley to move the blocks. The shape of the blocks is such that each requires four lifting points. Therefore, lifting would require either 8 strand jacks or 8 winches operating simultaneously.
Traditionally strand jacks would be the obvious choice. They are a simple, cost effective and lightweight method of lifting, and a proven technology with many thousands of units deployed on major lifting projects over the past 20 years. However, they are not the best option for this particular project. The reciprocating movement of the jacks can cause incremental movements of the load. Moreover, there are speed limitations with the jack after it has been unloaded, which could extend the time for the next lift. In addition, strand jacks have intensive maintenance requirements, most notably linked to the wedges and strand.
The service life of a strand is not linked to the number of bite marks that are applied by the wedges. It is the small residue of strand that is “scraped off” by the wedges, every time the strand passes through the strand jack. This “scrape off” will eventually contaminate the grips in such a way that the “teeth” of the grips are completely filled with the strand residue with the risk of slippage of the strand in the grips. At this point the wedges need to be replaced. With replacement possibly every 10 lifts, the cost and disruption to lifting operation could be significant.
Winches, on the other hand, are better suited to the project’s lift, move, and lower sequence. Compared with the reciprocating movements of the load using strand jacks, winches offer a better, smoother load movement. Moreover, although steel wire ropes on drum winches do require maintenance and even replacement, especially when exposed to the salty sea air for two years, in this particular application normal lubrication of the wire is all that’s required.
Enerpac selected a reeved winch system, based on 8 grooved drum winches with 500kN pulling force on the outer layer, will be used. The drum capacity is 800 meters based on a 52mm wire. The winches selected feature a grooved drum in compliance with the “Lebus Patent” for optimum spooling.
A significant advantage of winches in this application is the ability to adjust the winch speed. The winch’s hydraulic circuit is based on a closed loop system with “load controlled flow”. This means that when the hook is unloaded the speed of the winch can be doubled, allowing a shorter transition time between block lifts.
To skid or to trolley?
Once lifted, each block has to be moved into position. Longitudinal travelling of the gantry is a choice of either a skidding system or a trolley arrangement. Skidding involves a push-pull action by hydraulic cylinders as the gantry crane is moved along a track of laminated PTFE sliding tiles. Inevitably it uses incremental movement consistent with the push-pull cylinder stroke, in so doing it runs the risk of generating unnecessary fluctuating movements in the load. In addition, wear and replacement of the PTFE tiles needs to be factored into project scheduling and cost-of-ownership calculations.
For the Reunion Island project, Enerpac opted for a trolley system offering some clear advantages. There is continuous movement of the block and the speed is variable, enabling high speed positioning
is possible once the crane is unloaded. Moreover, there are very limited maintenance costs compared with the skidding system.
The trolley movement system comprises two trolleys per lifting beam, giving as total of 4 trolleys for the two beams. Each trolley comprises 4 wheels of which two are propelled with hydraulic drive motors and planetary gearboxes. The wheels run on two parallel rails bolted onto the runway beams of the barge.
Longitudinal length (railway for wheeled trolleys) is 80 meter. The average speed of the OHTC (trolleys) on the structure is variable from 0.0 cm/min up to:
- 100 cm/min when loaded,
- 200 cm/min when unloaded.
Each position of the cranes is entered into the lifting system’s programmable logic controller (PLC) with positioning accurate to 1 millimeter. Side shifting of the gantry hoists in a continuous movement is achieved by means of long stroke cylinders.
Overall, by opting for the winch and trolley arrangement, the Enerpac gantry crane provides a smoother handling of the concrete elements. It also allows quicker lifting and lowering. Using computer modeling techniques, Enerpac demonstrated that strand jacks and skidding would take 6 hours for one complete block lift and placing cycle. With the winch and trolley arrangement it will take only 3 hours.
There is a significant difference between the hydraulic power requirements for Strand jacks and Skidding vs Winches and Trolleys. The main difference is the installed power. To meet the required lifting speed a winch needs 55Kw while a strand jack operating at the same lifting speed will need 110kW. The same applies on the hydraulic system for skidding vs trolleys. Here the push-pull system requires twice the volume of oil to achieve the same speed as the trolley system.
Although the installed power on the skidding and strand jack system is twice the amount, the cost difference is minimal. The main reason for this is that the hydraulic system’s components (pumps and valves) needed for the winches and trolleys are much more expensive, because of the closed loop system and proportional valves, than the components needed for the strand jacks and skidding.
Synchronised Winch Operation
Enerpac is using a PLC-based control system to ensure accurate control and positioning of the hydraulic trolleys, 8 synchronized winches with four crane hooks, and side shifting cylinders on the beams. Together the 8 winches will provide a lifting capacity of 4800 tonnes. Without a load they will operate with a speed ranging from 0,0 cm/min up to 100 cm/min. When loaded the speed ranges from 0,0 cm/min up to 40 cm/min (lifting and lowering).
Technically the gantry crane’s operation relies on synchronizing the eight winches, two winches for each corner of the concrete components. Using winch control software enables configuration and monitoring of load and speed, minimum / maximum and synchronization alarms. In this way, the winches can be programmed to ensure safe and successful lifts. Enerpac has also developed a positioning system for the bridge pier concrete sections based on its experiences in placing bridges and offshore wind monopiles.