Hyundai, Spar lifts in 1999 enabled by strand jack technology

The offshore industry has seen two significant projects in 1999 performed with the use of strand jack and tower lift technology:

• Brent Spar Decommissioning project: After lifting more than 8,250 tons on one system to raise the Spar from the water, a second temporary sliding gantry system performed four lift and traverse operations with a load of up to 2,500 tons.
• Record onshore lift: Possibly a new world record for an onshore lift was the 12,000-ton deck lifting of the RBS 8-M semisubmersible at Hyundai Heavy Industries' Ulsan Yard. The lift aided Hyundai in introducing a new technique of fabricating semisubmersibles completely out of the drydock.

The strand jack and tower lift technology was developed and operated by PSC Heavy Lift Ltd. Strand jack technology is still considered specialist, but the lift process is becoming part of the mainstream engineering methodology in the fabrication of heavy structures. The equipment can either facilitate traditional construction methods or enable entirely new construction methods.

Strand operation

The benefits of strand jacks can be determined by understanding how the jacking system works and how the separate components, jacks, strand, power packs, control mechanisms, and support towers combine to create a jacking system.

The development of heavy lifting by strand jacks has its origins in the use of very high tensile stranded steel cables - "strands" - for developing very large forces in prestressed concrete structures. The high-pressure hydraulic jacks originally developed for tensioning prestressed concrete cables form the basis of the jacks used for heavy lifting.

The PSC strand jack lifts lower or move structures in a series of increments roughly equivalent to the stroke of the hydraulic jack. For the purpose of lifting or moving operations, the jack piston is simply extended and retracted in sequence. The wedge grip mechanisms automatically lock onto the lifting cable and pull it through the jack as the piston extends, and then locks it in its new position as the piston retracts to reset. A similar reverse sequence is used for lowering operations. Secondary hydraulic systems incorporated in the gripping mechanisms are used to release them and allow passage of the cable down through the anchor as the piston retracts, and no automatic lifting as the piston extends to reset.

Functionality

The jacks, which come in a range between 15 tons and 600 tons capacity, may be used individually, in pairs close together, or in groups to give any combination of lifting capacity required. They all have the following basic characteristics:

• Mechanically fail-safe gripping mechanism.
• Operating speeds up to 60 meters per hour.
• Final positioning to millimeter accuracy.
• Automatically synchronized operating speeds, regardless of load.
• Completely self contained.
• Load held mechanically, not hydraulically, if stopped with jack piston at part stroke.
• Full access to gripping mechanisms at all times.
• No requirement for re-lubrication of gripping mechanisms after initial installations.
• May be used in the vertical through to horizontal and even inverted position with equal efficiency.

The actual lifting capacity of any jack is directly related to the number of strands which are used with that particular jack, and is calculated based on a load factor of 2.5 on the breaking load of each strand. Thus typically, for the L300 jack, 19 strands having an individual breaking capacity of 38 tons and a total breaking capacity of 722 tons, are used to provide a safe working load of 290 tons.

Strand makeup

The strand now used is of 0.7 in. (18 mm) diameter, with 7 wires in 6/1 construction, which is die-formed after standing to further compact the wires. The 6/1 construction of strand is simply anchored and the problems associated with terminations of multi-layer ropes are easily avoided.

Although grouped in a "cable", all strands are anchored individually, which ensures the development of the full cable strength and, at the same time, ensures a high degree of redundancy in the cable. It is also significant that the strand is a very economical tension medium - possibly one of the most economical in existence.

Jack operation

To operate jacks continuously at reasonable speed requires hydraulic pumps. Larger jacks, larger numbers of jacks, higher speeds, or all of these require, very large hydraulic pumps.

The actual control and monitoring of the jack or system of jacks requires sophisticated arrangements of valves - manual, solenoid, or pilot operated - and gauges or transducers or electrical potentiometers.

The power packs used with strand jacks range from the smallest electric units, which are hand portable to units of 5-6 tons in weight, fitted with 180-hp diesel engines capable of operating a system of jacks lifting over 1,000 tons at a speed of 50 ft/hour.

Without going through all the features of hydraulic power packs, the minimum functional requirements of properly de sign ed heavy-lift power packs, and certainly those at PSC are as follows:

• Power packs should be designed so that all jacks in a system will operate at the same speed - even if they are not all carrying the same load. This keeps the lift in the right attitude and keeps the individual jack loads distributed as planned.
• It must be possible to pre-set a pumping pressure, and hence lifting force, to prevent inadvertent overload on each jack in a system.
• It must be possible also to adjust the pre-set intentionally.
• It must be possible to open or close the flow to any or all of the jacks in a system for lift adjustment.
• The actual pumping pressure, hence load, at each jack must be visible to the operator.
• The operator's functional controls must be simple and unambiguous.

Control mechanisms