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Walk on the wild side

Giant drilling rigs may soon be striding across British beaches. Dan Simpson reports

UK based marine geotechnical drilling and civil engineering contractor, Seacore, has developed a series of walking platforms which can provide safe accurate positioning in previously hard to reach areas such as rough inter tidal zones.

Seacore says the system increases productivity over conventional jack-up platforms, which require calm weather to float and move: bad weather often results in costly delays.

The company claims the new system eliminates weather related down time because it can be set up in calm water, such as a harbour, and then walk out into rough sea or strong currents.

'By developing a walking platform we have removed the vulnerability of floating to a new location and provided the added bonus of increasing working time and productivity by 30%, ' says Seacore managing director Robert Goodden. 'The Seawalker will be totally independent of sea conditions and provide a major advantage over traditional jack-ups'.

Designs have been completed for a 24m by 24m walking platform. The platform has 12 legs - four static jacking legs plus eight moveable legs configured as one pair on each side - and will be able to move back and forth, from side to side and diagonally.

Such a platform would be invaluable for work that requires precision movement, such as constructing foundations for wind farms and maintaining them.

The designs follow a 12m by 12m prototype developed by Seacore, which had two pairs of moveable legs and was the first in a range of eight legged, selfcontained, walking jack-ups.

The first Seawalker, which cost over £500,000 to develop, was successfully tested along exposed stretches of the north Cornwall coast in heavy seas, high winds, surf and on beaches.

Like traditional jack-ups, the prototype had four jacking legs, which could be raised and lowered from fixed jacking points at each end of the platform. The platform moved with another moveable pair of jacking legs on each side attached to a sliding frame, which could be jacked back and forth.

To operate, the four main fixed position legs are jacked down to the seabed to lift the platform clear of the water.

Walking begins by jacking forward the two sliding frames, each complete with their pair of legs, along the side of the deck.

The moveable legs are then jacked down to the seabed to support and take over the weight of the platform, allowing the main legs to be retracted and the entire platform jacked forward through the two side frames. The main legs are again jacked down to retake the weight for the simple walking cycle to be repeated.

Speeds of up to 25m/hr were achieved with the prototype, whereas the 12 legged platform, which walks using the same principles as the prototype, is expected to achieve speeds of 50m/hr.

Although successful, the prototype could only walk forwards and backwards with just enough slack in the system for the platform to turn a few degrees: 'A sideways displacement of approximately 2m over two rig lengths could be achieved', says Seacore project manager Tony Halliday.

This is suitable for linear operations such as trench digging and pipeline and cable laying but Seacore wanted to take the idea further and develop a platform that could move in all directions.

Goodden says the new 12 legged platform is 'the only realistic way of working in the surf zone for any prolonged period where accurate positioning is required'.

Seacore admits that the major problem in developing the platform is that the increased weight of the jacks decreases the payload of the platform: increasing the platform size does not directly increase the payload. It is a matter of finding the 'biggest platform size with the most efficient payload to make the platform feasible', says Halliday.

Only 30% of the total weight of the platform can be attributed to payload. A 24m by 24m platform can therefore only carry a payload of approximately 450t.

Increasing the payload by 75-100t would mean the total platform weight would have to be increased to about 2,000t.

The advantage of the one way walker is that it has a payload of approximately 50% of its total weight.

Seacore is considering increasing the payload by decreasing the weight of the platform without losing any strength through the use of higher tensile steel and by constructing the leg in fewer sections.

Limitations to the depth the platform can operate in are dictated by the worst case storm conditions; Seacore claims that safe depths of operation for the new platform are around 20m.

However, in calm water the platform can operate in greater depths.

A particularly useful aspect of the platform is that it can walk up the beach and onto dry land to start laying cable from the land connection, through the intertidal area and out to sea in an uninterrupted operation.

Although the previous prototype was developed entirely by Seacore, the 12 legged platform is a 50% joint venture with Trans Hex Group, a South African mining company, which is planning to use it for diamond mining.

Geotechnical uses for the platform include geotechnical site investigation drilling, trenching, pipeline and cable laying, drilling and blasting, and other underwater work, which would normally require specialist subsea machines.

Most of the platform has been designed in house by Seacore's Cornwall office, with the design of the specialist hydraulics contracted out to a company on mainland Europe.

Designs for the 12 legged walking platform have now been finalised and it is four to six weeks away from construction, which will begin as soon as a deployment date is set. It is anticipated that the platform, which has cost around £500,000 to develop so far, will be built and tested in South Africa.

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