SEAWALKER, Seacore's eightlegged walking jack-up platform, is playing a key role in a £750,000 project to improve the access channel to the Royal National Lifeboat Institution's station at Sennen Cove, Cornwall.
Designed by consultant Posford Haskoning for the RNLI's shoreworks department at Poole, the 120m long, flat-bottomed channel will improve conditions for the lifeboat crews and prepare the way for modern craft due on station in 2009.
The channel is being excavated in granite using drill and blast. It has a trapezoidal cross section with 25° sides and is 20m wide at the slipway and 50m wide at the seaward end. The finished channel bottom will be 1.75m below chart datum.
As well as jacking legs at each corner of the platform, Seawalker has two moveable jacking legs on on two sides. These four auxiliary legs are connected to a frame which slides horizontally along the side of the platform deck, allowing the rig to 'walk' between positions.
Capable of working in up to 9m of water, Seawalker avoids the delays often experienced when floating conventional platforms between working positions.
The platform has been working in breaking waves up to 5.5m high in stormy weather at Sennen Cove. Seacore is excavating the channel in 10m wide, 120m long strips, working away from the slipway.
Two Boart Longyear HD 150 rock drills mounted in a moveable cradle on the stern of the platform drill the blast holes before divers place charges. Typically 1.15m depth of granite is blasted.
When it reaches the end of the channel, Seawalker makes its way back to shore, with an excavator on the bow digging up the blasted rock, placing it to the side of the channel.
At low water spring tide, a temporary roadway allows long reach excavators access to the site.
These load the granite into dump trucks for disposal off-site. In all, Seacore will excavate between 5000t and 6000t of rock.
Work began at the end of March and is due to finish this month, depending on the weather.
Seawalker will then be shipped to southern Africa where it will be used to dredge and mine diamonds in the raging surf of the west coast.