The monopiles and new marker lights are part of an approximate SEK500M project by the Swedish Maritime Administration, Sjofaktsverket.

The vast port improvement scheme, which started at the beginning of 2003, also included dredging about 11Mm 3of clay and drilling and blasting approximately 0.5Mm3 of rock from the channels and port entrance. Bottoleden, which is 6km long and relatively straight and wide, was not deep enough for vessels with a draught exceeding 10m. The main Torshamnsleden channel is 10km long and deep enough for Panamax ships, but it is narrow and with a tight bend has been difficult to navigate.

Seacore, operating in up to 25m of water from its Deep Diver 1 jack up platform, provided its specialist monopile installation and beacon gantry erection service to main beacon design and build contractor M T Hojgaard, Denmark's biggest civil engineering contractor.

Working around the clock in two 12 hour shifts, Seacore used its own Teredo T8 reverse circulation drill and a hired PTC 175 vibratory pile driver to install the project's three types of monopiles, ranging from 1.5m to 1.8m diameter.

The Type 1 piles, as they are known, involved drilling a rock socket through little or no overburden and into between 8m and 9m of very hard granite with quartz inclusions. This was done by holding a reusable conductor tube in a pair of special circular gripping waling gates over the pile location and resting on the seabed. The pile gates, which operate just like a pair of mechanical hands, can move independently of each other and are mounted on a frame supported by a pair of cantilevered finger pontoons on the stern of Deep Diver . The T8 reverse circulation drill rig was gripped on to the top of the conductor and the drill bit lowered down inside to drill the socket in the up to 200 MPa compressive strength rock.

On completion the drill rig and conductor were removed and the appropriate pile, complete with temporary bungs in each end, was towed out from the Arendal quay in Gothenburg to Deep Diver for the jack up's crane to perform a buoyant lift. The pile was gradually flooded until the water levels were equalised and pile vertical. The lower bung was removed and the pile lowered into the drilled clearance rock socket and plumbed and levelled prior to grouting up the annulus between pile wall and the hole.

An extension piece was flange bolted on by divers to take the pile above sea level and the cantilevered gantry attached prior to M T Hojgaard completing the beacon.

The Type 2 piles were installed using a combination of the T8 drill and the PTC 175 vibratory pile driver in a 'drive, drill, drive' operation. A particular pile was towed out from the quay and positioned and held in the pile gates and slowly lowered into the seabed overburden. A reusable conductor extension tube was bolted on and vibrator attached to drive the pile down to rock head.

The vibrator was replaced by the T8 drill, which cleaned out the overburden inside the pile and continued drilling an under reaming socket slightly bigger than the pile diameter into the underlying rock. The drill was replaced by the vibrator and the pile driven to level into the blind hole. Some of these piles have additional support at the seabed provided by a large circular steel frame resembling a doughnut, which is lowered down the pile by Seacore and filled with ballast and grouted by M T Hojgaard.

The conductor was then removed and replaced with the pile extension and beacon gantry.

The third or Type 3 piles were pitched and held in the special gates and with a reusable conductor extension, driven with the vibrator to a penetration of up to 24m into the seabed through a mixture of overburden, loose sands and clays.

All the Type 3 piles were also stabilised with the ballasted doughnuts at the seabed. In addition all the project's 34 monopile foundations incorporate a special flange bolted 'weak link' just above seabed level. In the event of the navigation beacons being hit by a vessel they have been designed to snap off, leaving the solid seabed foundation undisturbed. The use of rigid beacons, compared with the traditional marker lights fixed to moving floating buoys and anchored by chains to the seabed, is expected to considerably reduce the amount of maintenance dredging of the channels.

MJ Information No: 19801