Caissons cast in Poland for a windfarm in Denmark have proved a victory for gravity foundations.
At the end of May, the last consignment of massive gravity foundations for Denmark's $255M Nysted offshore windfarm started its 200km journey from a casting yard on Poland's Baltic coast.
This event marked the final phase of Danish contractor Per Aarsleff's E40M contract for client consortium Energi E2, DONG and Sydkraft to build and install 73, 1,300t concrete caisson foundations for the windfarm 9km off Denmark's east coast.
Once they arrived at R dsand in the Femer Baelt, close to Lolland, a Per Aarsleff team placed and ballasted the caissons on prepared stone pads on the seabed. All were on course to be in place by the end of June, three weeks ahead of schedule, enabling turbine erection to start.
The caissons, standing in between 7.5m and 12.75m of water and each ballasted with 500t of rock and sand, are arranged in eight rows of nine, 850m apart.
The 72 main foundations will support 2.2MW Bonus Energy turbines on top of bolted-on tapered tubular steel towers.
The three-bladed generator nacelles will sit 69m above sea level. The remaining caisson will support the transformer substation for the 158.4MW windfarm, which covers an area of 24km 2.'The project has gone very well and during February and March we have had the least windy weather here for 25 years, ' says client foundation supervising engineer John Nielsen.
Per Aarsleff built the caissons in a casting yard in a free bonded area of the Odraport at Swinoujscie for two reasons: the sheltered basin was ideal for caisson construction and local labour is significantly cheaper than it would be in Denmark.
The heavily reinforced foundations were built directly on the decks of four 30m by 90m flattop barges, each 10,000t capacity vessel capable of accommodating four foundations.
The caissons have a hollow hexagonal base with internal walls to create six inner cells. At the centre is a hollow central shaft with a flaring 'ice-cream cone' top.
Per Aarsleff used the barges like giant floating pallets, pulled round the casting basin in phases.
First, lean-mix concrete foundation mat templates were placed on each barge deck to match the 16.5m caisson bases.
On each, a geotextile membrane was laid, followed by assembly of reinforcement for the 550mm thick caisson slab and placing of starter bars for the 2.4m high main outer and six inner cell walls and first section of the 4.25m diameter central shaft.
Formwork was erected for the slab, cast in one continuous 115m 3pour using a C45 mix with 32mm aggregate.
A central temporary steel tower was erected inside the shaft with a perpendicular laser inside, fixed to the caisson base, used for all setting out and surveying to compensate for any movement of the floating barge.
Reinforcement was fixed for the walls and central hollow shaft up to 1m below the start of the icecream cone.
Moving to the second position in the basin, the circular inner and outer steel shutters for the shaft and the shutters for the walls were fixed and filled in one continuous pour. Some 120m 3of C45 concrete went in, with smaller 16mm aggregate to cope with the small gaps between the more dense reinforcement.
After curing, the forms were stripped and as before all joints cleaned. The remaining central inner shaft forms were fixed, followed by six petal shaped segments, pinned together and fanning out to form the 7.2m high and 55infinity inverted ice-cream cone.
Reinforcement cages prefabricated on the quayside were placed and fixed inside the cone shutter together with a circuit of cooling water pipes.
A prefabricated double ring of wind turbine tower fixing bolts was also placed and levelled to a -1mm tolerance, before filling the form with 285m 3of the C45 mix in a continuous 20 hour pour, from the Elba and Liebherr onsite batchers.
The barges then progressed to the third location, where the cone shuttering was stripped and curing monitored. Before departure for Denmark, bollards, railings and cable ducts were mounted and the concrete gravity foundations secured for the voyage.
'We worked closely with Copenhagen-based consulting engineer Cowi to produce detailed design for the caissons, which range over 20 different heights from 11m to 16.25m, ' says Per Aarsleff production manager Niels Coff.
'There was concern about the possibility of the concrete cracking as the difference in temperature between the slab and shaft and the shaft and cone should not exceed 15infinityC. But a combination of formwork insulation during the winter months, and pumping cooling water through the pipeline cast into the concrete, worked very well and we have not had any cracking.'
It took between 26 and 29 working days to complete four foundations. 'The first barge, with its caissons, left last October and we aimed to have one barge en route to and from Denmark with the other three in the basin, ' adds Coff.
'When we started last summer we planned for a loaded barge to leave Swinoujscie every 18 days, but after a while we achieved a 13 to 15 day cycle and as we approached the end of the project we got down to 10 days, ' he says.
Meanwhile, in Gedser on the south-eastern tip of Denmark, the team was busy preparing the seabed to receive the caissons.
Using GPS-positioned and controlled vessels working in turn at each wind turbine foundation location, an average of 1.5m to 2m of sand or soft clay was dug out to create a circular excavation with a 22m bottom diameter and 1:2 sloping sides to expose the stiff underlying moraine clay.
Five cone penetrometer tests were carried out in the excavation, which, following client approval, was surveyed to prove the -200mm tolerance specified.
Divers placed a steel plate with a vertical steel pin at the centre of the flat-bottomed crater with a similar one on the seabed next to it. Temporary lattice steel masts with GPS antennas on top recorded positions and depths. A 16m diameter Tshaped steel ring beam was placed concentric to the central pin in the excavated crater and a GPS-controlled excavator with a clamshell crab then placed 2mm to 63mm stone in the bottom. A long steel blade, pivoting round the central vertical steel pin and running on the outer T-ring beam, was guided by divers and pulled round by winches to form a smooth, flat, 300mm thick stone pad.
'The exceptionally tight -20mm tolerance on level was achieved by measuring the pressure difference from transducers placed on the bottom of the central steel pin's base plate and the rotating scraper blade, ' explains Per Aarsleff engineer Christian Krarup Smith.
'The ring beam and central plate and pin were then removed from the crater ready for the completed stone pad to receive the caisson foundation.'
When a barge arrived on site, a special 1,800t capacity crane barge with giant crab-like claw grabbed the cone and lifted the caisson before towing it to its location for final positioning.
Computer software developed by Per Aarsleff constantly compared and displayed on screen the actual position of the caisson in relation to its final theoretical heading and position on the seabed.
Location, heading and depth of the stone pad, recorded from the GPS antennas was stored in the program. Data from four more GPS antennas on the crane, plus measurements from a distance laser on the crane to the caisson top, were also fed in to position the crane on to the correct heading and target. Once on target a spud leg was dropped from the crane barge into the seabed and final adjustments made.
'The crane lowered the caisson to 1m off the stone, and roll, pitch, heading and depth were checked before lowering it on to the pad, ' explains Per Aarsleff surveyor Lars-Georg Rodel.
'Alignment and level of the turbine tower holding down bolts, heading and target position were all double checked prior to releasing the caisson.'
Placing of each foundation took about three hours. Depending on weather conditions, all four caissons were placed in two to four days.
Per Aarsleff then used its Stonejack jackup barge with a GPS guided excavator to place up to 750kg granite boulders to the top of the walls in the caisson's six cells. Sand inside the central hollow shaft brought total ballast to 500t. An initial layer of scour protection rock was placed in the foundation crater and half way up the caisson base slab. Electrical cable ducts on the seabed were installed by divers, followed by a final scour protection layer of 60kg to 300kg stone.
A follow-on contractor will erect the steel towers, turbines and cabling before the windfarm goes on-stream in October.
'Our concept of minimising floating crane operations by casting relatively light concrete foundations on barges and ballasting them after placing has proven itself, ' says Per Aarsleff manager Lars M Carlsen.
'Our concept must have moved the boundaries a little bit in favour of the gravity foundation.'