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Lillugrund offshore project is located between 7km west of Klagshman and 9km east of Dragör. Wind farm is proposed to electrify 60,000 Swedish homes with 48 Wind turbines of 2.3 MW each manufactured by Siemens. The water depth in wind farm site varies from 6m to 12m, tides xxx, ocean current xxx. Siemens Wind turbines were chosen by conducting open bid based on profitability, proven technology, Experience, Warranty. Similarly for laying foundation certain major criteria were put forward to choose a contractor were Foundation Design, Foundation Fabrication, Seabed preparation, Installation, Permits & conditions, Planning, Extended warranty, Qualification. Pihl-Hochtief Joint venture put a favourable bid for gravity based foundations. Electrical cables were purchased from ABB. Baltic offshore AB were in charge for laying inner array cables in-between turbines and transformer station. Svensk Sjöentreprenad AB was responsible for post lay inspection of cables by divers. Seløy Undervannsservice A/S was involved in Dredging, laying cables from transformer station and onshore substation at Bunkeflo.
The services have comprised among others determination of ice, waveand current forces, structural design, geotechnical design and design of scour protection as well as all appurtenances.
Siemens's service organization is responsible for the service
during the first five years. During these years, Vattenfall's technicians are working under
Siemens's management. Vattenfall manages the operation of Lillgrund from the beginning.
During day-time, Lillgrund is operated from a local control centre and during nights and
weekends, Lillgrund operates remotely from Vattenfall's operation centre in Esbjerg.
There were three different types of cables used for inner array cable. They were 95mm2, 185 mm2, 244 mm2 of 36kV. Larger diameter cables were used near the transformer stations. Export cables were of diameter of 400 mm2 and 630 mm2 (145kV) to transport electricity from offshore transformer station to onshore transformer station at Bunkeflo. The transformer at offshore are set up to convert AC to DC currents and transported through HVDC (High Voltage Direct Current) cables. Length of the inner array cables and export cable were 23.8km and 7km respectively.
Cables were buried 1 meter under the sea bed. Prior survey was done by using multi beam echo sounding system to look out for obstacles like pipelines, electrical cables, etc., followed by trenching, cable laying and post laying inspection by sea divers. Due to bad weather conditions
Daily weather log to claim for bad weather compensation
Logistics hold the key role of the offshore project, as it needs careful planning and effective use to transport vessel. Since
Sea power (Self-propelled ship-shaped semi jack--up) vessel was used during the construction, which can carry 3 entire turbines at a time. Nacelle and rotor blades were transported from Jutland (Siemens Manufacturing unit at Denmark), Tower were supplied by two sub-supplier at Jutland, and transported to Nyborg port by Trucks. Nyborg port was used as assembly place, where rotor blades were fitted into hub and loaded into Sea Power vessel as shown in the figure xxx
It took five days for the vessel to reach back to port to load the turbine again. One day to load the turbines on the vessel, one day to travel from Nyborg port to Lillgrund site, two days to install the three turbines and a day to reach back to Nyborg port for reloading. This process went on 24x7 as long as weather conditions were good to work. The vessel uses it jack up legs to raise above the sea level to form a stable working platform and it can be stable up to a maximum tide height of 1 meters.
Apart from this, there were 5 boats used during constructions to transport technicians from Seimens site office at Dragör and Vattenfall site office at Klagshman. Its 35 minute travel from onshore site offices to offshore site and 10-15 minutes travel in-between turbines.
The communication between the work vessels and to site office were by use of Mobile phone, Marine VHF with P-channel, stationary and portable, Internet access (ADSL) with E-mail, AIS (Automatic identification system, showing the positions of the vessels on a
The wind farm at Lillgrund poses a potential hazard to shipping. The wind farm is located
outside of designated traffic channels, but in the event of a ship losing its manoeuvrability
capabilities, there is a risk of collision with a wind turbine. Large ships without navigation
control, coming from the west or the south, will run aground before they reach the wind
farm. Even prior to the installation of the wind farm, sea charts covering the area showed
the location of the wind farm, thus warning ships against passage, and therefore, getting
them accustomed to avoiding that area. According to a risk assessment study by COWI,
the possibility of ship impact on one or more wind turbines is one in every 6,000 years. The
largest environmental danger during a ship collision is the risk of oil leakage from ships and
wind turbines, but these wind turbines contain their oil in sealed systems, reducing the risk
of oil-spills from a collision or operational activities.
During the construction period all foundations were fitted with a small, solar-powered
flashlight showing the position of the wind farm during the night.
All vessel movements and the identification numbers of personnel onboard and all transfers
of personnel have to be logged by the vessel and transmitted to the work vessel
Pihl- Hochtief delivered the foundations
Baltic Offshore AB- inner array cables
Peter Madsen Rederi A/S - transportation
Svensk Sjöentreprenad AB - diving transportation
Seløy Undervannsservice A/S- export cable installation work
Swinoujscie, Poland - harbour was rented by pihl to construct foundations
Wight is 2000 tonns
Export cable, inner array cables
Types of cable used
Procedures for laying cable, climatic limitations for cabling
Post lay inspection by divers
5 year maintanace by seimens (30% employee r from vattenfall for first 3 years)
4th & 5th years 50 % of employee from vatennfall
during nigts & weekends operated remotely by vatenfall
HOCHTIEF Construction's Civil Engineering and Marine Works business
unit laid the heavy-weight foundations for the wind power facilities. This involved the high-precision countersinking of 1,400 precast concrete units with a weight of 1,200 tonnes each. For this purpose, the jack-up platform Odin, especially developed for offshore works and other large-scale maritime projects, was used again.
Precise positioning was achieved with the aid of a four-anchor system and a global positioning
The capability to operate in high wind and waves are very important to be able to maximise
the amount of hours of work each week.
Need stable in high waves
The advantage of using a catamaran instead of a one-hull boat is the increased stability.
This is an important factor when boarding a wind turbine or loading and unloading of
equipment. The two-hull design increases the possibilities to form the bow to fit the form of
the wind turbine foundations. Figure 6 shows the specially designed bow of m/s Lillgrund.
In every foundation there was a j-tube from the seabed to the top of the foundation, which
was designed for the cable. One meter below the seabed, a bell-mouth with an enclosure
cap, was mounted on the j-tube. The enclosure cap had to be removed before the cable
pull-in, figure 23. Mud had filled in outside of the enclosure cap, requiring divers to vacuum
the space before every pull-in so the cap could be removed. Because of the water current
direction, clearing the space in front of the caps took 15 - 30 minutes each on the south
sides of the foundations and up to two hours on the north sides of the foundations.
A workboat pulled the cable to minimise the risk for a kink
it is recommended to schedule the cable installation work during statistically
good weather seasons, attempting to avoid winter by a comfortable margin.