Svetsaren 1/2008 - Esab

(+ flange) and the monopiles. The design temperature
of the transition piece was -10°C (above
LAT - Lowest Anticipated Tide) and 0° (below
LAT) for the monopiles, whereas the lowest CVN
test temperature was -50°C, valid for the thickest
wall sections of the transition pieces. The
construction was subject to GL Rules &
Regulations IV Part 2: Regulations for the
certification of Offshore Wind Energy Conversion
Systems Edition 1999.
Figure 3. Monopile under construction. Note the longitudinal en circumferential welds.
modern production lines - a process that is still
ongoing. This policy has been extremely successful,
judging by the impressive list of offshore wind
farms in Western Europe in which the company
has been involved. More than 80% of the installed
offshore wind farms rely on steel foundations
fabricated by Sif Group bv, amongst them the
Horns Rev project in Denmark (the second largest
farm to date) and the Q7 project.
Sif Group bv maintains an effective quality
management system certified in accordance with
the ISO 9001: 2000 standard and with the
implementation of EN-ISO 3834-2 comprehensive
quality requirements for welding. Additional
international approvals and authorisations include:
• Structural tubulars: API Spec. 2B
• Pressure vessel parts: ASME U stamp, ASME
U2 stamp, ASME S stamp, PED 97/23
• Dynamically loaded Steel Structures:
DIN 18800-7 Class E – Ü stamp.
Dimensions, material grades and mechanical
requirements.
The challenging Q7 project involved the manufacture
of 61 mono piles and 61 transition pieces
(60 for the wind farm and one for the transformer
station). Both are tubular structures; the monopiles
are straight and the transition pieces slightly conical.
Figure 3 shows a monopile under construction.
The principal weld connections, the longitudinal
and circumferential welds are clearly visible. The
individual cans are 3–3.5m in length and 4m in
diameter with the longitudinal welds staggered at
180° intervals from can to can. The wall thickness
varies over the length of the monopile, from 45mm
for the thinnest section, to 86mm. Transitions
between differing wall thickness were smoothed by
chamfering (1:5) and/or weld build-up.
Table 1 gives an overview of steel grades and
CVN impact requirements for the several thickness
ranges, both for the transition pieces
Sif fabrication of monopiles and transition
pieces.
The production line starts with beveling by flame cutting
or machining and subsequent cold rolling of
plates to a ring section. With two bending machines,
Sif Group bv can roll plate with a thickness of
20-150mm to shells with a diameter of 0.6 to 8m
and a maximum width of 4.2m (Figure 4). The rolling
process is performed in several steps to achieve the
specified dimensions and roundness; also to facilitate
perfect alignment for high productivity welding.
Tubular structures, in general, and monopiles, in
particular, are straightforward constructions with
heavy longitudinal and circumferential welds. SAW
makes up more than 90% of all welding. Serial
production depends on an efficient factory lay-out
where fabrication is performed in a logical
sequence, minimizing internal transportation of
components. Factory lay-out is also important to
achieve the full production potential offered by the
Table 1. Material grades, thickness and impact requirements
Application Thickness Structural
Category
Material
grade specified
Test
temperature
Impact energy
energy requirements
Transition shell
T< 45 primary S355J2G3- -30°C 34J av. (L)
Td = -10°C
EN10025
24J av. (T)
45