Environmental Engineering Reference
In-Depth Information
shows an examp le of the fab rication of concrete su bstructures and the transf er t o a
heavy -lift crane vesse l for t he Thorton B ank offshore wind farm off the B elgian
coa st .
Carrying out the construction work directly on a floating pontoon requires the different
loading situations caused by the progress of the work to be taken into account when
checking the stability in the floating condition. In addition, the effects of waves plus the
rise and fall of the tide must be considered.
Building the substructure in a dry dock is worth considering when it is to be floated out
to its offshore location. However, it should be remembered that in most cases the
draught available is limited.
5.4.2.3 Transport and erection
S ub s t r u c t u r e s t h a t d o no t f l o a t a r e t r a n s po r t e d w i t h t h e h e l p o f h e avy- l i f t c r a ne
vessels or launch barges. The a dvantage of cranes is that they can position t he
subst ructure very accurately a t i ts offsh o re loca ti on and then l owe r it int o po si ti on
(Figure 5.17).
Alternatively, launch barges can be used for transport, which are often able to carry
several substructures to an offshore location simultaneously. Such barges are some-
times fitted with lifting equipment and so a substructure can be lowered into position
directly from
the vessel. In other cases, a heavy-lift crane vessel will
lift
the
substructure off the ship and lower it into position on the seabed.
Logistics c oncepts involving semi-subm ersible vessels have also been used. Such a
ship c an b al la st i ts b ow wi th wa te r t o s uc h an extent t hat the floating su bstructure can
b e f l oa t e d o u t o f t h e ve s s e l a n d p i c ke d u p b y a h e avy- l i f t c r a n e ve s s e l , f o r ex a m p l e .
Substructures can be designed to float and thus avoid the use of expensive heavy-lift
crane vessels altogether; several tugs can tow such a substructure from its place of
Fig. 5.17 Transporting a concrete substructure for the Thorton Bank offshore wind farm with a
heavy-lift crane vessel. (source:
www.c-power.be
)
