| Transportation & Installation
Transportation, either by transport barge or the
installation vessel itself, is foreseen with the anchor lying horizontally as
opposed to the vertical configuration when lowering to seabed for installation.
Also, sea fastening requirements will be limited due to the low
center-of-gravity and structural capacity of the anchor itself.
Installation can be performed by anchor
handling type vessels (AHT). These typically have winch capacities in the
region of 200-600t and in excess of 1000m of wire. They also have large deck
areas for transport of several anchors, and thus provide a very competitive
installation solution. The anchor can be transferred into the water by simple
skidding with use of tugger winches or A-frame. Anchor upending, when passing
over the stern, can also be controlled by a tugger winch and A-frame in combination. The shape and mass of
the anchor makes it insensitive to hydrodynamic loading when passing through
the splash zone, and during lowering to the seabed. Any type of construction
vessel can also perform installation with a suitable crane i.e. most diving
support vessels and multipurpose vessels (DSV/ MSV), either monohulls or semis,
in addition to specialized lifting vessels.
A permanent mooring line is attached to the
anchor prior to installation. With an adequately sized installation vessel, the
permanent mooring line can be handled in addition to the installation wire.
Otherwise a separate vessel is required, which, may also be an AHT.
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| Installation using two vessels for ultra deep waters. |
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A special connection arrange- ment is located between the
temporary installation wire and the permanent chain section from the anchor
top, designed to allow controlled release of the anchor when in correct
position and elevation above the seabed for dropping. This will have a certain
mass and may also have built-in damping to counteract the spring back effect
when the anchor is released.
The figure on the left shows the anchor arrangement when ready for
dropping, with sufficient slack in the permanent mooring line shown hanging
from the connection element at the end of the installation wire.
Another major issue for anchor installation is the near
seabed positioning for drop. This requires seabed positioning by use of a
transponder on the connection element at the end of the installation wire, and
station keeping and adjustment ability of the vessel. An inspection ROV will be
utilized for pre-survey monitoring during positioning for dropping and for
post-drop survey. This can be operated from the AHT.
Verification that
the anchor has reached
sufficient depth is achieved by inspecting the length of chain
protruding above
the seabed, when tightened after drop. Verticality is documented
through instru- mentation that is retrieved after drop. The anchor
itself can be left in place
upon completed service since it is located well below mudline; one only
needs
to cut the anchor line. alternatively the anchor can be removed using a
special technique to reduce anchor wall friction.
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