The geometry of the anchor
While searching
for the best simplified anchor geometry, knowledge of the most
common
anchors on the market is needed. Vrijhof anchors will give a good
overview of the most common anchors.
Two types of anchors can be considered, horizontal load anchors and vertical
load anchors.
The vertical load anchor can withstand both horizontal and vertical mooring
forces.
The horizontal anchor or drag embedment anchor can only resist the horizontal
loads.
The drag embedment anchor is mainly used for catenary moorings where the mooring
line arrives the seabed horizontally. The vertical load anchor is used in taut
leg mooring
systems where the mooring line arrives at a certain angle the seabed.
A good starting-point
for this case is to analyze the horizontal load anchor, because
this anchor is often used and will form an adequate challenge.
To determine a simplified geometry of this horizontal anchor an actual figure
of this
anchor is needed. In the figures below, a sketch of the selected anchor can
be found.
The actual blade
(fluke) on the anchor that will penetrate the soil is represented by the
horizontal part of the above given figure. The shank is represented by the other
part of
the anchor. On the end of the anchor you will find an anchor shackle.
A first simplification
will be made by modeling the anchor as a 2D model. Hereby all the
calculations will be made easier, but the geometry is still to complex to determine
all
the forces. A second simplification can be made by supposing the anchor as two
straight
lines as can be seen in the figure.
This simplification is allowed, because we are working in a conceptual (first)
model.
When all the forces and the penetration curve of this simple model are known,
a much
more complicated model can be made.

For the
rest of the case a few assumptions must be made. First, the type of soil will
be
sand, this way the cohesion and adhesion effects can be neglected. As a next
assumption
we consider that the anchor penetrates the soil at a very low velocity. Therefore
inertia
and water tension can also be neglected.
Several constrains where also made to simplify the 3D force analysis into a
2D analysis.
This way several shear zones can be neglected. As a final assumption, the force
acting
on the point of the fluke will be neglected. This force is low considering a
big anchor
and will be fully cancelled by the force perpendicular on the shank.