Handbook of best practices

1.3.3.2 Mooring line anchoring systems
The anchoring system of an oceanographic mooring line usually consists of the anchoring
ballast and an acoustic releaser attached at a safe distance from the ballast in order to avoid
any damage during the deployment of the line. The ballast can be an ordinary ship anchor, a
reinforced concrete or iron block, railway wheels and sometimes large chains. The safe
anchor mass should be determined taking account the overall positive buoyancy of the
mooring line, the type of the seafloor and the environmental conditions of the deployment
site. In some cases, where strong winds (surface) or strong currents are presence a
secondary smaller anchor can be used to increase the drag resistance of the anchoring
system.
1.3.3.3 Mooring design software
Mooring lines must be designed according to static and dynamic behaviour predicted by a
software model for the extreme storm conditions and the fatigue cycles (frequency domain at
list if linear behaviour). Such software are proposed by software engineering companies for
instance Orcaflex by Orcina Ltd. v The risk of more complex behaviour such as Vortex
Induced Vibrations may be estimated by hydrodynamic specialists from their experience or
from experiment in towing tank.
A useful tool to design and evaluate mooring lines for oceanographic observatories is the
MATLAB toolbox Mooring Design and Dynamics. This set of routines developed by Richard
K. Dewey is freeware license software that allows the user to build his own libraries of
materials in order to design a customized mooring line. The package can be used to design
and evaluate single point surface, including S-shaped moorings, and sub-surface moorings
lines by placing all the components of the line in the appropriate order through the graphic
user interface. The tension and the shape of the mooring line under the influence of wind
and currents, and the simulation of mooring component positions when forced by timedependent
currents can be simulated. The static model will predict, apart from the tension
and tilt of each component, the anchor safe mass to be used. The recommended mass of
the anchor can be different taking under consideration the choice of the anchor material e.g.,
steel or concrete. The solution is calculated by solving a set of force balance equations
under the influence of time dependent 3 dimensional currents. The user can define the
current velocity direction and perform evaluation for different scenarios. Here is illustrated
the solution provided by Mooring Design and Dynamics for a 1400 meters long submerged
mooring line hosting 2 sediment traps in the E1M3A observatory (Cretan sea). The first
graph shows the behaviour of the mooring line under the mean current velocities and the
second graph is the line behaviour under the maximum current velocities. For both cases the
safe anchor mass has been calculated.
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