The mooring keeps the rearing structure in position. For this, it requires not only a certain mechanical resistance but also a certain liberty of movement.
The resistance supports the combined efforts of the wind, the current and the waves : the mooring lines mus not break under tension. The liberty of movement allows the moored construction to move under the effect of individual waves ; the strain is in this way diminished.
What does it consist of?
A mooring is generally made up of foundations (anchor, dead weight, posts) of lines (rope, chain), of rtensioners(ballast, floats) and surface buoys.
The tensioners, which are only used with ropes, have the role of keeping the lines in tension. They are generally floats or ballast. The surface buoys are for dealing with forces on the mooring the horizontal plane with a rope which connects the buoy to the floating support. This limits the effect of significant vertical pull on the base and hence avoids the risk of immersion. Generally, it is recommended that the tensioners and the buoys are fixed at a sole point on the lines and that they are not placed in series on the line where they would be affected by forces on the mooring. The main mooring types are chain mooring or rope mooring.
Their foundations are preferably anchors. The chain must be sufficiently long to have its furthest point lying along the ground, even under maximum tension, this is so that the anchor is always in the optimum position.
Mooring with two ropes
This type of mooring is currently used in aquaculture. Each line includes a rope at depth and a rope at the surface. The finet connects the foundation with the sub-surface float. The second connects the float to the buoy. The function of the float is to continuously keep the mooring line in tension (Fig.1).
The main advantage of this mooring is its permanent tension, therefore there is little risk of weak tension. Furthermore, it allows for all variations in the height of the water (swell, tide...).
It must be checked, therefore, that the float will not penetrate the surface of the water when the water level is at its lowest. This would cause a loss of tension, shocks and risks of chafing where the float is fixed. It must be checked also that during the movements of the structure (current, swell...) all the ropes are in tension.
Mooring with three ropes
This mooring is used in three cases:
- When an anchoring length in unstable ground is required, this mooring provides a length on the ground which is less than that of mooring chain or using two ropes.
- When a mooring adapted to very sloping marine bottoms is required.
- In the search for a mooring allowing big horizontal and vertical movements of the floating base.
Each mooring line with three ropes is composed of a rope at depth, an intermediate rope and a surface rope. The first connects the foundation to the float, the second connects the ballast to the float and the third connects the ballast to the surface buoy. The float and the ballast keeps the mooring taut.
It must be checked that the float does not penetrate the surface of the water and that the ballast does not find itself on the ground at the lowest tide tide. This could also bring about zero tension, shocks in the mooring and chaffing. It must be checked also that, at the time of movement of the structure (current, waves...) that there is always significant tension in the ropes.
Rules to follow
The mooring must always be taut, in preference, because lines that are momentarily slack may chafe or provoke dangerous strain the moment that tension is put back on. The risk of weak tension is particularly high in low water ( low tide, wave troughs, high atmospheric pressure...)
The mooring runners, that is to say the subsurface floats or the ballast placed on the lines, must always stay, if possible, completely immersed.
It is recommended, moreover, that the mooring does not provoke the immersion of the floating base. For this, the mooring must operate horizontally to the floating base. Indeed, vertical forces on the base may provoke partial immersion, which is uncontrollable and unwanted. The risk of immersion of the floating base is especially high at the highest water levels (high tide, the crests of waves, atmospheric depression...).
In addition, the mooring must be sufficiently flexible to not restrain the movement of the base with each wave.
It is sometimes difficult to estimate the length of the lines which will fulfil the two conditions mentioned, a method to estimate this is therefore recommended here.
The concept of a mooring requires many stages involving the calculation of the following components :
- The length of the line components (ropes, chains),
- Forces and resistance of the lines,
- Foundations (dead-weight, anchors, posts).
The length of the lines
Before becoming concerned with the mechanical resistance of the mooring and its behaviour in the environment, particularly in the swell, it is necessary to determine the length of the lines. This estimation is sometimes difficult, for example when the tidal range component is large, the bottom is on a steep slope or when the site is shallow.
The length of the lines must be estimated with care so that the mooring respects the two preceding rules: Lines are always taut at lowest water and the strain is not too great at high water. To estimate the length of the lines which avoid weak tension, the following method is advised : at lowest waters, that is at low tide, at the trough of the wave and at the time of high anticyclonic pressure, imagine that the floating support base has been pushed as far to the right as it will go. This maximum is reached when the mooring line at the left is completely taut. In this position the line which is the least taut, that which is situated at the right, must become taut again, if only slightly. This position therefore enforces a certain length of the right hand line. Of course, the same reasoning applied for the left.
To estimate the lengths of the ropes which will help to avoid strains which are too big, we recommend the following method; at high water, that is to say at high tide, at the peak of the wave and at the time of meteorological depression, imagine that one pushes the floating base as far to the left as it will go, then towards the right. These maximum positions are reached, as before, when the mooring line at the right, and then the left are completely taut. The distance between the two positions must be larger than the orbit of the waves so that the movement of the base with the waves is not restrained by the mooring and there no risk of creating too great a strain.
IFREMER (DITI) has computer software which estimates the length of the lines for chain mooring and two rope and three rope mooring.
Exertion and resistance of the lines
It is essential to assess the strain on the structure to estimate the resistance of the lines. This strain is mainly due to the swell, the current and the wind.
The horizontal movement of a floating base can be broken down into three main components :
- average drift,
- linear response to the swell
- low frequency oscillations at the characteristic period of the anchoring/support system.
Classically, an anchorage is dimensioned only by considering the first two components, the third being taken into account by safety factors.
Experience shows that many moorings of rearing structures in the sea may drag under the effects of the swell, the current or the wind. This risk needs to be especially taken into account, because if a line drags, the division of the strain in the other lines will be different to what was calculated. This is why a mooring with a snapped line must remain sufficiently resistant.
The evaluation of the average forces exerted on an anchorage is obtained by summation of the forces due to extreme wind, extreme current and the maximum value of the drift of the swell.
The anchorage lines are therefore dimensioned from the static strains (drift) and movement imposed by the swell.
The strain caused by the drift can be estimated (example : pt. A fig. 2). The rigidity of the mooring therefore gives the average displacement of the support (pt. B). Then, the linear response allows the minimal and maximal displacement to be estimated (pt. C). They give the average position of the base which is more or less the amplitude of the swell orbit. The displacements and the rigidity therefore finally give the minimal and maximum strains(pt.D)
Th : horizontal strain (N),
p : linear weight of the chain in the water (N/m)
d : depth of the water (m)
x : horizontal displacement of the floating support (m)
A : average strain on the mooring (N)
B : average displacement of the floating support (m)
C : extreme displacements of the support (m)
D : extreme strain on the mooring
When a more exact estimation has to be done, the use of calculation codes of the forces is advised.
Four fundamental conditions are considered for the calculation of the mooring lines :
a) normal working conditions, and the most severe environmental stimulus corresponding to this condition,
b) as in a) but with the breaking of a line.
c) survival conditions in the most serious anticipated environmental conditions,
d) as in c) but with the breaking of a line,
For small structures maintained by a mooring system with one or two lines, situations b) and d) are omitted,
Table 1: Security factors of the mooring.
For use in static or near-static analysis according to the operating conditions and the structure (Rules for the classification of mobile offshore units, Bureau Veritas 1993).
The main foundations are dead weights, posts and anchors. They must be able to withstand the vertical and horizontal strain exerted by the anchorage lines. These foundations are affected in a cyclical manner by the swell. It is therefore the same for the ground surrounding these components. The stress on an unstable ground must in general always allow for a considerable proportion of the pressure.
An alternating push and pull would engender a repositioning of the soil grains and a risk of liquefaction and a total loss of resistance.
For foundations of small dimensions, the paper "Designing structures for the rearing of fish in the sea" (IFREMER, 1996) may serve as a reference.
To limit the effect of penetrating the ground with the dead weight and the risk of it overturning, the dead weight must be as thick as possible.
For reasons of fabrication and mechanical staying power, the ratio between its height and base must not be much under 1/4. The base of the dead weight can be fitted with 'spades' which are metallic pieces placed under and perpendicularly to the base of the dead weight.
When the resistance to shear on the surface of the soil is weak, the spades can penetrate deep (some cms to dms) where the resistance is better. They therefore avoid the sliding of the dead weight on a surface layer which has poor characteristics. These spades show maximum efficiency when their length is close to a tenth of their spacing. Three to four spades seem enough.
For good efficieny of the the dead weight, its mass must be enough so that it does not turn over, it does not slip and it avoids local and total detachment. On the other hand the mass must not be so great that it penetrates the ground.
Anchors and posts
For the estimation of anchors, the papers written by the manufacturers can be referred to as they generally provide curves of resistance according to the weight of the anchors and the type of ground.
For the calculation of the large size posts, it is preferable to consult specialists.
This type of steel floating device is especially recommended to support the drawplates in open sea especially when the drawplates are exposed to severe weather.
- It is well adapted to the frequent variations in the depth because it is designed to withstand strong pressure.
- In a strong current after the drawplate has settled, the floating device re-ascends intact when other floats implode or deteriorate.
- Signal buoys
- High performance wind generator, 12 V and 24 V,
15 to 20 knot winds
- Light towers (7 m to 12 m, modulable)
Study depending on the sites, by a design bureau specialising in composite materials
- Projector 12, 24 or 240 V, halogen or sodium-vapor.
Corniche du Bois Sacré
83500 La Seyne sur Mer
Tél. : 04 94 10 26 26
Fax : 04 94 10 26 30