A mooring system consists out of a number of lines and anchors which can be applied in numerous ways. The next section discusses some options which can be considered for the container terminal.
Numerous mooring systems are nowadays available. In the previous phase of this project some of the different options were discussed. The most interesting ones are shortly discussed.
Spread mooring
This is a relatively cheap and conventional option, which is used a lot all around the world. The main disadvantage of this option is that all the anchors probably have to be replaced when rotating the caisson, in order to keep the ships on the long lee side of the caisson. If the anchors can not be replaced the caisson should be installed perpendicular to the prevailing wave direction. In that way the terminal can be used most of the time, but will experience a lot of downtime compared to other systems.
Figure 2: spread mooring (catenary)
Single buoy mooring
The advantage of a single buoy mooring is that one can turn the caisson relatively easily. It is the same principle as a barge which is weathervaning, but the difference is that this caisson should be kept perpendicular to the wave direction. Therefore mooring lines can be added on the other end of the barge, to keep it in the correct position. A disadvantage is that the anchors probably have to be replaced when rotating the caisson; therefore it might be cheaper to replace them with thrusters. During survival conditions the caisson automatically turns its short side parallel to the wave crests; thus minimizing the load on the caisson.
Figure 3: Single Buoy mooring
Internal Turret
In this case the terminal can simply turn around the turret which is installed in the middle of the terminal. The caisson will automatically turn into such a position that the long side of the terminal is parallel to wave crests; thus the terminal always serves as a breakwater for ships who attend it. During survival conditions the caisson can turn its short side parallel to the wave crests; thus minimizing the load on the caisson. This can be done by thrusters.
Figure 4: Internal turret
Dynamic positioning
The caisson can be kept in position by a dynamic positioning system. Dynamic positioning results in a very flexible system, always capable of assuming the optimal position with respect to the wave crests, and continuously adjusting for that
This means an extreme high Opex, which is not realistic so this option won’t be considered any longer.
Figure 5: Dynamic Positioning
Mooring lines are usually constructed out of one or more parts of three different materials; chain, steel wire rope or synthetic fibre rope.
Chain is the most common product used for mooring lines. Two different types of chain can be recognised; studless and studlink chain. Studless chain is moor suited for this problem because it is better suited for permanent moorings.
Steel wire rope has a lower weight and a higher elasticity than chain, for the same breakingload. This can be an advantage in deeper water, when using longer lines. Because of the fact that offshore activities moved into deeper water, an even lighter product was developed; synthetic fibre rope.
Most mooring lines consist out of several different materials; for instance chain at the top and bottom of a line and in between synthetic fibre rope. This way the characteristics of each material are used in their best way.
Mooring lines can mainly be used in two different ways, namely the catenary mooring or the taut leg mooring.
With a catenary mooring, the last part of the mooring line is resting on the seabed. The main advantage is that therefore the anchors only need to withstand a horizontal force; the disadvantage is that the system has a quite large footprint. In deeper water the weight of the lines starts to play a role so this type of mooring is not suited for very deep water.
With taut leg mooring the footprint is a lot smaller because the lines are under constant tension in a straight line from the fair leaves to the anchors. Disadvantage is that the anchors thus need to handle a vertical force as well as a horizontal force. This type of mooring is better suited for very deep water.
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Figure 6: catenary and taut leg mooring
Not surprisingly different types of anchors are available nowadays. Five main types can be identified; the dead weight, drag embedment anchor, pile, suction anchor and the vertical load anchor (VLA). The dead weight is not applicable in this case because of the vast dimensions of the terminal; the VLA is also not applicable because the seabed is not suitable for this type of anchor. This leaves us with only thee types.
Drag embedment Anchor
The drag embedment anchor has been designed to penetrate into the seabed. The holding capacity is generated by the resistance of the soil in front of the anchor. The anchor is very well suited to withstand horizontal forces but cannot handle vertical forces.
Pile
The pile is a hollow steel pip which is hammered or vibrated into the seabed. The holding capacity of the pile is generated by friction of the soil along the pile and lateral soil resistance; it can thus resist both vertical and horizontal forces.
Suction anchor
The suction anchor is a steel pipe of which its top is closed. On top of the pipe a pump is installed which is capable of lowering the pressure inside the pile. This way the pile is pressed into the seabed by the hydrostatic head of water on top of the pile. Like the pile; the holding capacity of the suction anchor is generated by friction of the soil along the anchor and lateral soil resistance; and it can also thus resist both vertical and horizontal forces.
Before going into a detailed design of the mooring lines and anchors, a choice has to be made concerning the type mooring system what is going to be used.
The type of lines which where discussed can all be used in every concept. The choice for a certain material or the way the lines are suspended is mainly determined by economics. The same holds for the choice which type of anchor is going to be used.
This means that at this moment a choice must be made between the different mooring systems which were discussed in section 3.1. Several reasons are important in this discussion.
· During operational conditions waves can come from any direction. This means that the terminal must be able to rotate quite easily into the desired position. During extreme survival conditions waves will approach the terminal almost certainly from somewhere within the window, which is drawn in figure 1. When the spread mooring system is chosen, it is best that the terminal is positioned in the same way as in figure 1; that way the solicitation on the structure is as small as possible. However in that same position the terminal can’t function as a breakwater so it needs to be turned 90 degrees and set in that position. That will result in the highest loading on the structure which is possible and thus result in a very heavy and expensive mooring system. On top of that; during operational conditions waves can also come from outside that window, this can result in a high downtime because the terminal cannot always function as a breakwater.
When one does want to turn the terminal after the spread mooring is installed, one has to relocate the anchors which is a very expensive operation because a lot of anchor handling equipment and people are needed to do so.
For above mentioned reasons it is decided that the spread mooring is no longer considered.
Now a choice has to be made between the Single Buoy mooring and the mooring system with an internal turret.
· In spite of the fact that an internal turret is more expensive than a single buoy mooring, it is preferred to choose an internal turret because it lasts longer. The total lifetime of any mooring system is 25 years; that is just as long as the lifetime for an internal turret, whereas a single buoy mooring only lasts 15 years at an expense of only 2 million euros less than the internal turret. Their maintenance costs are equal.
· When a single buoy mooring system is chosen; the OPEX will be quite a bit higher as opposed to choosing an internal turret. Because of the fact that the internal turret is in the middle of the caisson, it will automatically turn its long side parallel tot the wave crests, one only needs thrusters during survival conditions to turn the caisson with its short side parallel to the wave crest in order to minimize the load on the terminal. In the case of a single buoy mooring, thrusters continuously need to keep the caisson with its long side parallel to the wave crests because of the fact that the buoy is located at the end of the terminal.
For above mentioned reasons it is decided that the Internal Turret mooring system will be further analysed.