To get to an optimal design in terms of costs and performance; the design which was used in the previous section needs to be adjusted. This can be done in several ways.
· The layout of the moorings lines needs to be smarter. Now smaller clusters of lines positioned in different directions can achieve better results than one radially spread cluster of mooring lines. Still all mooring lines need to origin at the centre of the caisson since that the location of the turret.
· The lines them self need to be adjusted. Line of just one material (polyester) cannot be used because the specific conditions of the splash zone and the seabed ask for other materials.
When a suitable design is established; it is used as an basis for the design for the 45 meters water depth
Putting all mooring lines in a radial pattern is not a very efficient way to moor this caisson, since only a small number of lines will resist a load which is coming from a certain direction. The survival load can be expected from within a 90 degree window from which a significant wave height of 15 meters can be expected. Since the direction of this window is know, it is more efficient to put in more lines in the opposite direction of that window. Of course mooring lines need to be added in order to withstand forces during operational conditions which can come from any direction. The resulting mooring pattern consists out of 38 mooring lines which can be found in figure 10. The mooring line composition can be found in table 4
mooringline composition |
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|
|
length [m] |
weight [N/m] |
strength [kN] |
EA [MN] |
material |
50 |
2462,3 |
11118,0 |
inf. |
chain |
300 |
278,0 |
11768,0 |
78,4 |
HMPE |
50 |
617,8 |
10885,8 |
915,0 |
Steel wire rope |
Table 4: mooring line composition
Figure 10: final mooring lay-out
The load-displacement characteristics have to be evaluated on two points, the performance at operational in any direction; and the performance at survival conditions when the load is coming from the 90 degree window. The demands are summarized in table 5, while the performance is shown in table 6. One can see that it meets its demands; the design forces can be counteracted as well as the displacements caused by the low frequency and high frequency motions.
load - displacement
demands |
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|||
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|
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|
|
|
static forces [MN] |
HF motion [m] |
LF motion [m] |
|
operational
conditions |
|
19,36 |
0,35 |
9,00 |
||
survival conditions |
|
76,16 |
1,5 |
20,00 |
||
Table 5: load-displacement demands
load - displacement
charcteristics |
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|
operational
conditions |
|
|
survival conditions |
|
angle |
max. strenght [MN] |
max. disp. [m] |
disp. [m] @ 19 MN |
disp. [m] @ 76 MN |
0 |
163,08 |
43,80 |
5,20 |
20,00 |
30 |
158,10 |
42,00 |
5,00 |
19,20 |
45 |
152,96 |
40,80 |
4,80 |
19,20 |
60 |
137,50 |
39,00 |
4,60 |
- |
90 |
90,50 |
35,20 |
4,80 |
- |
120 |
74,57 |
39,00 |
6,40 |
- |
150 |
51,45 |
31,80 |
8,40 |
- |
180 |
46,20 |
30,80 |
9,20 |
- |
Table 6: load-displacement characteristics
During operational conditions as well as during survival conditions the tension at the winches stays below the maximum value of 600 tonnes (5,88 MN); this is shown in table 7.
Ttop in winches |
|
|
|
|
|
angle |
max. Ttop @ survival load
[MN] |
|
0 |
5,30 |
|
30 |
5,42 |
|
45 |
5,76 |
|
Table 7: Tension at winches
Considering this mooring configuration one can see that vertical forces on the anchors are present; since the seabed is not suited to use drag embedded vertical load anchors the correct solution, in this case, must be the use of suction piles. The vertical forces on the anchors are shown in table 8.
Forces on anchors |
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|
|
|
|
angle |
max. Fv anchor @ survival load [MN] |
|
0 |
1,60 |
|
30 |
1,63 |
|
45 |
1,73 |
|
Table 8: vertical forces at the anchors