An concrete offshore floating container terminal is designed. It’s purpose is to store, load and offload containers. The terminal has a length of 800 meters, a beam of 240 meter and a draft of 16 meters; its deck is 8 meters above the still waterline so the total height of the terminal is 24 meters.
The problem is how to station the caisson. Can it be stationed in such a way that the downtime of the terminal is as small as possible? Since it also have to serves as a breakwater, the caisson must be able to turn in different directions in order keep its long side parallel to the wave crests, but its position must always be maintained. The maximum quasi-static rotational speed of the caisson should not exceed 1 degree per 10 minutes while a ship is approaching to berth alongside. During all other times, the caisson may rotate at up to 5 degrees per minute.
In order to determine the size and number of mooring lines, one needs to know the environmental conditions from which the loading on the caisson is derived. The caisson should be able to withstand North Sea conditions; as such it probably also be able to handle most other offshore area’s in the world.
Containers will be stacked up to 3 high, which results in a height of 7,5 meters. In calculating the wind load on the caisson a height of 8 meters above the deck area will be used. This should thus be added up to the height of the deck above the still water level. A wind speed of 15 m/s can be expected from any direction during operational conditions. The wind speed for survival conditions will be 30 m/s but will come from a sector which is 140 degrees wide, that is centred on the survival wave direction.
The constant component of the second order wave drift force is the biggest contribution to the entire load on the caisson. It is proportional to the square of the wave height, which is at max 3,5 meters during operating conditions and 15 meters for extreme survival conditions.
The time-dependent component of the second order wave drift force results in a low-frequency motion which has an amplitude of 9 meters during maximum operating conditions; during extreme survival conditions its amplitude is 20 meters.
On top of the low-frequency movement, first order wave forces will cause horizontal movement of no more than 0,2 times the undisturbed incident wave amplitude. This is true for both the operational and the survival case. The total horizontal movement of the caisson during survival conditions is thus 20 plus 0,2 times Hs.
The survival condition waves, with a height between 10 and 15 meters approach the terminal from a 90 degree sector. All waves with a height under 10 meters approach the terminal from any direction.
The design current will be 0,75 m/s for operational conditions and 1,5 m/s for survival conditions. These currents can come from any direction relative to the wind.
The seabed conditions are sand and sandy clay. They are appropriate for any anchors except the Stevmanta, which is a Vertical Load Anchor (VLA).
The water depth in this case is 150 meters; the mooring system will also be evaluated on 45 meters of water depth.
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Figure 1 view of orientation of survival conditions
|
Operational conditions |
Survival Conditions |
Wind |
15 m/s |
30 m/s |
Waves |
0 – 3,5 meters |
10 – 15 meters |
Current |
0,75 m/s |
1,5 m/s |
Table 1: summary of environmental data