5          Basic design

5.1         Introduction

Since the type of mooring system is now known, as well as the solicitation on the caisson; an analysis can be done to investigate how many lines are needed in order to keep the terminal station.

At first three identical configurations will be analysed for a water depth of 150 meters. These are radial distributed mooring systems; one consisting fully out of lines of chain, one consisting out of lines of steel wire rope and one consisting out of line of synthetic fibre.

Through these analyses the cost for different materials can be determined. Finally a design is chosen and optimized in the next chapter.

 

The analysis is done with help of a program called XSmoorings version 2.0. This program calculates displacements, tensions, breaking loads while increasing the environmental loads. When using the program, first details about the moored object, the environment, the safety factors and mooring lines and configurations have to be imported into the program. Once this is done, the program can analyze the mooring system and the results are shown. After evaluation the system can be optimized.

5.2         Evaluation of alternatives

The results of this first analysis are analysed to see whether the load-displacement can handle the survival condition of 76 MN at 23m displacement.

5.2.1        Chain

Iteratively a mooring system consisting out of 36 lines was realised. The system can handle forces up to 84 MN at a displacement of  21,8m. The lines have a length of 300 meters and a diameter of 185 mm., the pre-tension is 1900 kN. In figure 7 the outline of the system can be found.

Figure 7: mooring layout for a chain mooring


5.2.2        Steel wire rope

Iteratively a mooring system consisting out of 36 lines was realised. The system can handle forces up to 88 MN at a displacement of  3,8m. The lines have a length of 200 meters, consist out of six strand steel wire rope and have a diameter of 152 mm., the pre-tension is 500 kN. In figure 8 the outline of the system can be found.

Figure 8: mooring layout for a steel wire rope mooring

5.2.3        Synthetic lines

Iteratively a mooring system consisting out of 44 lines was realised. The system can handle forces up to 125 MN at a displacement of  35,3m. The lines have a length of 200 meters, consist out of polyester (20% of MBL) and have a diameter of 213 mm., the pre-tension is 1000 kN. In figure 9 the outline of the system can be found.

Figure 9: mooring layout for a synthetic line mooring


5.3         Conclusion

An actual cost data sheet is in this stage not yet made because the three designs considered in this phase are not very realistic, but merely a tool to indicate the influence of different materials on the mooring design. Some things concerning that choice can be said.

·         All three design need a large amount of line to be kept station but the difference between the individual designs is not very big

·         The design with the synthetic lines has the highest survival load; yet does stays within the 23 meters displacement at an load of 76 MN.

·         In all three cases vertical reaction forces occur at the anchors. Line sizing out of the vertical reaction force region can be done when using chain but one would need a line length as large as about 600 meters; that is very expensive. This means that no matter what material is chosen, suction cans or piles must be used to anchor the caisson.

·         The design using chain has also got the longest lines

·         Chain is the cheapest material per kilo, but one needs more kilo’s to moor the caisson than when one is using steel wire rope or polyester. Furthermore, the lifespan of chain is only 7 years, this means that every line has to be renewed four times during the mooring time of the caisson. This makes it an expensive solution.

·         Polyester lines last for a period of 25 years and thus do not need replacement during the operational years of the caisson. This makes a less expensive solution in spite of the fact that the material is somewhat more expensive per kilo.

·         Specially coated steel wire rope also lasts 25 years, but is slightly more expensive per kilo as polyester; again on needs more kilos of steel wire rope than polyester so all in all it is a more expensive solution.

 

Based on the above findings it is concluded that a detailed design is going to be made with polyester lines. Other materials will probably also be used because synthetic lines are for instance not allowed to be used near the seabed, in order to prevent them from picking up and entrapping abrasive soil particles between the fibers.

continu to chapter 6: final design

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