Some Aspects of Very Large Offshore Structures 



drift force and the characteristics of the anchor system, the motions 

 will be non-linear. Therefore, model tests are indispensible to in- 

 vestigate the anchoring of large structures. 



The above example has shown, however, that enormous pro- 

 blems are involved with the anchoring of very large structures with 

 a small length to breadth ratio. Therefore, in rather shallow water, 

 a structure fixed to the bottom, will be preferred in many cases. If 

 a floating structure is required - for instance because there exists 

 a risk of earthquakes - or if the structure has to be more or less 

 mobile, it is desirable to choose a shape with a minimum drift force, 

 as for example a ship- shaped structure moored to a single point moo- 

 ring system or a semi- submersible structure. 



IV. MOORING OF A SHIP TO A LARGE STRUCTURE 



For the oil storage tanks which are now in use or under con- 

 struction, a concept was selected by which the loading tanker is not 

 moored immediately to the storage tank, but to a separate single 

 buoy mooring system. 



If we consider the wave pattern around the circular tank, as 

 given in figure 9, regions where the waves are higher, as well as 

 regions where the waves are lower than the incident waves, can be 

 observed. For other wave lengths, the wave pattern changes, but the- 

 re is always an area behind the structure where the waves are lower 

 than the incident waves. It can therefore be expected, that the diffrac- 

 tion of waves by a large fixed structure will be advantageous when a 

 ship is moored immediately behind it. 



In order to investigate the behaviour of a tanker, moored to 

 a storage tank by means of a bowhawser, a model test program was 

 performed at the Netherlands Ship Model Basin with the cylindrical 

 model - discussed already in a previous section - and a model of a 

 tanker with a displacement of approximately 100, 000 ton. The main 

 particulars of the tanker are given in Table I, while figure 14 shows 

 a small scale body plan. The weight distribution and stability charac- 

 teristics of the tanker were reproduced to scale. The tanker was moo- 

 red to the storage tank by means of a single bowhawser, represen- 

 ting a nylon mooring line with a breaking strength of 150 ton and a 

 length of 50 m. The load -elongation characteristic of this bowhawser 

 is given in figure 15. 



The following tests were preformed : 

 a- Measurement of the wave height in regular and irregular seas 



969 



