Some Aspects of Very Large Offshore Structures 



wall, the chance that a peak load occurs is about two per cent. Wave 

 induced impact forces only occur, when the wave breaks just at the 

 wall, while trapping a thin lense of air. Apparently, the energy of the 

 impact is stored in the compression of the air cushion. Therefore, it 

 is very unlikely that peak forces will occur if the surface of the object 

 is curved. In the case of large structures with flat or practically flat 

 walls, the possibility that peak loads occur due to breaking waves, 

 must be taken into account. The magnitude of the peak loads can only 

 be found by means of experiments. 



III. THE ANCHORING OF FLOATING STRUCTURES 



The anchoring of very large floating structures involves tre- 

 mendous problems, since the anchor system must be able to survive 

 the severest weather conditions. In high waves the drift force becomes 

 very important and causes a high mean load in the anchor lines. Due 

 to the non-linear characteristic of the anchor system - which is sche- 

 matically shown in figure 12 - the spring constant increases conside- 

 rably by this mean load and consequently the oscillating motion of the 

 structure induces high oscillating forces in the anchor lines. 



Let us consider, as an example, a circular storage tank - 

 120 m in diameter, with a draft of 25 m and a displacement v/eight of 

 approximately 2 90, 000 ton - which is anchored in a water depth of 

 40 m. It was calculated that, in a design wave with a height of 20 m 

 and a period of 1 9 seconds, this structure is subjected to a drift force 

 of 4, 73 ton and an oscillating force with an amplitude of 58, 900 ton. 

 If it is assumed that the motion of the structure is a pure surge mo- 

 tion and that the damping can be neglected, the motion can be descri- 

 bed by : 



m x + ex = F . e~ lLdt (22) 



v xa x ' 



in which : 



m = the virtual mass 



v 

 c = the spring constant in x-direction of the anchor 



system 



F = the amplitude of the oscillating wave excited force 



xa . m 



m x-direction 



Since the relation between the force and excursion of the 

 anchor system is non-linear, this equation has no simple analytical 

 solution. 



Due to the drift force, the motion of the structure will be 



967 



