In this paper the results are presented of _a model study into the behaviour of anchor lines as used in the offshore mooring of a wide variety of floating structures. For this study model tests have been carried out with a buoy moored to four chains. The effect of model scale, pre-tension in the lines, wave height and wave period on the line tensions is discussed. Special attention is given to the dynamic behaviour of the mooring system, resulting into higher tensions than would follow from-calculations based upon the static catenary theory. The existence and significance of dynamic effects is confirmed by the results of forced oscillation tests with model anchor-chains in calm water.


For offshore floating structures, which are exposed to environmental conditions, station keeping above a fixed point is an important requirement. To resist the resulting external forces these structures are, in the majority of cases, anchored to the sea bottom. It will be clear that a proper design of the anchoring system is of great importance. Inaccuracies in the design of the system and in the estimation of line tensions can have serious consequences [1].

In analysing forces and tensions in anchor lines a static and a dynamic part can be discerned. The static part covers the calculation based upon the classic catenary theory, a method which is generally applied in the design of systems capable to balance the external forces- [21 and [3]. According to Alexandrov [4], this procedure yields reliable results. Also field tests have indicated that the results of such static calculations conform very well to the measurements [5], which implies that cable dynamics have a negligible contribution to the total forces.

The approximation of anchor line forces based on statics is generally valid for lines attached to structures, whose motions come close to the static case. To meet this condition it is essential for the structure to have large natural periods of motion and small amplitude responses in the frequency range of wave components. For certain structures, however, the behaviour in waves is much different. Owing to small natural periods the relatively high :frequencies of the wave components cause large amplitudes of motion.

Recent model tests in waves, carried out at the Netherlands Ship Model Basin, showed that the resulting behaviour of the mooring lines may no longer be approximated statically. Larger mooring line tensions were observed than would follow from static catenary calculations, particularly for the weather-side lines.

Dynamic effects may be analysed mathematically through application of a finite difference method, [6], [7] and [8]. Although some insight might be obtained in this way, the sensitivity of the dynamic effect to the various mooring line parameters is still not fully explained.

Therefore, two series of systematic model experiments were carried out. The first series comprised tests in waves with a cylindrical buoy moored to four chains.

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