Tension of anchor lines of single gravity grid mooring cage under combining effects of wave and current

Abstract

The characters of tension in the mooring lines of a single HDPE gravity cylindrical cage were studied by experimental tests.The proto type of cage had a perimeter of 40 m and a height of 10 m.Based on the Dixon rule of gear tests,two scales were chosen for the experimental model,i.e.λ= 1/10 and λ'= 1/2.The experiments were carried out in Ocean Engineering Basin of Institute of Industrial Science of the University of Tokyo.The grid mooring system used in the experiments was geometrically similar to the proto type.The horizontal projections of the front mooring lines were in the same direction of current or incident waves.Uniform current was produced in the whole basin at a velocity of 0 cm/s,10 cm/s and 15 cm/s.Regular waves with a period of the range from 1 s to 2.2 s and a height varying from 10 cm to 30 cm were generated.Tension in the mooring lines wwa measured.The following conclusions were made from the analysis of measured data:(1) Under the actions of current only,the tension in the front mooring lines was greater than that in rear mooring lines.(2) Under the actions of waves only,the tension in the rear mooring lines was greater than that in the front mooring lines.An empirical formula for the maximum tension(Tmax) in the rear mooring lines was deduced.It turned out that the maximum tension was line-arly related to the wave height(H),i.e.Tmax=C1H+C2(C1 and C2 are constants).(3) Under the combining effects of waves and current,the tension of the front anchor line was again greater than that in the rear one.Another empiri-cal formula was deduced.It showed that the maximum tension in the front mooring lines was exponentially in-creased with the wave height,i.e.(C3 and C4 are constants).