Page 71 - Failure Analysis Case Studies II

56
-

Fig. 8. Characteristic damage induced in spiral strand by pulling out a hockle or kink.

Rope:
construction 6 x 36 with IWRC
diameter 64 mm
mass in seawater 10 kg/m
C1 0.085 from [4]
c2 0.187 from [4]
c3 0.000531 from [4]
(Note: in the expression for torque/twist units of N and mm have been used throughout and twist
(d$/ dz) is defined in radians/mm.)

Chain:
diameter 89 mm
mass in seawater 150 kg/m
water depth 1000 m.
The calculation has been made on an incremental basis for each 50 m of chain raised, calculating
the degree of twist in the corresponding 50 m of rope wound onto the AHV winch. This twist is
based on the mean calculated for the start and finish of each increment, and is determined by
considering the twist at each tension associated with torque value of zero, the boundary condition
determined from an assumption of zero tension where the chain touches the seabed. The calculation
indicates that a total of 94 turns will have accumulated in the chain on the seabed by the time the
end of the chain reaches the surface. On the assumption that the chain can accommodate 3" of
rotation per link, then, the number of turns subsequently transferred to the spiral strand will be
reduced by 19, to 75. In practice the chain may develop some resistance to the progressive transfer
of so many turns, possibly lifting knotted clumps above the seabed, which would further increase