Fabrication and Installation - Offshore Structures: Design, Construction and Maintenance - page 341

Bending Efficiency Factor
Where any rope is bent round a shackle, trunnion, padear or crane hook, the
breaking load should be assumed to be the calculated rope breaking load multi-
plied by a bending efficiency factor

= − :

5/ ð P d /r d Þ 0

:

Bending efficiency factor 1 0 5 (5.10)

where P d is the pin diameter and r d is the rope diameter.
This results in the bending efficiency factors shown in Table 5.6 .

Sling breaking load = rope breaking load × bending efficiency factor (5.11)

Grommets
Grommets require special consideration, to ensure that the rope breaking load
and bending efficiency have been correctly taken into account.
The core of a grommet should be discounted when computing breaking
load. The breaking load of each part of a grommet is therefore usually taken
as 6 times the unit rope breaking load, with a factor to account for the spinning
losses in cabling. This factor is normally taken as 0.85.

breaking load of unit rope
(5.12)
Typically, a grommet will be used with one end over the crane hook and
the other end connected to a padeye by a shackle. The bending efficiency
factors at each end may differ, and the more severe value should be taken.
Bending efficiency is derived as before, where rope diameter is the single-
part grommet diameter. The total breaking load of the grommet used in
this manner is: 2

Grommet breaking load ð each part Þ = 0 : 85 × 6 ×

× (single-part grommet breaking load) × (more severe bend-

ing efficiency factor).

Shackle Safety Factors
The minimum shackle breaking load, where this can be reliable determined,
should the largest value of the minimum required sling breaking load as calcu-
lated before from Equation (5.11) ,or

Sling force from Equation ð 5 : 6 Þ= DAF

so that the suitable required shackle size can then be defined.

TABLE 5.6 Bending Efficiency Factor

P d / r d 0.8 0.9 1.0 1.5 2.0 3.0 4.0 5.0

Factor 0.44 0.47 0.50 0.59 0.65 0.71 0.75 0.78