Marking of single-sheave blocks

• Pages: 476-480

476

Appendix H

Appendix H

Marking of single-sheave blocks

H.1. General provisions

H.1.1. The safe working load (SWL) of a single-sheave block

in a derrick rig should be marked in accordance with the following

indications. For the sake of simplicity, the effect of friction and

rope stiffness (i.e. the effort required to bend the rope round the

sheave) has been ignored. In practice, the assessment of the SWL

of the block, as in paragraph H.2.2, ignores friction and rope stiff-

ness. These factors should nevertheless be taken into account

when determining the resultant force on the head fittings of

the heel blocks, span gear blocks and other equipment. This is

the responsibility of the competent person who prepares the

ship’s rigging plan.

H.2. Method

H.2.1. A single-sheave block may be rigged at various po-

sitions in the derrick rig, for example in the span gear, upper and

lower cargo blocks or heel blocks, and may be used with or without

a becket.

H.2.2. The SWL of a single-sheave block is always assessed in

accordance with one fundamental condition of loading, i.e. where

the block is suspended by its head fitting and the dead weight or

cargo load is attached to a wire rope passing round the sheave in

such a way that the hauling part is parallel to the part to which the

load is attached (figure H1). The SWL marked on the block is the

dead weight (M tonnes) that can be safely lifted by the block when

rigged in this way.

H.2.3. When the block is rigged as in paragraph H.2.2, the

resultant force on the head fitting is twice the SWL marked on

the block, i.e. 2M tonnes. The block manufacturer should design