3.3.1 Where force
diagrams are prepared, the vertical load at the derrick boom head
is to be taken as the SWL of the rig plus an allowance for the weight
of the boom and tackle as required by Ch 2, 2.3 Weight of boom and tackle.
3.3.2 The tension
in the cargo runner is to be calculated using the factors given in Table 2.2.1 Coefficients of rope
tension for the appropriate friction
allowance for the blocks and number of parts of the runner in each
section of the purchase.
3.3.3 Using this
information, a polygon of forces may be drawn and the resultant force
in the span tackle and thrust in the derrick boom determined.
3.3.5 Where the
cargo runner is parallel to the span tackle between the boom head
and mast head, the tension in the runner provides partial support
for the system. This force, called the ‘span relief’ is
deducted from the total span force in order to determine the load
in the span tackle. Consequently, in order to find the maximum load
in the span tackle, the system is to be considered in the lowering
operation and the span relief is to be W x P
2. All other forces in the rig are to be calculated for the
hoisting operation.
3.3.6 The required
breaking load of each rope is found by multiplying the maximum calculated
tension by the appropriate factor of safety from Table 2.2.2 Factors for wire ropes.
3.3.8 The resultant
forces in the system may be determined by direct calculation as an
alternative to the preparation of force diagrams.