Page 295 - Ship Construction.DJ Eyres 6Ed

Ch24-H8070.fm Page 284 Wednesday, October 18, 2006 7:00 AM
284 Ship Construction
In determining the span loads and boom thrusts, not only is the derrick
safe working load considered to be supported by the span, but also the
weight of the cargo purchase and half the boom weight. The other half of
the boom weight is supported by the gooseneck fitting.
Allowances must be made for the frictional resistance of the blocks when
determining the forces. This includes an allowance for the rope friction, i.e.
the effort required to bend and unbend the rope around the pulley, as well
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as an allowance for journal friction. Shipbuilders using British Standards
adopt the following assumed cumulative friction values.

Small and medium 8 per cent sheave with bushed plain bearings
5 per cent sheave with ball or roller bearings
sheaves 4 per cent sheave with ball or roller bearingsาวี
งานห้องสมุด ศูนย์ฝกพาณิชย์น
Large diameter 6 per cent sheave with bushed plain bearings
sheaves
Derrick exceeding 80 5 per cent sheave with bushed plain bearings
tonnes SWL 3 per cent sheave with ball or roller bearings

Force diagrams which are more involved than those for the single swing-
ing derrick are prepared for the union purchase rig. These diagrams indi-
ึ
cate the safe working load of the rig, the ‘limiting height’, the boom thrusts
which are greater with this rig, and the optimum guy leads. The ‘limiting
height’ is that height below which all positions of the lifted weight will result
in an included angle between the outboard and inboard runners of less than
120°. At 120° if the boom heads are level the inboard and outboard runners
will experience a force equivalent to the cargo weight (see Figure 24.5).
Usually the runner size determines the safe working load in union pur-
chase, but the thrust experienced by the derrick boom can determine this
value where only light derricks are fitted. The positioning of the guys can be
important to the loads experienced by the span and the guys themselves. If
these are at too narrow an angle to the boom, excessive tension in the guys
will result; a good lead is therefore essential. Unfortunately in practice the
magnitude of the guy loads is not always appreciated, but more attention
has been paid to this problem of late and preventers are now often set up to
reduce the load in the guy. There is available a suitable preventer for this
purpose; the use of old runners, etc., as preventers should not be tolerated.
In union purchase rigs it is possible to obtain a condition where the load
comes off the outboard span, and the boom may then close to the mast
under load. This condition is referred to as ‘jack-knifing’, and may be
apparent from the force diagram prepared for the rig, since the triangle of
forces does not close. At the design stage the guy positions can be adjusted
to avoid this happening. In practice this condition appears to occur occa-
sionally where derricks are used in union purchase at the bridge front. Here
the positioning of the guys is made difficult by the presence of the bridge