Page 292 - Ship Construction.DJ Eyres 6Ed

Ch24-H8070.fm Page 281 Wednesday, October 18, 2006 7:00 AM
Lifting Arrangements 281
Both the ‘butterfly’ and ‘yo-yo’ rigs give a load pattern similar to the
‘union purchase’ and ‘single swinging derrick’ rigs for which calculations are
made, but the guy loads with each can be particularly severe.
Patent derricks are generally of the single swinging type with some form
of powered slewing. The Hallen swinging derrick is shown diagrammatically
in Figure 24.3(a). This type of derrick may be installed at the ship’s centre
line to reach outboard on both sides of the ship and is controlled by a single
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operator in a manner not unlike the operation of a mechanical crane. As a rule
the safe working load of this type of derrick is between 10 and 80 tonnes.
Of particular note in the very heavy lift range is the patent Stülken
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derrick (Figure 24.3(b)) marketed by Blohm and Voss AG, which may have
a safe working load of between 80 and 300 tonnes. One advantage that this
derrick has is its ability to serve two hatches, the boom swinging through an
arc between the posts in the fore and aft direction.

FORCES IN DERRICK RIGS The geometry of the derrick rig will to a
large extent influence the loads carried by the rig components. Those
dimensions which have the greatest influence are the length of boom, the
distance between the boom heel and the masthead span connection (height
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of suspension), and the angle at which the boom is topped.
When the ratio between boom length and height of suspension is
increased the boom thrust will be higher; therefore should a long boom be
required the height of suspension must be adequate. It is not unusual how-
ever for shipowners to object to having posts at the bridge front and if the
height of suspension is then restricted there is some limitation on the boom
length, which can make working cargo from that position difficult. The
angle at which the derrick is topped has no effect on the axial thrust, but the
lead from the cargo purchase often increases the thrust as it is led parallel
to the boom on all except heavy lift derricks.
Loads carried by the span are dependent on both the ratio of boom
length to height of suspension and the angle at which the derrick is topped.
The span load is greater at a lower angle to the horizontal, and increases
with longer booms for a given suspension height.
To determine these forces simple space and force diagrams may be drawn
and the resultant forces determined to give the required wire sizes, block
and connection safe working loads, and the thrust experienced by the boom.
The horizontal and vertical components of the span load and boom thrust
are also used to determine the mast scantlings. Force diagrams are shown for
the rig components of the single swinging derrick illustrated in Figure 24.4.
For a safe working load of 15 tonnes or less the forces may be calculated
with the derrick at angles of 30° and 70° to the horizontal unless the owner
specifies that the derrick is to be used at a lower angle (not less than 15°).
At safe working loads greater than 15 tonnes the forces may be calculated