4.1 General
4.1.1 Where the derricks are arranged for operation in union purchase the maximum resultant loads in the system are to be determined in accordance with the requirements of this Section.
4.1.2 The scantlings and arrangements of the derrick system are to be determined for union purchase operation and for operation as single slewing derricks. Each part of the rig is to be suitable for the most severe loading to which it may be subjected.
4.1.3 The union purchase rig is, generally, to be designed so that operation is possible on either side of the ship. A typical rig is shown in Fig. 2.4.1.
4.1.4 Union purchase rigs may be designed on the basis of either:
(a) minimum headroom below the triangle plate; or (b) maximum included angle between the cargo runners.
4.1.5 The following criteria are to be complied with at all times:
(a) Minimum operating angle of either derrick is to be not less than 15° to the horizontal, and it is recommended this angle is not less than 30º.
(b) Minimum headroom to the triangle plate is to be not less than 4,0 m where the SWL(U) of the rig does not exceed 2,0 t, or 5,0 m for higher values of SWL(U).
(c) The maximum included angle between the cargo runners is not to exceed 12° .
(d) The outreach beyond the midship breadth of the ship is to be not less than 4,0 m.
4.1.6 The minimum headroom is defined as the least vertical distance, at any stage in the operating cycle of the rig, from the highest point of the ship's deck structure (usually the top of the hatch coaming or the ship side bulwark or rails) to the centre of the triangle plate.
4.2 Working range of the rig
4.2.1 The derrick booms are to be of sufficient length and to be so positioned as to cover the required working area of the hatch while complying with the criteria given in 4.1.5.
4.2.2 For this purpose the booms are to be such that the boom heads may be located at the positions listed in Table 2.4.1. These arrangements are illustrated in Fig. 2.4.2(a) and (b) for hatches with one pair and two pairs of derricks respectively.
4.2.3 Calculations of the forces and resultant loads in the system are to be made as follows:
(a) Based on maximum headroom:
Outboard boom, see Table 2.4.1.
Inboard boom, see Table 2.4.1, Case 1.
(b) Based on maximum included runner angle:
Outboard boom, see Table 2.4.1.
Inboard boom, see Table 2.4.1, Cases 1 and 2 (that is, two calculations are required).
4.2.4 Where, at the request of the Owner, the rig is to operate over a working range different from that given in 4.2.2, including instances where the rig is designed for use in one fixed position only, calculations are to be made for the extreme positions of the specified range. The boom positions are to be clearly defined in the 'Register of Ship's Cargo Gear and Lifting Appliances'. In all instances the arrangement is to comply with the limiting criteria given in 4.1.5.
4.2.5 It may be assumed that the maximum forces will be associated with the extreme positions of the rig. Intermediate positions within the working range need not, in general, be examined.
4.2.6 The derrick booms are to be restrained by the use of preventer guys which are to be attached to eyeplates at the boom head or looped over the boom. The slewing guys are to be slackened off once the rig is set up and only the preventer guys are to be taken into account in the calculation of forces in the rig.
Sections 3 & 4
Table 2.4.1 Boom head positions for union purchase calculations
Number of pairs of Boom and
Symbol booms at the hatch direction
One Two
Dimension, in metres Outboard boom:
Transverse yo 4 4
Longitudinal xo 0,25lh 0,20lh
Inboard boom:
Case 1 Transverse yi 1 1
Longitudinal xi 0,75lh 0,67lh
Case 2 Transverse yi 0,25bh 0,25bh
Longitudinal xi 0,375lh 0,33lh
NOTES
1. For illustration of positions and symbols used, see Fig. 2.4.2.
2. Case 2 is applicable to operation in the maximum included angle mode only.
3. lhand bhare defined as the length and breadth respectively of the hatch openings, in metres.
4.3 Calculation of forces
4.3.1 The dimensions and other particulars required for the calculation of forces in the rig are indicated in Fig. 2.4.3.
4.2.7 It is recommended that the deck eyeplate for the preventer to the inboard derrick should be approximately abreast the derrick head. The eyeplate for the preventer to the outboard derrick is to be positioned so as to obviate the risk of jack-knif-ing, see Fig. 2.4.5, and to avoid excessive guy tension.
4.2.8 The boom heads are to be connected by a boom head or schooner guy, the safe working load of which is to be as required by 8.4.6. Alternative arrangements will be considered.
Boon head guy or schooner guy Span rope
Span rope Preventer
guy
Preventer guy Slewing guy
Slewing guy pendant
Slewing guy tackle Bulwark stays or brackets in way of guy eye plates
Bulwark or rails Cargo hook
Cargo triangle plate
Stevedore's slings
Inboard cargo runner
Outboard cargo runner
Section 4
Fig. 2.4.1 Typical union purchase rig
4.3.2 Where the forces are to be determined by the construction of force diagrams it is recommended that the following procedure be adopted, corresponding to the parts of the typical diagrams as labelled in Fig. 2.4.4:
(a) Projected plan of the rig.
(b) True side elevations of the derrick booms.
(c) Lines of the cargo runners at the minimum headroom position.
4.3.3 The force diagrams may be constructed on these geometrical diagrams as follows. For illustration, values corresponding to a unit load on the rig are shown on Fig. 2.4.4.
(a) The runner tensions and components of tension at the boom heads are determined from Fig. 2.4.4(c).
(b) The horizontal component of runner tension is marked off and the horizontal components of boom thrust and of guy tension are derived from Fig. 2.4.4(a).
(c) The vertical component of boom thrust and the guy tension may now be determined, see Fig. 2.4.4(a).
(d) The compression in the boom and the tension in the span rope are determined from Fig. 2.4.4(d) using the horizontal component of boom thrust and the total of the vertical forces, which are:
• Vertical component of runner tension, as in (a).
• Vertical component of guy tension, as in (c).
• Weight of boom and tackle, see 2.3.1.
(a) One pair of derrick at hatch
(b) Two pairs of derricks at hatch
Working range in way of hatch Outward boom
of ship
xo
yo
bh
yi xi
lh Inboard boom
Half breadth of ship amidships CL
Outward boom
xo
yo
bn
yi xi
lh Inboard boom
Half breadth of ship amidships of ship
CL
Section 4
4.3.4 Where a diagram of the character shown in Fig. 2.4.5 is obtained, that is where the diagram does not 'close', the boom is in danger of jack-knifing. The boom will be in danger of jack-knifing if the total vertical load is less than (span tension x tan α), where α is the angle of the boom to the horizontal. Where this situation arises the eyeplates for the pre-venter guys are to be repositioned. In general, it is recommended that the position of the guy eyeplate for the outboard boom is such that the load due to the vertical components of the runner and guy forces (but not the boom weight) is at least equal to span tension x tan α.
4.3.5 As an alternative to the graphical procedure, the forces in the rig may be determined by direct calculation.
Fig. 2.4.2 Positions of derricks for union purchase
Section 4
H=
From heel pin to span eye
Z=
N=
G=
1m C=
A=
J= M=
K=
F=
B= E=
Datum level
N is to be measured at half length of hatch N may be above or below datum level
L = Fr
om heel pin to span eye
Eye plate for main guy
Eye plate for main guy
Hatch or bulwark Vertical distances above datum level are positive Vertical distances below datum level are negative
0,5xi
0,25bh lh=
Hatch
of ship
bh=
xi
SWL of runner...
No. of available booms...
Distance guy eyes from datum level
Inboard boom...
Outboard boom...
Hatch No...
Is hatch forward or aft of mast or derrick post?...
Half bredth of
ship amidships...
SWL (U)...
SWL of boom...
Name or Yard No. of ship...
CL
Fig. 2.4.3 Typical data sheet
Section 4
(a)
(b) Inboar
d boom
Outboar d boom
Datum
m
n
(c) (d)
radius r
0,34
0,890,83 0,66
Outboar d runner
Load (1t) Limiting
headroom Inboar
d runner 1,07
n
Datum Hatch or bulwark
Inboar d boom
1,09 0,46
0,34
0,46 (0,5 weight of boom) 0,89
Outboar d boom
Span load0,66 of ship
Inboar d boom
Outboar d boom
(m) Cargo runners
1,09
0,83 Plan view of guy
True length of guy
0,56 0,73
0,56 0,46
Height of guy ( n) CL
Fig. 2.4.4 Typical union purchase calculations