Section 4 Union purchase arrangements

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 the operation is possible on either side of the ship. A typical rig is shown in Figure 2.4.1 Typical union purchase rig.

Figure 2.4.1 Typical union purchase rig

4.1.4 Union purchase rigs may be designed on the basis of either:

1. minimum headroom below the triangle plate; or

2. maximum included angle between the cargo runners.

4.1.5 The following criteria are to be complied with at all times:

1. 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 30°.

2. 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).

3. The maximum included angle between the cargo runners is not to exceed 120°.

4. 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 in 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.

4.2.1 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 Ch 2, 4.1 General 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 Boom head positions for union purchase calculations. These arrangements are illustrated in Figure 2.4.2 Positions of derricks for union purchase and Figure 2.4.2 Positions of derricks for union purchase for hatches with one pair and two pairs of derricks respectively.

4.2.4 Calculations of the forces and resultant loads in the system are to be made as follows:

1. Based on maximum headroom:

   		Outboard boom, see Table 2.4.1 Boom head positions for union purchase calculations.
   		Inboard boom, see Table 2.4.1 Boom head positions for union purchase calculations, Case 1

2. Based on maximum included runner angle:

   		Outboard boom, see Table 2.4.1 Boom head positions for union purchase calculations.
   		Inboard boom, see Table 2.4.1 Boom head positions for union purchase calculations, Cases 1 and 2 (that is, two calculations are required).

4.2.5 Where, at the request of the Owner, the rig is to operate over a working range different from that given in Ch 2, 4.2 Working range of the rig 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 Lifting Appliances and Cargo Handling Gear. In all instances, the arrangement is to comply with the limiting criteria given in Ch 2, 4.1 General 4.1.5.

4.2.6 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.

4.2.7 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 with regard to the calculation of forces in the rig.

4.2.8 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-knifing, see Figure 2.4.5 Form of diagram when beam is in danger of jack-knifing, and to avoid excessive guy tension.

4.2.9 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 Ch 2, 8.4 Slewing and preventer guys 8.4.6. Alternative arrangements will be considered.

4.3.1 The dimensions and other particulars required for the calculation of forces in the rig are indicated in Figure 2.4.3 Typical data sheet.

Figure 2.4.3 Typical data sheet

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 Figure 2.4.4 Typical union purchase calculations:

1. Projected plan of the rig.

2. True side elevations of the derrick booms.

3. Lines of the cargo runners at the minimum headroom position.

Figure 2.4.4 Typical union purchase calculations

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 in Figure 2.4.4 Typical union purchase calculations.

1. The runner tensions and components of tension at the boom heads are determined from Figure 2.4.4 Typical union purchase calculations.

2. The horizontal component of runner tension is marked off and the horizontal components of boom thrust and of guy tension are derived from Figure 2.4.4 Typical union purchase calculations.

3. The vertical component of boom thrust and the guy tension may now be determined, see Figure 2.4.4 Typical union purchase calculations.

4. The compression in the boom and the tension in the span rope are determined from Figure 2.4.4 Typical union purchase calculations, using the horizontal component of boom thrust and the total of the vertical forces, which are:

   • Vertical component of runner tension, as in Figure 2.4.4 Typical union purchase calculations.
   • Vertical component of guy tension, as in Figure 2.4.4 Typical union purchase calculations.
   • Weight of boom and tackle, see Ch 2, 2.3 Weight of boom and tackle 2.3.1.

4.3.4 Where a diagram of the character shown in Figure 2.4.5 Form of diagram when beam is in danger of jack-knifing is obtained (this 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 the span tension x tan α, where α is the angle of the boom to the horizontal. Where this situation arises, the eyeplates for the preventer 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.

Figure 2.4.5 Form of diagram when beam is in danger of jack-knifing

4.3.5 As an alternative to the graphical procedure, the forces in the rig may be determined by direct calculation.