Section 6 Derrick booms

6.1.1 The requirements of this Section are applicable to the following types of tubular derrick boom:

1. Parallel boom. A boom which is of uniform diameter and thickness over its full length.

2. Tapered boom. A boom which has a mid-section of uniform diameter and thickness welded to tapered end sections.

3. Stepped boom. A boom which has a mid-section of uniform diameter and thickness reducing towards the ends in one or more steps. The change of section may be obtained by cressed, lapped or stepped construction.

6.1.2 Proportions for tapered and stepped derrick’s booms are illustrated in Figure 2.6.1 Forms of booms. Alternative arrangements will, however, be accepted where it may be shown that the resulting strength of the boom is adequate. Derricks of unusual design or incorporating special features will be considered on the general basis of these requirements.

6.1.3 The scantlings of derrick booms are to be determined on the basis of the axial thrust and the maximum combination of bending moments applied to the boom, as derived from the force diagrams and calculations for the derrick system.

6.1.4 Steel for derrick booms is to comply with the requirements of Ch 2, 1.5 Materials 1.5.3. Derrick booms intended for a SWL of 60t or more are to be suitably heat-treated after welding where considered necessary. For mild steel booms, this may normally be confined to the region of highly loaded welded connections. The requirements for higher tensile steel booms will depend on the materials specification.

6.2.1 The axial thrust in the derrick boom is to be determined from the force diagrams or calculations of forces for the derrick system, see Ch 2, 3 Swinging derrick systems, Ch 2, 4 Union purchase arrangements and Ch 2, 5 Derrick cranes and derricks of special design.

6.2.2 The bending moments acting on the boom are to be calculated for conditions with the boom in its lowest and its highest working positions.

6.2.3 The determination of vertical end bending moments for typical derrick head arrangements is illustrated in Figure 2.6.2 Determination of end bending moment. The bending moment is to be calculated approximately at the point of intersection of the axis of the boom with the line joining the points of action of the cargo and span loads and this may be taken as reducing linearly from this point to zero at the derrick heel.

6.2.4 For built-in sheaves, where the sheave pin intersects the axis of the boom, the value of h is to be taken as zero.

6.2.5 Horizontal bending moments arising from slewing or preventer guys attached to the derrick boom head may generally be neglected.

6.2.6 Where the derrick is fitted with a cross-head or outrigger, consideration is to be given to the bending and torsional moments which may arise where the span force is not equally distributed between the span tackles. In general, the boom is to be designed to resist the bending moments induced when supported by one span tackle only.

6.3.1 The following symbols are used in this sub-Section:

6.3.2 The slenderness ratio of the boom is not to exceed 180. It is recommended that the slenderness ratio be less than 150.

6.3.3 The wall thickness of the boom at mid length is to be not less than:

The wall thickness at ends is to be such that the cross-sectional area of material is not less than 0,75A.

6.3.4 The moment of inertia at the derrick head, , is to be not less than 0,40.

6.3.5 Where the derrick boom is tapered or stepped, an effective radius of gyration, r, is to be determined from:

Where l₁ is greater than 0,5l the value of c is to be obtained by interpolation between c calculated for l₁ = 0,5l and c = 1 at l₁ = 0,8l.

6.3.6 The bending moment acting on the boom is to be taken as:

1. At mid length, the sum of:

   1. The self-weight bending moment, taken as:

      

   2. One half of the applied bending moment in the vertical plane acting at the boom head.

2. At the boom head:

   The applied bending moment in the vertical plane, see Ch 2, 2.3 Weight of boom and tackle 2.3.1 and Ch 2, 6.2 Determination of forces 6.2.3.

6.3.7 Where significant horizontal bending moments are likely to be applied to the boom, they are to be taken into account in the calculation.

6.3.8 The critical stress, σ, in N/mm² for the derrick boom is to be determined from:

6.3.9 The maximum allowable thrust, T, in the boom is to be determined from the critical stress and a factor of safety, F, such that:

6.3.10 Where T lies between 10 and 160 tonnes, the value T for the mid length region may be determined from:

6.3.11 For the purpose of making a first approximation to the required scantlings for a boom, thrust coefficients are tabulated in Table 2.6.1 Boom thrust coefficients (mild steel σ_y = 235 N/mm²) and Table 2.6.2 Boom thrust coefficients (high tensile steel σ_y = 355 N/mm²) . The maximum allowable thrust, before making allowance for applied bending moments, is approximately:

Having selected a suitable boom diameter and wall thickness, the allowable thrust, taking into account the applied bending moment, is to be determined in accordance with the method given above.

6.4.1 In way of head and heel fittings, the wall thickness is to be not less than 5,0 mm or 0,025 x tube diameter at that point, whichever is the greater. Proposals to fit stiffening in lieu of increased plate thickness to meet this requirement will be considered.

6.4.2 Where internal access for welding of the derrick tube is impracticable, means of obtaining full penetration welds are to be agreed.

6.4.3 Lap joints are to be not less than half the boom diameter at that point in extent. Where the lap is welded, this is to be achieved by slot welds of at least 75 mm length and twice the boom wall thickness (but not less than 25 mm) in breadth. Where step joints are adopted, the inner tube is to extend into the outer tube a distance of not less than the diameter of the outer tube or 450 mm, whichever is the lesser. The end of the inner tube is to be stiffened by a steel ring not less than 40 mm in width and of thickness sufficient to give a sliding fit within the outer tube, see Figure 2.6.3 Steps and joints in booms.

6.4.4 Welds are to be sound, uniform and substantially free from defects. The throat thickness of fillet welds on lapped joints is to be not less than 0,7 times the thickness of the inner plate forming the joint.

6.4.5 Derrick booms are to be sealed to minimise corrosion to their internal surfaces. Where practicable, derrick booms are to be painted internally or otherwise treated to reduce corrosion, after the completion of all welding.

6.4.6 Derrick boom cross-heads, brackets for cargo runner and span tackle blocks and similar structures are to be of such a design that the combined stress does not exceed 0,56σ_y.