Section 2 Design criteria

2.1.1 Calculations are to be made for derrick systems with the derrick booms at the maximum and minimum angles for which the system is to be certified. These angles are to be taken as follows:

1. Maximum angle to the horizontal:

   70° generally, but a greater angle may be specified.

2. Minimum angle to the horizontal:

   1. SWL not exceeding 15 t:

      30° generally, but a lesser angle may be specified. In no case is the angle to be less than 15°.

   2. SWL exceeding 15 t:

      30° generally, but a greater angle may be specified. In no case is the angle to be greater than 45°.

2.1.2 The length of the derrick boom is to be such as to give adequate coverage of the cargo hatch and sufficient outreach beyond the ship’s side, within the limiting angles given in Ch 2, 2.1 Operating range for derricks 2.1.1.

2.1.3 The lengths of derrick booms for union purchase systems are to be sufficient to provide the coverage of the hatch and outreach beyond the ship’s side as required by Ch 2, 4.2 Working range of the rig 4.2.1. The angles of inclination of the booms are to remain within the limits given in Ch 2, 2.1 Operating range for derricks 2.1.1.

2.2.1 For swinging derrick and union purchase systems, a basic angle of heel of 5° and a trim of 2° are assumed for the ship. Provided these angles are not exceeded, they may generally be ignored in the calculation of forces and tensions in the derrick system and in the masts and derrick posts.

2.2.2 The angles of heel and trim of the ship with its largest loaded derrick (or derricks if more than one can be used at one time) swung fully outboard are to be calculated. Where the calculation shows that the ship would have a greater angle of heel or trim than 5° or 2° respectively, the actual angles are to be taken into consideration. This calculation is NOT to be taken to mean that LR accepts responsibility for the stability of the ship. Stability is a matter to be agreed between the Builder, designer and Owner and may be subject to National Regulations.

2.2.3 For derrick cranes, a basic angle of heel of 5° and a trim of 2° is generally to be included in the calculations. Greater or lesser angles may be specified provided these angles are clearly stated in the certificates.

2.3.1 The weight of the derrick boom and tackle is to be included in the calculations for all union purchase rigs and for swinging derricks and derrick cranes where the SWL exceeds 15t. Where available, the actual weight of the derrick boom and tackle is to be used. Alternatively, an estimated value equal to 10 per cent of the SWL of the system when rigged as a swinging derrick or derrick crane is to be applied at the derrick boom head.

2.4.1 For calculation purposes, a combined allowance for sheave friction and wire stiffness is to be made as follows:

2.4.2 The appropriate percentage is to be applied cumulatively to the parts of the rope supporting the load. Coefficients for estimating the rope tension are given in Table 2.2.1 Coefficients of rope tension and in association with Figure 2.2.1 Coefficients of rope tension.

2.4.3 As an alternative, the coefficients of rope tensions may be determined as follows:

2.5.1 Wire ropes are to have a breaking load not less than the maximum tension in the rope multiplied by a factor obtained from Table 2.2.2 Factors for wire ropes.

2.5.2 Natural fibre ropes used in derrick systems, where permitted, are to have a breaking load not less than the maximum tension in the rope multiplied by 8.

2.5.3 Man-made fibre ropes, where permitted for standing rigging, are to have a breaking load not less than the value obtained from Table 2.2.2 Factors for wire ropes multiplied by 1,25.