Section 5 Derrick cranes and derricks of special design

5.1.1 In general, a derrick crane may be described as a conventional derrick fitted with one of the following modifications:

1. Twin span tackles so designed that the derrick can be slewed without the use of separate guys.

2. A system for topping (luffing) the derrick boom other than by means of span ropes.

3. The cargo and/or the span winches built into the derrick boom and moving with it.

4. A system for slewing the derrick boom by applying a torque to a slew ring or trunnion.

Where more than one of the above modifications is fitted, the system will normally be considered as a deck crane and the requirements of Ch 4 Cranes and Submersible Lifting Appliances will apply.

5.1.2 Derricks of special design but not designated as derrick cranes are to comply with the requirements of this Section where they are applicable.

5.1.3 Attention is drawn to the fact that many of the designs and variations of design incorporated in derrick cranes and special derrick systems incorporate patent features.

5.1.4 Derrick cranes and derricks of special design are to be examined for the operating range and angles of inclination of the ship as specified in Ch 2, 2 Design criteria. Force diagrams or calculations are to be prepared as required by Ch 2, 3 Swinging derrick systems. More detailed calculations may, however, be required to support proposals for unusual systems.

5.2.1 Twin span tackles are to include:

1. Two separate span tackles each attached to the head of the boom, directly or via outriggers, and operated by independent winches.

2. Two span tackles led to a topping winch and a slewing winch. The topping winch shortens equally on both spans and the slewing winch shortens one span while paying out the other, see Figure 2.5.1 Span tackles with winch.

The two sections of the tackles may be attached to the cross trees of a mast or they may be fitted to two separate derrick posts. Where two separate posts are used, the boom is sometimes arranged so that it can pass through the vertical position between the posts and so be available to work hatches both forward and aft of the posts.

Figure 2.5.1 Span tackles with winch

5.2.2 Where twin span tackles are fitted, they are to be so arranged that the minimum distance of the span tackle from the vertical through the boom gooseneck is not less than one ninth of the boom length. This distance is to be measured horizontally at right angles to the line of action of the span tackle, or to the section of the span tackle under consideration, see Figure 2.5.2 Minimum separation. This minimum separation can be achieved by one or more of the following means:

1. by limiting the slewing angle of the boom; or

2. by fitting outriggers to the boom head; or

3. by fitting hinged or fixed outriggers, or their equivalent to the derrick posts, mast house or cross tree; or

4. by a combination of one or more of the above.

Figure 2.5.2 Minimum separation

5.2.3 Alternatively, the stability of the system may be demonstrated by calculation or by model tests.

5.2.4 Where the slewing angle of the boom is to be restricted, this may be done by moving the position of the mast head span eyeplate away from the transverse plane through the gooseneck into a position that will be vertically above the derrick boom when the boom is in its limiting position. Alternatively, limit switches may be installed to provide automatic cut-out of the slewing system. Limiting the slewing angle of the boom by means of a stop fitted to the boom heel or by allowing the boom to come up against a shroud or other obstruction cannot normally be recommended as these methods tend to induce large transverse bending moments in the boom or to cause local indentations and consequential failure of the boom tube.

5.2.5 In general, where twin span tackles are fitted, the strength of each is to be sufficient to support the boom plus the safe working load in the boom’s fully outboard position. Where, however, it can be shown that it is not possible for either span tackle to become slack in service, the strength of each tackle may be based on the maximum calculated span tension but is to be taken as not less than two thirds of the total span tension. Calculations in this respect are to be submitted by the manufacturer of the derrick crane.

5.2.6 Where the derrick boom is fitted with a cross-head and the span tackles are each connected by strops to both ends of the cross-head, then the required breaking load of each of the strops may be based upon 80 per cent of the maximum load in the span tackle.

5.3.1 Slewing guys may be fitted in addition to twin span tackles and may take any one of the following forms:

1. Guys of the normal type.

2. Guys which are so rigged that topping the boom will not alter the transverse position of the boom head relative to the centreline of the ship.

3. One or more bights of the span wire led down from the boom head span block, around a block at deck or bulwark level and back up to the boom head span block again.

5.3.2 Where a guy is led from an eyeplate on the deck around a block at the boom head and then to an eyeplate on the mast, a calculation or diagram of forces is to be prepared with the boom in its highest working position in order to check that the boom is not liable to jack-knife.

5.3.3 The required safe working load of the slewing guys of normal type is to be determined from Ch 2, 8.4 Slewing and preventer guys on the basis of the SWL of the derrick system. Where a derrick crane is of the type described in Ch 2, 5.1 General 5.1.1.(c), the nominal SWL for this purpose is to be increased by 25 per cent.

5.4.1 The scantlings of derrick booms are generally to be determined in accordance with Ch 2, 6 Derrick booms.