Derricks are one of the earliest of the cargo handling gear used on ships. The Derrick type of gallows in turn got its name from Thomas Derrick, an English executioner from Elizabeth era. There are various types of Derricks used on ships. The rigging of all these Derricks for its operational use, which includes setting up the ancillary parts of the Derricks will differ from different types of Derricks.
Before we proceed to safe working practices, care and maintenance, let us first understand some terms and parts of Derrick and cranes.
Parts of Derrick
- Cargo Boom: A spar extending from a mast or a king post, used as a derick arm to handle cargo. It is a cylindrical spar made of wood or steel by which cargo is loaded and discharged. It is supported by masts or king posts.
- Cargo Runner: The term used to describe cargo lifting wire used on Derrick.
- Topping Lift: The rig that supports the cargo boom at any desired angle from the deck and the tackle that rises and lowers the boom. There could be a single topping lift or a multiple topping lifts used for the derick.
- A span topping lift: It is a stationary lift shackled into the head of the boom and into the head of a king post. The boom remains at one certain angle at all times.
- Guys: The lines or tackle used to steady or swing cargo booms are known as guys. When led to a source of power, guys are called vangs. Guys may be outboard, inboard or leading athwartship. A athwartship guy is sometimes called the lay guy or the schooner guy. A schooner guy joins the spider bands at the Derrick heads of a 'Union Purchase Rig'. Outboard guys are made fast to the outboard side of the head of the booms and to fittings on the deck or bulwark. These guys are referred to as the working guys because they are under greatest stress. The stress on the guys occurs when the load is being transferred athwartship or when it is being supported anywhere between the two boom heads. Inboard guys are made fast to the inboard side of the head of the booms and to fittings on the deck or bulwark. Since the load on the cargo hook is always between the heads of the two booms or directly under one of them, there is title or no stress on inboard guys.
- Head of boom: It is the upper end of the boom.
- Heel of the boom: it is the lower end of the boom.
- Bail: It is triangular piece of steel or boiler plate used in connection with the single topping lift. The lower end of the single topping lift is shackled into the upper corner of the bail; the topping lift chain and the bull rope are shackled into the shackled into the topping lift eye, the bail served to separate them.
- Side whip or side runner: It is runner on boom positioned over the side of the ship.
- Hatch whip or hatch runner: It is runner on the boom positioned over the hatch.
- Jumbo Derrick: It is a heavy lift boom of handling weights from 5 to 50 tons or more. These heavy booms are supported directly from the deck. The location of the jumbo boom depends on the location on the largest hatches.
- Pedestal: It is the fitting which takes the gooseneck of the jumbo boom.
- Tabernacle: It is a watertight structure built around masts or between king posts, in which gear may be stowed or winches are housed.
- Preventer: Any piece of gear rigged in addition to the regular gear to prevent it from carrying away; the best known is the preventer guy which is simply a single wire shackled into the head of the boom and led down to the deck and made fast.
- Stopper: It is a piece of line or chain used to stop off a hauling part while it is being belated (made fast).
- Gooseneck: The bearing and swivel fitment, found at the heel of a Derrick which allows the Derrick to slew from port to starboard, and luff up and down when in operation.
- Hounds Band: A lugged steel band that straps around a 'mast' used to shackle on shrouds and stays. It is also employed to secure 'Preventer Backstays' when a heavy Derrick is being deployed in order to provide additional strength to the mast structure when making the heavy lift.
- King Posts: Also called Samson posts, are vertical supports, usually made of steel, one on each side of the centerline of the ship used to support booms.
- Luffing: Movement of crane jib or boom up and down.
- Masts: masts are the supports for cargo booms, and also signal lights, antennas, and crow's nests. On some ships, mast may be fitted with a crosstree to which the topping lift blocks are secured, and a mast house, which supports the heel of the boom.
- Shrouds: These are used to provide athwartship support for the mast or king posts. Two or more shrouds are used on either side of a mast or king post and are secured to the deck or bulwark in a fore and aft direction to provide maximum support.
- Snatch block: This is a single sheave block, often employed to change the direction of lead, of wire or rope. The block has a hinged clamp situated over the 'swallow' which allows the bight of a wire or rope to be set into the block without having to pull the end through.
- Spider band: A steel lugged strap found around the head of a Derrick which thee rigging, such as the topping lift and guys are checked onto. The equivalent on a mast structure is known as a 'Hounds Band'
- Standing Rigging: Term used to describe fixed steel wire rope supports.
- Stays and backstays: These are heavy wire ropes, found at the mast where the jumbo boom is located. When they support the mast or king posts from a forward direction, they are called back stays. Additional stays and back stays may have to be rigged when unusually heavy lifts are being loaded and discharged.
- Bull rope: Wire rope used on a single topping lift to top and lower the boom.
- Trunion: An arrangement similar to the 'gooseneck' of a small Derrick, normally found on intermediate size Derricks of 40 tonnes or over. They are usually manufactured in cast steel and allow freedom of movement from the lower heel position of the derrick.
- Tumbler: A securing swivel connection found attached to the 'Samson Post' to support the topping lift blocks of the span tackle.
- Union Plate: A triangular steel plate set with three eyelets used in 'Union Rig' to join the cargo runners and hook arrangement. It can also be used with a single span, topping lift derrick to couple the downhaul with the chain preventer and bull wire, sometimes referred to as 'Monkey Face Plate'
Safe working Practices with derricks:
Ships’ derricks should be properly rigged. Derrick rigging should be checked and
maintained with plan. Rigging plans contains the following information:-
- position and size of deck eye-plates
- position of inboard and outboard booms
- maximum headroom (i.e. permissible height of cargo hook above hatch coaming)
- maximum angle between runners
- position, size and safe working load of blocks
- length, size and safe working load of runners, topping lifts, guys and preventers
- safe working load of shackles
- position of derricks producing maximum forces
- optimum position for guy and preventers to resist maximum forces
- combined load diagrams showing forces for a load of 1 tonne or the safe working load.
Guidance on maintenance of Derrick
- Before raising, lowering and adjusting a derrick, the hauling part of the topping life should be flaked down the deck clear of the operational area, all persons standing clear. Someone should be available to assist the person controlling the wire on the drum and keeping the wire clear of turns and in making fast to the bitts or cleats. Where the hauling part of a topping lift purchase is led to a derrick span winch, the bull-wire should be handled in the same way.
- When a single span derrick is being raised, lowered or adjusted, the hauling part of the topping lift or bull-wire (i.e. winch end whip) should be adequately secured to the drum end. Where topping lifts are secured to bitts, 3 complete turns should be taken before the 4 cross turns on top. A light lashing shall be placed to prevent spring off of the wire.
The derrick head should be lowered to the crutch or to deck level for safety whenever the rig is to be changed.
- The pawl of winch requires to be lifted to allow the derrick to be lowered. This
shall be done with extreme care.
- Personnel should be detailed for lifting and holding the pawl bar, ready to release it should the need arise; the pawl should be fully engaged before the topping lift purchase or bull-wire is released. The person employed on this duty should not attempt or be given any other task until this operation is complete; in no circumstances should the pawl bar be wedged or lashed up.
- The winch speed should be in consistent with the safe handling of the guys.Winch driver should have the full awareness regarding the load and where required take instructions from a single controller with a clear view of operation.
- Cargo runners should be secured to winch barrels by U bolts and minimum 3 turns remaining on the barrel when fully extended.
- The runner should be used direct from the heel block via snatch blocks when dragging heavy cargo from tween deck. Where rollers are fitted to runner guides, they should rotate freely.
- Runner guides should be checked in position fitted to all derricks so that when the runner is slack, the bight is not a hazard to persons walking along the decks.
- A heel block should be secured additionally by means of a chain or wire so that the block will be pulled into position under load but does not drop when the load is released.
- All persons on deck in the vicinity should be warned before a derrick is raised or lowered, so that no person stands in danger from, bights of wire and other ropes.
- When steam winches are being operated, winch drivers must ensure that the winches and steam pipes are drained of water before work commences and after any period during which the winches have remained idle.
- Makeshift extensions to winch control, particularly for the purpose of operating two winches by one man, should not be permitted.
- Inexperienced persons should not operate winches unless under the direct supervision of a competent driver.
Maintenance of Derrick System
- Wires, blocks, sheaves, heel pins and other moving parts are greased in accordance with shipboard PMS.
- Testing is carried out annually / quadrennial (four yearly).
- All test certificates for new equipment like wires, shackles, sheaves, ropes, blocks etc maintained on board.
- While inspecting blocks / sheaves, following are checked:
-Misalignment of sheaves
-Heavily scored sheaves
-Incorrect sheave groove to wire diameter ratio
-Frozen guide rollers.
-Inadequate internal lubrication
- While inspecting wires, following are checked:
-Inadequate internal / external lubrication.
-Broken/ brittle strands.
-Limit switches are tested as provided.