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Light Derricks on a General Cargo Vessel

Most general cargo ships have union purchase installed onboard for cargo operations. It is also called married gear, span and tackle or twin gear system. The core principle, to find tension on hauling part being the vector addition of forces (Parallelogram Law or Lami’s Theorem). Out of the two tips of derrick booms, one is placed over wharf from where the load is picked up. The other tip is over the position in hold or deck where the load is landed. Boom is held in position by the tensioned guys and purchases in three directions, in addition to topping lift. These being the inboard flexible guy, outboard flexible guy and the preventer or lazy guy.

The topping lift is a purchase system of steel wire rope. In case the outboard guy parts, the preventer guy (Wire rope plus chain), which is stronger, rigid but slightly slacker than flexible guy can take over. In case the inboard flexible guy parts, the weight on hook will prevent from violent swinging out of booms. This is the reason that there is no preventer on the inboard side of a boom. Both flexible side guys and preventers are to be rigged and attended to. As the derrick is being lowered or topped the guys are to be heaved up or slackened. It must be ensured that no wire rope or guy gets tightened when topping up the derrick. The person attending to the lock (on dolly winch) should be attentive and at the slightest doubt about the speed or range of topping/ lowering he has to apply the lock. So that the derrick is prevented from having a free fall. Lowering of the derrick should be within limits as set out in the derrick rigging plan. While parking the derrick, the control over the side guys should be especially good since with an warranted swing the boom is liable to damage other structures. The derricks should not be lowered or topped if the ship is rolling as this would make controlling the derrick very difficult.

The end rope of the controlling flexible side guys should be held after a full turn on the rams horn or eye made for this purpose and there should be adequate clear slack. These horns are made on gunwale or bulwark stays. If a ram’s horn is not available then other suitable points may be used, however, the railings should be avoided. Derricks are secured either on a horizontal crutch (light derricks) or vertically with clamping to the mast. Prior to lowering the derrick the following are to be inspected and if any are found wanting attendance the same to be provided:

All the sheaves must turn freely. This can be checked visually, whenever attending to derricks. The crutch post and the bracket at the base are to be inspected for wear and tear. The grommet attached to the eye pad (for the gynfall wire) should be inspected. The crutch wood sheathing must be in good condition. After the derrick is parked, the crutch clamp is to be fitted and the locking arrangement fixed. There should be no play. The side guys are to be tightened and fixed on either side, the extra rope of the guys should be secured well.

Derricks are long hollow steel booms rotating on swivels (heel). The lower part ends in a goose neck, allowing the boom to turn in horizontal as well as vertical axes.

The booms have a part rope guy and a steel pendant which is used for heaving and positioning the derrick and also to keep the derrick in place. The polypropylene rope is used in a wooden tackle and can absorb sudden shocks, which come on the derrick while in operation. On each boom a preventer guy made of strong wire rope (or rope plus chain) is fitted, which is kept slightly slacker than the rope guy. This enables the rope guy to stretch before any load comes on the preventer guy. This preventer is the shock and strain absorber. If the preventer is weak, it can part with disastrous consequences. So maintaining the preventer in good condition is important.

When the two derricks are used together such that one derrick is positioned just above the loading area on the jetty and the other is positioned above the unloading position over the appropriate part of hatch square, and the gynfall (runner) wire ropes are joined together, the arrangement is called a union purchase.

This is the fastest method of working cargo. The manageable loads are light weights of about 1.5 to 2 tonnes. During the cargo operation the angle between the runner should never increase more than 1200. This makes the tension in each of the runners becoming more than the weight lifted. This may cause accident and even part the runner wire rope. Thus, while the Union Purchase may be the fastest method, it requires careful rigging of the derricks as well as an experienced winch man to handle the operation. The duty officer must keep an alert watch on the working of the same.

Cargo blocks are maintained during the voyage, but due to various reasons especially with bush bearing sheaves, the bearing may burn out. Upon hearing any unhealthy sound from purchase, winch, etc the cargo work must be stopped and the cause investigated.

The handling of the cargo gear also needs to be supervised. Any extreme rough handling should be stopped. Where the gynfall wire rubs against the hatch coaming or gunwale suitable padding should be placed. The derricks should be properly rigged and the preventer wire should, if it has been rigged properly, stretch when the load is in between the two derricks (in case of union purchase). With no load the preventer should be with some slack.

An approved rigging plan showing the arrangement of lifting appliances shall be provided. In the case of derricks and cranes the rigging plan should diagrammatically, show the positions of guys, preventers, (showing their lead) and the blocks. The resultant force on blocks, guys, wire ropes and booms must be indicated. It should be appropriate in guiding a mate to rig a lifting appliance correctly and completely from start to finish. The SWL and types of blocks, sheeves, shackles; size and type of guys; etc are clearly provided to rule out any possibility of a weak link or non compatible component in the system. Working range of crane, union purchase; limiting angle w.r.t. deck or vertical; etc are provided for all lifting appliance. If a derrick must be used as swinging derrick, in yoyo, in union purchase, as gun tackle or as luff tackle, the provision and details must be provided in the rigging plan.

Rigging a Gun Tackle:
The rigging is done as per rigging plan provided. The modification as gun tackle or luff tackle, to handle heavier weights can be by rigging. To modify into gun tackle the end of gynfall rope is attached to an eye at the derrick head and then taken to a floating block and then rove through derrick head block back to the heel block. On a mast house that has 4 booms / winches, only one boom and 3 winches will be used. On the side facing the cargo unit, one winch will be used for cargo runner and the other for slewing on one side. One of the winches on the other side will be used for slewing on opposite side. Thus, two winch men will have to operate in perfect coordination. All three winches must work in perfect coordination. The three booms on masthouse, not in operation are secured in vertical position. I will discuss the rigging for heavy lifts in another article.

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