Dismantling a derrick after a long campaign in a corrosive environment
A derrick has been installed for 14 months at a coastal fertiliser plant. The atmosphere is humid and salty, and the plant handles ammonia. The derrick is due to be dismantled and moved to another site. The dismantling plan was written for a two-day job and assumes the pins and bolts will come apart with normal hand tools. On the first day, the riggers find that several mast section pins are seized, one backstay turnbuckle is badly corroded, and the base bolts show surface rust and some pitting. The supervisor suggests heating the pins with a torch and using a large breaker bar to free them, and wants to lower the mast by cutting the backstays with a disc cutter to save time. The crew has a 4 hour window before the plant's ammonia line is purged.
1. Identify the hazards in the proposed dismantling method and state what should happen.
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The seized pins, corroded turnbuckle, and pitted base bolts mean the structure's condition is unknown and the planned sequence cannot be relied on. Heating a pin with a torch can change the metallurgy of a high-strength pin and may cause it to fail or gall; a breaker bar applies uncontrolled torque and can shear the pin or slip and injure the rigger. Cutting the backstays is worse: the backstays are holding the mast, and cutting them releases stored energy and removes the stability system without a controlled lowering sequence. The mast could fall, and the crew is inside the fall zone. The ammonia environment adds a respiratory and corrosion hazard, and the 4 hour window pressures the crew into shortcuts.
Work should stop. The dismantling plan must be revised by a competent person after an inspection of the actual condition, with a controlled sequence for freeing seized components and for lowering the mast. The plant must confirm the area is safe before work resumes. The time window is not a reason to accept an uncontrolled method.
2. Outline the sequence you would require for lowering the mast safely, and the documentation needed afterwards.
Read the worked answer
First, inspect and record the condition of all pins, bolts, turnbuckles, and backstays, and identify which components are seized or corroded. Second, design a lowering sequence that keeps the mast supported at all times, using the crane or a controlled winch and a planned release of the backstays, with the load path calculated. Third, free seized components using approved methods, such as penetrating oil, controlled heat within the manufacturer's limits, or replacement, not impact or cutting under load. Fourth, barricade the fall zone and exclude all personnel not essential to the operation. Fifth, lower the mast in steps, checking stability at each stage, and land it on prepared supports.
Afterwards, document the inspection findings, the revised plan, the sequence actually followed, any components replaced, and the condition of the crane for transport and storage. Record the plant's clearance of the area and the sign-off by the competent person. The documentation should support the next site's pre-installation inspection.