Harsh conditions in the offshore oil industry put severe demands on equipment, the reliability of which is not improved when access for maintenance is difficult. A pair of explosion proof EOT cranes in the well bay of the Scott offshore oil platform in the North Sea Sector, to the north east of England, have been refurbished and upgraded to better cope with their working environment.
R Stahl, in conjunction with Sparrows Offshore Services, carried out the work on the 15t SWL cranes earlier this year. Client for the project was Amerada Hess, the US oil company.
Due to an unacceptable number of breakdowns and the cranes’ inaccessibility – scaffolding was required each time there was a problem – it was clear that action had to be taken. An initial survey established the viability of refurbishing the cranes to a higher specification using components better able to withstand the harsh Zone 1 area classification environment.
The cranes, known as the east crane and the west crane respectively, each had a span of 5m and 15m height of lift. They also featured: limit switches on travel, traverse, hoisting and lowering; an anti-collision device between the cranes and for well bay risers; and an overload device.
Inspection of the east crane
The east crane was isolated during inspection and so crane functions were not tested. The inspection found that a down shop conductor festoon system provided the electrical supply to the crane. The feed cabling was suspended from hangars fixed to trolleys which ran on the bottom flanges of the I-beam, running the length of the well bay, parallel to the gantry beam. These cable hangers stacked the cable one above the other and the design allowed the stacked cable to rotate.
There were no rubber buffers on the trolleys and no tension strain relief cable ties fitted between the trolleys. During long travel operation the feeder cables touched each other as the system ambushed and this acted as buffer stops because the cable bending radius exceeded the trolley length. As the system extended along the gantry, tension was applied with cable loops of about 1m. Cable depth could have been increased to 1.5m, and the number of trolleys reduced, to benefit the system when ambushed.
Terminal boxes at the end of the gantry beam had IP 55 enclosure ratings and gland entries were found to be in good order.
The inspector reported some confusion regarding the supply cable. Available wiring diagrams stated the cable’s cross-sectional area as 25mm2 but on inspection these cables were only 4mm2 or 6mm2. Stahl estimated a minimum requirement of 16mm2.
The main long travel panel was mounted on the crane beam and was a direct entry (Exd) enclosure to IP 55. Gland entries were in satisfactory condition. Additional sealing tape had been applied over the panel lid to prevent further ingress of water. The panel was not opened during the inspection. Feeder cabling to the panel was looped off the down shop festoon system towing arm and laid along the crane beam, in some areas without mechanical protection. Anti-condensation heaters had been fitted inside the panel but did not work because of the crane isolation procedure.
Long travel motors were centrally mounted on each undercarriage. Output shafts of the foot mounted geared units drove a large pinion which in turn drove onto machined racks welded to the gantry beam. Enclosure and cabling details were as found on the long travel panel.
The travel limit switch was a non-adjustable cruciform arm arrangement operated by striker bars on the gantry beam. Enclosure and cabling was as before. Anti-collision protection between the cranes and the well riser was an IP 55 (Exd) limit switch actuated by a taut wire across the crane bridge. This system was prone to problems due to the area and working environment. Hoist overload protection was a direct entry IP 55 enclosure limit switch on the hoist rope.
The switch for the top hoisting limit was considered inadequate for the operational duty demanded of it. It was actuated by the bottom hook block contacting an insubstantial operating arm which allowed the top limit operational stop position to vary depending on the hoist speed and load.
Due to the circuit design, actuation of this switch isolated all crane motions including the lowering function which had caused problems when this had happened in inaccessible areas. A further limit switch was the emergency hoisting cut-out in case the main hoisting limit switch failed. This was a geared switch integrated with the hoist unit.
Cross-crane feed cabling and control lines were supported with a similar festoon trolley system to the one previously described. The main difference was that the trolleys ran internally on the C-rail section across the crane span. This small section, combined with tracking joists, was prone to snagging and seizure. A broken trolley support spindle allowed the cables to hang loose.
Cabling for the mobile control system pendant was like the others and considered inadequate for the working conditions. Cables to the control unit hung directly from the support trolley. Strain wires were fitted to support the pendant but the pendant’s weight and size were considered unsuitable both from an operational and a safety point of view.
The hoisting panel, an integral part of the hoist unit, was mounted on the crab. Enclosure rating was IP 55 (Exd) like before. This panel was opened for inspection and found to be damp inside due to water ingress. Oxidation had left the panel in poor condition. The lid and enclosure face, which formed part of the flame path was also in poor condition. The anti-condensation heaters were considered inadequate because of their size and position.
Inspection of the west crane
The west crane was not scaffolded but power was available, so observations of the crane’s operation could be made. Operation of both the long and cross travel functions was smooth and speed changes both up and down were deemed acceptable with normal load swing. Hoisting and lowering was also tested but, like the other functions, limit switches were not actuated.
The pendant’s strainer wire support was broken and the pendant trolley was wedged in a joint of the C-rail track. Even with the pendant supported by strainer wires, safe supervision of the load while handling and operating the pendant was difficult. The deck operating level was about 15m below the crane and there were no audible or visual warnings to indicate approaching travel, collision or hoisting limits.
Summary
The inspection report established that the electrical installation did not meet requirements but there were several other problems:
•inaccessibility for maintenance
• inability to safely control the crane with existing equipment
• electrical enclosures not protected against water ingress
• activation of the top hoisting limit switch mid-span rendered the crane inaccessible
• and deterioration of the Exd control enclosure’s flame path required consideration that use of the crane should be confined to safe areas only.
Mechanically and structurally the cranes were generally in good condition, apart from the trolley systems which needed to be replaced.
Recommendations and action
Due to the position of the cranes, their working environment, the breakdown frequency with attendant costs and downtime, and the lack of access, it was recommended that provision should be made to improve access for preventive maintenance work.
The operating environment nec-essitated an upgrade of all the electrical en-closures including the integral panel and hoist unit from the existing IP 55 to a minimum rating of IP 66 for the hazardous area classification. Round, flame retardant cables and glands were used because flat form cable and glands are not available to IP 66.
A hand-held push button radio control system was specified to eliminate the cumbersome and possibly unsafe existing pendant system. The new control system also included an explosion proof interlocked wall-mounted push button station for each crane to give an alternative control method in the event of a radio system failure. An audible or visible warning operates before the top hoist limit switch is actuated to give added protection for the operator, who is 15m below the crane bridge.
The Stahl explosion proof electric wire rope hoist has a fabricated steel barrel, machined with a dual groove to allow a true vertical lift. The barrel has a minimum of three complete turns of rope remaining in the grooves when the hook is in its lowest position. When it is fully raised all rope is contained within the grooves. A two piece rope guide is used to ensure complete layering into the grooves and to maintain rope tension.
All motors and brakes, for hoisting, cross travel and long travel, are contained within Eex d flameproof housings with electrical connections made via Eex e increased safety enclosures. Motors are a taper sliding rotor design with an integral conical disc brake. To monitor temperature, thermal sensors are embedded into each phase of the motor windings. The geared motor units were mounted on the existing crab unit structure using adapter plates.
Each motion is controlled by separate contactor equipment operating at 42V. Hoist and traverse controls are mounted in the integral hoist panel located at one end of the unit. A separate enclosure houses the long travel controls, mains contactor and transformer. Anti-condensation 48V panel heaters are included which operate when the mains supply is disconnected or when the emergency stop button is pressed.
A hoist overload protection device stops the hoisting motion at 110% of the rated load. No other motions are affected and it is still possible to lower the load. A three second time delay is incorporated so that normal acceleration forces are not mistaken for an overload condition. Self resetting ultimate and operational hoist and lower limit switches are fitted. They are operated by movement of the rope guide and also provide protection against phase reversal of the power supply. Limit switches on the traverse and long travel motions prevent collision with the end stops.
Collisions between cranes, and between a crane and the well head are prevented by infra-red photocells. Two photocells with reflectors are fitted to the crane bridge which gives a beam across the complete crane span. The transmitter beam is reflected back by the separate reflectors. Interruption of the beam disconnects one direction of the long travel motion.
Cable trolleys have large diameter cable formers so that minimum bending radii are not exceeded and are long enough to ensure that cables do not touch each other when in an ambush situation. Tension relief wires between cable trolleys prevent strain on the electrical cables. The long travel festoon runs on the existing I-beam but for the cross travel a new I-beam replaces the old C-rail system. Cables are round type HO7RNF flame retardant rubber, harmonised and suitable for operation in hazardous areas. They are not armoured or braided due to the continual flexing of the festoon system. For an explanation of the explosion proofing category system see HOIST Issue 6 (Q3 1999) p34-5.
