A giant makeover

The crane is mounted on a single circular track to provide polar motion referred to as long travel. There were two crabs running on the upper beams of the crane, one for the main hoist (25t capacity) and one for the auxiliary hoist (7.5t capacity). All motions are electrically powered. To enable decommissioning of the reactor, the crane was required to be modified and upgraded for the DFR projects and future operations mainly associated with defuelling, decommissioning and dismantling of equipment. The refurbishment was to bring the crane up to modern standards for lifting radioactive material, referred to as nuclear lifts.

The specification

A refurbishment specification was issued by the UK Atomic Energy Authority (UKAEA) outlining the removal and replacement of the two existing crabs, electrical system, anti-collision system and part of the polar (long) travel drive system. As part of the refurbishment, the reach of the crane had to be increased to permit the hook to operate at a greater radius to reach parts of the facility now required for decommissioning work. The overall load capacity of the crane and the hook operating elevations needed to be maintained. A new maintenance access platform was to be added to the Goliath structure to assist in undertaking crab maintenance.

The specification set out the design requirements and following a tender exercise and negotiations, a fixed price contract between UKAEA and Weir Strachan & Henshaw was let. Some changes to the technical requirements were agreed during negotiations for the adoption of inverter drives enabling better control of the motor drives. A single crab was designed to provide the duty previously carried out by two crabs. This required the new crane hook to achieve the same reach at the cab end of the Goliath as the previous hook, and an extended reach at the non-cab end of the Goliath. In addition, the hook elevation below floor level had to match the previous hook elevations. The additional requirement to maintain all of the equipment from the upper side of the crane, combined with the difficulty in using any access equipment from below, determined that all equipment on the crab had to be designed for access from above.

The design approach for the equipment was to design to BS2573 with appropriate duty factors and exceptional loading cases. The mechanisms are fault tolerant and the crane has to remain operational should a single rope break when in use.

The crane was fitted with an electrical control system and a separate protection system and the protection system attains a Safety Integrity Level of level 2. The philosophy behind the protection system is to provide a system with fault tolerance and minimal interaction between the control and protection systems.

Mechanical design

– Crab structure

The crab structure was manufactured from carbon steel and joined together to form a welded structure. All equipment connected to the structure was mounted on machined pads that were full penetration welded into the structural members. All of the material was supplied with copies of the mill certificates for mechanical properties and chemical composition to fulfil the quality assurance requirements.

– Hoist reeving and drum

Several reeving arrangements were considered in order to use a multiple rope system. Variations in rope size and return pulley locations above and below the crab structure were considered. Due to the limited space available, and the need to access equipment from above for maintenance, a four rope system was selected. The solution increased the torque rating of the gearbox, but eliminated the need for return pulleys, decreasing the height of the crab and providing a compact design.

With the four rope reeving system, two ropes were wound in grooves at each end of the drum on opposite helices. At each end of the drum the grooves had the helix geometry of a twin start thread and the ropes were laid in adjacent grooves. In this way the two ropes pay off of adjacent grooves as a pair of parallel ropes. After leaving the drum the ropes go down to the hook block and over return pulleys to come back up to the crab where they are connected to a four rope termination balance beam. On the drum the rope ends are fixed to the drum using rope clamps that are bolted radially into the circumference of the drum. When fully raised there is one ‘free groove’ and, when fully lowered, there are three turns of the rope left on the drum (dead turns) before the rope reaches the rope clamps.

The rope drum is machined from welded carbon steel fabrications that consist of the main drum and a stub axle end piece. At the non-drive end of the drum there is an emergency brake disc formed integrally with the main drum fabrication. The drum itself is supported in spherical roller bearings in two independently mounted plumber blocks, which are supported on cross members of the main crab fabrication. The drive to the coupling is direct from the speed reducing gearbox located on the crab fabrication adjacent to the drum. At the non-drive end of the drum there is a connection to the Stromag switch box that is used in the control system for over-raised and over-lowered, and to provide a single contact into the protection system for over-lowered when the hoist is passed down to the fan gallery, the lowest area in the sphere to be accessed.

– Rope balance beam

The balance beam is used to adjust the initial set up of the ropes and to accommodate variations in rope length due to rope tolerance, rope stretch, and manufacturing tolerances on the drum and the hook block pulleys.

Each of the ropes has its own termination on the balance beam which consists of a spherical bearing, guide rod, load cell, thrust bearing and adjusting nut assembly. Each termination allows its associated rope to freely orientate itself as the balance beam moves to keep the pull on the assembly aligned with the rope, and to provide an axial load onto the termination load cell. In this way all four ropes are balanced and take equal load. The four rope termination load cell signals are summated to provide an indication of the load carried by the ropes.

The balance beam is fitted with a number of safety limit switches which operate when the balance beam rotates out of design specification. Operation of any limit switch will trip the hoist and prevent any further raising or lowering under powered operation.

– Hoist drive line

The main speed reducing gearbox for the hoist drive is a five-shaft helical spur gearbox using side-by-side gearing. The drive into the gearbox is from a high speed line comprising a brake module and a drive motor. The standby and service brakes sit between the gearbox and the motor. The drum brakes are operated by thruster units and fitted with manual override mechanisms.

Between the gearbox and brake module there is an emergency hand-wind facility that can be used to lower the load supported by the crab. Hand lowering is only used in an emergency where there has been a total loss of power and it was desired to lower the load safely to the floor.

The main drive motor was a squirrel cage motor fitted with a forced cooling fan and was controlled by a variable speed microprocessor-based controller located in the control panel on the bridge of the Goliath.

At the non-drive end of the hoist drum the emergency brake disc passed through two emergency brakes. The brakes are opposite each other on either side of the drum and supported on pads formed integral with the crab fabrication. The emergency disc brakes are DC power operated with the brakes powered off and sprung set providing fail safe operation upon power failure.

– Crab access

Access onto the crab is provided via walkways that are bolted to the main crab fabrication and which overhang the fabrication on either side of the crab across the span of the crane rails. Each walkway is fitted with handrails that can be folded down into the clearance envelope defined by the arch structure of the Goliath and the sphere. The handrails are folded down whilst the crab is in operation and are raised and locked into position when access is required for maintenance.

– Cross travel drive

The cross travel drive system comprises a squirrel cage motor directly mounted on a speed reducing gearbox bolted to the crab fabrication. The output shafts from the speed reducing gearbox go to two wheels on one side of the crab via gear couplings and intermediate shafts. Two of the four crab wheels are powered by one motor which incorporates an integral disc brake unit on the outboard end of the motor.

The travel capability of the crab was increased during the design phase in order to reach the radial position required for the removal of fuel from the reactor. To accommodate the increased travel the existing crane rails had to be extended. Due to the reach of the crab, the curvature of the sphere, and the need to minimise the weight, a low level buffer arrangement was designed and fitted.

– Long travel drive

The two (polar) long travel drives are diametrically opposite, one being located on each end carriage. Each new motor gearbox unit comprises a speed reducing gearbox and motor drive.

As the crane rotates around the sphere’s vertical centreline the end carriages travel in an area that may be occupied by personnel. In order to protect personnel from the crane as it travels there are warning lights and audible alarms fitted at each of the four goliath end carriages. Adjacent to the warning lights and audible alarms there is an anti-collision barrier system which has protection switches mounted in the mechanism. Movement of the barrier from the vertical position will activate the protection switches which will stop the long travel drive.

In addition to barriers to protect personnel, the crane may encounter objects of such a size as to fit under the barrier and become trapped in the wheel. In order to provide protection, each corner of the crane is fitted with ‘finger’ limit switches that extend below floor level adjacent to the crane rail. Operation of any of these switches stops the long travel drive.

Electrical design

A new electrical supply and control system has been installed on the crane. The new supply cable delivers the power to a suite of panels mounted on the main bridge beam on the top of the Goliath structure. The suite of panels comprises the incoming power distribution panel, electrical protection panel, crane control panels and motor brake resistor panels. The cables run on their associated cable tray systems providing segregation of cabling where practical. Power, control and protection cables are delivered to the crab via a drag chain located in the centre opening of the crane adjacent to one of the main Goliath beams.

Control System

The control system comprises a main control panel on top of the Goliath, with operating stations in three locations.

The main operating desk control station is located in the control cab. From this position the crane operator can view most of the operations from above, although some restriction in visibility is imposed by the structure and geometry of the equipment within the sphere. To overcome the difficulties of visibility for some of the lifting operations an additional operating station comprising a portable pendant control box and umbilical cable has been provided. This will plug into either of two control points, one located on each of the two main drive carriages, and allows the operator to control the crane motion from floor level.

Control of the crane is only possible from one control station at any time and the stations are interlocked via a key interlock system. The philosophy behind the control system is that under supervision the operator may move the load to any desired position.

Provided the crane is operating in the safe working envelope the protection system is not required to operate. Should the crane motions, or the load carried by the hook, exceed the normal operating limits, the protection system will be challenged and shut down the systems as necessary. Manual operation does not override the control system limits and it is not possible for manual operation to drive the crane to the protection limit until it has moved through the associated control limit.

The control system is such that when the system reaches the control limit the associated drive is stopped in a controlled manner and prevented from continued operation in that direction. However, it may be used to move away from the limit condition at any time. When a control limit has been reached an indication light is provided on the operator’s control panel to warn the operator of the end of motion. Continued operation, or failure of the control system, brings the motion to the protection limit which causes the protection system to stop the motion and bring the equipment to a halt. Activation of the protection limit is given by illumination of a light on the operator’s control desk. Motion is then inhibited in either direction until a key override system is operated. Overriding the protection system will only allow the motion away from the limit, and is carried out under managerial supervision. Once the crane returns to the control zone the protection system can be reset manually.

Motor controls

The existing crane control system used the standard series and shunt type electrical controls that have been in use on cranes for a number of years. These systems were well suited for use with motor controls where interruption of the power supply to the motor and its easy directional reversing by phase switching is possible. For modern cranes many of the drive systems, including the ones supplied for this refurbishment, utilise inverter drive technology. The inverter drive technology provides the power connection to the motor which does not have a standard three-phase electrical wave form. The inverter uses frequency control to provide the direction and speed control to the motor. The inverter also provides the facility to apply a constant torque which improves the control and dynamic response for the crane operation. The removal of power from the inverter will remove power to the brake units and the brakes will be applied immediately.

Hoist systems

The hoist drive motor is inverter controlled and in order to operate at sustained creep speed, it incorporates an independent forced cooling motor driven fan. The motor is fitted with an encoder that feeds a speed signal back to the inverter drive. Inverter drives are used to provide controlled speed and acceleration that can be easily adjusted. A feature of an inverter drive system is the ability to hold the load stationary permitting the motor to take up load before the brakes are released. The inverter controlled motor will decelerate the motion to rest before the mechanical brake is applied.

The hoist drive system is fitted with two separate service and standby drum brakes located between the motor and the speed reduction gearbox. When the hoist is at rest the service and standby brakes are applied.

Each of the mechanical brakes are powered off and sprung on and each one is capable of stopping the hoist. The brakes are thruster operated, with the service brake being applied immediately and the standby brake operating with a slight time delay introduced by a mechanical dashpot on the thruster unit.

The control limits for the hoist are fully-lowered or fully-raised. The protection for the hoist is initiated by over-raising, over-lowering, over-loading, under-loading or overspeed of the hook. Over-loading of the hook is provided for loads applied that exceed the safe working load of the crane. Under-loading of the hoist corresponds to the beginning of slack rope operation, as determined by a reduction in the indicated load below zero indication on the display. Should any of these over-conditions exist the protection system operates and de-energises the emergency brake callipers that operate directly onto the brake disc integral with the rope drum.

Cross travel system

The cross travel drive uses an inverter controlled motor fitted with an integral shaft extension disc brake. The motor has two speeds for cross travel operation, slow and fast.

Due to the extended reach requirements of the crab it was discovered during an early evaluation of the stopping distances that using the normal deceleration rates would result in the crab having insufficient stopping distance before running into the sphere, should the control system fail and a full speed trip be required. To overcome this, and to permit the crab to come closer to the sphere shell, a creep speed was introduced to the crab for use, solely by the control system as the limits of travel are approached. This operates by enforcing creep speed once the crab approaches the end of travel. This creep speed is enforced irrespective of the speed selected by the operator. Indicator lights are provided on the operator’s control panel for when the crab passes through the control limit or the protection system operates.

Long travel drive system

The two long travel drives are diametrically opposite, one being located on each end carriage adjacent to legs numbered one and four (Fig 1). Rotation of the crane takes place over an angle of 440 degrees. Each of the long travel drive motors are operated from an inverter control unit located in the control panel on bridge beam of the crane. Crane motion is controlled by the operator at the control station, or at one of the control pendants.

Having selected the direction and speed of operation the crane is operated manually until it reaches the control stop, at which point the control system limit is reached and the inverter will decelerate the crane and bring it to rest. Operation of the limit is indicated to the operator by the way of an illuminated light on the control desk, or by a control light adjacent to the pendant.

Operation towards the limit is now inhibited. However, operator control of the crane in a direction away from the limit is possible.

The anti-collision barrier and rail protection switches are each wired into the long travel protection contactor. Operation of any of these switches will trip the contactor in the power supply to the long travel drives, stopping the long travel motion.

The crane has a small power distribution system on it in order to provide power to tool sockets at strategic locations. In addition, a new lighting system has been added to the crane. These are located on the main walkway at the upper level in order to illuminate the area that is masked as the crane travels under the main sphere lights. Four sodium lights are provided, equally spaced across the central span to achieve this. They can be retracted up to the walkway for maintenance.

The refurbishment is to bring the crane up to modern standards for lifting radioactive material, referred to as nuclear lifts nukes 1 As part of the refurbishment, the reach of the crane had to be increased to permit the hook to operate at a greater radius to reach parts of the facility now required for decommissioning work nukes 2 The overall load capacity of the crane and the hook operating elevations needed to be maintained nukes 3 The new crane hook to achieve the same reach at the cab end of the Goliath as the previous hook, and an extended reach at the non-cab end nukes 4 The hook elevation below floor level had to match the previous hook elevations nukes 5 The design approach for the equipment was to design to BS2573 with appropriate duty factors and exceptional loading cases nukes 6 Fig 1 – The crane is mounted on a single circular track to provide polar motion referred to as long travel nukes 7 Fig 2 – The crab structure is manufactured from carbon steel and joined together to form a welded structure nukes 8