Nassco Automated Its Steel Handling Procedure Around Existing Equipment. Production Was More Than Doubled With Fewer Men Required.
National Steel and Shipbuilding Company, San Diego, Calif., completed its current major program of expansion and modernization in 1968. With the completion of this program, Nassco has more than doubled its shipbuilding capacity and is now ranked as one of the largest U.S. shipbuilders.
The objectives of this program were to achieve the maximum building capacity within geographical boundary limitations and to minimize costs of production.
These objectives have been met. The construction of shipways 3 and 4, somewhat larger than ways 1 and 2, more than doubles the yard's building capacity. The installation of an integrated, centrally controlled system for the handling of shipbuilding steel from receipt of material through subassembly has increased the capacity for these operations to a possible peak equivalent to 2,000 tons per week and reduced the 20-man force previously used in direct material handling for a lower production rate to a maximum of seven men for the much higher capacity.
Together with this increase in building capacity, it was necessary to provide new outfitting berths, warehouses, cranes and increased capabilities in supporting shops.
The concept of the steel storage and fabrication area modernization was developed on the basis that no new land areas would be available. In essentially the same steel yard, plate shop and subassembly area, it was required to double the throughput capacity - tons per unit of time. The concept of the mechanical handling system was to achieve this capacity by moving materials rapidly, continuously and along preselected lines.
Each of these attributes were studied by Nassco engineers in order to put them in their proper perspective and to determine equipment requirements.
Rapidly - By moving material rapidly, less time is spent in transit between operations. More of the unit time is left for production operations.
Continuously - Time spent in storage between operations is time spent in neither transit, nor production, nor anything else. Time is wasted. Time is spent in unnecessary rehandling.
Preselected Lines - An item of material withdrawn from storage and issued to the work must be started on a specific predetermined path. A centrally directed system approaches the attributes of continuous process manufacturing.
The material handling system is designed to best serve an existing plate shop and subassembly area. This is an important distinction as opposed to building a complete new facility incorporating a material handling system. The shop buildings, platens, and craneways are there. The principal tools are there.
To the existing facility and existing tools were added two new prefabrication capabilities: 1. Flame Planer - For edge preparation of plate and cutting plate into strips. 2. Beam Welder - Fabrication of welded shapes from stripped plate.
With these new tools and existing tools established, it was then necessary to develop material flow patterns and the equipment to move the material. The system was developed by Nassco's staff and H. Nielson and Son who supplied Via Nova conveyor units and control system.
The total conveyor system is divided into seven functional sections:
Section 1 - Storage yards and surface preparation. Cranes load plates and shapes on the conveyor which has a speed of 100 fpm throughout the system. The conveyor takes the material to the wheelabrator and paint booth. During this operation, the conveyor speed is controlled by the operator for the most effective surface preparation and painting. In normal operation, all material handling, shot-blasting and painting in Section 1 is fully automatic.
Section 2 - Distribution point. Material leaving the painting operation is loaded automatically on a transfer car that distributes it automatically to the next operation.
Section 3 - Collocator. The Collocator is a mobile unit running on 1,000 feet of track. The unit can deliver and deposit material on the ground at any of 18 preselected stations. The control of this unit is complex, involving power supply and 50 or more signal circuits. All previous units were captive units, tethered by messenger cables. This arrangement presented problems: How to fold and unfold 1,000-feet of multi-conductor cables? How to do it at 600 fpm to maintain cycle time? The solution involved some pioneering with and by the vendor. The car was freed from cable tethers by using crane-type conductors and collectors. Two additional conductors were provided. Control signals are handled by multi-frequency transmission - all signals using only the two conductors.
Section 4 - Flame planer. The flame planer performs edge preparation of plates or stripping of plates into flanges and webs for the beam-welder. All burning tables are arranged with at least one roller conveyor on feed and discharge sides. On the feed side, this enables the transfer car to deliver a plate, whether the burning table is working or not; and, it also provides for a ready plate to run in on the burning table without waiting for transfer car delivery. On the discharge side there is a place to run the plate off without waiting for the transfer car.
Section 5 - Beam (T) welder. Plate cut into strips is delivered to a holding conveyor. This unit is essentially a vertical switch to receive material for the welder. The completed installation is one integrated mechanical operation that cross-feeds stock, forms the beam, centers and flushes the ends to finished dimensions, continuously welds the beams head to toe, runs it out on a cooling table and cross-feeds the finished beam into racks. This installation is 104-feet long and 25-feet wide. In this operation the plates to form the flange and web are delivered on two levels. The unit moves the flange plate into a horizontal position and then slides the web plate in position, vertically, on the flange. Power rolls flush the ends, centers the web, applies pressure to the two pieces to form a tight joint, and advances the material to the Ogden Engineering Company's special welding machines. The flange and web move through the stationary welding machines as the weld is made.
Section 6 - Collection and distribution. A transfer car receives material from the flame planer and profile cutting machines and delivers it to Section 3 for distribution to subassembly stages or delivers stripped plate to the beam welder. It can also deliver material to the hand layout and burn area. This transfer car is equipped with divided rollers to enable port and starboard pieces cut from the same plate to be delivered to two subassembly locations.
Section 7 - Profile cutting. This section serves the CM-60 and CM-56 burning machines. Profile cutting is relatively time consuming, so holding stations are provided for material. A small magnet crane serves this section.
System Controls: The mechanical material handling plant is an arrangement of fixed and mobile specialized equipment. The components are integrated into a whole by coordinated control. The conveyors and cars provide mechanical capability. The controls make it a system. There are three modes of control. In descending order they are: central automatic control, local automatic control, and local manual control.
Central control is located in an elevated tower which permits visual observation of the full travel of the transfer cars and the Collocator. The central control console contains a back-lighted graphic display giving continuous indication of position and status of all system units.
Each section has a local control console for that section. All consoles, local and central control, are connected by a closed intercom system. A section may request local control from the central control station.
In operation the control system memory has two important characteristics: 1. Once the operator establishes the ordered sequence, he does not need to be further concerned about it. The control system will carry out the sequence as soon as it is mechanically possible to do so. 2. In a complicated sequence, each step will be taken as soon as the way is clear. The movement will always be as far along the sequence as possible.
Cranes: The plate yard is a rearrangement and extension of an existing facility. The existing crane was equipped with a ViaNova magnet beam and associated controls. At the other end of the system, the subassembly platen is served by two H. Nielson and Son ViaNova cranes. These cranes are also magnet beam equipped. The magnet beams are equipped with a row of magnets for shapes, with rotation of the shape about its long axis, controlled from the cab. The cranes also include an auxiliary hoist to provide conventional hook service for subassembly.
The integrated, centrally controlled material handling system installed by National Steel and Shipbuilding meets its requirements. The system at Nassco works.