The conductors of the Electricity Grid are carried across the River Thames at Dagenham, Essex, by two 487-feet steel towers. The cables had to be raised into position by an ingenious method which caused the least possible interference to the busy river traffic
THE NORTH SUSPENSION TOWER of the Thames Grid crossing in an early stage of construction. The steel structure in the centre was temporary, and was built to carry a 3-tons derrick crane which placed the main bracings in position. The tower was built up to a height of some 130 feet by this means. Later, wooden derrick poles were fixed to the main legs and used for hoisting the upper members of the tower.
THE national system of power transmission, generally known as the Grid, links up generating stations all over Great Britain and transmits power to large consumers and to nearly every city and town in the country. The scheme has necessitated the building of many thousands of towers to support the overhead conductors which comprise by far the greater part of the system. Wherever possible these overhead conductors take the shortest possible route between two points, and this has involved the crossing of many rivers.
The most interesting of these crossings is that across the River Thames at Dagenham, Essex. The six conductors and one earth wire of the Grid had to be suspended across the river at a minimum height of 250 feet, to give adequate clearance for shipping. This necessitated the building of the highest transmission line towers in the world. The work was carried out for the Central Electricity Board by Callender’s Cable and Construction Co., Ltd.
It was essential that the Grid should cross the Thames Estuary, as it was necessary to provide a connecting link between the two portions of the southeastern section of the Grid. The site finally selected lay between Dagenham on the north bank and Abbey Wood on the south. The width of the river at this point is 2,600 feet.
The surrounding country is flat, and this flatness made it impossible for the engineers to make use of the effect of a valley. Many suspension towers which carry cables across rivers have been built on hills overlooking the rivers, and thus it has been a simple matter to provide the necessary clearance. At Dagenham it was possible to obtain the requisite clearance of 250 feet only by building two towers 487 feet high, and the total length of the crossing span was 3,060 feet. The main towers are really suspension towers — the conductors being in equal tension on either side of them. The wires pass over the suspension towers and are terminated on an anchor tower on either side, the distance of the anchor towers from the suspension towers being 1,530 feet. The total route length of the section of conductor used for the crossing is therefore 6,120 feet.
The work of providing a link between the two sections of the Grid involved the solution of two separate problems — that of building suitable towers and that of raising the conductors into position.
All the towers are of lattice steel construction and, because of the abnormal height of the suspension towers, all the members up to a height of 400 feet are riveted. The top sections and cross-arms, however, are bolted together, and all the steelwork has been galvanized to avoid the necessity of painting.
The suspension towers are 120 feet square at their bases, and each of them weighs approximately 290 tons. They were designed with a factor of safety of 2½ to carry the conductors in the maximum loading conditions of wind and ice. These conditions were fixed at a wind load of 15 lb. per square foot and a coating of ice with a radial thickness of half an inch. The towers were designed also to resist a wind pressure varying from 25 lb. per square foot at the bottom to 60 lb. per square foot at the top.
Special features of the suspension towers include provision for lifts which can be installed, if necessary, in the clear space up the centres of the towers. Stairways and ladders are installed, and these are separated by “resting” platforms. All the cross-arms are fitted with rails extending from the centres of the towers to the suspension insulators, and these rails are arranged to carry a travelling cradle from which easy access to the insulators is possible. “Working” platforms, extending for the whole width of the towers, have also been placed in position about eight feet below each of the cross-arms. Because of the great weight and size of many of the members for the suspension towers, normal methods of building them could not be used. A temporary steel structure 35 feet square and 89 feet high was built centrally on the first tower site, and it was arranged to carry a circular track carrying a 3-tons derrick crane with a jib 55 feet long. Outriggers were fixed to the top of the staging, and against these the four main tower legs, each 90 feet long, were supported. Adjustment was provided so that these main legs could be brought into their final positions to receive the main bracings.
Distortion Obviated
The bracings were erected by means of the derrick, and all the remaining members were fixed to complete the assembly of the bottom section of the tower, rising to a height of 87 feet. From this point the building of the tower was carried on with the crane to a height of some 130 feet. After this wooden derrick poles were fixed to the main legs and were used for the hoisting of the upper members.
Four gangs of men were employed, two working on the main legs and bracings and the other two on the secondary members. The riveting gangs followed as closely as possible behind the erectors, thus obviating the risk of distortion of the structure through the weakness of temporary bolted connexions.
When the work was completed, the maximum amount of lateral error in the positions of the tops of the towers was 1¾ in. — a figure which speaks well for the accuracy of building. The northern tower was built first, the total time taken being eighteen weeks. The temporary structure used for starting the building was then transferred to the south bank and used for the other tower. The experience gained with the northern tower enabled the southern tower to be built in twelve weeks. Each of the suspension towers contains 17,000 pieces of steelwork, 35,000 rivets and 7,000 bolts.
The design of the anchor towers, which are 104 feet high and weigh 24 tons each, had to take into account the terminations of the main crossing conductors on one side, with or without the effect of the incoming Grid lines on the other. These were the normal longitudinal loads. Allowance was also made, however, for the simultaneous breakage of three conductors in either span. With normal loading the factor of safety for the anchor towers is 2½; with possible breakage of conductors taken into account it is reduced to 1½.
The installing of the foundaitions of all four towers was carried out under a separate contract, and as the soil in the neighbourhood was alluvial and of poor resisting qualities, the footings were built on reinforced concrete piles. Steel bases for the towers were secured by anchor bolts to blocks of concrete carried on the piles, and the bases were designed to provide direct attachments for the members of the towers.
PETROL-DRIVEN WINCH on the south suspension tower, where it was used for hauling up the cables. A steam winch, situated behind the south anchor tower, was also used for this work. Six conductors, each of which weighs about 6 tons, are suspended across the Thames by the two great towers, which are 3,060 feet apart.
An important factor which influenced the design of the towers was the size of the conductors. The problem here was mechanical rather than electrical, the voltage drop across the span of 6,120 feet being relatively unimportant. The conductors had to be large enough to prevent serious corona discharge — an electrical problem which the Grid engineers have successfully solved — and they had to be of sufficient conductivity to prevent heating losses.
Each conductor is made up of ninety-one individual wires. The inner strands are of cadmium copper, to give conductivity, and the outer strands are of phosphor bronze, which is particularly suitable because of its resistance to corrosion and fatigue. The conductors weigh about 2 lb. per foot and have a breaking strength of approximately 57,600 lb. No joints were permitted in the individual wires, and each strand was drawn from a single billet 2 feet long into a final length of some 6,400 feet. Each of the complete conductors, after stranding, had a total weight of about 6 tons, and it is believed that these conductors are the largest yet made in phosphor bronze.
The insulators necessary to support conductors of this type present certain problems of their own. Those used at Dagenham are of a size which cannot possibly be appreciated by one who has not climbed the towers or who did not see the insulator strings on the ground before they were hoisted into position. Each of the six conductors is supported by insulators consisting of two strings in parallel, each string consisting of eleven units. The length of each string is approximately 6 feet.
The separate units of which the strings are built up have a diameter of 11 in, and are designed for a breaking load of more than twelve tons and a puncture value of 150,000 volts. The completed strings of insulators have flash-over values of approximately 690,000 volts in the dry state and of 600,000 volts when wet.
When all the towers had been built and the time had come for the erecting of the conductors, the insulators were in position. Each of the cross-arms, however, bore a sheave, or grooved wheel resembling a huge pulley; the conductors were to be erected over the sheaves and transferred, later, to the insulators, when sag adjustment and tensioning had been carried out.
Several methods of hoisting the conductors were possible, and the various advantages and disadvantages of each method were carefully considered. One of the chief factors was the river traffic, which, at the site of the crossing, is always heavy except for periods of an hour or two at low tide. Whatever scheme was adopted, it was of vital importance that no hitch in the proceedings should be likely to interfere with this river traffic.
Raised From the River Bed
One scheme that was considered was the use of pilot cables between the two towers, these cables being used to pull out the conductors, always under sufficient tension to maintain the necessary clearance for shipping to pass underneath. The advantage of such a scheme is that once the pilot cable has been slung into position, all future operations are independent of river traffic. The difficulty of maintaining a tension of 6½ tons, however, during the whole process of pulling the conductor out from one side of the river to the other is a distinct disadvantage. Furthermore the simultaneous pulling in of a conductor on one side of the river and paying out on the other calls for close communication between the two sides, and any slight hitch in the system might cause a serious accident.
For these reasons the pilot cable method was not adopted, although it had been seriously considered. The alternative method was to carry the ends of all the cables over the sheaves on one of the towers and then to lay them all at the same time in the bed of the river. The other ends could then be raised and passed over the other suspension tower and the conductors raised individually from the bed of the river and adjusted to their proper sag.
SOUTH ANCHOR TOWER, with insulator strings being slung into position. The two anchor towers are 104 feet high and each of them weighs 24 tons. The main suspension towers, one of which is illustrated on page 73, are 487 feet high ; each of them weighs 290 tons.
During the latter operation it was necessary to interfere with the normal passage of shipping. Such interference was possible only for one or two hours at low tide, when the volume of traffic, in any event, would be small. A programme was worked out which provided for the raising of one conductor each day for a week. Plans were made to the approval of the Port of London Authority, which arranged to notify all shipping using the Thames of the dates and times at which the river would be closed to traffic. This is believed to be the only instance on record of the closing of the Thames to shipping, and long-term notices were issued to directors of navigation all over the world, to all harbour masters of the British Isles and to all shipping companies using the Port of London.
While the raising operations were in progress, P.L.A. launches patrolled the river on either side of the site, turning back all shipping. Communication between the launches and the shore was carried out by a prearranged code of flag signals. First the drums of cable were mounted in a lighter, which was anchored near the northern suspension tower. The drums were so mounted that they could rotate on spindles which allowed the conductors to be paid out over rollers in the stern of the lighter. The ends of the conductors were then hauled over the sheaves by pilot cables of 30 in. diameter, suspended from the cross-arms of the northern tower. Thence they were taken back to the northern anchor tower, where they were terminated on the insulators and other fittings, in their final positions.
Adjustment of Tension
The next operation was to tow the lighter across the river, and to pay out the conductors so that they sank to the bed of the river. When the lighter had been moved to a point adjacent to the southern suspension tower, the remainder of the conductors was paid off from the drums, and the ends were hoisted by pilot cable over the sheaves on the southern tower.
It now remained to haul each conductor, in turn, to the correct height, the sag having been calculated beforehand. A haulage cable was attached to the first conductor, pulling it up approximately to the correct sag by a steam winch situated behind the southern anchor tower.
The conductors were next adjusted to the correct degree of sag, and were finally made off on the insulators and fittings on the southern crossing tower. The remaining task — a delicate one — was to transfer each conductor from its sheave to the suspension saddle on the insulator string.
The whole programme was carried out without a hitch, beginning on Sunday, September 18, 1932. The conductors were laid in the river at high tide, as it was desirable to take the lighter as close as possible to the shores. The time occupied was only fourteen minutes, and no interference was caused to shipping by this first operation.
On September 20 the first conductor was raised by the steam winch and steel pilot cable, and on September 21 the second was pulled into position. It was realized at this stage that it would be possible to accelerate the work. September 22 and 23 were spent in earnest consultation, which was justified by the raising of the other conductors in the next two days. The average time for the pulling up of each conductor was forty minutes.
The transference of the conductors from the sheaves, on the suspension towers, to their insulator strings was carried out by a specially built steel cradle. This ingenious device took the weight of the conductor at two points — one on either side of the sheave — while the sheave was lowered out of the way and the insulator string brought into position. Ordinary clamps at the lower end of the insulator strings held the conductor in position, snail clamps being used only on the anchor towers.
It is a great tribute to the engineers in charge that the work went through from beginning to end without a hitch, and that no trouble has been experienced since the crossing was built in 1932. It is a large-scale work of engineering, almost comparable with the building of a great bridge. In a sense it is a bridge, but it carries an invisible load — electrical power — from one part of the country to another.
WORKING 450 FEET ABOVE THE GROUND, men had to remove the wood packing from the insulator strings after they had been slung into position. Each of the conductors is supported by two such insulator strings connected in parallel. The strings consist of eleven units and they are designed for a breaking load of over 12 tons and a flash-over voltage of 600,000-690,000.
