The Strépy-Thieu boat lift
As long ago as the 19th century, the link between the Meuse and Scheldt basins by means of a navigable waterway proved essential for the transport of coal and, in general, for the economic development of the province of Hainaut. The construction of the canal du Centre was decided by the Law of 4 August 1879. The canal, opened to shipping in August 1917, was designed for barges of up to 300 tonnes. In 1963, modernisation work started in order to bring the canal up to the European 1350 tonne standard. But how could the 70 m difference in level between La Louvière and Thieu be overcome? The previous canal overcame this drop by means of 2 locks and 4 hydraulic lifts each rising 16 m, and brought into service between 1888 and 1919. To replace the six old structures, and taking account of the lie of the land, the solution of a vertical lift compensating a 73 m drop was ultimately adopted. The "steering" of the whole project was carried out by the General Directorate for Waterways Infrastructure, with the cooperation of the Technical Supervision Division for the geotechnical studies. The Electricity, Electromechanics, Computer and Telecommunications Division was responsible for the mechanical, oleohydraulic and electromechanical parts of the lift:
Two independent counterweighted lifts
The structure consists of two independent counterweighted lifts. Each one comprises a mobile cage which travels vertically between the upstream reach and the downstream reach. The mass of each water-filled cage is balanced by 16 suspension counterweights and 8 control counterweights, connected to the cage by a system of steel cables (112 suspension cables and 32 control cables).
A cage/counterweight subsystem is set in motion by means of 8 winches. Each of these has a "low speed" (LS) reduction gear, driving 2 drums onto which the control cables are wound. A synchronisation loop connects the LS reduction gears to 4 high speed (HS) reduction gears driven by 4 electric motors.
The cages and the reaches of water are fitted with vertical gates allowing a cage to be connected with the reach of water opposite which it is located. This equipment is supplemented by watertight seals between the cages and reaches of water, and the lifting mechanics of the vertical gates between them.
Although the operation of the two cages is completely independent, the
boat lift has a unique structure in order to give the whole unit
sufficient rigidity and low vulnerability to deformation. In essence,
the concrete structure consists of a watertight monolithic floor, a
central tower, extending over the whole length of the lift, and the
central zone of the slab at level 131.15 m forming the mid-point of the
floor of the machine room. The two side sections of this floor consist
of metal girders supported on metal columns on one side and the central
floor on the other.
The machine room for manoeuvring the cages is located in the upper part of the tower. In the centre of the roof of the machine room is a twin-column structure on which the sides of the roof and the facades are supported. The solutions adopted were guided by the concern to make the system isostatic. The total mass of the whole building is approximately 200,000 tonnes. The overall dimensions are 117 m high, 130 m long and 81 m wide. Each canal bridge consists of 4 aerial spans of 42 m, resting on the central lift tower, on three piers and an abutment with the canal upstream. The mass of one span is 420 tonnes. The piers and the abutment are made of reinforced concrete.
Each of the gates of the canal bridges is duplicated by a guard gate to prevent the upstream reach being emptied accidentally in the event of an incident on one of the lifts. The cages are made of a welded construction, with the longitudinal rigidity provided by two box girders 8 m high. These box girders are connected together by braces in the form of a double T, approximately 2.9 m apart, which support the stiffened base.
Between the braces, there are two rooms housing the instrumentation necessary to power and control the electrical and oleohydraulic equipment located on the cage.
The masses of the cage and the water that it contains are balanced by 8
suspension counterweights and 8 control counterweights. Balancing of
the cage for average operating conditions (cage filled with 3.75 m of
water) has proven the most economic solution: the motor torque to be
developed by the mechanisms may change direction from one manoeuvre to
the next, and even during the course of a manoeuvre (variation of the
imbalance due to the weight of the cables). So connections between the
winches and counterweights are essential.
In addition, the terminal gear of the speed-reduction gears can only bear a reduced overload. The winches are linked together by a rigid synchronisation loop, and the spread of the torque between the various winches at the input to the winches is undefined. So the torque transmitted by each winch must be controlled and limited at the output.
With a single
counterweight for control and suspension, the spread of the weight
between the control and suspension cables could vary in proportions
such that the overloads might cause irreparable damage to the terminal
gear of the speed-reduction gears. So it was necessary to separate the
two functions clearly.
The control counterweight travels freely in a housing located inside the suspension counterweight. The mass of the control counterweight was calculated so that the minimum traction power in each control cable of the cage should never be lower than 100 kN.
In the machine room, the mechanisms are divided into 8 groups, each consisting of 2 rows of 7 suspension idler pulleys, 2 hoisting drums equipped with their own locking brake and a low-speed reducing gear. The choice of spooling diameter of the hoisting cables was the result of a compromise between the lifetime of a cable and the cost of manufacture of the drums and pulleys. The diameter of the drums and pulleys (4800 mm) remains compatible with the manufacturing possibilities, and in combination with an 85 mm diameter cable, it allows a number of cables per cage limited to 144.
Each suspension counterweight is connected to the cage by means of two
sheets of 7 steel cables. These cables are attached directly to the
counterweight by means of cable lugs, passing through idler pulleys
4800 mm in diameter, and leading down to the cage.
The connection between the counterweight and the cage creates a high level of redundancy, which necessitates the use of a system for spreading the load between the cables. This system consists of oleohydraulic jacks fitted between the end of each cable and the corresponding fixing lug of the cage. The jack piston rod is fitted with a tension handle, enabling the length of the connection to be adjusted. An anti-roll system prevents the rotation of the cable on its own axis, the tension handle or the jack piston road completes the systems.
The uniform distribution
of the load between the cables is obtained by interconnecting, in
parallel, the 500 upper sections of the jacks in groups of 7.
Interconnecting the suspension jacks of a counterweight actually forms
an oleohydraulic spreader beam.
The counterweights are made from a metal carcass, attached to suspension lugs, submerged in a mass of ballasted concrete with a density of approximately 3. As they move between the metal columns, the counterweights are guided vertically on each side by means of 4 spring-mounted rollers. A fixed mechanical stop limits the longitudinal movement of the counterweight. Inside the central tower, the counterweights are sheltered from the wind, and are not guided.
- Link between drum/cage/counterweight: the hoisting winches consist of 2 drums 4800 mm in diameter, driven by a low-speed reducing gear. The rotation speed of the drums is 0.796 rpm and that of the motors is 1000 rpm. 4 cables are wound on each drum, two of which are connected to the control counterweight, and the other two to the cage. The cable/control counterweight connection is via triangular mechanical spreader beams, while the connection to the cage uses oleohydraulic jacks and tension handles, as with the suspension cables. In operation, the mode of interconnection of the control jacks producers a symmetrical double oleohydraulic spreader beam by interconnecting the jacks of the control cables of the two winches or a quarter of a cage in two independent groups: the 1st group comprises the 4 outer jacks of each winch, and the second the 4 inner jacks.
- Construction of the drum: the hoisting drum is the most delicate component of the lift to design and produce. The model required 4,018 equations to be solved. The two critical points in the construction of the hoisting drums were, on the one hand, the problems of straining posed by the welds between web and ferrules, and on the other hand, the machining tolerance of the winding area of the cable, which must be less than 0.5 mm for a reference diameter of 4800 mm. The winding area is entirely grooved and consists, for each cable, of two round turns and five useful turns: the cables are clamped inside the drums. The output gear of the low-speed reducing gear is fitted onto the axis of the drum, and the corresponding bearing is located in the centre of the reducing gear. The mass of the drum/toothed wheel assembly is 72 tonnes.
- Braking: braking of the whole mechanical system and the cage occurs in 2 stages: deceleration until stopping point, and maintaining zero speed regardless of the torque by means of the regulation of the asynchronous motors (electric braking) and the application of the locking brakes on the drums, after stopping. In the event of a defect in the electric braking, a disk brake fitted onto the motor drive shaft takes over and brings the cage to a halt. The braking torque is determined so that it never exceeds the calculated maximum power transmissible by the low-speed reducing gear, which is 335 kW. The blocking torque would enable a barge loaded with 1350 t of steel which had sunk upstream to be held in a cage connected to the reach, with the water level at 4.15 m.
- The high-speed and low-speed reducing gears: to refresh your memory, the rotation speed of the motors is approximately 1000 rpm, and that of the hoisting drums is 0.796, so that the overall reduction ratio is 1248:1. Each high-speed reducing gear reduces the rotation from 993.4 rpm to 98.058 rpm and is coupled to two low-speed reducing gears, in order to deliver the required final speed of the drum, for a cage manoeuvring speed of 0.2 m/s during the lifting procedure. The web/crown assembly of the toothed wheels of the reducing gears was produced using electron beam welding. The crowns were made of 34 Cr Mo4 steel and are forged in a single piece on a circular mill; they are bulk annealed before machining.
These cables are of 85 mm nominal diameter, of the Nuflex type, made of 18 strands of 26 Lang extruded zinc-plated steel wires. For the control and the suspension of the 2 cages, the construction of the lift required the use of 36.7 km of cables. That length is subdivided into 224 sections of 98 m for the suspension, 64 sections of 116.2 m for the control of the cages, and 64 sections of 13.7 m for the counterweights.
At the extreme positions of the travel of the cage, the docking manoeuvres impose a series of movements. In addition, whatever problems may occur with an item of equipment, the cage must be secured. Long-lasting stoppages are also provided for, downstream, for maintenance operations, or repairs to a serious fault. These various functions are carried out by 8 securing systems, 4 upstream and 4 downstream.
The jacks of the locking brakes on each cage hoisting winch are
controlled by an oleohydraulic unit consisting of two identical motor
pump units, each of which powers a jack. The lock manoeuvring jack unit
of each securing device downstream is fitted with two identical motor
pump units which normally function in parallel. The control of the
securing jacks upstream, the suspension and control cables is carried
out by 4 oleohydraulic units located in pairs in 2 rooms installed
under the bottom of the cage. One unit powers the jacks relating to one
quarter of the cage, or two corresponding hoisting winches and the
securing device upstream.
The control and suspension jacks have the specific feature of moving very little, but they must remain completely watertight. This objective was achieved by nickel-plating and chrome-plating of the piston rods, and use of a Teflon piston/cylinder seal. Tests showed the great consistency of manufacture. The maximum stress in the cylinder is 310 N/mm², under a test pressure of 335 bar.
Guidance of the cages
The cage is guided only from the central tower quarter-way and
three-quarter way along its length. Each guide rail forms an assembly
- a T-shaped support;
- 3 raceways which support the transverse guide rollers;
- two rails for the longitudinal guide rollers.
Manoeuvring the gates
The ends of the cages and the reaches of water upstream and downstream are closed off by 3 groups of gates per cage:
- a group downstream consisting of the gate of the reach and the corresponding cage gate. The 2 gates are raised simultaneously by a common mechanism;
- a group upstream, similar to the downstream group;
- a guard gate, placed in front of the upstream reach, manoeuvred by a specific mechanism.
- movement of the link rods which come into contact with the shoes fitted to the gate of the cage, and force the two gates to leave their emplacement;
- raising at low speed (15 mm/s) during the time necessary for the water levels in the cage and the reach to equalise;
- raising at a nominal speed of 200 mm/s.
The reach gates and
those of the cages are fitted with a device to absorb any impacts from
boats, which can stop a 2000 t loaded barge travelling at 5 km/h. The
leaf of each gate is fitted with systems to keep the cage/reach
watertight, and to empty out the water between the gates.
The boat lift of Strépy-Thieu consists of a range of items electrical equipment that can be subdivided into five categories:
- power supply (HV and LV distribution and the emergency generator);
- the manoeuvre equipment;
- the low voltage equipment for management, sequences and controlling the operation of the structures;
- the auxiliary equipment (waterway signalling, external lighting, etc.);
- communications between the various parts of the installation and with the barges.
The boat lift of Strépy-Thieu is powered by an aluminium cable of a
cross-section of 3x400 mm². The HV connection is designed for a power
of 10 MVA at a nominal voltage of 10500V. The short-circuit power of
the network is 500 MVA.
- one transformer 350 kVA - 10500 V to supply the outbuildings;
- a transformer 200 kVA - 4500/10500 V for the emergency generator;
- 4 transformers 630 kVA - 10500/380 V for the auxiliary equipment for the cages and the common areas;
- 8 transformers 630 kVA - 10500/880 V for the control of the cage hosting winch motors and the gates;
- one transformer for the equipment for tourist purposes.
- Lifts and gates: The law of movement of the cages imposes very severe constraints on the converters, particularly the torque inverters during the manoeuvre, the need to keep the cage stationary, the very low speed approach to the canal reach and the braking on the network. The development of cycle converters with variable pulse width (PWM) enabled speed lower than 5 Hz to be controlled. The simulation of the characteristic curves of the motor, by means of a microprocessor, makes it possible to control torque at very low speeds and when stationary. The great robustness of the asynchronous motor, its reduced maintenance and low price, combined with the performance of the PWM cycle converters are arguments which militated against a possible direct current solution. The power circuit therefore consists of 8 units each consisting of a transformer, a frequency converter and an asynchronous squirrel-cage motor. Per cage, 2 of these transformer/frequency converter units serve to power the 3 motors for raising the gates (cage/reach gate upstream, cage/reach gates downstream and guard gates);
- Motors: to develop the manoeuvre torque necessary for a cage movement, 4 motors are provided. The motors chosen are machines 1000V - 1000 rpm - 50 Hz permanently powered at 670 V. In normal operation, the frequency converters are powered at 880 V, from the 10500 V network via the 10500 V/880 V transformers. The maximum torque of a motor is 5300 Nm. When operating on the emergency generator, the transformers are powered by a reduced voltage of 4500V, and under these conditions, the secondary voltage is 380 V.
- Frequency changers (converters): At the moment when a cage is being raised, the motor operates to take the weight of the cage and the cables; during the movement of the cage, the equilibrium is reversed; the weight of the cables becomes greater on the counterweight side and the system must function in "braking" mode: the converters which power the motors must therefore be capable of, on the one hand, controlling a movement at zero torque and, on the other hand, functioning by recovering energy from the network (the motor functions as a generator). At the end of the lift movement, the cage is maintained stationary (motor speed zero), with the load still suspended from the cables (full motor torque), in order to enable the various docking manoeuvres.
Low voltage equipment
The management of the structure, which includes the regulation of the
manoeuvring of the cages and gates, is broken down into two levels: the
upper level (NHS) and lower level (NHI). The NHS is charged with the
display, the control interface, the surveillance of the coherence of
the redundant information of the NHI and the testing of redundant
controls, in order to detect any fault. The NHI is designed to meet the
requirements of system safety and availability and carries out the
functions of control of the equipment and safety controls. The NHI
consists of groups of 3 redundant automatic control systems, and
critical actions have to be validated by at least 2 systems out of 3.
These automatic control systems are known as decentralised control
units (UCD). Per cage, 3 UCDs are installed in the computer room and 3
UCDs are installed in the electrical room under the cage. The various
units of the NHS and the NHI communicate with each other.
- the control desk: this is where the traffic, the movement of the cage and the auxiliary systems relating to it are controlled (signalling, etc.);
- the control desk: this is where the surveillance of the structure is carried out, and it is possible to operate the system from this desk;
- a series of local controls dispersed in the structure for adjustment, maintenance or repair of various items of equipment.
The various types of images enable:
- the equipment of the plant to be shown schematically;
- display of the progress of sequences in the form of graphs;
- display of the status of the regulation loops;
- the necessary commands to be executed;
- the history of the structure to be maintained.
- Auxiliary equipment
- waterway signalling,
- the cage-mounted fire extinguishing system,
- the half-gantry cranes in the machine room.
- Miscellaneous equipment
- three passenger lifts and two goods lifts,
- lighting equipment for the inside of the tower, for the cages and for the approaches to the structure,
- fire detection and extinguishing equipment in rooms and technical premises,
- telephony, intercom and utility television equipment,
- access monitoring and anti-intrusion equipment,
- conventional electric equipment for building premises (distribution panels, indoor lighting, heating, plugs, etc.).