A special type of lock

Rebuild a lock without hindering marine traffic for a customer seeking a special type of lock gate and state-of-the-art design technology: The expansion of the largest lock in Scandinavia presents plenty of challenges. The existing lock in Södertälje is 135 metres long and 20 metres wide. The lock chamber is to be enlarged to 170 metres long and 25.3 metres wide. MH Poly and Iv-Infra, a joint venture under the name of S3P, will provide the design for the new lock heads, including the segment gates, mechanical components and the fender beam constructions in the lock heads for protecting the lock gate arms.

Duplex segment gates

The lock gates will be segment gates made of duplex steel. This type of gate is rarely used for locks in The Netherlands. Duplex steel is a type of stainless steel with corrosion-resistant properties. The benefit of using this material is that the maintenance costs during the structure life are very limited due to the absence of necessary conservation.

The design of the lock gates

The skin plate of the segment gate has a circular shape and is provided with longitudinal reinforcements. The geometry of the skin plate on the underside has been adjusted to create an optimum flow profile when in the levelling position. The cross girders transfer the load to two tubular shaped main girders on both sides with openings that allow water to flow in and out. The gate has buoyancy tanks on both sides meaning that the operational weight is virtually nil. The buoyancy tanks are transversely positioned to ensure a self-closing moment in all positions. In the event of a complete power failure, the gate is able to close itself. The gate has two end stops integrated into the threshold to guarantee the correct position of the lock gate when in the closed position.

The connections for the arms are positioned at the ends of the main girders. On each side, there are two arms that meet at the main axle, which has a tubular profile. The main axle rotates on two bearings that are mounted in a steel frame. A console for mounting a hydraulic cylinder is fitted between the primary and secondary bearing. The console creates an operating arm from the hydraulic cylinder to the main axle. This controls the position of the gate and creates the facility to lock the gate in different positions. These constructions will be carried out symmetrically, thus with a drive system on both sides of the gate.

Challenges in the design

The segment gates are designed in reverse directions on both lock heads. Should a collision take place when entering the lock chamber, the impact will always occur on the concave side of the gate with the arms bearing the reaction forces. Due to availability requirements, a third gate and arms are to be manufactured and stored on the lock complex. The gates and arms are interchangeable.

Fatigue is an important aspect in the design of the steel structure, especially for the detailing of the buoyancy tanks, arms and the main axles. The buoyancy tanks are moved from the waterline to the bottom of the lock during every movement cycle. This provides a major load change on the structure and particularly the stiffener plates of the buoyancy tanks. It has been chosen to use internal ring stiffeners which are in keeping with the longitudinal stiffeners on the skin plate. On each side, there are six separate compartments, each with an access gate for inspection and maintenance. For possible future adjustments to the gate or greater growth than expected, ballast weight (approximately 2.5% of the gate weight) will be installed in the buoyancy tanks.

The seals form an important part of the functioning of the lock gate. Leakages must be kept to a minimum and moreover, a certain lifespan must be guaranteed. A double lip seal will be applied on both sides in a circular form. To reduce the wear caused by gate movements, the centre point of the seal has been specifically selected so that when the gate moves, in addition to the tangential velocity, a slight radial movement occurs through which the length of contact between the seal and the collapsed steel plate is reduced.

The design of the bottom seal turned out to be a difficult task due to occurring deformations, construction tolerances and stiffness requirements. Gate deformations due to deterioration are relatively great, therefore the contact area of the embedded plate of the threshold is parallel to the deformation direction. The occurring deformations of the threshold and construction tolerances of the lock gate and the threshold have all been taken into account when considering the deformation capacity of the bottom seal. The design is based on the principle that the gate must always make contact with the end stops when closing. This maximises the spring stiffness of the bottom seal.

Would you like to know more about Södertälje? Pieter will be pleased to tell you all about it. Contact him by email or call via 088 943 3200.

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