Twiflex locking devices provide reliable shaft holding on ice class tugboat
A major OEM of marine propulsion gearboxes has commissioned Twiflex to manufacture and supply a reliable locking device for use on a new ice class, ultra-shallow draft tugboat. The challenging specification required a design that provided a locking torque of 11.6 kNm on each of the tug’s dual shaftlines, whilst also meeting a compact footprint.
Stretching some 1,200 km from Russia in the north to Iran in the south, the Caspian Sea is the world’s largest inland body of water. In 2013 the EIA estimated reserves of some 48 billion barrels of crude oil and 292 trillion cubic feet of natural gas. This makes it an important resource for the former Soviet states of Azerbaijan, Kazakhstan and Turkmenistan on its eastern and western shores, all seeking to develop their national oil and natural gas industries.
With these three countries in particular looking to deploy offshore oil rigs and natural gas platforms in the Caspian Sea, tugboats capable of providing the required towing and anchor handling duties have an increasingly important role to play.
It is not just the region’s oil and gas industrials who depend on these small but powerful vessels. With ice being a hazard to vessels in the winter across the northernmost reaches of the Caspian Sea, there is a growing call for ice class vessels, with stronger hulls and more durable propulsion systems. At the same time there is demand for shallow draft tugs, designed to operate in shallow waters. In this situation every kilogram of weight counts, which places stringent requirements on the powertrain components.
In one particular case, a Turkish OEM was supplying the power transmission system for a new ice class, ultra-shallow draft tugboat. The 30 meter long, twin-propeller vessel features two gas engines, each rated at 970 kW (1800 rpm), and is designed with a draft of just 1.8 m. The tug has a 25 tonne bollard pull capacity and is also equipped for anchor handling.
The requirement was for a mechanical lock on each of the two propulsion shafts so the drivelines could be secured for maintenance. In addition, the locking device provides protection if one of the propulsion shafts becomes disabled or damaged while at sea. If it’s not locked, drag from the operational propeller can damage the non-operational redundant shaft components.
The specification was particularly challenging both in providing a minimum locking torque of 11.6 kNm for each shaftline and meeting the restricted space envelope for the installation.
Twiflex offers a range of locking devices with either axial or radial engagement to provide up to 500 kN locking force. These have been designed for a more cost effective and compact solution when holding is the only requirement. All devices can be supplied as either base or face mount and include lock on / lock off status sensors with positive lever actuation which remains locked when engaged.
In this application Twiflex engineers selected the radial locking RL65 to act on the 470mm diameter, 30mm thick customer-supplied split locking disc on each shaftline. Once engaged the lock provides 17.6 kNm locking torque so the shaft can be secured for maintenance.
With the tugs operating regularly to meet demanding towing requirements, reliability is key in this challenging environment. The success of the Twiflex RL65 has resulted in the OEM looking to work with Twiflex on future vessels which incorporate similar propulsion systems.
Twiflex, a brand of Altra Industrial Motion, has over 70 years’ expertise in the design and manufacture of advanced braking and locking technologies. It is an acknowledged leader in the supply of equipment to the marine industry, including Turning, Locking and Braking (TLB) systems for propeller shafts.
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