Purpose of a Mooring Winch

Mooring winches hold a ship securely to the shore via a mooring line (rope or cable). They haul in, or release the line as the ship moves up and down due to load changes, wave, or tide action. When hauling in, the line tension (pull force) is set by the hydraulic pressure. Once that pressure is reached, many mooring winches apply the band brake and turn off the electric motor. The brake is preset to release the line at a tension that is slightly higher than the tension set by the hydraulic pressure. When the ship moves down due to the tide going out, or increased load, the line tension goes down. A load cell senses this reduced tension and starts up the electric motor again. This cycle is repeated over and over again.

Possible Problem with Brake System

When the band brake is properly adjusted, this provides a reasonable method of tension control under normal circumstances; that is, when the level of the ship changes slowly due to loading, unloading, or tide changes. However, during a storm, these changes are more sudden and the tension can vary drastically. In some cases, this has caused significant damage to the winch frame. After that, proper tension control is now lost and at the worst possible time; during a storm. This increases the likelihood of further damage to other winches and possibly the ship.

As cars are rolling off the ship, the ship rises & mooring line is released in order to maintain reasonable line tension.

Possible Alternative

For a hydraulically driven winch, a very good alternative is to leave the hydraulics on and let the hydraulic system control the tension. The system senses the tension and makes adjustments continuously, automatically.  I’ve had some very positive experiences with closed loop hydraulics and mooring control. It worked very well, but it’s a bit more complicated than an open loop hydraulic circuit.

Hydraulic Mooring Control

With a hydraulic drive, the pressure is directly proportional to the tension. So we really just need to sense and control the hydraulic pressure.

The Rexroth A10V pump is available with a mooring control option. This is essentially a pressure control where the pump is allowed to go over centre.  That means, as the winch is pulling in to tighten the cable, the pump is on the positive side of centre. When a wave is raising the ship, it is pulling cable off the winch. Since the pump is simply controlling the pressure, it will go over centre to allow the drum to turn in the opposite direction, to let cable out. It does this while maintaining the same, preset pressure and therefore we have the same tension in the cable at all times. This happens smoothly without any sudden increase or decrease in tension.

Proper Control

This is proper “control”. In fact, it’s closed loop control, because we are controlling pressure with a feedback signal. Pressure translates into torque and torque translates into tension. 

The disadvantage of this method is, that the system needs to be running while the ship is moored. The brake system can be shut off. However, the pump would be operating near zero stroke most of the time, so the energy consumption would be small. If this extra energy expenditure is a concern, then we can change the system to a Rexroth pump with the DFEn control (Sytronix). I discuss this control in my previous article on capturing energy when lowering a load.

Possible Energy Recovery

Similarly, we can capture the energy with this winch, when the ship is rising and pulling cable off the winch drum.  When the ship is stationary, of just moving slowly due to load, or tide changes, the pump would be rotating around 300 rpm, using very little energy (often around 1 kW, possibly less).  An extra bonus is that there would be very little noise and we still have full control of the tension.

Is Hydraulic Mooring Control for You?

Mooring winches with brakes have been around for many years. They’ve done a great job in many applications, but if good tension control is important to you, then hydraulic mooring control is definitely worth investigating.

What has been your experience with mooring winches? Good or bad, I’m interested in learning from the experiences of others. Please leave your comments, or questions below.