Woolwich Free Ferry

Description

AMCO called upon Fairfields to oversee the design and delivery of a replacement control system for the linkspans; one that would provide automatic operation rather than the original manual operation. The new solution needed to provide a reliable and robust fail-safe method of operation and help to optimise operation cycle times and increase the efficiency of the ferry crossing’s loading and unloading operations.

Thanks to the upgraded control system, these requirements have been successfully achieved and there has been a dramatic reduction in reactive and preventative maintenance tasks on the control system. Current safety regulation standards are now being met and the control system for the Woolwich Ferry has been future-proofed until at least 2024.

Oliver Mangham, Fairfield’s Applications Director, explains:

“The original control system was a complex mechanical design, including a single hoist motor and winch assembly for each of the four linkspans incorporating a set of counter weights and a system of compensator beams. The compensator beams allowed the linkspans to rise, fall and twist as the ferry moved under the transfer of heavy vehicles and the wash from passing river vessels. The linkspan control system had to take into account these movements and the complexities associated with the large tidal range of the Thames at Woolwich, which can be as much as 8 meters.

“While considered as an effective solution when first engineered over 50 years ago, modern demands and the passage of time had rendered the legacy mechanical systems obsolete, as well as increasingly troublesome to maintain.”

A controlled solution

The new design incorporates a suite of redundant Siemens safety PLC and communication equipment. Each linkspan contains a S7-1500 safety PLC which communicates to a number of ET200SP I/O units for signals such as the status of linkspan detection sensors, barrier positions and traffic control. The communication backbone is provided by a series of managed Scalance switches configured in a redundant ProfiNet ring. An important feature of the new system is to be able to accommodate single points of failure, so that any occurrence will not result in the suspension of the service. The system provides ease of operation and a number of operator graphical interfaces. In addition to the operational benefits, access to detailed alarms, diagnostics and reporting information better assists the maintenance teams.

As part of the new lifting system, each linkspan utilises two Siemens motors controlled by S120 Sinamic motion control drives.  These are connected to the load through a gearbox, via a chain and counterweight arrangement designed to accommodate high dynamic loads. Incremental encoders on the rear of the motors provide millimetre accuracy and allow the system to anticipate the ferry position for a smooth landing. Profibus absolute position encoders on the gearbox are used for independent safety positioning and governance.

Each of the ferries has a local Siemens ET200s PLC providing ferry diagnostics to the terminal buildings and the ability to operate the terminal from the ferries themselves.  This is provided by a series of redundant wireless links between the terminals and the ferries. To ensure a ferry can only communicate with the anticipated linkspan, the ferry PLC utilises a GPS position to confirm its location.

The overall solution has drastically reduced the number of moving mechanical components, thereby also reducing the ongoing maintenance requirements, whilst at the same time dramatically improving the availability status of the service.

Challenges

One linkspan at each terminal in turn was taken out of service for the refurbishment works and a critical element of the control system installation was the requirement to carry out the works without causing any disruption to the other operating linkspans servicing the ferry.

Oliver Mangham explains:

“Fairfields was required to install and commission the upgraded control systems at both the North and South terminals whilst maintaining the service with the legacy system and within a constricted time period.  This was a significant challenge for the whole team, but one that we successfully overcame thanks to a collaborative approach with AMCO, as the Principal Contractor, and Briggs, as the vessel operators and overall Project Manager.  I am happy to say that the control system upgrade was completed with only minimal disruption and the Woolwich ferry crossing operated a continuous service throughout the changeover period.”

Outcome

Now that the overall two-year linkspan refurbishment project is complete, the newly installed control system is helping to deliver a number of key benefits for the ferry operators.

Oliver Mangham concludes:

“Since installation, significant improvements have been seen. These include a vast reduction in the amount of reactive and preventative maintenance tasks that need to be undertaken on the controls systems.  In addition, the ferry operators have a far more simplified system to manage, as well as enjoying improved ability through optimised performance of the linkspans.  With safety a priority consideration, the new control system also conforms to all safety standard requirements under current regulations.  Finally, as a result, the management team at Briggs has been able to reduce operating costs and now have a control system that will be fully functional and fit for purpose until 2024 at the earliest.”

The upgrade project has entailed the most extensive mechanical, electrical, instrumentation control and automation refurbishment of the four linkspans since they were first built, with the overall linkspan refurbishment project being led by AMCO Engineering as part of the wider multi-million pound life extension project by TfL to secure the operational future of the ferry crossing service for the next generation.

The Woolwich Project was recently awarded first place in its category at the London RICS Awards, beating the Tottenham Court Road Tube Station upgrade and Crossrail Canary Wharf projects.