Remote diagnostics help ships offload cargo

Unloading sand and gravel collected offshore for building materials is demanding work that usually has to be completed to a tight timetable.

Building projects for the London Olympics are driving demand for aggregates through the roof, so dredger captains need to discharge their cargos efficiently so that they can get back out to sea without missing a tide.

Remote diagnostics developed by Barwits Control Systems for United Marine Dredging (UMD) are helping are helping with this Olympian effort.

Unloading sand and gravel collected offshore for building materials is demanding work that usually has to be completed to a tight timetable so that ship can get out to sea again and maintain operations.

So if any part of the complex scraper discharge system fails, a rapid repair is vital.

While Tunbridge Wells in the heart of Kent may not be so well known for its maritime heritage, it is the home of Barwit Control Systems, which over the years has built up a reputation for being the foremost provider of dredger discharge control systems.

Mechanically the system consists of two 12m3, 20 tonne scraper buckets, which are lowered into the cargo hold and scraped across the load and then winched over a conveyor and up-ended.

The synchronisation and motion profile of the buckets is complicated to set up and critical to the success of the unloading operation, and they must be maintained throughout the entire 5000-6000 tonne unloading operation.

"We've just completed our fifth vessel", says Colin Jeffreys, Barwit's cOntrol Systems Designer.

"Each one is bespoke to the project, and each is a development from the previous system".

"Over the years we have particularly come to rely on Mitsubishi Electric control hardware, which is robust and reliable even in this most arduous of sea-going applications, and which is backed by excellent service".

The whole control system, which extends throughout the full length of the ship, is based on a single Mitsubishi Q series PLC (programmable logic controller).

The Q series is Mitsubishi's flagship industrial controller, and the fact that Barwit does not feel obliged to have a second unit in hot standby is testament to its reliability.

The Q is designed to offer a high level of redundancy for system-critical applications and uses dual CPUs, power supplies and bases.

It combines an out of the box system building using standard components with customisation by simple parameter setting.

It has a level of simplicity normally expected of smaller PLCs and is configured for total data tracking and extensive physical redundancy so that system availability can be guaranteed.

Configured with the Q on the dredger system over a remote I/O network are three Mitsubishi E1151 HMIs (human machine interfaces) and a GPRS remote communications unit.

"The system can be interrogated from on board the ship via any of the HMIs", explains Jeffreys.

"The GRPS takes this capability a big step further by allowing specialist to 'dial in' from anywhere in the world".

"This means that if there is the slightest glitch during unloading, the crew can instantly contact us or Mitsubishi for expert advice".

"We reckon that the first such phone call will pay for the GPRS several times over, such is the loss of earnings if a ship is stranded overtide".

The system also includes several Mitsubishi variable speed drives.

One of their critical functions is to optimise the speed of the conveyor depending on the material being unloaded.

The winch motors controlling the scraper buckets are DC-controlled, and their speed is also defined by the materials being handled.

"The shipboard communications is based on Profibus (although the HMIs are connected by a dedicated Ethernet cable), which is immune to the high levels of electrical noise to be expected on maritime systems".

"Originally we used fibre optics, but this was too slow to replace in the event of a breakdown".

"On jobs like this, reliability and back-up are critical and we are delighted with the relationship we have formed with Barwit and Mitsubishi" according to Mark Williams.

Superintendent City of London UMD.

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