The DIS displays a vessel’s position, real time water levels and data from detailed bathymetric charts.
In 2001, the St. Lawrence Seaway authorities started a study which would determine the maximum load ships can carry while maintaining a safe Under-Keel Clearance (UKC), which has led to the maximum draft of vessels transiting the Seaway in the MLO Montreal to Lake Ontario and the Welland Canal gradually increasing.
At the opening of the Seaway in 1959, the maximum draft for ships was set at 6.85 metres (22 feet 6 inches). This maximum draft is now set at 8.08 metres (26 feet 6 inches). However, changes in water levels and a phenomenon called ship sinkage or 'squat' made adjusting the maximum draft again more complicated.
Draft is measured prior to departure but a moving ship actually sits much lower in the water, particularly in shallow or constrained channels. How much a ship 'squats' depends on factors such as the size and speed of the ship, shape of the channel, depth of the water, currents, wind, and even the presence of other ships.
Undertaken at the request of the St. Lawrence Seaway Management Corporation and industry partners that included the Canadian Ship Owners Association and the Shipping Federation of Canada, the aforementioned study resulted in standard squat models for the various types of vessels transiting the Seaway.
Over the past year and a half, the St. Lawrence Seaway authorities have developed a functionality description of a Draft Information System that incorporates the Seaway squat models, which was finally approved by all stake holders in March of this year and was then issued to the public.
In accordance with this specification, Transas developed its own DIS. An independent functionality verification and assessment was performed by Lloyds Register in June of this year, after which the system was installed on board Algoma Central Corporation's M/V Algoma Spririt for final approval by the St. Lawrence Seaway authorities.
The DIS can run as a stand-alone system or in a network with the Transas NS4000 ECDIS, and is designed to calculate and display the under-keel clearance (UKC) based on a range of data.
This includes high resolution Bathymetry data provided by the Canadian Hydrographic Service (CHS) and water levels received automatically via Seaway AIS stations and a network of water level gauge stations, or set manually by the user.
Ship forward and after draft data is also used, set manually by the user, while ship squat data is based on Seaway Squat Models for the vessel and channel type.
In dangerous navigational situations or under system fault conditions, relevant notifications will be provided for the operator.
When the DIS is connected to the ECDIS network, an ECDIS can be set up as the DIS for back-up purposes in the same way as the DIS can be used as the ECDIS back-up.
A Draught Information System button in the 'UKC Data' display is intended for switching between the DIS mode and standard ECDIS mode. When the DIS mode is turned off, the MFD can operate as an additional work station.
The DIS will indicate whether certain areas are safe to pass or require speed adjustments to decrease the squat of the vessel. A required safety margin of 30cm of UKC has to be maintained. The HD chart contours are then filled in with colours indicating safe passage areas.
Transas says that the availability of this data on the bridge should enable most vessels to maximise their draft up to 26 feet 9 inches, compared with the current Seaway permissible draft of 26 feet 6 inches.
This increase of 3 inches would mean that the average Great Lakes vessel can carry an additional 250 to 400 tons of cargo.
“The use of the Transas DIS will allow for the safe and effective deep loading of our vessels to optimise the full available water column in the Seaway,” commented Tom Anderson, director, ports & harbours, navigation, Algoma Central Corporation.
“The use by our vessel personnel of the Transas DIS integrated with shipboard ECDIS/ECS and the supporting Seaway AIS network is an example of an e-Navigation initiative that was developed for a specific area user that has further potential for use in other ports and their connecting waterways.”