Non-invasive draught measurement technology from Canada

Typical draught measurement systems require installation of pressure transducers on pipe probes extending through the hull of a vessel at different positions. In principle, these systems estimate and display the draught based on the pressure readings at predefined locations of the hull.

This approach for measuring draught has one major drawback - the inaccessibility of the probes. As a result, the installation and repair for this system is only possible when the vessel is in the shipyard and the hull is exposed. In other words, the installation and troubleshooting must be scheduled during the built process or when the vessel goes for major hull repair. To address this issue, in 1993 Weir-Jones Engineering introduced an innovative method for measuring draught. This non-invasive draught measurement technology, which has been continually updated in the past 18 years, is known as Automated Draught Indicator System (ADIS).

The ADIS system is equipped with four ultrasonic transceivers. In the standard configuration, each transceiver generates short bursts of ultrasonic energy and captures the reflections from the water surface. The embedded microprocessor in the sensor measures the travelling time of the generated ultrasonic wave and accurately determines the distance of water surface from position to the transceiver’s mounting.

In addition, the sensor takes advantage of an internal temperature sensor to compensate for temperature effects on the measurements. The sensor digitally sends the measured level values that define the position of the water plane relative to the vessel to the central processing unit. In the central processing unit, the position of any part of the ship relative to the water plane is accurately calculated. The draught of the vessel is determined based on measurements taken of the port and starboard freeboard, fore and aft. The depth of the keel below the water plane is calculated using the dimensions of the vessel and proprietary algorithms, which ensure accuracy by correcting for the presence of propeller wash in dock and waves while underway.

Two bridge display outputs are available for double-ended vessels or for alternate display locations. The bridge display provides six digital readings; draught forward and aft, draught and freeboard amidships, port and starboard. Data can also be routed to other data storage systems such as hull condition monitoring and voyage data monitoring systems.

The ADIS installation requires no underwater hull penetrations and can be completed in as little as eight hours on existing vessels. It is therefore claimed to be significantly less costly than systems requiring underwater hull penetrations and is also essentially maintenance- free.

The installation consists of four high precision sensors mounted fore and aft, port and starboard above the waterline. Where rubbing bands exist, these can provide convenient mounting points; other locations are below the bridge wings or on unobtrusive brackets at or below deck level.

For vessels that may hog or sag during loading and unloading of cargo, an ADIS configuration that uses six transceivers is available. In this way, cargo distribution can be optimised for trim purposes or for keel clearance in locks.

The first generation of ADIS used two draught measurement sensors, and a system was installed in 1993 on the BC Ferries vessel Queen of Victoria for a three-year test period. After more than 20,000 hours of successful operation, the first generation of ADIS was made available to marine operators and constructors as a proven commercial product. As a result, ADIS was adopted as a standard draught indicating system on the BC Ferries fleet of vehicle ferries based on Canada’s west coast.

Since its initial development, the system has been continuously upgraded and more features have been added. In 2004, the second generation system with four sensors was developed, and in 2010, a third generation was introduced that includes a GPS interface which enables ADIS to deactivate the alarm feature once the vessel is underway. The mean time between failures for the third generation has been found to be in excess of 50,000 hours.

Basically, the ADIS system offers vessel operators accurate draught measurements, with a claimed accuracy of ±10mm, which is believed to be crucial during the loading process. In addition, it provides real-time loading information about hogging, sagging and trim. The standard ADIS digital bridge display shows the draught of the vessel, port and starboard, forward, amidships and aft, and cargo tonnes to go.

A key benefit claimed for the system is increased safety of ships and other marine structures. When installed on displacement hulls, high-speed and naval vessels, and marine structures such as semi-submersible drilling platforms, ADIS enables operators to record precisely how much cargo has been loaded or unloaded to avoid overloading or instability. This enables the operator to improve the handling of vessels by precisely adjusting the trim of the ship.

On cargo vessels and ferries, the system should rapidly pay for itself by improving fuel efficiency, eliminating disputes about the exact weight of cargo loaded and discharged, and optimising loading.

In addition to enhanced stability and passenger/ cargo safety, on ocean going vessels ADIS is said to offer two tangible benefits that are becoming increasingly important in terms of environmental considerations. These are the improved fuel efficiency and reduced CO₂ emissions that can be achieved by optimising the vessel’s trim. In the case of tankers, this is usually only practical when the vessel is in ballast, but the cost savings and emission reductions are significant, and result in the cost to the owner of installing ADIS being recovered in less than two month’s sailing in ballast.

Consider a 300,000dwt Capesize tanker sailing in ballast at about 17 knots. A 30-day voyage would consume 2,400 tonnes of fuel, i.e. 80 tonne/day. At a bunker cost of $700/tonne, this is a total fuel bill of $1.68 million.

Recent studies in Europe and by Mitsui in Japan indicate that fuel consumption efficiencies of between 5% and 7% can be achieved by optimising the trim of the vessel. Using the lower figure of 5% produces a fuel saving of approximately $80,000 per trip in ballast, with commensurate reduction in CO₂ emissions of more than 300 tonnes. On this basis, the capital cost of an ADIS system on a tanker in the 300,000dwt category is an investment with a payback period of slightly more than a month of sailing in ballast. This does not take into account the associated benefits of reducing CO₂ emissions by almost 400 tonnes per trip.