Question

With reference to oil content monitoring of bilge or tanker discharges:

(a) Describe with the aid of a sketch the general arrangement of an oil monitoring system. State the inputs that are recorded; (7)

(b) Differentiate between monitoring chambers using direct light and those using scattered light. (7)

(c) Explain the problem to be overcome in the design of monitoring systems. (6)

Answer 1

With reference to oil content monitoring of bilge or tanker discharges:

Part (a)

Describe with the aid of a sketch the general arrangement of an oil monitoring system. State the inputs that are recorded.

Part (b)

Differentiate between monitoring chambers using direct light and those using scattered light.

Part (c)

Explain the problem to be overcome in the design of monitoring systems.

Part (a)

General arrangement of an oil monitoring system (ODMCS / oil content monitor):

Sketch: The oil monitoring system consists of a sample line taken from the discharge line (the overboard discharge), a sample pump, a monitoring chamber (the sensor), a control unit, and a recorder. The sample of the discharge water is drawn continuously from the discharge line, passed through the monitoring chamber where the oil content is measured (by the optical/ultrasonic sensor), and the reading is displayed and recorded. The control unit operates an alarm and an automatic stopping device (a valve) that stops the discharge if the oil content exceeds the limit (e.g. 15 ppm for bilge, or the 30 ppm/15 ppm for tanker discharges).

Inputs recorded:

The oil content of the discharge (ppm).
The flow rate of the discharge.
The date, time and position of the discharge.
The ship's speed and the discharge conditions.
The total quantity discharged.
The operation of the alarm/stopping device.

These are recorded in the Oil Record Book and the monitoring system's recorder.

Part (b)

Direct light vs scattered light monitoring chambers:

Direct light (transmission/obscuration) method: A light source (e.g. an infrared or visible light) is shone through the sample, and a photocell on the opposite side measures the light transmitted. When oil is present, it absorbs/scatters the light, reducing the transmitted light; the reduction is proportional to the oil content. This method measures the light blocked by the oil.
Scattered light method: A light source is shone into the sample, and a photocell is placed at an angle (e.g. 90 degrees) to the beam. When oil droplets are present, they scatter the light onto the photocell; the scattered light is proportional to the oil content. This method measures the light scattered by the oil droplets.

The direct-light method is affected by the colour/absorption of the oil and by solids, while the scattered-light method is more sensitive to the droplet size and is less affected by the oil colour; both require calibration and are affected by fouling.

Part (c)

Problem to be overcome in the design of monitoring systems:

The main problem is to measure the oil content accurately and reliably in a variable discharge (different oil types, emulsions, temperatures, flow rates, and the presence of solids/chemicals), and to provide a reading that is representative of the discharge. The system must:

Be insensitive to the oil type, colour, droplet size and the presence of solids.
Be resistant to fouling (the sensor and the sample line must not be blocked by oil/sludge).
Be calibrated and stable over time.
Respond quickly to changes in the oil content.
Operate the alarm and the automatic stopping device reliably.
Be type-approved and meet the MARPOL requirements.

The design must overcome the fouling, the calibration drift, and the variability of the discharge to provide a reliable measurement and to prevent an over-limit discharge.

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