Hydrographic Report

Under Keel Clearance - in real-time

by Maurice Perwick (Eliot Sinclair & Partners Ltd) and Gary Chisholm (Trimble NZ)

What is the optimum safe tolerance for Under Keel Clearance?

Typically a channel is dredged to a defined depth and any deep draft vessel exercises a margin of safety such as entering port in high tide, or exiting with a lighter load.

It has been suggested that a way to save money and the possible environmental impact of dredging, while still making ports accessable to large vessels, is to reduce the Under Keel Clearance tolerance without compromising safety.

There are a few technologies available for this measurement...

One technique is to characterise the performance of each class of vessel in the channel. This is carried out by using precise GPS (centimetre accurate Real-time Kinematic GPS - see following report) while sailing in and out. Then for following port entries use that data plus wave bouy information, nominal draft, vessel speed and wind data and report in real-time on the actual draft - a technique called Dynamic Under Keel Clearance.

Another technique is to install Real Time Kinematic (RTK) GPS receivers on deep draft vessels so that the precise absolute height (depth) of the keel is known independent of tide gauges and changing vessel draft. When combined with an accurate terrain model of the navigable depth of the channel then the Under Keel Clearance can be determined and the tolerance could be minimised. This expands the window of opportunity for vessels to enter and exit the port and minimises dredging whilst maintaining safety. The caveat here is that both the RTK system onboard must be reliable and quality controlled and the terrain model of the channel is both accurate and current.

Companies in NZ, Australia and worldwide are progressing design of such systems. The following report explains some recent work on this topic in NZ.

Nelson Harbour using Trimble RTK-GPS Techniques.

Eliot Sinclair and Partners Ltd have recently completed six transits of Nelson Harbour over a six week period. They measured the roll, pitch, and squat of large container ships, log vessels, and petroleum tankers at various states of the tide to minimise the under keel clearance and therefore maximise the effects for OMC Pty Ltd of Victoria to carry out analysis for under-keel clearance prediction.

A control survey was carried out around Tasman Bay to determine the WGS-84 - Geoid separation and inclination in order that the real-time GPS measurements were in terms of the Port height datum. Positions were logged every one second to sub-decimetre accuracy.

The surveys were carried in all weathers, and over the whole 24hr day with gear being lifted onto and off the ship onto the pilot boat by hand hauling up the side of the ships.

Preparing a Trimble 4700 RTK receiver (centimetre accurate) on the bridge of a ship approaching Nelson Port.

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