Speed matters



Mike Harrison, director, Inshore Systems, Cirencester, UK


Speed (or more precisely the translational velocity and angular speed) is the most important factor for the safe berthing of ships. Today there are accurate, user-friendly and inexpensive solutions for the reliable and precise measurement of berthing speed, yaw and approach angle, using state of the art GPS technologies. Some ports take risks unknowingly; they accept inefficiencies and a lack of accountability. Progressive operators make Portable Piloting Units (PPUs) work for them. These advanced navigation and berthing aids give ports a competitive edge and enhance safety. This article explores how some imperceptibly small changes during ship approach can mean the difference between a normal berthing and serious damage. It looks at the technology now available to ports and pilots which can improve skills, reduce risks and improve profits.

Considering kinetic energy

The laws of physics are clear. The kinetic energy of a ship is a function of its mass, its speed and rotation. During berthing some or all of that energy must be absorbed by the fenders. If fenders are over-compressed then loads rise until something breaks, buckles or collapses to absorb the surplus. A split fender and distorted steelwork may be the result, but the consequences are often worse with punctured hulls and/or berths closed for unscheduled major repairs. These risks can be virtually eliminated for the cost of less than one fender system. Engineers have always considered the lateral (translational) berthing velocity of ships in their designs. But they invariably ignore the rotational speed (yaw) component despite its large influence on the ship’s kinetic energy. Yaw is caused by wind or currents, sometimes by equipment failure or human error. Whatever the reason, yaw is likely the culprit in many berthing incidents which result in hefty costs for ship owners and their insurers.

The limitations of visual checks

Consider this: follow the minute hand of a clock turning at six degrees per minute and ponder the difficulties a pilot faces when trying to judge a one degree per minute rate of turn while aboard the rotating ship. Add in an un-forecast fog bank or snow storm, perhaps a squall or tug failure. Yet one degree per minute raises the ship’s normal berthing energy by 40 per cent. Increase that to 3 degrees a minute and the berthing energy is easily double the normal value ‘assumed’ by the dock designer – quickly exceeding any design safety factor for the fenders and structure. More surprising is that these modest yaw rates occur routinely just metres from the fenders. Most are caught and corrected, some are not. Why have engineers ignored yaw? Probably a combination things: the lack of historic data; most design codes assume  = 0; maybe a misunderstanding about how yaw and translational speeds are related; lack of awareness about the tools and aids available to simultaneously and accurately measured speed and yaw on the ship and/or from shore.

This is no longer the case. Affordable MEMS gyroscopes and rate of turn indicators can detect and measure ship’s heading to better than 0.2 degrees accuracy and rate of turn to less than 0.5 degrees a minute. A well trained eye might just about detect this, but never 100 per cent of the time for a busy, multi-tasking pilot. And even the very best pilots couldn’t hope to judge a ship’s exact bow position, hidden from view and perhaps 200 metres distant with two centimetres accuracy. Or estimate speeds and vectors as slow as 2.5 centimetres per second (0.05 knot) without any visible reference points.

The bridge in a box

This is routine stuff for a PPU which combines GPS with differential or real time kinematic (RTK) corrections, integrated with a microelectromechanical systems (MEMS) gyroscope and rate of turn sensors. PPUs weigh only a few kilogrammes, come packaged in a compact box and have a 15 hour battery life – often described as a ship’s ‘bridge in a box’. One button operation, graphical and familiar software, laptop or tablet displays are optimised for daylight viewing and night operations. A modern PPU system tells the pilot at a glance exactly what the ship is doing at any time, triggering alarms when recommended limits are exceeded, but without taking the pilot’s attention away from other key duties.


To read the full article download PDF

Cookie Policy. This website uses cookies to ensure you get the best experience on our website.