Dynamic Under Keel Clearance (DUKC©) Jonathon Pearce (OMC International)

July 18th, 2013 by JCB
Posted in Articles, Features, Technical and Training | No Comments »

INTRODUCTION

Traditionally, ports have operated under fixed rules which govern the minimum under keel clearance (UKC) to permit safe transit along port approach channels. These rules may have been in existence for many years and in most cases they have an unblemished safety record but as they are based on the assumption that this clearance is sufficient regardless of the prevailing environmental conditions they can fail, often with disastrous consequences and evidence shows that up to five percent of transits are marginal, even unsafe.

Technology is now available to effectively mitigate the risk of grounding.  These systems model vessels dynamically in real time to accurately calculate the UKC which  ensures that a vessel cannot transit unless it is safe to do so.  There are also productivity gains in using such systems because, since the fixed rules are designed to cover the worst possible passage conditions likely to be encountered in a particular port, a vessel can usually sail with increased draught.

This technology is used operationally in the Dynamic Under-Keel Clearance System (DUKC®) developed by Melbourne based, OMC International.  It is a real time UKC system used by ports and shallow waterways to maximise port productivity and safety. The DUKC® considers all factors that effect a vessel transiting a channel to determine the minimum safe UKC requirements and seamlessly interfaces probabilistic UKC planning (maximum draught & tidal windows) up to 12 months in advance with short term transit planning utilising real time environmental and vessel specific information which also includes UKC monitoring throughout the transit to deep water.  With a track record of 20 years and more than 90,000 vessel transits globally without incident, DUKC® has a strong history as an operational tool.

The latest version, DUKC® Series 5, integrates proven core calculation engines with a web interface thus allowing easy accessibility to the system for approved users world-wide who are thus able to successfully execute UKC related tasks via the web. The web interface has gained acceptance by pilots as it allows on-board calculations through a laptop or tablet, such as an iPad.

The DUKC® system does not use the vessel’s draught as the baseline, but a predetermined safety limit which must not be breached. Added to this limit are the vessel’s dynamic movements which are modelled using the predicted environmental conditions and this gives the minimum water level that is required to ensure safety at all times throughout a planned transit.

The methodology behind DUKC® has been internationally recognised, and the improved certainty and information that dynamic systems can deliver has seen regulatory bodies regarding such systems as an essential aid to navigation  and these bodies are in the process of developing standards for dynamic UKC systems.

OMC is currently the only specialist maritime firm in the world whose core focus is providing proven technology for determining and managing real-time UKC in depth-restricted waterways.  Increasing international recognition is being given to the significant benefits which dynamic determination of UKC provides as a risk mitigation tool and many ports have become increasingly interested in installing DUKC® systems.

OMC’s experience and knowledge of hydrodynamics  offers a paradigm change in this critical area of maritime safety and DUKC® has been recognised as a core e-Navigation concept, which is available and operational today.

STATIC RULES

The traditional static rules were devised when vessels were smaller, their speeds lower, ship/shore communications poor and technology generally unavailable to determine ship motions accurately.   Therefore, there needed to be a simple method of calculating a safe UKC, and the generally accepted draft to depth ratio was 10% unless conditions dictated otherwise.

The Permanent International Association of Navigation Congresses (PIANC) guidelines recognise this ratio, but it is often forgotten that this is a minimum suggested safety clearance and is for calm waters only, and that twenty, even fifty, percent may be better, especially for ports that are subjected to wave motions.

The static rule tries to capture all anticipated factors in a single allowance.  Essentially the only controllable factors is the tide height (and therefore transit time) and speed (which determines the amount of squat).  Where there is deep water with little or no wave motions this may be suitable, but where depths are critical, and conditions more variable, there may be times when the allowance is marginal. Some ports do try to assess other factors, but whilst some of these factors can be precalculated, in practical terms elements such as wave motions are undeterminable once a transit commences.  To address this issue, some ports apply a predetermined roll/pitch angle to give the ship-handler an indication of loss of UKC due to wave motion.

Speed is an absolutely critical element in maintaining safe UKC. Evidence has shown that pilots do not always maintain vessel speeds within the planned limits. If the transit is too fast, the ship will squat and heel in excess of the predicted amounts. Both effects are approximately proportional to the square of the speed so, if too slow the ship will not reach way points at required times and in tidal waterways, may therefore “lose” more water than predicted. Once underway these elements can be difficult to assess and can often be overlooked.  Most ports will use a single squat formula, but there are many formulae in existence; the most appropriate formula will depend on the bathymetry, channel design and the type of vessel.

The biggest drawback with static rules is that they are wholly dependent on the environmental conditions.  If they are too optimistic safety could be jeopardised; too conservative and they become uneconomic and this means that a port cannot maximise efficiency.

 

DUKC®

By contrast, dynamic UKC’s are determined based on the actual vessel and its stability parameters along with the following real-time conditions:

Wave height

Wave period and direction

Water levels

Currents

Tidal plane

Wind

The vessel’s transit speed and waterway configuration.

Wave spectra, ship speed and water depths vary along the transit and the effect of these variations is computed by the numerical ship motion model used in each DUKC® system. In addition, wave spectra and tidal residuals will change over time, and these effects are also accounted for. With respect to squat, individual ships and the pertinent characteristics of the complete approach channel are modelled using the most appropriate squat formula, and include the effect of temporal and spatial variation of tidal currents during the transit.

Dynamic systems can be viewed as a “bottom up” approach.  The system has, at its core, a minimum limit that must not be breached.  Each of the computed factors is then added until the minimum tide height is found that ensures a safe transit.  Thus when the conditions are favourable, vessels may have greater tidal windows and can sail with a deeper draught; but when conditions are not, then tidal windows are reduced and may even be closed or a vessel may be able to proceed but with a lighter draught.

The system is also predictive, so if a pilot wishes to adapt his transit plan (especially the transit leg speeds), or if there is an unforeseen event (e.g. a berth delay), or a change in the environmental conditions the system will automatically update the safe transit windows

OMC’s DUKC® system is now a mature product.  The day-to-day operation of DUKC®, in preference to static rules for UKC, has moved the system from academic theory into a best practice in the real world.  Integration of the sophisticated numerical calculations with real time environmental data ensures the integrity and quality of the dynamic data.

The accuracy of the numerical models used in the DUKC® System has been validated by undertaking more than 300 ship transits world-wide to obtain full-scale measurements of vessel speed, track and vertical displacements.  These validation tests have been undertaken for a wide variety of channel widths, configurations and lengths, vessel types, sizes and stability conditions, vessel speeds, wave conditions, tidal regimes and current speeds. This modelling guarantees the accuracy and applicability of the models using customised numerical models to calculate the UKC requirements of a particular ship sailing in a particular waterway with respect to the environmental conditions at the particular time.

The system has also been rigorously and independently tested by specialist risk management consultants to ensure that it satisfies internationally-accepted levels of risk for safely managing the UKC of vessel transits.

The DUKC® product suite is continually being adapted in response to customer feedback and availability of new software technologies. New applications include the integration of the technology onto portable devices carried by pilots and into VTS Centres enabling vessel speed and predicted under keel clearance ahead to be monitored on board and ashore.

DUKC® SERIES 5: Product Overview

OMC International’s DUKC 5 product suite, integrates the proven core calculation engines into a web interface permitting users to successfully execute UKC related tasks via the web rather than the traditional desktop-based user interface.

The software consists of several modules integrated behind a single web portal which can be arranged and configured to help manage UKC related problems ranging from long-term voyage planning to real-time onboard pilotage applications and to the monitoring of numerous vessels in real-time within a VTS centre.

At the heart of the DUKC® Series 5 software suite is a pair of critical engines: An Environmental Forecast Engine and a UKC Calculation Engine. Each engine consists of tested and proven sub-components. Built on top of the core engines are services which provide DUKC® applications with both web and non-web access to the underlying engines and each system can be customised to user requirements.

Networks of external data sources such as met-ocean sensors, AIS data streams and GPS positions are provided to the DUKC® system as required. All DUKC® outputs, diagnostics and statistics can be logged, queried or distributed in real-time to users.

The UKC Calculation Engine 

This engine and its subcomponents manage everything from complex vessel motion calculations to the logic of transit planning. Its purpose is to compute and solve UKC formulae.

The Environmental Forecast Engine 

This is the centre for all met-ocean inputs Its three primary functions are:

-To quality assure and filter all met-ocean inputs.

-To integrate all available met-ocean measured and predicted data, ranging from astronomical predictions to real-time sensor measurements to third-party (inc. national weather service) forecasts.

-To produce short term, medium term and long-term met-ocean forecasts.

-To predict met-ocean conditions in between sensor positions. For example, the engine computes tidal heights and streams between tide gauges and current meters.

WEB SERVICES

Voyage planning service

This calculates the probability of waves and tides from astronomical tide forecasts and historical wave and tide statistics, and uses these to calculate the most likely tidal window or maximum draught for a vessel.  The level of probability is set to ensure that the vessel can transit but a user can decide on a greater ‘risk profile’ which would allow a greater draught but with the risk of missing a tide. This allows a scheduler to specify the level of certainty for the ship to transit without delay.

 

Met-ocean service

This allows interactions with the met-ocean engine described above and handles all requests related to met-ocean data.

 

Vessel service

This service provides access to a comprehensive list of recognised vessels and their particulars which can be linked up to external vessel data sources such as port information systems.

 

Transit planning service

The transit planning service manages real-time to short-term predictions of UKC which are automatically updated from latest met-ocean observations. The Transit Planning Service is used to plan vessel transits through the specified waterway using the latest met-ocean observations and accurate vessel load state information and AIS positions.   This service is the core module for safe passage planning and allows accurate pre-planning, monitoring and contingency planning of a transit.

 

Optimiser Planning service

This service allows the optimisation of multiple vessel departures on a single tide whilst considering constraints such as tug availability, current restrictions and booking priorities.

Transit Monitoring Service On-board

OMC on-board solution provides UKC related information displayed within a charting package environment as overlay on top of a chart. This allows UKC information to be displayed more intuitively to pilots and allows horizontal navigation aspects to be included in the assessment of under-keel clearance.

The chart overlay display of UKC information allows pilots to make on the fly navigation decisions from an under-keel clearance perspective. For example,

making decisions about safe passing areas.

Assessment of the impact of speed increases on the safe travelling corridor.

Assessment of local shoals on the safe travelling corridor

The picture below shows a screen shots of a vessel transiting a channel.  Red areas indicate ‘no-go’ regions where the DUKC® predicts the vessel to have insufficient under-keel clearance due to dynamic ship motions, which in this example is primarily due to squat.

Operations – Transit Monitoring Service Shore-based

The transit monitoring service automatically tracks and monitors the UKC of ‘active’ transit plans. Users who have been assigned permissions to access monitoring functionality can track the under-keel clearance of one or more vessels simultaneously.

The UKC information is updated continuously based using the latest met-ocean observations, vessel load state information and AIS positions. Tracking of vessels occurs automatically once a transit plan has been activated.

Reporting service

Allows searching and retrieval of archived outputs, previous calculations, errors and diagnostics.  All calculations, errors, system messages and diagnostics are logged and can be queried if desired. Underlying the core engines and services are the various data storage components. Data storage structures include databases for vessel details, voyage plans, transit plans, met-ocean data and business messages.

 

Conclusions

The use of static rules at many ports needs serious consideration about whether they are suitable and if all factors are understood.   The paradox of the static rules is that without an incident a port’s static rules may appear validated and considered safe.  In reality, where UKC limits are critical (which is increasingly the case with ever larger vessels being launched), and conditions variable, there may be times when the clearance is marginal and the port has experienced an unknown “near miss”.

Recent developments in navigation technology make possible accurate planning and the continual monitoring, and control of the UKC of large vessels during transit along shallow waterways.  These decision support tools, and the integration into navigation systems, such as a pilot’s PPU, also allow the effect of alternative speed/sailing options on under keel clearance to be quickly investigated by pilots and masters in situation where the passage does not proceed as planned.  The information that is now available through a dynamic system enhances the decision making processes of the port and pilot and complements the master/pilot information exchange.

DUKC® systems have a proven track record and since the methodology builds on the concept of a minimum clearance limit that must not be breached, DUKC® systems effectively control the risk of a touch-bottom/grounding incident. This level of risk cannot be achieved with static rules because the clearances vary and are determined by the environment present on the day.

For more information

Website: OMC International