thans for your article which is very interesting and very actual.
Seems to me that apart of European experience, in US waters you can find well established procedure of navigation with negative “dynamic” UKC. Various ports in US Gulf (e.g. Houston, Texas City, Corpus Christi, Lake Charles, Port Neches) accepting tankers loaded to the static draft equal to controlled depth in area and moving them irrespective of tide.
The only explanation to this is muddy navigation, but shipping companies prefer to pretend that they are navigating with sufficient UKC instead of acknowledging that fact.

Nice that you throw some light on the issue.

Regards

Y. Shakh

I was very pleased with the attention paid by the online edition dated 25 July 2008 of “The Pilot” to the research on ship behaviour in muddy navigation areas at Ghent University and Flanders Hydraulics Research. This research project – which is presently still continuing – has not only given new insight into ship hydrodynamics in these very particular conditions, but also had very practical consequences for bottom survey, maintenance dredging, navigation and access policy for the harbour of Zeebrugge. I would like to emphasize the role of the Zeebrugge pilots in this research project: their constructive co-operation during the execution and interpretation of the real-time simulation runs was highly appreciated, as in this way maximum advantage was taken of their skills and experience.

There are a few topics addressed in the article I would like to comment on, if you allow me.

• About the definition of nautical bottom: “This definition is somewhat vague…”. I agree; this definition suggests a philosophy to be followed, but does not give a ready-to-use practical solution. Moreover, this definition is generally valid, not only in muddy areas. In case of a rocky bottom, or a bottom covered with boulders, the highest peak will determine the nautical bottom, and bottom touch will lead to damage; in muddy areas, not damage but controllability will be an issue.

• “… the researchers … have concluded that the important factor is the density of the mud in suspension and have established that a density of 1200 kg/m3 can safely be navigated by all vessels.”

Most waterways authorities that are confronted with mud issues use a density level as a practical criterion for nautical bottom survey, and in many cases 1200 ton/m3 is selected as a critical value. However, density as such is not an important parameter. In fact, the nautical bottom in muddy areas should be defined as the level where the “fluid” mud stops and the “solid” mud begins. Instead of density, a “rheologic” criterion should be used that determines whether mud is to be considered as a fluid or as a solid material. Of course there is a link with density: the more solid material suspended in water, the higher the density, and if the concentration of solid material exceeds a certain critical value, the flow characteristics of mud will change significantly. Unfortunately, a fixed density value separating fluid and solid mud cannot be defined, as this depends on many parameters, such as the sand content of the mud; as a result, mud with a density of e.g. 1250 kg/m3 can be either “black water” or a very sticky material, depending on the composition. If a density value is selected to determine the nautical bottom, it has only local and temporal validity; for instance, the nautical bottom density in Zeebrugge used to be 1150 kg/m3, but has been increased to 1200 kg/m3, as a result of changes of the mud properties. The only reason why density is used instead of rheology, is related to survey techniques: a density profile is much easier to determine than a rheologic profile, as rheology measurements are very sensitive to the used device, the measuring procedure and even the analysis method. Ideally, the density criterion should – sooner or later – be replaced by a rheology criterion.

• Another point in the conclusions I would like to reply on is the following: “If the ship’s keel penetrates by more than 10% of her draft into low density mud layers, this may result into unacceptable situations”. This is surprising, and in contradiction with another conclusion stating that 12% negative under keel clearance is acceptable in case sufficient tug assistance were available. Therefore, in my opinion this sentence could be based on a misinterpretation. Of course, there is a limit for penetration into the mud layer; in the particular case of deep-drafted container vessels in the harbour of Zeebrugge, the pilots accept a penetration of 7% into the mud layer if sufficient tug assistance is guaranteed.

• I have some doubt about paint being removed by navigating through fluid mud layers. Indeed, in the speed range ships manoeuvre in Zeebrugge, shear stresses caused by contact between a ship’s keel and the upper mud layers are comparable to shear stresses at full speed in sea water. Apart from this consideration, it appears to be hardly feasible for a freight ferry – unless she really is in trouble – to touch the mud layer in the harbour of Zeebrugge, as the interface of the fluid mud layer is typically located at about -13 m under the low water level.

Yours sincerely,

Prof. Marc VANTORRE

Ghent University
IR15 – Maritime Technology Division
Technologiepark Zwijnaarde 904, B 9052 GENT (Belgium)
Tel +32 (0)9 2645555
Fax +32 (0)9 2645843
marc.vantorre@ugent.be

c/o Flanders Hydraulics Research
Knowledge centre “Manoeuvring in shallow and confined water”
Berchemlei 115, B 2140 ANTWERPEN (Belgium)
Tel + 32 (0)3 224 6956 (direct)
Tel + 32 (0)3 224 6035 (reception)
Fax + 32 (0)3 224 6036
marc.vantorre@mow.vlaanderen.be

Mobile phone +32 (0)478 349971