SQUAT: PART 2: MUD NAVIGATION & NEGATIVE UNDER KEEL CLEARANCE

Whilst wading through the various documents to produce the article on squat in the January issue, I came across several references to the linked topic of muddy water navigation and the concept of negative under keel clearance (UKC).

I must admit that until I read the research I had no understanding of this form of navigation and felt that it must be another theoretical area of research with no feasible practical application because we have enough trouble presenting masters with passage plans using minimum UKC and would therefore have no chance trying to explain to a stressed out Captain that the passage plan would involve navigating through areas where the draft would be greater than the charted depth!! However, there are several ports where the liquid mud in suspension is sufficiently fluid to be navigable and the difference between the echo sounder depth and the solid mud depth can be considerable and therefore the commercial advantage of accurately measuring the navigable mud layer can be considerable. The depth where the navigable mud becomes non navigable is called the “Nautical Bottom”. One major port where this phenomena is present is Zeebrugge and it is therefore in Belgium where most research has been undertaken. As with squat, the maths and physics are complex and the following is therefore an attempt to de-mystify the concept.

Navigable mud can open the operational window for port operations. Photo JCB

To asses the navigability in muddy navigation areas the “nautical bottom” concept was introduced and in 1997 the Permanent International Association of Navigation Congresses (PIANC) formalised the following definition:

The nautical bottom is the level where physical characteristics of the bottom reach a critical limit beyond which contact with a ship’s keel causes either damage or unacceptable effects on controllability and manoeuvrability.

This definition is somewhat vague in that there are so many different ship types that what may be a critical limit for a laden bulk carrier might have no adverse effect on a fine lined containership. Fortunately the researchers have examined different ship types in detail and have concluded that the important factor is the density of the mud in suspension and have established that a density of 1200kg / m3 can safely be navigated by all vessels. However, although the nautical bottom can be established by density its effectiveness as a safe critical parameter is dependent upon the ability to continuously monitor the density of a mud layer and detailed knowledge of ship behaviour in muddy areas. This has been undertaken by, physical tank testing, mathematical modelling and by live trials mainly involving the pilots in Zeebrugge. The following information contains extracts from papers published on the internet by many different establishments but in particular:

Marc Vantorre: Ghent University, IR04 – Division of Maritime Technology

Michael J. Briggs: Coastal and Hydraulics Laboratory, U.S. Army Engineer Research and Development Center

Klemens Uliczka: Federal Waterways Engineering and Research Institute, Hamburg

Pierre Debaillon : Centre d’Etudes Techniques Maritimes Et Fluviales,

DEFINITION OF DEPTHS IN MUDDY AREAS

HYDRODYNAMIC DEPTH:

The exact level of the interface between moving muddy water and stationary mud.

PARAMETRIC DEPTH

The bed level determined by some material parameters e.g. target strength, shear strength etc.

OPERATIONAL DEPTH

The depth of a particular parameter relevant to some specific operation e.g. navigation.

In areas where the mud bed is firm these 3 definitions will coincide.

COMPARISON OF NORMAL SOUNDING DEPTH AND NAUTICAL BOTTOM

As an example of the difference that using the nautical bottom as opposed to normal sounding bottom can make, the following diagram shows real data from surveys on the EMS using different sounding frequencies, with the 210 khz being the standard for sea water and the 15khz to penetrate through to the 1200 interface. The results reveal an additional navigable depth of over two metres in places.

Muddy Navigation Areas

The presence of a fluid mud layer on the bottom of a channel has a significant influence on ship behaviour in general, and sinkage and trim in particular. Two effects play a dominant role:

The pressure field around the moving hull causes undulations of the water mud interface that modify the distribution of vertical forces over the length of the ship and, therefore, sinkage and trim.

If the ship’s keel penetrates into the mud layer, the hydrostatic (buoyancy) force acting on the submerged hull increases due to the higher density of the mud.

The interface deformation is a function of many parameters, such as ship speed, layer thickness, mud density and rheology, and the initial UKC with respect to the mud-water interface.

Contact between the ship’s keel and the mud layer depends mainly on the UKC, but is also influenced by the interface undulations and the ship’s sinkage. As a result, both effects are interrelated. Most of the information available on this subject is based on experimental research using models.

Experimental research

One of the major problems for reseach into mud layer navigation is producing an accurate model for the mud behaviour. Mud behaves in a complex manner and its characteristics vary with the depth. The model tests that have been carried out mostly use an artificial mud layer because it is difficult or even impossible to repeat several tests under the same natural mud conditions.

Additional problems with model tests are the scaling effects of the simulated mud with respect to the model and consequently for the port of Zeebrugge a new research program was initiated, consisting of captive manoeuvring tests in Flanders Hydraulics Research shallow water tank and both fast- and real-time simulation runs. The mud layer was simulated by means of a mixture of chlorinated paraffins and petroleum. Most runs were carried out with a model of a 6000 TEU container as this one was the standard type of vessel for the harbour of Zeebrugge at that time. Mud layer thicknesses were varied from 0.75m to 3.00m and under keel clearances referred to the water-mud interface from -12.2% till +21% of draught.

Mud-water Interface Undulations

A ship navigating above fluid mud layers will cause vertical interface motions

(internal waves and undulations) that are influenced by the ship’s forward speed as revealed in the following diagram:

Model tests showing conditions b) & c)

a) At very low speed the interface remains practically undisturbed.

b) At intermediate speed an interface sinkage is observed under the ship’s bow if the fluid mud layer is relatively thick. At a certain time, an internal hydraulic jump, perpendicular to the ship’s longitudinal axis, is observed. The front of this internal jump moves aft with increasing speed.

c) At higher speeds, the internal or interface jump occurs behind the stern

The sinkage for a ship sailing in a muddy bottom condition is decreased relative to the condition in which the mud layer is replaced by a solid bottom. This is because the ship can “feel” the hard bottom more than the softer, less dense, mud layer. If the mud layer is replaced by water (normal conditions without a mud layer) however, the sinkage would decrease relative to the condition with the mud layer. However, this does not take into account the effect of extra buoyancy (i.e., mud is denser than water), but this is only important in very dense mud layers and/or important penetration. In general, the influence on trim is more important than sinkage since the mud layer causes the ship to be dynamically trimmed by the stern over its complete speed range. Thus, the effect of mud layers on average sinkage is only marginal as trim is much more important.

Mathematical modelling

Obviously the results of model tank tests could not be immediately transferred to real ships for trials so the results needed to be transferred for use in a simulator which meant that mathematical models needed to be created. There were many complexities involved in this process and for those of you interested in this aspect of mud navigation full details can be found in the papers within the links at the end of this feature.

ZEEBRUGGE: REAL-TIME SIMULATION RUNS

The final purpose of the research program consisted in ascertaining the actual operational limits for mud navigation by means of live trials. As the pilots play a central role in the navigation to and from Zeebrugge, the input of their experience and assessment in this project was required. For a selection of bottom conditions, a real-time simulation programme was organised with Zeebrugge pilots at the full mission bridge simulator of Flanders Hydraulics Research, Antwerp. All runs were carried out with a container ship (length over all: 300.0 m; beam:40.25 m; draft: 13.5 m) calling at and departing from the harbour of Zeebrugge.

The simulation programme was composed paying attention to several aspects:

· Validation of the mathematical models: is the behaviour of the ship assessed as realistic during the simulation runs? In order to evaluate this aspect, simulations were carried out above a solid bottom and above muddy bottoms with reduced under keel clearance, according to existing or realistic situations.

· Determination of the limits of the controllability: according to the PIANC definition, contact between the nautical bottom and the ship’s keel causes unacceptable effects on controllability and manoeuvrability. In order to make an assessment in these matters, a series of simulation runs was carried out during which contact occurred between the ship’s keel and mud layers with higher density and viscosity.

· Evaluation of the navigability of mud layers: in case it is decided to determine the nautical bottom by means of a density level higher than the present 1.15 t/m³, the ship’s keel will possibly penetrate into mud layers with reduced density and viscosity. The ship’s behaviour in such conditions was assessed by a series of simulation runs.

In total, 63 runs were carried out by 15 pilots during 8 days.

These manoeuvres are typical for large container carriers calling at Zeebrugge, so that a feedback to the pilots’ experience was guaranteed; moreover, a broad range of hydrodynamic conditions (speeds ahead/astern, propeller rpm ahead/astern, drift angles, yaw rates, …) was covered during the simulation runs. During each single run, the bottom characteristics were assumed to be constant over the entire harbour area.

The access channel to the harbour, the Pas van het Zand, is characterised by important tidal currents in the zone beyond the breakwaters; at low tide, the magnitude of cross currents takes values of 2 to 2.5 knots. As these currents greatly affect the shipping traffic arriving and departing from Zeebrugge, realistic current patterns were introduced into the simulation environment.

All manoeuvres were carried out in frequently occurring, moderate wind conditions (SW, 4 Bf); during some runs, more severe winds were applied. Tug assistance was guaranteed by two tugs of 45 ton bollard pull each; during some runs the available tug power was increased.

Qualitative evaluation of the simulation runs

All pilots were requested to complete a questionnaire just after the simulation run; this resulted into a first, very important assessment of the manoeuvres. According to the opinion of a large majority of the pilots, the simulation of the outside view, the ship’s behaviour and the tug assistance could be considered as “good” to “very good”.

After each run, the pilot was asked whether it would be advisable to carry out the manoeuvre in reality. Based on this assessment, the conditions were classified as “acceptable”, “marginal” and “unacceptable”

Analysis based evaluation of the simulation runs

Taking account of the comments of the pilots on the simulated manoeuvres, it was clear that following criteria should be considered for assessing the bottom conditions:

· Speed: Is a departing ship able to reach a speed that is sufficient to compensate for the cross current acting beyond the breakwaters?

· Controllability by own means: Can a departing ship obtain a straight course without extreme use of rudder and propeller?

· Manoeuvrability with tug assistance: Are the ship’s rudder, propeller and the tug assistance sufficient to perform the manoeuvres safely within acceptable time limits?

Based on the pilots’ qualitative assessment, limits were determined to quantify these criteria:

· Speed: in order to keep within the fairway, a departing ship’s speed should be at least 8 knots, and preferably 10 knots. These values were selected as limits for unacceptable, marginal and acceptable conditions.

· For a departing ship’s controllability by own control devices, the standard deviation of the rate of turn to be an adequate indicator. For the different bottom conditions, this value is displayed as a function of the water depth to draft ratio. Taking account of the pilots’ evaluation, values of 5 and 6 deg/min were selected as critical limits.

· In order to evaluate the ship’s manoeuvrability with tug assistance in a quantitative way, the impulse of steering force was introduced, being the time integral of the sum of the lateral rudder and tug induced forces. The values of these impulses were calculated for each sub trajectory, and compared to the pilots’ evaluation of the adequacy of tug assistance. In this way, it was not only possible to quantify the third criterion, but extrapolations to assistance by more or less powerful tugs could be made as well.

CONCLUSIONS

As a result of the analysis of the real-time simulation runs with a small negative under keel clearance, it can be concluded that contact with mud layers of a density of 1,200 kg/m³ or more should be avoided, even if sufficient tug assistance is available.

However using a limit of 1200kg/m³ was considered safe for navigation provided that tugs were available as per the following table:

  • 0% under keel clearance using 2 tugs of 30 ton bollard pull and less;
  • -7% under keel clearance if 2×45 ton bollard pull tug were available;
  • -12% under keel clearance in case of 2×60 ton bollard pull tugs were available.

These conclusions are only valid in moderate wind conditions for 6000 TEU container carriers. However the methodology can be applied to any vessel or harbour. The new critical limit led to the admittance of deeper drafted vessels and an optimization of the maintenance dredging works in the harbour Zeebrugge, without jeopardizing the safety of navigation.

However, a warning was made that pilots should always be aware of the level of the water-mud interface, which should be indicated on the nautical charts as well, for several reasons:

· If the ship’s keel penetrates by more than 10% of her draft into low density mud layers, this may result into unacceptable situations.

· Small positive under keel clearances relative to the mud-water interface may result into a modification of the ship’s behaviour and controllability.

· A major conclusion of the simulation study was the importance of available tug assistance. If insufficient tug power is available contact with the mud layer should be avoided so that the nautical bottom is moved to the mud-water interface; if more powerful tugs can assist the ship, the pilot may decide to allow a larger negative under keel clearance. In the near future, the tracks, controls and tug assistance of deep-drafted containers ships arriving at and departing from Zeebrugge at low tide will be recorded by the pilots in order to provide a feedback to the simulation study. After an evaluation phase, it will be decided whether the new criteria for the determination of the nautical bottom will be applied in practice.

Other considerations

Although the above analysis reveals that mud navigation is feasible, such navigation results in other physical effects on the ship. Some of my colleagues who served on freight ferries running regularly to Zeebrugge have informed me that navigating the mud layer doesn’t just keep the hull clear of growth but also removes the paint and dry docking reveals a clean metal hull and the propellers also become highly polished. The other obvious problem is with engine cooling systems which are not designed for cooling by muddy water.

JCB

3 Responses to “SQUAT: PART 2: MUD NAVIGATION & NEGATIVE UNDER KEEL CLEARANCE”

October 13th, 2008 at 13:25

Sir,

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

 
February 26th, 2010 at 18:21

Lots of Good information in your post, I favorited your blog post so I can visit again in the near future, Thanks

 


Capt. Yuriy Shakh
January 3rd, 2011 at 01:07

Dear Sir,

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

 

Leave a Reply

UK Maritime Pilots' Association
European Maritime Pilots' Association
Internation Pilots' Association SITE SPONSORS
Navicom Dynamics
OMC International