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The Rheocable Method

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The RheoCable method allows for the rapid assessment of the true nautical depth providing information about the transition zone between the fluid mud and the solid mud (the nautical depth)

The RheoCable Method (TRM) is a new and totally unique method for determining the nautical depth in the presence of fluid mud. Breaking away from traditional acoustic and density based systems, TRM relies on viscosity changes at the fluid mud / solid mud interface to precisely map this rheological boundary. The RheoCable Method was developed by THV Nautic on behalf of the Belgian port authorities (Afdeling Maritieme Toegang, Belgium), subsidised by IWT (Instituut voor Wetenschappelijk en Technologisch Onderzoek, Belgium) and proved during a pilot program undertaken in the Port of Zeebrugge. OEMG Global has partnered with THV Nautic, developer of the Rheocable method to bring the system to the Australasian region.

What is the significance of Nautical depth?

The behaviour of ships sailing and manoeuvring above a fluid mud layer with restricted keel clearance or even negative keel clearance has been a worldwide concern for captains, pilots and harbour managers (Clandillon-Baker 2008). The true nautical depth (nautical depth) is defined as “…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” (Clandillon-Baker 2008, Delefortrie, G.; Vantorre, M. 2005a, PIANC 1997). In muddy environments, nautical depth is defined here as the discontinuity in the vertical viscosity profile below which, given sufficient contact between a ships keel and the solid mud, ships may become dangerously uncontrollable.

When the TSHD Vlaanderen 18 (pictured here), during the 1989 trials in Zeebrugge, was put in contact with the consolidating mud, the observation was that the ship was out of control. She followed her own course, was not reacting to rudder and / or propeller manoeuvres.

The actual depth of this discontinuity is very difficult to determine using classical acoustic methods (dual frequency echo sounders) and density measurements (density probes) (Clandillon-Baker 2008, Claeys 2008, Claeys 2006, Delefortrie, G. and Vantorre, M. 2005a). The failure of these methods is due primarily to the vertical discontinuity, or the horizon between the fluid / solid mud interface, representing a change in viscosity rather than a change in density. Though this rheological property of the discontinuity has been recognised, until now, a practical, accurate and reproducible method for mapping the horizon outside of the laboratory has not been available.

In recognition of the variable (and questionable) results obtained from dual frequency echo sounders in mud environments (Malherbe et al 1986), and that ships can safely navigate through liquid mud without compromising vessel manoeuvrability, significant research has been undertaken to relate nautical depth to mud density. However, as rheological characteristics and density relationships of mud will vary from port to port (Fontine et al 2006), appropriate mud density criterion must be determined for each port to define the approximate nautical depth. The result is that harbour managers must either dredge so that the top of the fluid mud, or a prescribed density of mud, is at the required nautical depth. However, both solutions fail to optimise dredging and the latter additionally increases the uncertainty that nautical depth has been achieved (Claeys 2008).

Overview of Current Systems

There are no current methods to map the rheological transition zone outside of laboratory analysis, however several methods are currently being used to approximate the nautical depth in the presence of fluid mud, including:

  • Dual frequency echo sounders
  • The NaviTracker
  • Prick probes
  • Hybrid methods.

These methods are briefly discussed in turn here, along with problems associated with each method.

Dual Frequency Echo Sounder

One of the most common methods currently used to approximate nautical depth is a dual frequency echo sounder operating at frequencies between approximately 33 kHz and 210 kHz. It is generally accepted that the 210 kHz signal reflects primarily off the top of the fluid mud layer (Delefortrie, G. and Vantorre, M. 2005b) and thus provides a reliable indicator of the water / fluid mud boundary. The lower frequency 33 kHz signal does penetrate the fluid mud layer, however the results tend to be unstable and unreliable as the signal reflects, apparently randomly, from different reflectors within the mud and indeed behaves differently depending on the rheological characteristics of the mud present.

The NaviTracker

The NaviTracker is a device developed since 1987 by Baggerwerken Decloedt NV, capable of measuring density values within the fluid mud layer. Based on these density measurements the height of the nautical depth in various ports has been defined in Europe and around the world with examples provided in Table 1.

Table 1 Permissible mud densities that supposedly define nautical depths within European ports

Port Mud density at nautical depth (kg/m3)
Zeebrugge 1200
Rotterdam 1200
Nantes, Bordeaux 1200
Germany 1180 – 1250

The NaviTracker, however, is not capable of determining the level of the solid mud underlying the fluid mud layer. The probe would be damaged or lost if it were to penetrate the solid mud layer. Also, as discussed, density measurements are not always appropriate for determining the nautical depth within a fluid mud / solid mud regime as in many situations the rheological discontinuity does not coincide with a density discontinuity.

Typical density logs showing the echo sounder (210 and 33kHz) and rheocable (Nautical Depth) results demonstrating that neither density logs nor the 33kHz results are able to provide results suitable for optimizing dredging (Druyts etal 2010).

Prick probes

These probes are lowered through the fluid mud layer while simultaneously measuring density or viscosity values. As discussed density profiles cannot determine the true nautical depth (rheological transition level) but only vertical density variations. Viscosity profiles as measured, for example, using the RheoTune can be considered more appropriate and accurate in defining the true nautical depth as compared to density profiles. However, as viscosity probe measurements tend to be time consuming and cumbersome, they are less suited for routine survey campaigns in ports.

Hybrid methods

Methods have been developed that utilise both prick probe measurements and acoustic signals. The aim is to determine the rheological transition level by using prick probe results as a calibration for the acoustic measurements after advanced processing. Examples of such hybrid methods are provided by Stema BV (Silas software and DensiTune probe) and Norbert Greiser (Admodus – survey system). However hybrid methods have yet to prove successful in the determination of the nautical depth in the presence of fluid mud.

Principles of Operation

A reliable, reproducible and quick method to determine the nautical depth as defined by the rheological transition level should be based upon the rheological properties (viscosity) of the mud present rather than density. For routine applications the method additionally needs to be applied in a continuous manner at normal survey speeds (3 to 5 knots). TRM comprises the Nautic Sounding Array (Figure 1) in addition to processing and acquisition software.

The Nautic Sounding Array

The Nautic Sounding Array of the Rheocable method.

A sensor package is dragged behind the survey vessel at the end of an umbilical (data) cable. The sensor package comprises a pressure gauge and weights. The pressure sensor is placed in a sealed pressure pod with 2 circulation tubes reaching above the fluid mud layer to ensure the correct translation of pressure measurements into water depths based on the known density of seawater. The water density is continuously measured at several levels along the umbilical cable using CTD (Conductivity, Temperature and Depth) probes (during post processing, pressure is further compensated for atmospheric pressure). Following the sensor package is a short resistivity cable. The resistivity cable is used to verify that the sensor package is travelling on the fluid / solid mud interface and not floating above it.

 

Table 2 Typical resistivity indicators determined during the survey and in post processing.

Medium Resistivity (Ohmm)
Seawater 0.26
Fluid Mud 0.26 – 0.32
Solid Mud >0.32

The key to the Rheocable method is the real time verification that the sensor package is travelling along the rheological interface. Verification is achieved by monitoring real time resistivity values returned to the topside via the umbilical. If the cable is located on the solid mud layer relatively high resistivity values are measured. However, should the cable be floating within or above the solid/fluid mud interface, lower resistivity values are observed, corresponding to fluid mud and/or seawater. The vessel speed is adjusted to allow for the maximum survey speed without the cable floating. The high viscosity of the solid mud keeps the moving sensor package on the top the solid mud layer. Resistivity readings are gathered every 2 seconds and therefore the vertical position of the sensor package is continually monitored and vessel speeds can quickly be adjusted if the array starts to float.

Data Processing and Interpretation

As discussed, the Nautic Sounding Array is continually registering depth via the pressure sensor The position of Nautic Sounding Array, relative to the Nautic depth, is monitored via resistivity measurements. At the conclusion of the survey, the key parameters at the port are determined (Table 2 and Figure 4).

Sorted resistivity values indicating the interface at 0.32Ohmm

These results are derived when resistivity values measured over a limited period of time. The results are sorted from low to high values. This curve shows a distinct discontinuity with low values corresponding to 0.26 Ohmm seawater resistivity value measured with a cable floating above the solid/fluid mud interface and resistivity values above 0.32 Ohmm measured with the cable on the solid/fluid mud interface. Resistivity values between 0.26 and 0.32 Ohmm are rare or absent. Nautic depth determinations measured in combination with resistivity values greater than 0.32 Ohmm can thus be safely considered to represent data acquired with the pressure sensor on the solid/fluid mud interface.

Here the dredge level is mapped in 3D (red) with the fluid mud fraction (yellow) and the solid mud fraction (orange) above.

Once the key parameters are determined for the port, resistivity values for the survey are mapped to ensure that the pressure sensor readings were collected at the nautical bottom (Figure 5). Any pressure readings not collected at the nautical bottom can either be corrected or discarded.

Once it has been determined that pressure readings have been collected at the nautical bottom, pressure readings can then be corrected for tide and barometric pressure and then hung to the required datum (Figure 6). Finally, based on the derived nautical depth (Figure 6) and the required depth, volumes can be determined.

Conclusions

The RheoCable Method is a proven and efficient method for determining the nautical depth as defined by the discontinuity in the vertical viscosity profile, separating fluid mud and solid mud. The method allows, for the first time, port managers to confidently optimise dredging operations by knowing exactly where the solid mud begins. In addition, if 210KHz single beam data is collected utilising an echo sounder, the thickness of the mud layer can also be determined.

The Future of Dredge Management in the Presence of Fluid Mud?

The unique ability of the RheoCable method to map the true nautical depth in the presence of fluid mud opes the possibility for new and efficient dredge management. A suggested process could be:

  1. An “in-survey” combined with a 210kHz survey would provide the thickness and depth of, and to, the liquid mud layer as well as the depth to the nautical bottom. A limited geotechnical study could then be undertaken to confirm the RheoCable results.
  2. Once the RheoCable results are confirmed, the volume of sediment to be extracted (i.e. only the consolidated mud, not the fluid mud) to achieve the required nautical depth can be calculated.
  3. The dredge requirements can then be put to tender and the appropriate contractor engaged.
  4. At the completion of the survey a RheoCable survey can be rerun (out-survey). If the required nautical depth has been achieved, throughout the survey area, the volume dredged can be calculated and the contractor paid accordingly.

References

Claeys, S. (2006). Evaluation and combination of techniques used to determine the nautical bottom: a call for rheology based instruments, in: (2006). Evolutions in hydrography, 6th – 9th November 2006, Provincial House Antwerp, Belgium: Proceedings of the 15th International Congress of the International Federation of Hydrographic Societies. Special Publication of the Hydrographic Society, 55: pp. 141-144.
Claeys, S (2008). Evaluation of nautical bottom detection techniques (2008). Sea Tecnnology, Compass Publications, Arlington VA, USA. July 2008.
Clandillon-Baker, J (2008). Squat – Part 2 Mud Navigation and negative under keel clearance. The Pilot, United Kingdom Maritime Pilot’s Association. April 2008 N0 293.
Delefortrie, G and Vantorre, M (2005)a. Modelling navigation in muddy areas, in: (2005). Hydronav’05 – Manoeuvring ’05: Joint 16th International Conference on Hydrodynamics in Ship Design and 3rd International Symposium on Ship Manoeuvring, Ostroda, Poland, September 2005. pp. 49-58
Delefortrie, G. and Vantorre, M. (2005)b. Revision of the nautical bottom concept of Zeebrugge based on the manoeuvrability of deep-drafted container ships. Proceedings of the CEDA Dredging Days 2005.
Druyts M, Brabers P and Errey J (2010). The Rheo Cable Method: Determining Nautical Depth in the Presence of Fluid Mud.THV Nautec
PIANC (1997) Approach channels – A guide for design, Final report of the joint Working Group PIANC and IAPH, in cooperation with IMPA and IALA. Supplement to PIANC Bulletin, No. 95, 108 pp.

Additional Resources

MDCE bvba and Demco nv are partners in the joint venture ‘THV Nautic’, OEMG operates the system in the Australasian market. This joint venture promotes and commercialises the innovative survey system jointly developed by the partners for the detection of the Nautical Depth in areas where fluid mud and consolidating mud are present and where this areas need to be dredged regularly. Papers presented here (apart from Driyts. M, Brabers. P & Errey. J (2010)) are translations from the original Dutch texts written during the development of the method.

Druyts. M, Brabers. P & Errey. J (2010) The Rheocable Method: Determining Nautical Depth in the Presence of Fluid Mud. Download the article here.

 

 

 

Druyts, M & Brabers P (2010) Test survey of the Nautical Bottom in Zeebrugge (Belgium), witnessed by the surveyors of the Flemish Hydrographic Services, with the Rheocable survey method: final report. (Dutch). Download the article here.

 

 

Druyts, M & Brabers P (2010) Presentation of Rheocable sounding of Nautical Depth. Download the article here.

Druyts, M & Brabers P (2010) Measuring the Rheological Behaviour Transition: Notes on Scientific Background and Mathematical Cable Model. Download the article here.

 

 

Druyts, M & Brabers P (2010) Measuring the rheological transition by the Rheocable sounding method: Report of the tests carried out in the laboratories of Flanders Hydraulics Research. Download the article here.

 

 

Druyts, M & Brabers P (2010) Rheological transition RheoCable dredging strategy. A new dredging strategy based on the application of the RheoCable method. Download the article here.