OEMG Fluid Mud Mapping is a new and unique method for determining the nautical depth in the presence of fluid mud. Breaking away from traditional acoustic and density based systems, OEMG Fluid Mud Mapping relies on viscosity changes at the fluid mud / solid mud interface to precisely map this rheological boundary.
What is the significance of Nautical depth?
The behaviour of ships sailing and maneuvering 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 maneuvers.
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 maneuverability, 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 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)|
|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.
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.
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). OEMG Fluid Mud Mapping comprises the Nautic Sounding Array (Figure 1) in addition to processing and acquisition software.
The sounding array
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 traveling on the fluid / solid mud interface and not floating above it.
Table 2 Typical resistivity indicators determined during the survey and in post processing.
|Fluid Mud||0.26 – 0.32|
The key to OEMG’s Fluid Mud Mapping is the real time verification that the sensor package is traveling 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 approximately 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 Sounding Array is continually registering depth via the pressure sensor The position of 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).
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.
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.
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.