Nautical depth: RheoTune a portable density probe and yield strenght

Nautical depth: RheoTune a portable density probe and yield strenght

The portable Rheotune provides end users with the most accurate nautical depth measurement by using the combination of echo sounders
and density and yieldstrenght measurement equipment.

: Density and yield stress measurement with Rheology

Deliverables:
• Properties of fluid mud
• Yield stress profile
• Density profiles

The RheoTune is a versatile and portable system that provides both density as well as
yield stress profiles of fluid mud simultaneously.

The fact that the RheoTune is pre-calibrated makes it easy and quick to deploy, enabling swift and accurate
data acquisition. Running proprietary software for both acquisition and processing results in a detailed almost
real time display of the profile that can be imported into sub–bottom profiling processing suites enabling a full
and comprehensive picture of the fluid mud and its characteristics.
To increase productivity further an automated electrical winch or manual light weight winch with slip ring is
available. The Tune system is optimized to integrate with the Silas Seismic Processing Suite.

Key features
• Constant accurate results
• Easy to deploy and operate
• Pre-calibrated

• Manual operation from the smallest of vessels
• 24V battery operation
• wifi data transmission

The RheoTune serves to:

• Determine the nautical depth in navigation channels.
• Determine density and yield stress of silt layers in dredge & dump areas.
• Accurate monitoring of siltation in ports and marine traffic areas.

The RheoTune is optimized to integrate with the SILAS acoustic profiling system. The SILAS on-line density mapper allows rapid tracing of critical locations, that require in-situ probing.

Optional there is the Intelliwinch, a winch developed for time-scheduled automated measurements.

NAUTICAL DEPTH

General principle

Navigational channels and harbors are often covered with a fluid mud layer, which is characterized
by a low density and weak shear stress (Delefortrie et. al., 2004). When this water-mud interface is considered
as the actual bottom the navigation depth will frequently be much shallower than the one required.
In these conditions it is better to define a nautical depth 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 maneuverability” (PIANC, 1997).
This depth is usually defined by a physical level of a certain density within the fluid mud layer (Van
Craenenbroeck et. al., 1998). As the physical characteristics of mud vary, the critical density used to
determine the nautical depth is site specific. For instance muds from Guyana, Southern America show
a very low yield stress at densities as high as 1.5 ton/m3, while North European harbours show a significant
yield stress increase at density levels varying between 1.15 and 1.25 ton/m3 (Fontein, Werner,
Van Der Wal, 2006). Therefore the decision to pass through a particular mud should not be based on its
density alone, but by yield stress values as well, as will be explained later in this paper. Due to influences
of nature, the actual nautical depth will vary over time as well and a proper monitoring system
is required.
Traditional survey techniques are not capable of detecting multiple density levels or yield stress and
will only acquire the top of the fluid mud layer. An ultra-high-resolution sub-bottom profiler in combination
with in-situ density and yield stress measurements will be able to capture the full profile and
detect all required density levels. The recommended operating principle is discussed below.

Recommended operating principle
Traditional survey techniques consist of using a Single beam or Multi beam echo-sounder with a relative
high frequency which is unable to penetrate to the bottom of the fluid mud layer. Figure 1 displays the
clear difference in penetration between traditional Multi beam surveys (black and green lines) and
the preferred use of a high-resolution sub-bottom profiler (blue and red lines). In the morning the top of
the fluid mud layer is marked by the blue line, while the base is marked by the red line, according to the
sub-bottom profiler. The Multi beam echo sounder measures only the top (black line), but is inconsistent
and penetration of top of the fluid mud is too high. On the other hand it is unable to penetrate towards
the bottom of the fluid mud layer. In the afternoon the top of the fluid mud is marked by the blue circles,
while the base is marked by the red circles, according to the sub-bottom profiler. The red circles align with
the red line, showing consistency over time in measurements of the bottom of the fluid mud with this
technique. The green line represents the Multi beam echo sounder measurements in the afternoon. At
this time the Multi beam has too little penetration to detect even the top of the fluid mud. The difference
between both Multi beam surveys – morning (black line) and afternoon (green line) – shows that this traditional
survey can give variable outcomes in one day only and should not be considered reliable. The way
forward is to use a combination of a high-resolution sub-bottom profiler and an in-situ density profiler
which is able to measure both density and yield stress.

Download White Paper on Nautical depth