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Basics of Estimating Water Depth In Deep Water
Highly accurate mean water depth measurements are needed in a number of maritime applications, for example, in connection with the design of seafloor structures and tendons for deepwater Tension Leg Platforms.  Errors in water depth estimates can lead to costly errors in construction.  Woods Hole Group tackles the task of depth measurement using a different methodology depending on the water depth and required accuracy of the result, aiming to achieve the most cost-effective accurate solution.

For floating production platforms, the accuracy of the measurement is expected to be within a foot or less in water depths of up to 10,000 ft (3,000 m). This requires a depth measurement accuracy of 0.01%. This requires specialized measurement techniques expertise, and has to be supported by realistic estimates of uncertainty.  The problem is not straightforward since there is no simple direct measurement technique (i.e., long tape measure), so indirect measurement techniques need to be used.  A second complication is that water depth at a location is not fixed in time.  Water depth fluctuations occur at time scales of seconds (waves), hours (atmospheric pressure, tides), days to weeks (eddy or topographic wave passage), seasons (heating and cooling), years (global warming).

One accepted ‘indirect’ measurement techniques for determining water depth is by measuring the transit time of sound through the water column with an echo sounder.   However, this technique is not applicable when high accuracy measurements are required because an echo sounder in deep water is typically accurate to only 0.2% of full depth, which would translate into 7-foot uncertainty if the water depth is about 1000m.

A preferred solution can be achieved using a temperature-compensated pressure sensor such as a Paroscientific DigiQuartz sensor.  This type of pressure sensor provides accuracy of pressure measurement up to 0.01% of full scale range, and repeatability of 0.005% of full range.  If the average water density in known to the same accuracy, then the depth can be calculated. The fact that the sensor accuracy formally satisfies the requirements of the industry does not, however, guarantee that depth measurements using this sensor will have the same level of accuracy.  Other factors, such as varying water density and atmospheric pressure, and spatial changes in the acceleration due to gravity, add error and uncertainty as well.  Specialized experience is required to overcome and understand the various sources of error and uncertainty.

For instance, the average density of the water column is a function of the temperature and salinity profile through the water column.  Modern high-precision CTD (conductivity / temperature / depth) sensors must be used to measure the density profile to remain within the required water depth accuracy range.  Using such an instrument limits the depth uncertainty due to the varying water density to about an inch.  Our experience shows that using monthly mean temperature and salinity profiles adds about a foot to possible error.

Precise in-situ measurements of atmospheric pressure are less critical since the value of the atmospheric pressure can typically be estimated from a weather map or a nearby weather station with sufficient accuracy.  The uncertainty of this estimate translates into a depth measurement error of less than one inch.

In an ideal environment; that is, when the bottom is relatively flat (i.e., there is no need to be precise in terms of the horizontal position of the pressure reading), the most cost-effective solution for estimating the water depth is to sample through the water column using a CTD equipped with an echo sounder to take a precise measurement of the distance to the seabed (mudline) when the probe is near the bottom.  However, in the real environment, collecting the data at exactly the right place is of a paramount importance; therefore, the CTD probe is usually installed on an ROV that is used to probe the water column.  Using an ROV makes it possible to bring the pressure sensor exactly to the position where the depth measurement has to be taken.  The absolute accuracy of such instantaneous water depth measurement will be limited to about one foot in approximately 1000 meters of water.

In most applications, however, collecting an instantaneous water depth measurement is not sufficient since the water depth at any location will vary in time due to a variety of physical processes.  Of importance to engineering design are the mean water depth and the range of variability about the mean, not an instantaneous depth.  The difference between the mean water depth and an instantaneous water depth is a superposition of tidal, meso-scale, seasonal, and interannual oscillations of the sea surface.  Among these processes, the barotropic tide and meso-scale eddies, both barotropic and baroclinic, play a major role.

Use of tidal models for estimating sea level at any specific moment in time is not recommended since the models for the deep ocean are not accurate enough for estimating the phase of the tide at any specific location.  Also, models simulate only a subset of the tidal constituents, while more constituents may be required to represent the actual tide.  Instead, a long-term deployment of a pressure sensor at the bottom can provide the needed information on tidal variability at the site.  The deployment duration depends on the number of tidal constituents to be resolved.  While deployment from one to several months allows the elimination of tides from the estimate of a mean depth, there are other processes occurring on longer time scales that need to be considered as well.

The water depth anomaly associated with meso-scale variability of the ocean (eddies and basin-scale currents) can be assessed using satellite altimetry.  These data have been routinely collected since 1993. Thus, an instantaneous water depth measurement can be referenced to the 15-year mean sea level.  Energetics of major physical processes have to be examined to provide an estimate of the range of water depth variability around the mean.

In summary, a combination of specialized scientific experience, equipment, and practical in-field skills is required to collect accurate water depth measurements for construction purposes in the deep ocean.

 

Leonid Ivanov, Ph.D., Physical Oceanographer
Dr. Ivanov has more than 20 years experience worldwide in open ocean and coastal oceanography. Dr. Ivanov has conducted field studies and research in the eastern Tropical Atlantic focusing on the investigation of physical mechanisms driving seasonal and meso-scale ocean variability, as well as baroclinic and barotropic tides, inertia-gravity waves and coastal fronts off West Africa. 


David Szabo, Houston Regional Manager/Senior Oceanographer
Mr. Szabo earned his M.S. in oceanography from Florida State University and his B.S. in meteorology and oceanography from New York University.  Between earning his degrees he completed a three year commission as an officer (LTJG) in the US Coast Guard and served as a Military Oceanographer.  Through his work experience Mr. Szabo has developed a broad practical knowledge of the application of metocean research in engineering applications.  His experiences range from design and management of major oceanographic measurement programs to validation of numerical wave and current models for engineering use.
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