A comparison of two measurement systems designed to provide real-time current information in support of navigation

Freeport LNG Development, LP is designing and constructing an LNG receiving terminal in Freeport, Texas. The Port of Freeport wants to install a near real-time current monitoring system near the port entrance to assist pilots navigating the waterway. Knowledge of currents at the approach to the Freeport Entrance Channel is critical for the safety of the approaching vessels. Longshore currents are redirected around these jetties, creating intensified longshore (cross channel) currents just offshore of the jetties which makes alignment of vessels difficult. Having current data available in near real-time will allow the pilots to make information-based decisions, thus helping ensure safe navigation and improving the efficiency of the port by maximizing tonnage throughput. This paper presents the results of a comparison trial conducted jointly by Fugro GEOS and NOAA CO-OPS. This cooperative effort came about when Fugro GEOS contacted NOAA after being contracted by Freeport LNG Development, LP to perform an oceanographic survey at the Freeport Entrance Channel. Data from a Nortek Aquadopp mounted in NOAA´s new clamparatus system on Navigation Buoy 6 at the Freeport Entrance Channel are compared with those from an RDI Workhorse acoustic Doppler current profiler mounted in a seabed frame nearby. Seven clamparatus systems have been deployed in the Chesapeake Bay as an enhancement to the PORTS® there, and this was the first system installed in Texas. The clamparatus system provides a more cost-effective solution for obtaining current profile information in real-time and provides data within the navigation channel which is of most interest to the professional mariner. However, the data require full verification. Errors can be introduced by the magnetic effect of the steel structure of the buoy and its movement. A brief review of the two sets of instrumentation, mounting and measurement techniques is given. A thorough comparison of the direction data collected from both systems is presented. The mean of the absolute value of the difference in directions at 6.5m below the surface is 13.8° (standard deviation = 19.4°) when all data are included. The difference decreases to 8.7° (standard deviation = 7°) when only points with speeds- greater than 0.15 m/s are considered. Recommendations are made for future trials and system improvements.