Rodney Riley

Enhancements to NDBC's Digital Directional Wave Module

The present paper describes advancements that the National Data Buoy Center (NDBC) has achieved recently in improving its Digital Directional Wave Module (DDWM). Using the 3DM-GX1® (MicroStrain®, inc.) in the DDWM has expanded wave-measuring capability by addition of a triaxial acceleration package in place of a single, along-mast accelerometer used in the older NDBC wave system called the Angular Rate Sensor (ARS). NDBC has initiated two important changes in onboard processing. First, we transform along-mast accelerations to true vertical accelerations using measurements of pitch and roll angle. This change eliminates potential errors arising from a constant buoy heel angle. Second, NDBC has changed from a linear noise correction to an ƒ −2 inverse power function. The NDBC Engineering Laboratory has carefully tested these changes using apparatus of the Dr. Ed Michelena Sensor Test Facility, NDBCs latest mechanical wave machine, the Desktop Wave Simulator (DTWS) and a detailed electronic computational wave model, the Directional Wave Simulation Software (DWSS). The DWSS was developed to examine extreme conditions that cannot be simulated in the laboratory. It also provides a method to obtain a known input for end-to-end system testing or evaluating future on-board processing modifications. In addition, triaxial accelerations are used to determine the average pitch and roll angles of the platform without need of a separate tilt sensor. The latest version of the DDWM can be used in either directional or nondirectional modes. A compact flash memory unit is now installed as a standard feature on all DDWM units, providing high-resolution records of raw measurements.

Performance of the nortek Aquadopp Z-Cell Profiler on a NOAA surface buoy

Observations of current velocity in near-surface and near-bottom boundary layers are critically important for many scientific, operational, and engineering applications. Nortek developed the Aquadopp Z-Cell Profiler, a dual-frequency, six-beam acoustic Doppler current profiler, to meet the needs of observing the complete water column velocity profile, including the near-surface or near-bottom currents. The Aquadopp Z-Cell Profiler employs three acoustic beams directed horizontally and spaced equally around the circumference of the profiler with 120 deg spacing between the beams. These beams measure the two-component horizontal currents at the level of the instrument (cell zero), thereby eliminating the common blanking distance associated with standard ADCPs. Near-surface and water column current velocity profile observations from a Z-Cell Profiler mounted on a NOAA NDBC 3 m discus buoy (located in the northern Gulf of Mexico) are compared with current velocity profile measurements from a bottom mounted 600 kHz Nortek AWAC and 600 kHz Teledyne RD Instruments Workhorse acoustic Doppler current profiler. A tidal analysis suggests that velocity data from the horizontal beams (cell zero) are of good quality and consistent in direction and magnitude with the velocity measurements in cells below, with the AWAC and Workhorse velocity, and with theory. Several cases are presented that indicate the measurements in cell zero are important to make independent of velocity lower in the water column in order to correctly characterize the flow regime. Current speed and direction differences between cell zero and lower cells project a horizontal spatial separation of water parcels as much as 20 km/day, with a mean separation of 8.5 km/day.

Improved quality of National Data Buoy Center (NDBC) Acoustic Doppler Current Profiler (ADCP) measurements

In 2006, NOAAs National Data Buoy Center (NDBC) began an effort to add ocean sensors (directional waves, surface currents, current profiles, and ocean temperature and salinity) to its fleet of weather buoys. In this paper, we report on the improvements in the quality of ocean currents collected from the Acoustic Doppler Current Profiler (ADCP). Initially, the ADCP was deployed in a cage that was suspended below the bridle of the buoy. It was an effort to remove the motion of the surface buoy from the current record. Measurements were made at intervals of 30 to 45 seconds over an hours time and averaged. The cage required a lengthy cable to carry the current information to the payload within the buoy. The movement (twisting, swinging, and upand-down motion) of the cage put stress on the cable, which was also exposed to long line gear and other fishing tackle, and the failure rate was high. Several iterations of cable were attempted, but the failure rate of the real-time currents remained high. Because of the averaging method, the currents were under-sampled and represented a considerable period of the changing tidal current. The ADCP was moved inside the bridle just below the buoy to shorten the exposure of the cable to the environment. An additional result of this method is to sample closer to the ocean surface. The sampling interval was increased to 0.5 or 1.0 Hz and the samples are averaged over five minutes. The resulting measurements represent a more appropriate instantaneous current speed and direction and average out the motion of the buoy. A comparison of identically configured buoyand bottom-mounted ADCPs at Station 41036 off the North Carolina Coast indicate that the methodology produces statistically equivalent currents at all but the surface and bottom bins [1]. ADCP data from a large number of oil and gas platforms in the northern Gulf of Mexico are collected, quality controlled, and disseminated to the public by NDBC. The quality control algorithms were developed by a consortium of oil industry, ADCP vendor, and Minerals Management Service experts and implemented by NDBC to quality control the realtime ADCP data in March 2006 [2]. The algorithms were developed for low frequency (38 and 75 kHz) Teledyne RD Instruments (TRDI) ADCPs. The algorithms have been expanded to include the 300, 600, and 1200 kHz TRDI ADCPs. The algorithms test for echo amplitude, percent good beams, error velocity, and horizontal velocities. Additionally, a test to determine the presence of the surface or bottom allows these bins to be included in the data stream as the tide changes. These algorithms have been implemented into NDBCs realtime processing stream.

Test results of the SeaKeepers ocean sensor module on remote platforms

The National Data Buoy Center (NDBC) and the nonprofit International SeaKeepers Society have adapted the SeaKeepers Ocean Sensor Module (OSM) to NDBC platforms and data collection systems. The reason for this effort was to develop a low maintenance system for oceanographic data collection and transmission from remote platforms. The OSM has been integrated into a 3-meter buoy and underwent nine months of dockside testing for performance and durability of the electronics and antifoulant system and to verify the quality of the data. It was deployed in the Gulf of Mexico near buoy station 42007 for verification of that stations meteorological data. Test results have shown that the OSM and antifoulant system performed well in remote, harsh conditions. The results are presented in this paper.

Adaptation of the Sea Keepers Ocean Sensor Module to NDBC platforms

The nonprofit International SeaKeepers Society has developed an automated marine observing system to collect atmospheric and oceanic data from ships while underway. These systems have been installed on large ocean going vessels from commercial shippers to private yachts. The system collects, processes, and transmits the data through a satellite system. The National Data Buoy Center (NDBC) has partnered with the SeaKeepers Society to adapt and integrate the ocean portion (Ocean Sensor Module) of this system to buoys, piers, and offshore towers. The Ocean Sensor Module is a waterproof box measuring 30/spl times/16/spl times/10 inches that can house up to five (5) multiparameter sensor units. The electronics (Computer Module) are housed in a separate container that measures 18/spl times/16/spl times/10 inches, and the water is sampled through a pumping system. The system is relatively maintenance free, and because of this, a pilot project was established to adapt the ocean sensor module for use on 3and 6-meter buoys. The new design includes reduction in power consumption and space and development of a low-maintenance, low-power pumping system. The sensor box and pump are externally mounted. An NDBC Oceanographic Sensor Interface Controller (NOSIC) replaces the shipboard NT operating system computer version; it will be internally mounted in the OSM and will interface with the buoy electronics. The NOSIC will interface with the sensors to collect, process, store, and transmit the data to the buoy payload for later satellite transmission through GOES or other satellite communication modes. The unit also contains an electrochemical halogen generating anti-fouling device that was developed by the University of Miamis Rosensteil School of Marine and Atmospheric Sciences. The generated halogen compounds will clean the wetted components, thus extending the cleaning cycle. The details of the design and the results of a fully integrated dockside test on a 3-meter buoy will be presented.