ADCP Deployments near Offshore Structures

A recent comparison study of ADCPs against a nearby mooring was carried out by the Woods Hole Group (WHG) under a contract to the combined Climate And Simulation of Eddies and Eddy Joint Industry Projects (CASE/EJIP) offshore industry groups. The ADCPs were deployed at 2,000 m depth in the Gulf of Mexico and positioned approximately 65 m from a platform riser, which was the limit of the rig ROV´s tether. The reference data was provided by a string of moored current meters located approximately 1 km from the platform. A goal of the comparison was to establish the profiling range of ADCPs in the low backscatter waters of the deep Gulf of Mexico. It was shown that the usable measurement range was reduced a substantially smaller amount than had been feared. The single-ping data collected on the ADCPs revealed the presence of multiple, substantial acoustic interference terms. We show that these terms can be reduced or eliminated with the use of the data quality screening criteria currently employed by the National Data Buoy Center on all offshore Gulf of Mexico ADCP data made available under a Notice to Lessees from the Minerals Management Service (MMS). In addition, and perhaps most importantly, the Long Ranger 75 kHz ADCPs were shown to have velocity measurements that were biased low in relation to the moored current meters. We will show that data collected with riser interference inside a 15deg cone of the ADCP transducer beam main lobe will cause this bias. We will illustrate through data collected in the study and data provided by Fugro GEOS how this interference biases the data and how it can be avoided through the orientation of the ADCP.