Integrated Marine Monitoring System - BMT Group

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7.3 Current <strong>Monitoring</strong> Three types of current monitoring will be discussed: • Current Profiles between the Surface and the bottom of the platform • Deep current profiles (below the Keel) • Deep Current Profiles from the sea floor up These options are shown schematically in Figure 5 for a semi-submersible. SEA SURFACE Water Depth approx. 1900m Long range horizontal ADCP MID-DEPTH. 28 measurement bins of 16m length Riser Mounted ADCP ADCP 468m ASB LMRP Mounted ADCP ADCP 20m ASB BOP & LMRP UPPER. ADCP 30m below MSL UPPER. First bin: 72m below MSL MID-DEPTH. Depth of last valid bin: 924m ASB / 976m below MSL MID-DEPTH. First bin: 492m ASB SEABED. Last valid bin: 476m ASB SEABED. First bin: 44m ASB Rio Oil & Gas Expo and Conference 2010 UPPER. 33 measurement bins of 32m length UPPER. Depth of last valid bin:1096m below MSL/ 804m ASB SEABED. 28 measurement bins of 16m length Figure 5. Example of possible options for Current <strong>Monitoring</strong> on a Semi-Submersible Surface to Platform Keel For Spars, it is very difficult to obtain an uncontaminated profile from the water surface to the bottom of the hard tank (–250 feet MSL). The flow field around the spar hull is disturbed significantly by its presence. Multiple Horizontal ADCPs that are intended to provide the current speed and direction in a river by “looking” out across the flow have been employed on numerous platforms. The range of these instruments is on the order of 200 meters. Current vectors in the plane of the device are relatively uncontaminated at or near the instrument’s extreme range. In some cases, the H-ADCPs have been installed on the hull within diver range of the surface or can be bundled with the downward looking profilers as is illustrated in Figure 6. In some cases, H-ADCPs have been installed with a small tilt to provide a profile that spans the hard tank with the full understanding that the observations from the bins close to the sensor may be contaminated by the wake of the hull. Such an installation is shown in Figure 7. For Truss Spars, the estimate of the current vectors from the hard tank to the bottom of the truss (soft tank) is less contaminated due to the sparse spacing of structural members. For TLPs and Semi Submersibles, the task of producing valid current measurements over the depth of the hull is much less difficult since the base line of the hulls are relatively shallow (50– 100 feet). 10
Rio Oil & Gas Expo and Conference 2010 Figure 6. Combination 38 kHz ADCP Deep Profiler and 300 kHz horizontal ADCP installation on West Africa FPSO Figure 7. Upward tilted 300 kHz H-ADCP mounted on Heave Plate at approximately 300 feet below the free surface of BP’s Mad Dog spar Platform Mounted Deep Profiles The 38 kHz ADCP from RD Instruments is the sensor that provides the deepest penetration available today in a single instrument and is recommended for installation on new platforms in deep water. The range of the instrument is nominally 1000 meters. The actual vertical penetration depends upon the inclination that may be required to prevent impingement of the beams on either the hull, production risers, mooring lines, steel catenary risers or umbilicals, etc. Figure 8 shows the final result of a beam pattern interference analysis for a typical installation on a Truss Spar. The resulting tilt of the sensor is 37 degrees and the actual vertical penetration below the sensor is 700 meters. Figure 9 is a photograph of a typical deployment fixture for a 38 kHz ADCP. Another alternative is to deploy a 75 KHz ADCP looking downward from a heave plate or pontoon. Figure 8. Example of Beam Pattern Interference Analysis for 38 kHz ADCP installation 11
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Integrated Marine Monitoring System - BMT Group

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