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Volume 7 Issue 1
September 2012

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Houston Spotlight

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Delaware Article Pic For offshore energy exploration and production, real-time deep water current data is an invaluable asset for effective and safe drilling operations. Beneath the surface of the ocean, discrete masses of water move at different speeds at different depths. These strong currents can present hazards to drilling operations, especially as drilling moves into increasingly deeper water. The forces on risers can increase dramatically when impacted by currents associated with the Loop Current and Loop Current Eddies in the Gulf of Mexico. Current data can also be used to develop defensible engineering parameters for the design of offshore facilities. Both real-time and historical oceanographic and meteorological data are critical components of daily drilling operations and long-term planning.

Woods Hole Group has a long history of designing, installing and operating environmental measurement systems on a variety of platforms. These include fixed production platforms and FPSOs on the continental shelf and semi-submersibles and drillships in deep water. Tradition continues with the award of contracts for ADCP systems on six newbuild drilling rigs in the Gulf of Mexico. In each case, the design of the ADCP system accounts for the features particular to each rig, such as system location and local vessel structures, so the system will collect high quality data at reasonable long term cost with limited maintenance and downtime.

Two recent projects included the design, fabrication, installation and commissioning of ADCP deployment systems on two drill ships. The operator has a long term charter of these drillships and is using them in the deep waters of the Gulf of Mexico. The large size of the moon pool on these drillships made a track mounted ADCP deployment system in the moon pool feasible. These deployment systems are improved versions of the unique moon pool deployment system first developed by Woods Hole Group in 2000 for the Transocean Discoverer Spirit.

The ADCP is mounted in a carriage, which rides a sled along the rails to its deployed position at the base of the moon pool. Compared to more conventional “over the side” systems, rail mounted moon pool systems provide more accurate data, require fewer service visits, and allow the ADCP to remain in place collecting operational data during severe storms. Another form of rail-based deployment system was installed in June 2012 on a new build drillship now operating in the Gulf of Mexico. The rail system in this case was mounted on the starboard side of the PSA and took advantage of existing hull-mounted but unused ROV fixtures. This system also enjoys the advantages of more accurate data and fewer service visits because of the strength and stability provided by the combination of rails and ADCP sled.

Another project was to design, fabricate, install, commission, monitor and service ADCP systems on two of Pride International’s (now Ensco) sixth generation, ultra deepwater drillships. One system was installed in the Gulf of Mexico, and the system was commissioned in Brazil, where it was supported by Woods Hole Group do Brasil. The second ADCP system will be installed in the Gulf of Mexico later this year.

The identical measurement systems on the Ensco drillships involve a downward-profiling TRDI 38kHz Ocean Observer ADCP and an upward-profiling TRDI 300kHz Workhorse ADCP. Together these sensors obtain current data from the near surface to a depth of approximately 1000 meters. The ADCPs are mounted on a rigid frame suspended from an articulated gantry mounted on a cantilevered platform projecting from an upper deck of the drillship. It is difficult to position the ADCPs far enough away to avoid the local distortion of the earth’s magnetic field caused by the steel hulls. That distortion would result in unknown but potentially large and variable compass direction errors. Therefore, these ADCP systems are suspended from two electro-mechanical cables with sufficient separation to hold the frame parallel to the hull. Similar to rail mounting, this allows the system to use the ship’s heading instead of the ADCP’s compass to determine current direction.

In the Gulf of Mexico the current profile data are transmitted in near real-time to the National Data Buoy Center (NDBC) in accordance with the requirements of Bureau of Safety and Environmental Enforcement (BSEE) NTL No. 2009-G02. Data are also transmitted to Woods Hole Group for system performance monitoring. These systems are among the first to utilize Teledyne-RDI’s underwater electronics housing for the 38kHz Ocean Observer. This new feature permits the use of robust electro-mechanical support cables, rather than separate suspension and electrical cables, providing increased reliability and ease of deployment. The current profiles collected with these ADCP systems will support drilling and handling operations in near real-time, provide reliable, trustworthy measurements of ocean characteristics for rig designers and safety planners, and fulfill the BSEE requirement to report current profiles from drilling and production platforms operating in water depths greater than 1000 meters in near real-time.

Delaware Article Pic Woods Hole Group is currently completing fabrication and testing of an ADCP system on a new build rig recently arrived in the Gulf of Mexico. Installation of the system is expected during the fall of 2012. The rig is a dynamically positioned, deep-water, semi-submersible drill rig. Because the dynamically positioned rig will normally be pointed into the current, the relative flow will usually be from bow to stern, allowing the use of a single-cable suspension system. The ADCP frame is positioned in “clean” water, below the wakes of the pontoons and the thrusters. The location is far enough from the components of the hull so the internal ADCP compass will reliably determine current direction. The latter is important because single-wire suspension does not prevent the frame from turning. Upward- and downward-looking ADCPs will be used to measure current profiles from the near-surface to a depth of approximately 1000 meters with near real-time transmission of the data to NDBC. In addition to systems on rigs and drill ships, Woods Hole Group is developing real-time data solutions using moorings and buoys, which will be featured in a future newsletter.


Submitted By:
David Szabo
Houston Manager

Contact Information:
Woods Hole Group – Houston
12705 South Kirkwood, Suite 100,
Stafford, TX 77477
P: (713) 468-5075
e: David Szabo, M.S.

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