Improved Current Profiling Systems for Offshore Rigs

By: Dr. Bruce A. Magnell, Scientific Director
Dr. Archie T. Morrison III, Senior Ocean Engineer
Oceanography and Measurement Systems
Woods Hole Group, Inc.

Exploratory oil rigs (Mobile Offshore Drilling Units or MODUs) and production platforms operating in the Gulf of Mexico are subject to the requirements of BSEE Notice to Lessees (NTL) 2009-G02. When operating in water depths greater than 400 m, operators must collect and deliver to a public archive, real-time current profiles covering near-surface to the seafloor or 1,000 m. The profiles support real-time operational decisions and are used by industry and government engineers designing offshore vessels and platforms. The industry is particularly interested in high-speed currents extending over a large depth range. Woods Hole Group works with offshore operators and manufacturers to develop safer, more reliable measurement systems.

The newest LARS installed on a semi-submersible and ready for deployment.

Profiling systems developed for the offshore industry over the past few decades commonly relied on the Teledyne RDI 38kHz Ocean Observer ADCP. The OO38 ADCP transducer unit is mounted on a frame (“sled”) and suspended in the water from a Launch and Recovery System (LARS). Single- and dual-cable LARS systems are used. Dual-cable sleds are assumed to align with the hull, allowing use of vessel heading in place of the ADCP compass, which may be compromised by the magnetic field of the MODU. An expensive, 47-conductor cable is used in most existing systems to connect the transducer to the TRDI Ocean Observer Deck Unit, which communicates with a networked computer. Raw binary data and meta-data are transmitted every 20 minutes to the National Data Buoy Center (NDBC) over the internet connection. Data displays may be provided at a limited number of locations on board.

Rig personnel are typically responsible for launch and recovery of the sled when the MODU changes location or maintenance is required. The OO38 cable is commonly attached to load-bearing wire rope with cable ties, manually applied during deployment and removed during recovery. Damage to the cable is not infrequent and the operation puts personnel at risk as they work at height over water. LARS winches, used to raise and lower the sled, can be difficult to maintain in the harsh marine environment. These safety, convenience, and reliability issues place a substantial burden on rig personnel. Cable damage, when it occurs, produces a data outage and consumes time and budget. The limited availability of real-time data displays on board (drill shack, bridge) and on-shore (operations centers, metocean contractor facilities) constrains data visualization, performance monitoring, and quality assurance.

Four years ago, Woods Hole Group began using TRDI’s new Under Water Electronics Assembly (UWEA), an underwater version of the OO38 Deck Unit. The UWEA is mounted on the sled and connected to the rest of the system with only 7 power and data conductors. Furthermore, we integrated those conductors and the load-bearing wire rope by using electro-mechanical cables and slip-ring equipped winches. These advances, unique to Woods Hole Group, minimize risk to personnel, substantially reduce the time required to launch and recover the sled, and reduce the previous high likelihood of cable damage.

The orange housing of the TRDI OO38 UWEA is shown mounted on the instrument sled with an upward-looking workhorse ADCP and the downward-looking OO38 transducer.

The use of the UWEA, electro-mechanical cables, and slip rings provides much greater reliability and ease of operation, but attention to winch characteristics becomes more important. The long-term reliability of the electro-mechanical cables requires a relatively large-diameter winch drum and a clean cable lay during recovery. Level-wind systems are complex mechanisms, difficult to maintain; a repeated poor lay, even of armored cable, can break internal data and power conductors. Variable elevation and azimuth wire angles at the winch complicate level-winding operation.

Recognizing these problems, Woods Hole Group engineers teamed with a specialized machining partner and developed a new design, specifically intended for current profiling systems. The winch utilizes a large-diameter, helically grooved drum, capable of storing the entire length of cable in a single layer. A large-diameter idler controls the wire angle approaching the drum and reliably forces a clean lay with one passively moving part, independent of sheave block position, with no operator intervention. Installed on Woods Hole Group’s latest current profiling system, the new design has proved reliable and greatly enhanced operator efficiency, safety and convenience. Personnel can operate the system with minimum risk and launch and recovery can be accomplished in minutes.

A view of the single lay, helically grooved drum and idler roller, a design unique to Woods Hole Group.

On most MODUs, a dedicated system control PC collects the instrument profiles, adds metadata, provides a display on the PC monitor, and transmits the measurements to NDBC. Unfortunately, the PC is seldom readily accessible to drillers or bridge officers, the personnel who stand to benefit the most from knowing the current structure under the keel in real-time. To help overcome this limitation, our systems provide a video output to the internal CCTV system, distributing text and graphic profiles throughout the rig, readily accessible to all personnel. Alternative time-series and other displays can be selected for CCTV distribution and the system supports data export in a variety of spreadsheet and graphical formats. In addition, Woods Hole Group personnel collaborate with client IT security personnel to establish secure remote access to the system control PC, enabling personnel to diagnose and resolve system issues quickly and without travel costs.

Rig personnel depend on real-time current measurements to guide operations and industry engineers and government regulatory agencies rely on historical data to develop design criteria and evaluate offshore structures. Woods Hole Group is working to enhance the operational and design utility of these data through the ongoing development of data visualization tools. These displays will be available on secure, client accessible websites, allowing client personnel anywhere, on or off the MODU, to visualize and analyze real-time and historical data. The displays created for the website also allow personnel to effectively monitor data quality and quickly resolve problems. The websites will also provide rig personnel with system documentation, including technical manuals and wiring schematics.

Looking forward, there are new instruments - such as the Nortek Signature 55 Acoustic Doppler Profiler - providing alternative solutions for MODU current profiling systems. Instrument selection requires consideration of factors such as profiling range, data quality, and cost. Woods Hole Group makes a point of designing systems and selecting instrumentation to successfully meet client measurement goals. Our team works closely with instrument manufacturers, to provide industry feedback and to understand the relative benefits of different products, so we can make the best choices for our clients.

Woods Hole Group is also designing a new single-cable LARS. These can be simpler and less expensive than dual-cable systems, but the ADCP is free to rotate, forcing dependence on the instrument compass, which is affected to an unknown degree by the magnetic field of the rig. To study this, we configured our most recent dual-cable system to record simultaneous rig and instrument heading measurements. Our team is actively analyzing this unique data set to determine the source and magnitude of compass errors. Our goal is accurate, empirically proven, current direction from a lower cost single-wire current profiling system. This will be one further step in Woods Hole Group’s continuing development of safer, more reliable oceanographic systems for the offshore oil and gas industry.