Radio beacon positioning for portable surveys, tracking
A minimum shift keying receiver that uses differential global positioning system data transmissions from marine radio beacons has been developed by Trimble Navigation of Sunnyvale, California (US) for portable operations.
The system is ideal for surveying, dredging operations, resource management, and operations that are conducted in transition zones where precise positioning is required. The system, known as ProBeacon, has an accuracy of less than one meter when using a Trimble 4000 Maxwell as a reference station or a GPS mobile receiver of similar capability.
The system's signal processing is based on a proprietary noise cancellation technique, which uses multiple channels to reject impulsive noise. It uses advanced computer logic, constantly monitoring message errors. When the signal degrades, the system will automatically switch to a different beacon.
ProBeacon uses broadcast beacon almanacs and GPS position data to find the nearest beacon. The system is equipped with an H-field antenna that does not need grounding. In addition to North American operations, the system can decode differential GPS transmissions broadcast in the UK and Ireland, as wells as most international standard broadcasts.
Deep submergence ROV begins operations
A remotely operated vehicle (ROV) dedicated to deepwater support operations where current velocities are high has begun work in the US Gulf of Mexico. The vehicle, known as the Maxrover, is a product of Deep Sea Systems International of Falmouth, Massachusetts (US). The vehicle has been tested twice on significant jobs:
- Drilling support: The first test of the vehicle was with American Oilfield Divers of Lafayette, Louisiana in support of drilling operations in Main Pass Block 255, where tidal currents provided a significant shakedown experience. The water depth at the site was 400 ft and currents up to 2.0 knots were experienced. Sea conditions up to sea state 5 were experienced.
The tasks included pushing and maneuvering casing strings into 30-in bell guides and inspection of various riser assemblies. The ROV also was used to inspect the wellhead during grouting operations. Deep Sea Systems engineers say the only unscheduled maintenance on the vehicle was a sheared propeller pin. The dual view cameras provide wide angle video for various tasks.
- Pipeline support: A pre-maintenance pipeline survey was the second major job. The ROV was deployed from a work boat using an A-frame. The Maxrover was flown down the riser leg of an adjacent platform, along the J-tube, and out the pipeline 1,400 ft to a crossover.
The pipeline also was at a depth of 400 ft, where currents were 0.75 knots. During the operation, the vehicle pulled 2,500 ft of neutral tether. The vessel was equipped to pull 6,000 ft of tether.
During the survey, the operators imaged with the scanning sonar debris near the pipeline, the location of an anchor cable, and overburden over the pipe.
Deep Sea Systems' Maxrover is capable of deepwater support operations in high velocity currents.
Camera features leak detector
A hand-held or fixed mount underwater video and search camera developed by J. W. Fishers of Berkley, Massachusetts (US) has a unique detector system that informs the diver and topside of water intrusion as small as one drop.
The DHC-1 camera can work in water depths up to 500 ft. An abrasion resistant urethane jacketed cable connects the camera to the surface. The video picture, black and white or color, can be displayed on any video monitor, television, or VCR.
Subsea mateable optical connector
A mating system for optical connections underwater has been developed by ECA of Toulon, France under contract for the European Commission. ECA has been involved with operations connections for electrohydraulic multiplex systems for remote control subsea wellheads and manifolds for over 10 years.
Long distance and multiphase pumping require data transmission standards that cannot be easily achieved by existing electrical connector capacities. The only solution is to shift to optical connectors. The ECA connector is dry mateable and can be used in up to 1,000-meter depths. It can be mated and unmated by diver or ROV. Rotating spheres and an optical gel serve to seal the connector, since water cannot be permitted to enter.
ECA's subsea mateable optical connector.
World record depth for reservoir monitor
Shell Offshore's A7 development well on the Auger prospect in the US Gulf of Mexico holds the world offshore depth record for a continuous reservoir monitoring system. The RCS system, built by Rohrback Cosasco Services of Broussard, Louisiana (US) was installed to a measured depth of 20,240 ft.
The electronic pressure monitoring installation surpassed a previous completion record on the A5 well, which was at 18,860 ft. The water depth at the Auger tension leg platform site is 2,860 ft.
The monitoring system uses a high resolution sensor to monitor reservoir pressure and temperature parameters, which are critical for reservoir management. The real-time parameters are monitored on site and at Shell Offshore's New Orleans facility.
Acoustic pig detectors installed in US Gulf
Subsea pig detectors, recently introduced by Fluenta of Norway, are being installed for the first time on two deepwater developments in the US Gulf of Mexico. The systems will be installed on Phillips Petroleum's Garden Banks 70/71 project and Enserch's Garden Banks 388 project.
- Phillips project: Seven units have been ordered for permanent installation. They will be integrated into a production control system to be installed by Kvaerner FSSL. Fluenta is supplying the transducers, electronics, and signal processing, which will be mounted on six-in. pipeline segments before being installed on the seabed. The production control system interfaces will be handled by Kvaerner FSSL.
- Enserch project: Five units will be positioned by an ROV on Enserch's floating production facility subsea systems. All will be mounted on the network of 12-in. gas condensate pipelines. The battery-operated detector remains in a passive condition until activated by an acoustic signal from the ROV. It can then detect the passage of a pig for up to 24 hours before being retrieved and returned to the surface. Detection of trains of up to five pigs are signaled to the ROV by means of coded light strobe pulses, with acoustic backup.
Through-metal acoustic telemetry
Strainstall Engineering Services of the UK is conducting tests of an acoustic telemetry system that acts through the metal of a platform or steel structure. The system is considered a replacement for expensive and vulnerable cabling and hydro-acoustic transponders that must overcome many noise problems.
The problem, according to Strainstall, is the splash zone, which presents many problems to conventional communications means. The firm has conducted a series of acoustic impulse tests on a platform and on one-km lengths of welded pipe. Power requirements during tests were low. Operators will need to transmit signals through floating platform tethers, pipelines, drill strings, and coiled tubing.
Virtually all continuous or welded steel structures will support acoustic telemetry, and so will bolted structures to a limited degree. Flanged and bolted structures with rubber or fiber sealing gaskets will attenuate the signal and are not considered reliable.
AUV units can cut costs on surveys
Fixed or periodic surveys on known routes, such as those conducted over pipeline routes, can be achieved by autonomous underwater vehicles (AUV) at less than one-third of the cost of remotely operated vehicles (ROV).
An AUV systems operates from a computer program, which provides positioning, navigation, direction, and depth readouts for the route. The program is enhanced by sensor feedback controls, which help to steer the AUV. The AUV is simply launched at a prescribed point where the survey begins and is picked up at the site where the survey ends.
By comparison, an ROV requires a pilot and tether management vessel on the surface to guide the ROV. Ken Collins of Applied Remote Technology points out that a survey of 200 miles of subsea pipeline with an AUV would cost about $29,000, compared with a cost of $104,000 for an ROV and crew.
| Parameter | ROV | AUV |
| Search speed(kt) | 1 | 4 |
| Bad weathertime | 50% | 20% |
| Survey days | 13 | 3 |
| Crew require | 5 | 4 |
| Crew cost | $39K | $8K |
| Ship cost | $39K | $9K |
| Vehicle cost | $26K | $12K |
| TOTAL | $104K | $29K |
Underwater acoustic telemetry more reliable
Datasonics, working with scientists at the Woods Hole Oceanographic Institution, has developed a system of underwater acoustic telemetry products that are much more reliable than the earlier systems.
The key to enhanced reliability lies in the use of advanced digital signal processing software algorithms which detect and correct errors which can occur when transmitting data in a noisy or acoustically difficult environment.
Conventional acoustic telemetry transmits at data rates generally no higher than 50 bits per second, using standard frequency shift keying technology. This data rate is adequate for most uses, but it generally results in a fairly high error rate, especially when used in a difficult multipath environment.
Datasonics' data modulation technology is based on a system where many frequencies are transmitted simultaneously and software routings are used to reconstruct the proper frequencies at the receiving end of the link. In this way, errors can be detected and corrected quickly.
The data rate can be as high as 2,400 bits per second, but more importantly, the data rate can be progressively reduced with each step reduction in data rate used to increase processing power, and therefore data reliability. This results in a system with enhanced reliability when operating at lower data rates. There are two major applications of the technology:
- Data can be recovered from pipeline instrumentation underwater. Pressure and temperature are measured, stored in a modem buffer memory, and transmitted to a surface buoy where the data is re-transmitted to radio link to a support vessel. Prior to use of acoustic modems, the vessel was forced to remain on station, with an electrical umbilical cable used to transmit data from the pipeline instrumentation to the vessel.
- Acoustic modems can be used to control instrumentation modules as part of research on tension-leg production platforms. In this application, the modems are used to provide synchronization of all the subsea instruments and to collect data from these locations.
Magnetic field evaluation determines weld cracking
An alternative method of structural weld crack detection is being upgraded to function over large weld sections and produce evaluations in real time. The method is the surface magnetic field measurment (SMFM) technique.
The method was presented in a technical paper in the proceedings of the International Offshore and Polar engineering conference in Asaka, Japan last year. The paper was presented by D. Mirshekar-Syahkal of the University of Essex (Colchester, UK) and S. H. H. Sadeghi of Amirkabir University (Tehran, Iran).
The method was developed to replace conventional methods in underwater structural applications where conditions are difficult and good readings are hard to obtain. Conventional methods include eddy current and potential drop (differential) technologies:
- The eddy current method requires frequent calibration of the measuring device, and must frequently be accompanied by a diver highly trained in calibration.
- The potential drop method requires good electrical contact with the metal, and extensive preparation of the work surface.
The SMFM method involves the placement of U-shaped wires or a rectangular coil carrying an alternating high frequency current. Current research on the method involves the development of linear or matrix arrays of sensors for fast 2D electronic scans.
The authors point out that electronic scanning is preferable to mechanical scanning because of reduced time underwater, less surface preparation, and much lower noise levels.
Sensor provides line tension data
A submersible in-line tension sensor has been developed by Billings Industries of North Falmouth, Massachusetts that provides tension load data to a remote data acquisition station.
The sensor provides a 0-5 volt output that is zero and gain adjustable and monitors tension loads to 10,000 lb at 6,000 meters of depth.
The sensor is fabricated of non-corrosive components and can be easily interfaced with standard cable, mooring, and rope configurations.
Global link provided for low rate messaging
Global satellite and marine stations exist to transmit continuous data or voice messaging, but few links exist for infrequent messages. Orbital Communications of Dulles, Virginia (US), in conjunction with IWL Communications of Friendswood, Texas (US) is developing a network to handle 1-2 messages per day communications.
Typical communications would include messages such as daily morning reports from drilling locations around the world, cathodic protection monitoring from platforms and offshore installations, tank level gauging, and electronic flow measurement at non-custodial transfer points. Remote transmissions units (RTU) and remote monitoring units (RMU) on wellheads will simply plug into the system.
A constellation of 36 small satellites, positioned only 500 miles up, will handle communications globally. IWL will integrate mobile and remote data collection devices with the Orbital Communications system.
Copyright 1995 Offshore. All Rights Reserved.
