Ocean bottom cable (OBC) use has grown steadily over the last decade for 4D survey acquisition and for reservoir management. Until recently, 4D surveys focused mainly on shallow waters (typically less than 200 m or 656 ft). Deploying geophones in shallow water is simple. The vessel moves along the survey line and geophones are launched where they are supposed to land on the seabed. This cannot be extrapolated readily into deeper water. Shallow-water deployment techniques do not provide feedback on the cable conditions as it sinks in the water column and touches down. Therefore, the installer cannot properly respond to a cable placement error on the seafloor.
As the water depth increases, ocean currents affect the position of the cable and sensors being deployed. In addition, the response time of the cable to changes in ship actions and cable payout is affected strongly by water depth. Depending on depth and sinking speed of the cable, there can be a delay between the time the ship modifies its speed/course and the time when those changes are reflected on the bottom. This response lag can make it impossible to correct the impending touchdown errors using last minute ship maneuvers.
The problems faced by the seismic industry as it moves into deepwater cable installations already have been overcome by the military and commercial submarine cable industry. Military cable installation requirements are similar to those of the seismic industry; both need to place sensors accurately on the seabed. In addition, the seismic industry requires that cables be retrieved. This is not trivial in deepwater where high retrieval tensions can drag the cable and sensors a long way across the bottom, making the array prone to fouling with bottom obstacles. Furthermore, constant dragging of the cable and sensors induces abrasion and reduces the life expectancy of the seismic array.
Technology developed to accurately install submarine cables with multiple in-line sensors for the military has proven successful. This technology has been adopted by the telecommunication industry since the late 1990s and over 80% of the commercial cable installers currently use it. The same technology is suited for offshore oil exploration in the deployment and retrieval of OBC cables in deepwater.
The technology uses a real-time cable installation control system that computes the geometry and forces acting on the suspended cable, and the cable touchdown position and bottom tension (or slack). This is done by accounting for cable characteristics (size/weight), ship velocity, bathymetry, currents, and all the other parameters affecting the dynamic position and accuracy of the cable lay. With such knowledge available at all times, immediate and accurate cable lay forecasts and command decisions can be made to account for any real-world situation, whether planned and unplanned.
This control system changes the focus of cable deployment control from the cable condition as it leaves the vessel (current practice for OBC) to its condition on the seafloor. This allows cable installers to focus on the most important issue in any cable lay – the installed condition of the cable on the seafloor. The computer model shows in near real-time the cable bottom conditions and can predict the results of future cable and ship actions on cable seafloor conditions. The result is an improvement in the installer’s knowledge of the cable’s condition on the seafloor and in his ability to predict and control touchdown conditions. Cable lays previously considered impossible have been done accurately and reliability using this approach.
Control system
The control system continuously interacts with the ship’s navigation and cable handling systems to gather the necessary inputs for the cable model and to publish the results to the helmsman and the cable operator. To properly compute the cable touchdown position and cable bottom slack (or tension), the control system must measure enough key parameters to obtain a mathematical solution of the cable shape behind the vessel. Data on bathymetry, planned route, cable tension/slack, targets along the route, and the physical characteristics of the cable are prepared in advance and stored in files accessed by the system during deployment or retrieval. Essential real-time data include ship position and the cable length as a function of time. Ship position is available from any of the ship positioning systems (e.g., DGPS) and cable length out is available from the cable engine counter and from marks (e.g., geophone locations) along the cable. If placement accuracy requirements are high, as for seismic lays, ocean current data throughout the water column can be measured using an acoustic doppler current profiler (ADCP) on the deployment vessel and then incorporated into the control system.
Dynamic 3D cable model used by the control system.
The control system monitors and controls all critical parameters of cable laying/retrieval, from cable payout speed to ship instructions (ship speed and course). The control system is multiple applications that run simultaneously and interact extensively to coordinate and communicate the raw and processed data. In addition to data gathering programs which interact with ship equipment, the system contains a 3D dynamic cable model to provide complete coverage of the suspended cable based on information supplied. The cable model divides the cable into finite a number of straight segments connected by perfect ball joints. The forces acting on the cable segment consist of tension, wet weight, normal and tangential drag, and inertial forces. Each time-step, the boundary conditions are updated based on the new ship positions and the cable payout. Using the newly computed boundary conditions and the cable shape from the previous time step, the new cable shape is updated.
Knowing the present cable solution, the system predicts cable lay dynamics, and these forecasts are used to determine, new ship (speed and course) and cable instructions to optimize cable laying/retrieval. Depending on accuracy requirements, new ship and cable instructions are issued every two to 10 minutes.
OBC incorporates transponders
While the control system is based on cable physics and its accuracy has been validated, that accuracy is limited by the quality of the input data, in particular the measurements of cable length paid out and ocean currents. Having accurate transponder measurements can compensate for some of these input errors. The additional cable position (x,y,z) information obtained from transponders attached to the cable can improve the overall cable shape and placement accuracy. This is important for OBC installations where transponders are placed at regular intervals along the arrays.
If highly accurate transponder data is available (RMS position error ≤ 0.25% slant range), the control system has the option to directly force the mathematical solution of cable shape to pass through the bottom most transponder position on the suspended cable. Since the position of this transponder is known, the bottom transponder acts as a “ship” closer to the bottom to improve the accuracy of the touchdown location.
If the transponder positional data is not accurate as transponders get closer to the seabed, the control system uses a Kalman filter to estimate the approximate currents acting on the cable based on the transponder measurements and the cable dynamic. These currents then can be used in the dynamic cable model to improve geophone placement accuracy.
The control system is optimized for OBC retrieval. As a result, the software can model more accurately the cable-seabed interactions and cable being dragged as a result of the seabed cable tension. Knowing the cable conditions on the seabed at all times allows for cable retrieval with lower seabed tensions, which in turn decreases cable dragging on the seabed. This helps minimize cable fouling with bottom outcrops and also cable abrasion to extend system life.
Validation results
The control system has successfully installed many deepwater (up to 7,000 m or 22,966 ft) cable systems including U.S. Navy tracking ranges and surveillance arrays, power and telecommunication cables, environmental sensors, and more recently OBC arrays. Over 140,000 km (86,992 mi) of cables have been installed in the last eight years with the control system.
Detailed validation has been completed in many of these projects where long base acoustic navigation systems accurately measure the final locations of transponders attached to the installed cable, and the measured positions compared with the calculated touchdown positions predicted by the control system. Final expected placement accuracy is a function of the water depth and the size/specific weight of the cable and in-line sensors. The figure below includes the validated results achieved in three different projects which cover the range of cables sizes used by OBC systems. The SOAR-2 and PTS are U.S. Navy projects where multiple in-line sensors were installed in waters from 200 m to 2,000 m (656 ft to 6,562 ft) deep. The HDWCP was a Department of Energy funded project whose goal was to validate the accuracy at which power cables could be installed in waters up to 2,000 m deep. Somewhat higher placement accuracies may be achieved by using the Kalman filter. This new system is being used by FairfieldNodal to deploy a large OBC system in the Red Sea for Saudi Aramco. This challenging project is in waters up to 1,000 m (3,281 ft) deep in areas with seabed slopes in excess of 60º.
Deepwater OBC installations
Using the control system benefits all phases of cable installation, from route and installation planning, lay simulation, operator training, to actual installation and retrieval. Cable planning is done in a GIS environment. The user can load a large number of databases to precisely place the cables relative to existing shorelines, bathymetry, existing seabed features, etc. Once the user has defined where the sensors are to go on the seabed and what values of seabed slack/tensions will be used on different portions of the route, he do detailed planning and simulation of the cable installation and retrieval.
Installation planning is done by computer simulated cable lays to dynamically model the cable, equipment, procedures, and environment of the actual lay. Such simulations allows for detailed analysis/selection of the equipment, techniques, and data inputs required to safely and reliably lay the cable along the planned route. The system can simulate real-world problems such as ship navigation errors, variations in ship and cable payout speed, unobserved changes in ocean currents and bathymetry, etc. This lets users investigate answers to questions before going to sea. These can include: Is a dynamic positioning system required on the vessel? Is it necessary to measure currents to achieve the desired placement accuracy? Do we need to attach transponders on the cable and how often? How much does the installation speed affect the placement accuracy of the sensors?
The system is a proven training tool, and it provides operators with as much experience as possible under different deployment scenarios before the real lay. The cable lay simulations allow the operator of the cable deployment control system to become familiar with how the cable behaves (e.g., response times and cable shapes) under different environmental conditions and helps develop a clear understanding of efficient techniques for cable placement and bottom tension/slack control.
Editor’s note: Contents of this article are from a technical paper submitted to the Society of Exploration Geophysicists (SEG) 2010 Conference (Copyright SEG), “A Real-time Control System for Installation and Retrieval of OBC Arrays” by Jose Andres, Venkata Jasti and Antoine Bon, Makai Ocean Engineering; Copies available at www.seg.org
Jose M. Andres
Makai Ocean Engineering
Offshore Articles Archives
View Oil and Gas Articles on PennEnergy.com
