Real-time controls aid seismic survey cable deployment and retrieval

Geometry and shape guide position development

J. M. Andres
Makai Ocean Engineering

Figure 1: Input and output parameters of the ICS. Image provided by Makai Ocean Engineering. [39,038 bytes]
Figure 2: 3D modeling of a cable lay. Image provided by Makai Ocean Engineering. [42,538 bytes]
Figure 3: Deployment of the portable tracking system for the U.S. Navy. Image provided by Makai Ocean Engineering. [42,330 bytes]
Figure 4: Deployment of the SOAR II Range. Image provided by Makai Ocean Engineering. [42,009 bytes]

The use of technology developed to accurately install fiber optic cables with in-line bodies for the military has been adopted recently by telecommunication companies to install the new generation of fiber optic cables. This technology is also well suited for offshore oil exploration in the deployment and retrieval of cables for 4D/4C seismic surveys in deep water.

This technology utilizes a sophisticated real-time cable installation control system which accurately computes the geometry and forces on the suspended cable during a lay to determine the cable touchdown position and the cable bottom slack (or tension).

The control system takes into account all parameters, which affect the cable shape behind the vessel: cable physical properties, ship velocity, bathymetry, currents (if available), and other parameters affecting the dynamics 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 that account for any complex real-world situation, both planned and unplanned.

This control system changes the focus of cable deployment control from the cable condition as it leaves the vessel (current practice) to its condition on the seafloor. It allows cable installers to focus on the most important issue in any cable lay: the installed condition of the cable on the seafloor.

The sophisticated computer model monitors in near real-time the cable bottom conditions in the recent past and can predict the results of future cable and ship actions on cable seafloor conditions. The result is a major 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 that were previously thought to be impossible have been performed with high accuracy and reliability.

This paper describes the cable deployment control system, its at-sea control and simulation capabilities, results from several at-sea operations, and its principal advantages.

Basic system

The Integrated Control System (ICS) for cable deployment and retrieval operations is a sophisticated software package running on a desktop workstation, which continuously interacts with the ship's navigation and cable handling systems. Figure 1 [39,038 bytes] summarizes the main input and output parameters of the ICS.

To properly compute the cable touchdown position and cable bottom slack (or tension), the control system requires the measurement of a sufficient number of key parameters to obtain a mathematical solution of the cable shape behind the vessel. Knowledge of bathymetry, planned route, cable tension/slack desired along the selected route, and the physical characteristics of the cable are prepared in advance and stored in files accessed by the system during deployment and retrieval operations.

Essential data that must be collected in real-time are the ship position and the cable length as a function of time. If placement accuracy and reliability requirements are high, as is the case for many military cable installations and 4D/4C seismic surveys, ocean current data throughout the water column and even sporadic position information of acoustic transponders attached to the cable can be incorporated into the system.

Real-time information on ocean currents throughout the water column can be readily obtained by use of an acoustic doppler current profiler (ADCP) installed on the deployment vessel. Existing ADCP technology allows measurements of currents from the surface to depths of 1,100 meters.

The ICS is the brain in a cable deployment operation. It monitors the critical parameters of the cable laying/retrieval process and exercises control by computing proper cable payout speed and ship instructions.

The heart of the ICS is a powerful and sophisticated 3-D dynamic cable model specifically developed for real-time cable laying control (Figure 2 [42,538 bytes]). Full 3D modeling of different cable types, in-line bodies, complex cable shapes that change with time, 2D or 3D bottom terrain, and currents that change with depth and time are all included in the program.

Once data gathering programs have collected data from the ship's equipment, the ICS's 3-D dynamic cable models provide a complete mathematical solution of the suspended cable shape based on the information supplied. These solutions, which usually are computed at 15 second to 2 minute intervals depending on the lay parameters, provide the ICS with its near real-time monitoring capabilities. The program knows the cable touchdown location and bottom slack or tension as recently as 15 seconds in the past.

Once the ICS knows the cable touchdown conditions in the recent past, it then looks ahead into the future in order to predict the outcome of ship course changes or cable payout changes on the cable's touchdown condition. Multiple future predictions of cable laying dynamics are computed by the cable models within the ICS. These forecasts are used to evaluate new ship and cable instructions in order to optimize cable laying/retrieval performance.

The ultimate output from the control system is an instruction that contains a new cable payout rate, ship speed and ship course. Depending on the accuracy requirements associated with the cable lay new ship and cable instructions are issued every 1-10 minutes. Accuracies achieved with the ICS are much better than those achieved with conventional cable lay techniques. Uncertainties about the touchdown point using the ICS can range from 2% to 20% of the water depth, depending upon the type of cable, deployment speed, and type and quality of the equipment used. The better accuracies are achieved when currents are measured during deployment.

Simulation capabilities

The ICS development also led to the development of the cable lay simulator (CLS). Developed initially as a test and training platform for the ICS, the CLS became a powerful pre-lay simulation and planning tool in its own right. The CLS allows the user to simulate, prior to a cable lay, the entire cable lay process including the ability of the ship, cable handling equipment, and instrumentation to properly place the cable on the seafloor.

With the CLS, the control process is simulated including the errors and uncertainty associated with input information, the varying environmental conditions, and the ability of the ship and cable payout operators to follow instructions. By operating this software, the installer can accomplish the following:

Evaluate the performance of his existing cable lay equipment and instrumentation to determine whether it is adequate for the cable lay installation at hand. For example, is a dynamically positioned ship required? Is it necessary to measure currents in real-time to achieve the desired placement accuracy? Is the navigation system adequate? How well do you need to know bathymetry?
The cable lay installation and control process is so accurately simulated such that a cable lay operator can not distinguish between a simulated operation and one at-sea. This process therefore trains the operator. The engineer in charge or the cable control operator can lay the cable along the particular route one or more times prior to actually performing the real cable lay.
Contingency operations can be simulated and practiced prior to going to sea. For example, under a sudden cable payout stop condition, what should the cable ship do to prevent undesirable bottom cable movement or tensions?

Recent installations

The ICS has been successfully used to install a variety of cable systems including U.S. Navy tracking ranges, power and communication cables, and environmental sensors. In the past two years alone this control software has been used to direct eight different commercial cable lays and one military lay.

Five of these lays involved the deployment of seafloor seismometers and tsunami detectors installed on very specific targets on the bottom in depths up to 2,600 meters with extremely rough bottom terrain and slopes at angles greater than 30°. Each of these lays was successfully completed with all cable bodies laid within their target circles.

Cable and Wireless Marine, the largest submarine cable installer in the world, has equipped two of its cable vessels, the C/V Venture, and its newest vessel, Cable Innovator, with these advanced systems.

They also make use of the CLS system to assist them in cable lay planning. Their most recent work includes the use of this software to monitor the 3000 km + Jasuraus cable lay which links Australia and Indonesia and the 6000 km + Gemini South trans-Atlantic cable lay. The same system will be used for the installation of the trans-Atlantic Gemini North lay in March-April 1998.

During October 1997, the ICS was used by the U.S. Navy to provide deployment control for the laying of a 70-mile-long portable tracking system (PTS) array in the Bahamas (Figure 3 [42,330 bytes]). A total of 51 in-line bodies were accurately placed in water depths ranging from 40 meters to 550 meters.

During this deployment, the ICS was also used to retrieve a portion of the system with minimum dragging of the previously laid cable. In earlier work, the ICS was used to deploy the SOAR II Acoustic Range in water depths of 800 meters to 1700 meters (Figure 4 [42,009 bytes]). Currently, the ICS is being used for the simulation and deployment planning of the Hawaii Shallow Water Tracking Range (HISWTR) to be installed between the Hawaiian Islands in 1999.

Advantages

The major advantages of using the ICS and CLS for the cable installer are:

(1) The system provides the installer with a real-time shipboard display of cable touchdown conditions (cable position and slack or tension on the bottom), thus eliminating a major uncertainty inherent in existing lay techniques.

(2) The CLS is an invaluable planning tool for the installation contractor. Through lay simulations, the impact of vessel selection, equipment choice and the properties of the cable systems can be accurately and inexpensively evaluated before commitments are made. Once onboard the lay vessel, the ICS gives the contractor accurate planning and control of future cable positions and cable bottom tension/slack as a function of ship course, ship speed and cable payout. Such planning, foresight and feedback allow the contractor a level of lay control never before possible.

(3) The knowledge and control provided by the ICS during unexpected lay events is far beyond what is currently available from existing cable lay techniques. Existing deployment plans typically make use of cable laying tables, which correlate the speed and course of the ship with the cable payout rate along the pre-established cable route. One of the main shortcomings of this approach is that it ignores the effect of ocean currents and assumes that the cable lay will proceed exactly as planned.

The pre-established tables cannot be easily regenerated or modified when a contingency event occurs. If the cable deployment operation deviates significantly from the original plan (cable engine malfunctions or ship stops), the cable loads, touchdown points, and slack will differ considerably from the planned values. Existing techniques do not allow the personnel in charge of the cable deployment operation to determine what the suspended cable is doing and how to effectively control it.

When a contingency occurs, the ICS continually predicts the suspended cable shape including changes in bottom slack and position and allows the personnel in charge of the operation to issue appropriate ship and cable commands as needed despite the lack of cable lay tables. The contingency control capabilities of the ICS have been demonstrated on several occasions during at-sea cable deployment operations in the past. In all cases the ICS has allowed the cable installer to resume the deployment operation with no adverse effects on the cable lay.

(4) The ICS interfaces with existing equipment installed on cable ships and can be used without introducing major changes to the conventional cable laying techniques. The addition of the ICS represents a small cost compared to the capital and operational costs associated with cable-lays. The total cost depends on the desired accuracy and the acceptable level of risk.

(5) In addition to cable deployment and retrieval operations with bottom slack and/or tension, the ICS can also model towed cable strings with in-line bodies, and incorporates modeling of cable dragging over the seabed. While cable dragging may be an acceptable technique to reposition a cable in shallow waters and smooth bottoms, as is done in some 4D/4C seismic surveys, dragging long lengths of cable in deepwater over irregular bottom terrain can be impractical and increases the risk of damaging the cable.

For cable recovery, it is desirable to minimize the cable movement over the bottom. By logging exactly where the cable went when first laid and then running the control system to lift the cable cleanly from the seabed, the ICS can minimize the amount of seabed cable movement and damage.

Author

Dr. Jose Andres is Vice President of Makai Ocean Engineering and head of the cable group. For the past 10 years, he has directed the development in cable deployment control systems and has participated in many of Makai's cable deployment operations.