ADVANCED ELECTRONICS Fourth generation DP set up on pipelay unit

Accuracy in deepwater pipeline touchdown product of sophisticated routing trajectories

Alan C. Herold
Nautronix

The Chickasaw, a 275 ft long by 80 ft wide reel pipe barge, is one of the first vessels to be equipped with a fourth generation dynamic positioning system. The vessel can follow sophisticated routing trajectories in deep water, commanding the vessel's location on straight and curved sections of track so that pipe touchdown at the seabed is precisely specified by the operator or contractor.

The Chickasaw, owned by Global Industries, currently is working in the US Gulf of Mexico. The vessel was retrofitted with a Nautronix ASK4002 dual-redundant DP system and dual forward-mounted steerable thrusters. Using the reel process to lay pipe, the barge is able to cut cost and time offshore, avoiding weather delays and high operating costs.

The Chickasaw lays pipe by spooling from its reel while the barge moves forward with thrusters. The barge can achieve installation rates of 50,000 ft in 24 hours. Conventional barges move forward on anchor wires in increments after each pipe weld, and are limited by the anchoring system to shallower depths.

The vessel can work in water depths beyond those of anchoring systems, an advantage in heavily congested pipeline fields common in shallow depths in the US Gulf of Mexico. The Chickasaw can reel up to 20,000 ft of 12-in. diameter pipe or 239,000 ft of four-in. pipe, or combinations of different sizes, including umbilicals.

Dynamic positioning

The Nautronix ASK4002 unit is based on a networking concept, with high reliability hardware and proven control algorithms. The architecture and processing schemes are state-of-the-art Honeywell ASK products. The primary components of the system are:

Two control console assemblies housing color graphic units, operator interface panels, high performance industrial grade digital processors, and power supplies.
Two bulkhead mounted cabinets containing industry standard analog, digital, and synchro signal conversion equipment, data hiway interfaces, and a power supply.
A remotely located standard rack accommodates the Starfix communications satellite based positioning fixing system, a differential global positioning system (DGPS), sensors, and a Nautronix data logger.
A second remotely located rack holding redundant Starfix and DGPS positioning systems.

The reel pipe lay barge, Chickasaw, received a fourth generation dynamic positioning system and steerable thrusters, allowing it to work easily in congested areas.

External equipment includes two vertical reference units, two wind sensors, two gyrocompasses, installed propulsion units, dual line printers, and dual uninterruptible power supplies.

The system receives and processes sensor subsystem signals corresponding to vessel position (related to a reference point), vessel attitude (pitch and roll), vessel heading, and wind speed and direction.

This data is processed in the ASK computer, where it is used to calculate and allocate the thruster control orders that maintain the vessel at the desired position. The computer also allows control panel input to the ASK system. The uninterruptible power supply provides backup for the electronics power.

Major technical innovations allow the Nautronix ASK 4002 to set fourth generation standards for DP. The Chickasaw barge is composed of dual distributed microprocessor-based components that provide several advantages:

Dual-redundant systems provide mirrored and fully backed-up operation of all systems at all times to maintain vessel control.
High speed centralized processing minimizes system transport delay.
Minimization of failure-prone system backplanes and discrete wiring improves reliability.
Modular growth across the product line supports future upgrading.
Maximum reliability through the use of industrial hardened equipment and ease of routine maintenance improves system reliability.

State-of-the-art control algorithms, including Kalman filtering and state variable techniques, are utilized to achieve optimum vessel control performance. All sensor measurements are processed simultaneously with automatic adjustment for sensor noise levels. In the event that all sensors should malfunction, operations continue based on an embedded vessel model.

The ASK 4002 provides integrated full-color graphics and text displays and a dedicated operator control panel protected against dust and moisture. Graphic representations are provided for easy monitoring of vessel performance. Alarms are color-coded and provided in simple language. Operator prompting and command status messages are provided.

Additional system level capabilities include the following:

General purpose sensor and thruster interfaces are included, allowing ready configuration for the vessel by installing appropriate signal modules and selection of database values.
Ease of functional reprogramming for custom processing features is ensured through the implementation of a common high-level software language.
System reliability is maximized by the use of signal isolation. Every sensor or thruster interface signal is optically isolated from internal circuits to eliminate the effects of external electrical noise and grounding currents.

Numerous advanced techniques were incorporated into the Chickasaw system to support the unique requirements of this application.

Thrust allocation

An artificial intelligence system was developed to provide thrust allocation logic which dynamically selects thruster levels and directions which minimize total operating costs for vessel propulsion. It does this by analyzing cost penalty factors for past, current, and predicted future thruster performance, and by then selecting the most efficient solution from the list of acceptable candidate solutions.

In addition, a new thrust biasing algorithm was developed which reduces thruster azimuthing and provides loading of the power plant for efficient operation during slack conditions. When available, diagonal thruster pairs are pointed toward each other and are used to expend a minimum thrust level as entered by the operator.

Fixed azimuth modes of operation of the thrusters are also available. In these modes, the thrusters are set to one of two possible pointing configurations and not moved from these angles. The forces and moment desired are met with no change of thruster angles up to the limits of the individual thrusters. If the desired forces cannot be achieved in these limited conditions, the thruster operational mode switches automatically to full azimuthing capability.

The console of the Nautronix ASK4002 system contains an artificial intelligence system that learns from past experiences in order to minimize thruster power consumption.

Vessel routing

Sophisticated schemes have been developed to support pipelay operations. Pre-computer waypoint data files, generated by the customer, are loaded into the system and stored for processing.

The waypoint file defines the start point, the end point, and the points of intersection (PI points) of the straight line segments for the intended pipe route. For each PI point, a turn radius is specified, allowing a smooth curved transition of the desired pipe track from one straight line segment to the next straight segment.

Touchdown distance data are supplied in the waypoint file to define the horizontal distance from the point the pipe is expected to be touching the seabed to the vessel location. This touchdown distance is used to compute the vessel route based on the instantaneous pipe route.

The computed vessel route commands the vessel to swing a wider curve than the calculated pipe route, which allows the pipe to be laid along the prescribed route.

The algorithms incrementally calculate the desired pipe track and associated vessel track, based on the waypoint data and an operator-entered track speed. Vessel heading is nominally calculated to be along the pipe track for straight segments and along the tangent to a curved section of the pipe route.

Heading offsets may be entered by the operator to minimize the pipe angle at the end of the stringer during high lateral current conditions. The operator sets the speed control to determine the speed of advance along the track, to stop at any point on the track, or to reverse the vessel motion back down the track.

Track-relative position offsets may be entered by the operator to allow the vessel to parallel the nominal, waypoint file-defined track. The track generation may be terminated at any point and resumed at any point along the nominal track, allowing the vessel to leave the intended track for weather or resupplying as needed.

As the end of the pipe track is approached, the vessel speed automatically slows to a stop at the end point, then switches to automatic position and heading hold control modes. This allows the operator the option of placing the end of the pipe in a fully controlled mode of operation.

Data playback

A comprehensive data recording and playback system has been developed for analysis of data from both systems of a dual-redundant configuration. The recording medium is a floptical disk, a hybrid technology combining magnetic disk storage with high accuracy optical laser read/write head positioning.

A variety of features are available for data post processing. These include selectable time histories of selectable subsets of the recorded data. ASCII data export capability for processing by Excel, Lotus, or other off-the-shelf data processing programs, statistical analyses, and hard copy output to a dot matrix or laser printer.

The data set is highly configurable. System input/output signals as well as internal program variables are recorded. The recording rate is operator-selectable at the DP consoles. Analog data and digital data may be set at independent record rates. At maximum recording rates, a single floptical diskette will record data for about an 8-hour shift. The data recording allows extensive display of performance during a pipelay as well as troubleshooting capability.

Positioning

Navigation-quality conversions are embedded in the DP system to allow calculation of northings and eastings from the position sensor receivers' reported spheroid latitude and longitude. Conversions are provided for a complete repertoire of local state plane conversion schemes. This approach essentially provides the navigation function embedded in the DP system, eliminating the requirement for an equivalent system external to the equipment.

The Chickasaw laid about 500,000 ft of pipe in the first 10 weeks of operation utilizing the ASK 4002. Average speed of deployment has been 3,000 ft per hour at a pipelay speed of about 0.7 knots. Deployment is suspended for anodes placed every 600 ft. Pipe tension has ranged from 15 kips up to 32 kips.

Operational efficiency has substantially improved, owing to a number of contributors:

Anchor deployment has been eliminated. The vessel is able to arrive on location and go directly to work deploying pipe.
Maneuvering near platforms is faster, easier, and more precise. If required, a smaller assist tug reduces day-rate expenses.
The stable platform realized with the barge under DP control allows very accurate regulation of pipe tension. Without tug-assist, tension was very difficult to regulate.
Improved pipe tension translates into very predictable pipe touchdown distance. This allows accurate deployment of the pipe in the right-of-way.
Stop and start can now be achieved on demand. The ASK 4002 unit allows abandonment and re-attachment of the barge to pipe even in the middle of a routing curve.

AUTHOR

Alan C. Herold is vessel controls business manager for Nautronix. He developed the architecture of the ASK4000 systems and was the principal engineer for the system design and software development for the ASK4000JS, ASK4001, and ASK4002 systems.

Previously, he was employed by Honeywell is a similar capacity. He holds an MS in Aero/Astronautics from the University of Washington and has done graduate studies in control theory and computer applications.