Abstract

Currently, system-level design of THESEUS has been completed, and detailed design is underway. This paper gives an overview of the THESEUS AUV, outlines its specifications, and describes key technical problems related to cable laying.

Introduction

The design of THESEUS is driven by the single goal of demonstrating an under-ice fiber optic cable laying capability. Environmental factors such as depth, currents, bottom and ice cover profiles, temperature and salinity impose constraints on the design. Logistical factors such as mission range, mission time, transportation to the mission site, and on-site operations also impose constraints.

The proof-of-concept trials entail THESEUS being launched through shore-fast ice in shallow water, leaving one end of the fiber optic cable at the launch site. Transiting at low altitude to the cable delivery site, THESEUS will deploy the cable on the seafloor. Once the cable has been delivered, THESEUS will return to the launch site for recovery.

Vehicle Design

THESEUS is being designed with the following key requirements:

A general arrangement of THESEUS is shown in Figure 1. Overall, THESEUS is 35' long, 50" in diameter, and consists of a pressure hull with free-flooding composite hull sections forward and aft. The aluminum pressure hull houses the energy storage and power distribution subsystems, as well as most of the dry electronics. The bow dome contains a ballast tank, obstacle avoidance sonar and other acoustic components. The sensor bay just aft of the pressure hull contains a doppler sonar, acoustic transponder, and attachments for an umbilical and antenna. The payload section contains the fiber optic cable and cable deployment components. The aft section contains another ballast tank and the thruster motor. Independently moveable hydroplanes at the bow and stern provide flight control.

Figure 1

The range and maneuvering requirements imposed on THESEUS require an efficient hull configuration. Towing tests with a ¼-scale model have been performed at the Institute for Marine Dynamics in St. John's, Newfoundland. These trials will confirm the drag estimates and will provide hydrodynamic derivatives to be used in a dynamic simulator currently under development.

To ensure that the pressure hull is capable of withstanding the forces to be encountered from hydrostatic pressure and handling, the hull components are being designed with a finite-element analysis package. Six hull sections with removable internal rings and two spherical endcaps make up the pressure hull structure.

Energy Storage

THESEUS is required to travel a long distance at a moderate speed, with a large payload. Nearly two-thirds of the total energy needed to complete a typical cable-laying mission is required for propulsion, with the remaining energy going to power onboard electronics, sensors, control surfaces and other components. The energy source will be required to power the vehicle throughout its mission, yet be compact enough to keep the overall vehicle dimensions reasonable.

As with most underwater vehicles, the choice of energy source is governed by: energy weight and volume densities, practicality, reliability and cost. For its under-ice cable-laying mission, THESEUS will use silver zinc (AgZn) cells. Although they are costly, have a limited cycle life and are more difficult to charge than NiCd or lead-acid cells, their high energy density makes them the preferred energy source. The AgZn cells are packaged into three 60V banks in the forward three pressure hull sections, and are arranged in spill-proof fibreglass boxes for easy removal.

The power requirement for each Theseus subsystem is shown in Table 1.

Navigation

Designing a navigation system which will allow an AUV to autonomously navigate under-ice more than 400km is a challenge. The presence of a permanent ice cover imposes several constraints on such a system. It follows that all sensors used to determine position must be located below the ice cover, but not necessarily on board the vehicle.

A hybrid solution to the navigation problem has been chosen: an onboard, medium-accuracy positioning system for outbound/inbound transits, and an external, but subsurface, terminal guidance acoustic positioning system for cable delivery and vehicle recovery.

On the outbound and inbound transit phases of its mission, Theseus will dead reckon using a medium-accuracy inertial navigation unit (INU) and a doppler sonar. The INU provides heading and attitude data, while the doppler sonar measures forward and lateral velocities, as well as altitude above the seafloor. This combination is expected to provide position with an error of approximately 3% of the distance travelled.To ensure that Theseus can navigate accurately to deliver the terminal end of the fiber optic cable, as well as return to the recovery ice hole, an acoustic positioning system will be used. This system has not yet been finalized, but a series of pre-positioned acoustic transponders appear to be a leading candidate.

Since the AUV will be operating in an unknown environment, it must be capable of detecting and avoiding obstacles in its path. An obstacle avoidance sonar developed by Sonatech has been chosen to provide this capability.

Control

Overall vehicle operation is managed by an MC68030 computer running Proteus, [current version: ACE 3.0] a control system developed by ISE Research for AUV control ². This software has a number of interesting features including layered control and subsumption. Proteus has been proven to be a very useful and flexible package for AUV control.

One of Proteus' most useful features is its ability to execute scripts. Scripts are usually small, self-contained "subroutines" which are written in a very "English-like" language. Scripts allow system designers to develop rules for operation of nearly all vehicle functions, without having to re-compile or even modify software source code. Scripts can be developed to run in parallel, at different priority levels, and have many useful operators (verbs).

The control software can be used to increase the fault tolerance of an AUV. In previous sea trials with the ARCS AUV, Proteus has been configured to switch to an alternate control strategy when it detects a failure in a hydroplane. For example, the foreplanes usually control vehicle depth. If a foreplane fails, the remaining foreplane can be set to a neutral position, and vehicle depth control can be accomplished using the aftplanes to adjust vehicle pitch.

Cable Deployment

As the vehicle moves forward above the seafloor, the fiber optic cable is dispensed out of the stern. Despite the small size of the cable (less than 0.1" OD), the cable deployment system occupies one-third of the enclosed volume of Theseus.

The cable is stored on a series of spools which are stacked longitudinally along the vehicle axis. The ends of each spool are spliced together prior to launch. The cable and splices wind off the spools from the inside-out, and exit through a tube in the stern. The tension on the cable (to keep it from free-spooling) is maintained through the use of a special glue applied to the cable during the winding process. To keep the system simple and reliable, no other active tensioning or dispensing devices are used.

As the cable leaves the vehicle, weight is lost. To prevent this from affecting vehicle trim, the loss in cable weight is counteracted using an automatic buoyancy compensation system. Surrounding each cable spool is a toroidal hard ballast tank. Starting the mission empty, each tank is filled with water as the cable is dispensed from its companion spool, keeping the net buoyancy of each spool/tank assembly near neutral. The rate at which each tank is filled is determined by the onboard computer.

Variable Ballast System

Theseus may be operated in waters with salinity varying from nearly fresh to very saline. In ice-covered waters, there is often a freshwater layer immediately below the ice. Consequently, a vehicle such as Theseus may change buoyancy by 400 lbs or more during a mission due to these salinity variations. Changes in buoyancy also occur with depth. Vehicle structures, including the hull, lose displacement as they compress. This effect is offset by the compression and consequent density increase of the water itself under pressure.

To deal with these changes in buoyancy, and to provide a means for changing vehicle buoyancy for launch and recovery, Theseus uses a variable ballast system. Hard ballast tanks are located in the free-flooding bow and tail sections. The tanks are toroidally shaped to make efficient use of the space available, as well as to allow the dispensed fiber optic cable to pass through. Since the two tanks are located fore and aft of the vehicle's centre of gravity, they can also be used to adjust pitch trim.

The contents of the tanks are controlled by the onboard computer. It monitors the volume of water in each tank, and activates pumps/valves to fill or empty the tanks as required.

Communications

By definition, an AUV does not require human intervention. This, however, does not preclude the requirement for monitoring vehicle activity whenever possible. The cable-laying mission introduces a unique opportunity for communication, namely the fiber optic cable itself. During the outbound phase of the mission, Theseus will use the cable as a communications channel to the operator console at the mission operations site. High speed, full-duplex, real-time communications will be available as long as the cable integrity holds.

During the inbound phase of the mission, Theseus will be capable of communicating with surface-based personnel using an acoustic telemetry system. Arctic field trials of a 50 bps system have yielded ranges of more than 4km in a shallow, high multipath environment, and 8km in deeper waters.

Design & Reliability

To help ensure that Theseus will be capable of completing a long-range, under-ice mission, it must be reliable. Reliability begins at the design stage. Theseus is being designed using a top-down, structured approach. The single role of under-ice cable laying drives the Vehicle Specifications, which then results in a System Specification. The AUV System can then be broken down into subsystems, or i ndividual components which are relatively self-contained. Individual designers can then concentrate on the detailed design of each subsystem. Good systems engineering techniques and continued communication between the designers helps ensure that the entire system operates as required.

To ensure an acceptable level of reliability, the AUV system has been assigned an estimate for the probability of mission success. This figure is then used as one of many parameters in the system design. A reliability network model of the entire system is developed, which allows each subsystem to be assigned a reliability requirement. Trade-offs in reliability between subsystems are permitted, as long as the resulting reliability of the entire system stays above the requirement. Subsystems or components of a subsystem which have an insufficient level of reliability may require improvement through redesign, selection of higher quality components, or by adding redundancy.

Logistics

Operating a large AUV in an arctic environment poses a number of logistical problems, including: transportation of equipment to the mission sites, field training of mission personnel, and handling and launch and recovery through ice. ISE Research is actively pursuing these problems through ongoing field trials.

Conclusion

Theseus is one of the largest in a new class of long range AUVs. Unlike many other AUVs, Theseus is being designed for a single requirement - laying fiber optic cable. ISE Research and Canada are committed to the ongoing development of AUV technologies in this area.

References