Hydroacoustic position reference

The medium of underwater acoustics is used for a great variety of functions, including echo sounding, survey sonar, fish-finding and military application sonar. Hydroacoustic position reference (HPR) is also a major position reference for DP purposes. A large number of DP vessels are equipped with HPR.

In addition to position-reference, acoustic techniques may be used for monitoring and control of underwater functions, thus a drillship might be using a long baseline acoustic system for positioning, and the system will incorporate acoustic control and monitoring of wellhead functions as a backup to hard-wired control. Similar acoustic telemetry and monitoring may be used in offshore loading terminals and submerged turret installations.

When used as a PRS for DP, two main principles are current, with some variations. The two main principles are ultra-short baseline (USBL) HPR and long baseline (LBL) HPR.

Both of these methods involve the transmission of interrogation acoustic pulses from a transducer mounted on the vessel's bottom, to be received and retransmitted from sea-floor transponders.

Reception of the transponder replies allows positional data to be determined.

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6.10.1 Ultra-short baseline HPR

Ultra-short baseline (USBL) principles are used in the majority of acoustic PRS applications. This principle is sometimes known as SSBL, or Super-short Baseline; the terms are synonymous. In the USBL system, a single transponder is laid upon the sea floor. This transponder is battery-powered, and is programmed to respond to interrogation 'pings' at particular coded frequencies. The ship is equipped with a transducer, typically located at the bottom of a probe or 'pole' projecting 4m to 5m below the vessel's keel level. Acoustic pings are generated within a transceiver unit, and transmitted from the transducer, travelling downward through the water. These are detected by the transponder, which replies after a known fixed time interval called the turn-around delay (TAD).

The reply is received at the transducer and passed to the transceiver for processing. The time-lapse measured determines the slant range to the transponder. The other piece of data required is directional; the transducer is directionally sensitive, able to determine an accurate, three-dimensional direction of the reply path.

This principle enables position-reference to be obtained from acoustic communication between the vessel transceiver/transducer and a single transponder placed upon the sea floor. The accuracy of position measurement is a variable factor, but in general USBL systems typically yield accuracies of around 1% to 2% of water depth. The acoustics themselves may well be of greater accuracy than this, but other factors intervene. Since the measurement frame is vessel-referenced, the measured data must be corrected for vessel attitude, ie heading, roll and pitch. Any errors in gyro or MRU (motion reference

Chapter 6 73 Position referencing

unit - roll and pitch data) will directly transfer into the error budget of the HPR system.

Nevertheless, in water depths of up to about 250m, USBL techniques constitute a useful PRS for DP. The horizontal range of such a USBL configuration is typically between 50% and 100% of the water depth. At greater horizontal ranges than this, significant errors begin to appear.

Although communication with only one transponder will give position-reference, greater reliability is obtained from the deployment of multiple transponders. Each of these will be interrogated in sequence, giving separate sets of position-reference data. The DP system will treat each transponder return as a separate PRS, but it is important that the DPO treats this array as a single PRS. He cannot count two transponders being interrogated by the same HPR system as two independent PRS; this configuration can only be considered to be one PRS as interrogation and reply processing is via a single transceiver.

6.10.2 The long baseline HPR system

In deeper water the accuracy of USBL systems may be insufficient for use as a PRS for DP. There are many deep-water developments worldwide, and alternative solutions are needed. One such is the long baseline principle.

The accuracy of USBL is limited by the resolution of the angular measurements at the transducer head. In the long baseline system we do away with the need to measure such angles.

In a long baseline system, acoustic communication is obtained between a transducer on the bottom of the ship, and a calibrated array of transponders located on the sea floor.

These transponders are all located to yield an acoustic ray-path between 20° and 40° to the vertical. All transponders are interrogated by a common transmission from the vessel transducer.

Each transponder replies after known, fixed, turn-around delays. Each reply arrives at the transducer at a different time, allowing a slant range to be determined for each transponder. Since the transponder locations are known, triangulation of the slant ranges allows determination of the vessel position.

Note that, with the LBL configuration, an array of transponders (eg four) yields a single position.

The array of transponders must be laid, tested and calibrated before use. This operation may be contracted out to a survey vessel, the resultant data communicated to the surveyor in the DP vessel prior to arrival on the worksite. The array configuration may then be entered into the vessel's HPR system complete with geographical co-ordinates of the transponders. If this is the case then the LBL system is effectively globally referenced.

Since the LBL system does not involve the measurement of vertically-referenced angles, the accuracy of position measurement is not affected by vessel roll and pitch. The accuracies obtainable from LBL systems are in the order of 0.2% to 0.4% of water depth.

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In a typical LBL configuration for a deep-water drillship, redundancy may be provided by operating three separate and fully independent HPR systems, each one using a separate transceiver and transducer, and each interrogating four or five sea-floor transponders. The total array on the sea floor may be between 15 and 20 transponders forming a 25° cone around the spud-in location for the vessel.

In any HPR system, the reference point being navigated is the transducer on the bottom of the vessel. The DP system must therefore be configured with offset values of the transducer from the CG allowing all position reference to relate to this CG datum.

6.10.3 Transponders and transponder deployment

The transponders deployed may belong to the ship and be deployed on a downline, or they might well be units belonging to a third party, eg a transponder laid by a pipelay barge marking the end of the pipeline, or a transponder marking a wellhead. If using such a third-party transponder, then care must be taken as the condition of the transponder may be unknown, particularly battery condition.

Chapter 6 75 Position referencing

Various types of transponder are available from the HPR system manufacturers. A standard transponder simply gives positional data, but many transponders are fitted with a variety of sensors giving further data. Transponders may be equipped with sensors such as temperature, density, depth, compass and inclinometers. Information from these devices may be displayed or incorporated in various ways. An inclinometer transponder may be used to monitor Riser angle in a drilling operation. Riser angle is a major factor in determining required position for the drillship.

Transponders may be deployed in suitable locations using a downline from the ship.

A common method is to attach the transponder (complete with buoyancy float) to a mudweight, and to lower it to the sea floor on a wire. Once located on the sea floor, the wire can be slacked 50 or 100m to give freedom of movement to the vessel. If the vessel movements result in the transponder becoming out of range, then it is a simple matter to 're-spot' the transponder by picking up the wire and redeploying the transponder.

The transponder must be de-selected from the HPR system before doing this, of course. Another method of transponder deployment is to buoy-off the downline. This may be preferable, but gives difficulties in transponder recovery. Another method is to lower the transponder by downline fitted with an off-load hook, allowing recovery of the downline.

The transponder will be of the type equipped for acoustic-command release of the mudweight.

Sending the release command should result in the transponder coming to the surface for recovery by boat.

Specialist transponders are used in some applications. The inclinometer transponder has been mentioned above. Another acoustic function is the tracking of ROVs and sea- floor tracked vehicles. It is possible to equip such vehicles with standard transponders, but a problem arises with the noise level at the vehicle. This may preclude the efficient reception of the interrogation commands. The alternative is to interrogate the transponder via hard-wire, ie the ROV umbilical.

This is known as 'electrical triggering' as opposed to 'acoustic triggering'. The device is known as a responder, and it solves the problem of noise interference at the ROV location. It must be noted that, in most circumstances a responder located on the ROV is not actually used as a position- reference, as it is a moving point. In this case the responder is designated mobile (instead of fixed) within the HPR system, the DP system thus simply displays it on-screen and does not include it in the PRS pool.

6.10.4 Underwater acoustics

Communication with underwater transponders is by its very nature a slightly hit-or-miss process.

The operator (or HPR system manufacturer) has no control of the quality of the water column.

There are many factors which may have detrimental effects upon acoustic communication. The biggest enemies of underwater acoustics are noise and aeration, and by far the greatest noise source in the DP theatre is propellers and thrusters. It may also be stated that controllable-pitch propellers and thrusters are orders of magnitude noisier than

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fixed-pitch units, mainly because controllable-pitch units are usually running at full rpm all the time, while fixed-pitch propellers are often running at relatively low revs. All DP vessels are equipped with an array of propellers and thrusters, thus noise is ever-present. The DPO must take steps to avert noise interference by locating transponders away from thrusters and thruster wash.

In this respect it is necessary that the DPO predicts the pattern of noise and aeration in the light of changing weather conditions and tidal conditions. It may be useful to deploy two transponders in different locations; one for use on the flood tide, the other for the ebb, switching between the two at the appropriate times.

The accuracy of positioning using acoustics is dependent upon the use of a correct value for sound velocity. An acoustic system may have a default sound velocity of typically 1480m/sec, and this may well be used uncorrected. If a more accurate value is available, it can be entered into the system, while it is also possible to download a sound profile. A sound profile is obtained from a 'T/P dip' in which a bathythermograph is lowered to the sea fioor and recovered. The device has recorded the water column profile of density and temperature. This data yields a sound velocity profile which can be used by the HPR system.

Typically, HPR systems use acoustic channels which incorporate frequencies within the 1 8 - 3 2 kHz band. Lower frequencies than these will suffer interference from noise, while higher frequencies have the penalty of limited range performance.

6.10.5 HPR System Hardware

A typical HPR system consists of a transceiver, an operator terminal, a transducer hull unit, plus the sea floor transponders. The operator terminal may be integral with the DP operator console, and is where the DPO will control and configure the HPR system from.

Chapter 6 77 Position referencing

A position-plot display shows the vessel and transponder deployment, while Microsoft Windows dialogues and menus allow full operation of the system.

The transducer will be mounted into a hull unit, which allows deployment and retraction of the pole. A sea chest enables the hull watertight integrity to be ensured when the pole is raised. A motorised raise and lower function can be operated locally or from the bridge. It is essential that procedures are followed ensuring that the sea chest valve is open before deploying the pole, retracting the pole after DP operations are concluded before getting underway, and closing the sea chest after retraction. With the pole retracted (and the sea chest closed) an inspection panel can be removed allowing access to the retracted transducer head.

Transducers will differ in characteristics. A typical modern unit is the HiPAP 500 transducer from Kongsberg Maritime, one of the largest suppliers of acoustic systems. This transducer unit is a golf-ball unit fitted within a 500mm sea chest. It incorporates 241 acoustic elements. When initiating acoustic communication with a transponder the unit will search at a 'wide-beam' configuration, the unit thus has a 100° beam-width around 360° of azimuth. This covers the entire underwater hemisphere. When a transponder is detected, the unit switches automatically to narrow beam focusing onto the position of that transponder. If the transponder moves relative to the vessel, the transducer automatically tracks the transponder.

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6.10.6 Hydroacoustic-aided inertial navigation

In deeper water, long baseline acoustics suffer a number of problems. The travel-time of the acoustic energy may mean a low update rate for the position, while poor acoustic conditions in the water may further reduce update rate. Under these conditions the HPR system may become less reliable and accurate. A possible remedy for this problem is given by incorporating an inertial measurement unit and processor within the HPR system. Kongsberg market such a system as hydroacoustic-aided inertial navigation (HAIN). A characteristic of inertial navigation systems is very low noise but significant drift over time. Conversely, HPR systems may suffer high noise and occasional data dropout. Combining these two principles allows the inertial system to provide infill positioning during periods of dropout or low update rate of the acoustics. This enables greater reliability of the system in deeper water situations.

6.10.7 Multi-user acoustic systems

A further concern in deep water fields is acoustic saturation. This is brought about by the limited variety of position references available in deeper water, added to which is the fact that nearly all floating installations will be using a form of DP. Everybody will be competing for acoustic channels, and acoustic interference is likely. Both the USBL and LBL principles are only applicable to single-user operation. To provide a solution to this problem, a number of manufacturers of acoustic systems have developed multi-user acoustics.

An example of such a multi-user system is the Nautronix NASNet, or Nautronix acoustic subsea network. This system is showing promising development, and consists of a calibrated local-area array of subsea acoustic stations. Each station transmits time- referenced acoustic signals which are detected by hydrophone in the vessel. A comparison between time of transmission and time of reception is enabled, giving a range to the subsea station. The system is, in effect a subsea GPS.

The vessel is passive, ie listening only, so the system may be used by any number of vessels without saturation. Also, since the subsea stations are globally calibrated, this system is suited for globally- referenced positioning (UTM co-ordinates).

6.10.8 Merits and limitations of HPR as a PRS

In general, HPR has proven to be a useful position reference system, and is widely fitted in DP-capable vessels. It does have a number of limitations, however. One of the most significant limitations relates to operation in shallow water. Acoustics perform poorly in shallow water due to the increased noise factors coupled with the difficulties in obtaining a suitable location for the transponder. It must also be mentioned that HPR is a complex system to operate, and that operator training is a necessity.

Chapter 6 79 Position referencing

The advantages and disadvantages of HPR as a position reference for DP are:

Advantages:

• HPR is a versatile system, with many functions additional to position reference

• HPR is an accurate position reference system

• The system is vessel-centred, ie no dependence on third parties

• Ability to track moving targets (eg ROV)

• May be configured as a global reference

• Various configurations available: USBL; LBL; multi-user systems Disadvantages and limitations of HPR:

• Expensive and complex

• Limited range capability

• Suffers degradation from interfering noise and aeration (particularly thrusters)

• Also degraded by water layering, turbulence and impurity

• Accuracy affected by presence of large underwater structures

• Ineffective in shallow water conditions

• Can suffer interference from other HPR users

• Operator training required

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