WORLD CLASS – through people, technology and … · – Fugro FASTRACK. KONGSBERG April 8, 2010 7...

1KONGSBERG April 8, 2010

WORLD CLASS – through people, technology and dedicationWORLD CLASS – through people, technology and dedication


Acoustic Positioning Systems

HydrofestAberdeen University

Wednesday 31st March

Ian Florence

Subsea Construction Manager

Kongsberg Maritime Ltd.


Acoustics are not a Black Art

Most people do not understand acoustics so think they need people with special skills and of course chicken bones to do acoustic jobs

Most acoustic jobs go wrong because they are specified with the wrong equipment for the task in hand.


CRITICAL TECHNOLOGY IN ALL ASPECTS OF OFFSHORE


POSITIONING SYSTEM OVERVIEW

Long Base Line (LBL)– Range measurements to subsea transponders

Short Base Line (SBL)– Range measurements from several transducers to a subsea transponder

Super Short or Ultra Short (SSBL or USBL)– Range and angle measurements to a subsea transponder

Integrated Inertial Positioning– Kongsberg Hydroacoustic Aided Inertial Navigation (Hain)– Fugro FASTRACK


Accuracy & Precision

Accuracy is probably one of the most misused words in the offshore poisonings industry

We must look at the difference between accuracy and precision

Also what inherent weaknesses and strengths each system has


"If you have only one watch, you always know exactly what time it is. If you have two watches,

you are never quite sure..."


Precision Vs Accuracy


Error Budgets

Each position system is made up of various components and processes

Each will have an inherent error

Each will have a systematic and random error

Sometimes called Coloured and white noise respectively


Typical Error Budget


Courtesy of Subsea7

Acoustic Positioning


ACCURACY

Each method has its own accuracy

We need to look at this word and define it more

Absolute accuracy

Relative accuracy

Repeatable accuracy


ABSOLUTE ACCURACY

This is when we need a subsea position with real world coordinates (e.g. latitude/longitude)

For example if the subsea position has to be related to an UKOA block boundary or other subsea objects

Usually consists of tying in a DGPS (Differential GPS) and subseapositioning system

You Are Here!

Lat: 56˚25’ 32.56”N

Long: 02˚15’ 12.56”W


RELATIVE ACCURACY

Usually used when you are not interested in where in the world you are

If you want to position a structure say 35.236 metres @ 124.76 ˚ from an existing wellhead for example


REPEATABLE ACCURACY

This is when you need to place something and come back to it

Say you “wet store” something and you have to return to it on a regular basis


CONSIDER THE ACCURACY REQUIREMENTS

Each position method will have an achievable accuracy

Each method will equate to an amount of equipment and more importantly an amount of vessel time

It is important that the accuracy requirements are not over-specified

Generally if you double the accuracy the cost to achieve that accuracy will increase four fold

Even more important will the increased accuracycost vessel time ?


USBL/SSBL TECHNIQUES

The signal is sent to the transponder and turned around so the range is a simple V=m/s equation. The phase is detected by the wave front hitting element 1 first and at a ∆T later hitting the reference element . This is done on the fore/aft and port/starboard axis.

The baseline is the distance between the elements (super short)

∆T x C

Φ

Baseline

Range =( ∆T x C)/2



Many companies manufacture USBL systems but they can not change the laws of physics

All USBL system work on this basic range / phase measurement principle



Advantages– Easy to calibrate– Most vessel are equipped with systems – Require only one transponder but can track multiple transponders

Disadvantages– As the depth (or distance to target) increases the accuracy

decreases– Can be prone to systematic errors– Requires Vertical reference unit and gyro compass – Deep water high accuracy = high$


LONG BASELINE TECHNIQUES

Long baseline acoustic positioning requires an array of seabed transponders



Advantages:– Used for higher accuracy and

redundancy – Decouples surface navigation– Mitigate the effects of gyro and

VRU error– Survey relative to known points

on the seabed– Independent of the depth of

water– Mitigates the problem with speed

of sound



Disadvantages– Requires the deployment of four or more transponders (costly in

deepwater)– Require lengthy calibration– Usually arrays are job specific and require recovery at the end of

the project– Usually only one user for each array


INTEGRATED INERTIAL SYSTEMS

Inertial systems are used to “smooth” the inherent white noise inaccuracies of USBL or LBL systems



Advantages– Used to augment USBL can avoid deploying an LBL array– Can be post processed to increase accuracy (precision)– Easy to use once set up



Disadvantages:– Inertial units are expensive ($120,000+)– Restricted exportability– Needs high-accuracy heading sensor for the vessel– Set up can be complicated– Pre-operation vessel calibration required


INS is on the increase !

INS systems have now become accepted

Kongsberg are still the leaders but a sign of success is companies manufacturing components and packages to fill this increasing market demand


MULTI-USER LBL


Nasnet

A digitally encoded system allowing multiple users

Long range requiring minimum assets to be deployed

Single seabed reference frame

Highly robust acoustic signalling to mitigate noisy vessels and ROVs

Long duration deployment

A simple to use system

Expandable system architecture providing multiple capabilities


Legacy Tone-Based Technology

• Each channel has its own individual frequency

• With separation guard bands between adjacent channels

• Hence limited number of channels active at any one time

Tone Signal – single frequency, no modulation


Correlation Processing - Tone Burst

Incoming Signal

Stored Replica

Correlation Response

DetectionThreshold

ReasonableTiming


• Many unique codes possible on a single frequency carrier

• Also known as Phase Shift Keying (PSK)

Wideband Signal – Unique digital code phase modulated onto the frequency carrier


Wideband – Range Correlation Processing

Incoming Signal

Stored Replica

Correlation Response

LargePeak

ValidationThreshold

VeryAccurateTiming

Perfect Match – other codes rejected

More Wideband

Wideband and DSP (Digital Signal Processing) is making real advances in navigation telemetry and data logging

This will only increase the accuracy that can be achieved in even deeper water.

Problem is the client will always want more –preferably cheaper



WIDEBAND (SPREAD SPECTRUM)

Allows more channels

Faster calibration time

Allows operations in a noisier environment

Give greater range accuracies

Allows high-accuracy measurements (e.g. jumper measurements) from standard transponder array


Too Many Choices ?

Read the procedure

Some poor project surveyor and bidding department have spent months working out all this for you.

They spent a long time getting the procedure accepted by the client take the time to read it and more importantly understand it.

If you don’t Know ASK


WORLD CLASS – through people, technology and dedicationWORLD CLASS – through people, technology and dedication

WORLD CLASS – through people, technology and … · – Fugro FASTRACK. KONGSBERG April 8, 2010 7...

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Transcript of WORLD CLASS – through people, technology and … · – Fugro FASTRACK. KONGSBERG April 8, 2010 7...