CUSTOMER :

Survey Operations

Multibeam Echosounder Work Instruction

GR-SRV-421Prepared by:C. AndersonProject Surveyor

Approved by:R MorganGroup Survey Manager

1.0Issue for UseFeb.22.04

Ver.Reason for IssueIssue DatePrepared byApproved by

This is an electronically generated document, which has been reviewed and approved in accordance with the Acergy Management System. An audit trail of review and approval is available within the electronic system. The screen version of this document is the CONTROLLED COPY at all times. When printed it is considered a FOR INFORMATION ONLY copy, and it is the holders responsibility that he / she holds the latest valid version. (, Acergy or a subsidiary thereof, Copyright 2006 and design right reserved. Copying and/or disclosure of the confidential information contained herein is prohibited without written permission of the proprietor.

TABLE OF CONTENTS

31.Scope

2.Objective33.References34.Definitions35.Responsibilities46.Procedures56.1System Objectives56.2System Description56.3System Installation and operation96.4system calibration tests156.5equipment specifications196.6data acquisition22

1. Scope

This Work Instruction covers the installation, operation and maintenance of Multibeam Echosounders.

2. Objective

The objective of this work instruction is to enable suitably qualified personnel to operate Multibeam Echosounder systems.

3. References

GR-SRV-301

Surface Navigation Practice

GR-SRV-311

Single Beam Echosounder Practice

GR-SRV-313

Tidal Prediction Practice

GR-SRV-322

Gyro Calibration and Verification Practice

GR-SRV-411

Heading Reference Unit Work Instruction

GR-SRV-412

Motion Reference Unit Work Instruction

GR-SRV-428

Bathymetric System Work Instruction

GR-SRV-431

Velocity Probe Work Instruction

GR-SRV-442

Tide Gauge Recording Work Instruction

4. Definitions

AUV

Autonomous Underwater Vehicles

BDU

Bottom Detection Unit

DGPS

Differential Global Positioning System

DVL

Doppler Velocity Log

EEZ

Exclusive Economic Zone

FFT

Fast Fourier Transform

GPS

Global Positioning System

IGS

Inertial Guidance System

IRM

Intervention, Repair and Maintenance

kHz

Kilohertz

LBL

Long Baseline

MRU

Motion Reference Unit

NOS

National Ocean Service

ROV

Remotely Operated Vehicle

SV

Sound Velocity

USBL

Ultra Short Baseline

UUV

Untethered Underwater Vehicles

5. Responsibilities

The Survey Manager is responsible for establishing and maintaining this document.

The Project Surveyor has the responsibility of implementing this procedure on relevant offshore projects.

It is the responsibility of all survey personnel involved in the installation and operation of surface navigation systems to comply with this procedure and provide any comments on improvements in this procedure to the Project Surveyor.

6. Procedures

6.1 System Objectives

The growth in the use of multibeam echosounders over the last ten years has been in part due to the ability of these instruments to cover wide swaths of the seafloor in a single pass.

With high-resolution bathymetry obtainable over wide areas and with acoustic frequencies ranging from 10kHz to over 500kHz, multibeam echosounders offer the potential of great accuracy and provide detailed seafloor imagery with scales of economy unavailable from traditional single-beam echosounders.

In commercial surveying, multibeams are employed for many diverse applications and are found installed on ships, survey launches and in towed fish, on ROVs (Remotely Operated Vehicles), in AUVs (Autonomous Underwater Vehicles) and UUVs (Untethered Underwater Vehicles).

Multibeams are used in offshore surveying to provide information in support of an engineering activity such as pipeline installation, jacket and template placement, and telecommunications cable installation. Offshore surveys are also conducted on a regular basis for IRM (Intervention, Repair and Maintenance) or monitoring purposes. Occasionally surveys are made for mapping purposes such as pre-seismic exploration reconnaissance, Exclusive Economic Zone (EEZ) mapping or for aggregates and seafloor mineral exploitation. In all these cases the overriding objective is to chart or image the seafloor in its entirety.

6.2 System Description

The multibeam echosounder system comprises of a transducer, a transceiver and a computer processing system (which integrates and controls all of the separate components). Additionally, a position and orientation sensor(s) is required together with a data storage system.

6.2.1 Transducer

The transducer converts electrical energy into acoustic energy and vice versa. The size of the transducer is designed according to the required beamwidth defined as twice the angular distance from nadir to the point where the expanding wavefront is reduced to half power (- 3 dB). Multibeams operate around a centre frequency and the transducer comprises an array of elements. The size of the array is determined according to the rule that beamwidth is inversely proportional to the number of wavelengths across the aperture; the narrower the beam-width, the longer the aperture required. The individual elements of the aperture are spaced at a maximum distance of 0.5 wavelengths. In high frequency multibeams, the elements themselves become larger than the element spacing. To cater for this the elements are staggered above and below each other in rows.

The transducer array transmits a pulse that is very narrow along-track and wide across-track. Typically, a system may have a transmit beamwidth less than 3( along-track and as large as 85( either side of the nadir. Multibeams without automatic transmit beam pitch-steering require a separate receive array set orthogonal to the transmit array in a configuration known as a Mills Cross.

6.2.2 Transceiver

The transceiver handles both the transmission and the reception of (electrical) signals and is where beam-steering and beam-forming occur; the two defining operations of a multibeam system. Depending on the sophistication of the transceiver, it may also perform pitch-stabilisation beam-steering on the transmit pulse. One beam is transmitted but many more are formed simultaneously to receive the reflected energy from the ensonified area of each transmitted pulse. The resulting pattern shows the single transmit beam intersecting the receive beams in areas called footprints.

A footprint is equivalent to the intersection of the area ensonified with the projection of the received beam pattern on the seafloor to a reference power level (nominally 3 dB). The combination of successive transmit and receive-cycles forms a swath. To determine the speed for the survey platform to achieve 100% coverage, two dependent factors need to be considered a) the transmit beamwidth and b) the transmit pulse-repetition rate. The speed of the survey platform must not exceed a velocity where successive transmit pulses no longer overlap; otherwise gaps will result in the data.

When meeting standards such as the International Hydrographic Organisations SP44, uncertainty in orientation, roll, refraction and bottom detection in the outer beams may reduce the usable swath width. Consequently, swath width may have to be reduced to meet the acceptable depth error specified and, in very shallow water, may not provide 100% cover near nadir regions.

6.2.3 Beam-Steering and Beam-Forming

Beam-steering can be applied to both transmit and receive pulses by orientating a beam in a particular direction. By inserting time delays in the elemental contributions in the transducer array, a virtual array is created whose face is perpendicular to the desired steering direction. As the beam is steered further away, the area of intersection between the beam and the seafloor becomes wider and takes on a parabolic shape. Increasing steering-angle results in the received beam looking at a broader angular sector and hence objects that would be detected in the smaller near-nadir beams are lost in the outer beams. Further factors that affect resolution are lengthening echoes with decreasing grazing angle and lower backscatter returns.

Beam-forming is the term used to describe how the product of the transmit and the receive-beams combine to form narrow pencil-like beams. In a pitch-steered system, the single transmit pulse is steered about the pitch axis of the survey platform maintaining bottom ensonification directly below the ship. Beam-steering is usually achieved through the summation of time-delayed hydrophone contributions across the transducer array. The transceiver can accomplish (receive) beam-steering through Fast Fourier Transform (FFT) beam-forming. In FFT beam-forming, the spatial wavelengths in the across-track array direction of each instantaneously received echo are analysed to determine the direction of contribution.

6.2.4 Processing System

The typical processors in a multibeam echosounder system are the bottom detection unit, integrator and operator unit. The bottom detection unit receives return data from the transceiver and calculates two-way travel times for given beam angles. The travel times are then passed to the integrator and grouped with the position and orientation data captured at the time of swath transmission of each swath. Once grouped, the data is sent to the data logger for storage and to the operator unit for real-time Quality Control of the data during the survey. Data post-processing preparation is normally offline to clean the data and apply the various correction parameters.

6.2.5 Bottom Detection

Most multibeams use two algorithms for bottom detection, dependent on the grazing angle from transceiver to seafloor. In the vertical, amplitude information is used, while in directions towards the horizontal, the reflected signals phase is used. Between the two extremes a weighted combination is the normal solution, however, much research is going on in this area of acoustic science. Amplitude detection and phase detection are applied sequentially on each beam; the systems processing software then selects the best solution.

As the bottom detection methods employed have a direct consequence on the quality and reliability of data acquired, the bottom detection process of a multibeam requires consideration before selection.

6.2.6 Position and Orientation Measurements

After each transmission, the main processor takes the sonar-relative times and the angles from the bottom detection unit (BDU), which are then de-skewed with the time-stamped position information. The position information comprises horizontal position, elevation, 3D orientation and water column information (sound velocity).

Horizontal position is determined from one or more positioning systems. For example:-

A ships position may be determined using a DGPS system.

An ROV, UUV or tow-fishs position may be determined using DGPS for the support vessel and USBL tracking system.

An AUVs position may be determined using an Inertial Guidance System.

Unless the vessel is operating with a high order DGPS system providing vertical accuracy better than 0.5m, the natural recourse for height determination is the ambient, tide reduced, sea surface. Five corrections are required:

a) Tidal height corrections, including met-ocean effects

b) Draft the distance from the transducer to the (static) water level. In the case of an ROV or other remote transducer housing, the separation between the sea surface and the transducer is Depth and is normally determined using a high-precision pressure sensor.

c) Settlement and squat to allow for the amount a vessel underway depresses the local sea surface (settlement) and to allow for the tendency for the vessels stern to move downwards (squat)

d) Velocity of sound to correct for the ever-changing speed of sound through the water

e) Heave, roll, pitch and heading to correct for the dynamics of the survey platform

6.2.7 Data Storage

Some of the latest multibeam systems generate enormous amounts of data. Storage considerations are important and are commonly addressed using mass-storage devices such as magnetic tapes and optical discs.

6.3 System Installation and operation

Special cases of the following apply to multibeam systems installed in towed vehicles, ROVs and AUVs/UUVs. The x, y, z, attitude, heading and timing issues of all these vehicles is just as critical as in a ship installation.

6.3.1 Transducer

The multibeam echosounder transducer assembly should be installed as near as practicable to the centreline of the ship and level about the roll axis. It should be aligned with the azimuth / centreline of the survey platform; this is particularly critical for multibeam systems that do not have beam-steering. In most cases, three-dimensional alignment of transducer arrays requires the dry-docking of a vessel to permit accurate measurements, using land survey techniques, to be made.

For temporary installations, multibeam systems are commonly mounted over the side of vessels. It is vital that the transducer is aligned as above and is tied into the vessels 3D geometry. Further, care must be taken that return signals are not masked from the transducer due to vessel roll. The performance of transducers mounted too far forward or aft can be seriously impeded by aeration, which reduces the signal-to-noise ratio affecting outer-beam performance. It has been found that over-the-side mounts can also suffer from vibration caused by unsupported lengths of tube or from tensioning lines and vortex shedding. It is therefore important that over-the-side mounts are checked at survey speeds before operations begin allowing sufficient time to rectify any problems.

6.3.2 Motion Reference Unit (Heave, Pitch and Roll Sensor)

Wherever possible, the MRU (Motion Reference Unit) should be installed on the centreline of the vessels frames and as near as practicable to the centre of gravity or intersection of the roll and pitch axis. Whenever possible, the same mounting angles should be used as for the transducer; the x-axis of the MRU should match the x-axis of the transducer.

Azimuth misalignment of the MRU will result in depth measurements being in error proportional to the water depth. Misalignment in yaw causes a roll error when pitching and a pitch error while rolling. MRU calibration should be as per manufacturers instructions to allow measurements to be performed with an error budget between 5cm to 20cm. In conventional MRU systems, heave is a difficult component to establish and requires careful observations. For detailed information on the calibration of the MRU, consult the Motion Reference Unit Work Instruction GR-SRV-412

6.3.3 Heading

The precision demand of heading sensors for multibeam echosounding operations is a function of the water depth and the accuracy demand of the survey. While 0.5( in shallow water may be perfectly adequate, the same precision in deeper water, say 2,000m, would result in an alignment offset in the outer beams of over 18m.

As a word of caution, there have been instances in the past, during calibration to determine the error of, for example, a gyrocompass, where the convergence between local Grid North and True North has been applied either the wrong way/wrong sign, or not applied at all.

Gyrocompasses

For many multibeam tasks, the conventional survey-quality gyrocompass is adequate. It is crucial to determine the error between the ships frames and the gyrocompass. Normal gyro-calibration procedures are required and should be performed on a regular basis, per common practice.

GPS Attitude Sensors

The availability of precision (