Page 1 of 62 - University of Southern Queensland · Page 16 of 62 Following is an extract from the...

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University of Southern Queensland

Faculty of Engineering and Surveying

Critical Review of Techniques for

Rigid Spoolpiece Metrology

Observation and Reduction

A dissertation submitted by

Mr. Jason Falken

In fulfilment of the requirements of

Course ENG4111 & ENG4112 Research Project

Bachelor of Surveying

27th October 2005

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ABSTRACT

The dissertation investigated the current procedure and techniques used for subsea rigid

spoolpiece metrology. There was a clear need to quantify sources of error, correct use

and application of measurements taken, effects of sensor accuracy and precision on final

reductions and the adequacy of the existing techniques.

The research was based, for paradigmatic and illustrative purposes, on an actual project

undertaken by the author for the sponsor-company.

The research-intensive dissertation drew heavily on the quality of the available

literature. The literature review was thus a major undertaking of the dissertation. The

method was to research and review the appropriate available literature, and determine

the requirements, aims, techniques and problems associated with a typical metrology

project.

An analysis was drawn of the compiled information, highlighting the best techniques

and discussing methods of improvement. The research outcome was thus a scientific

review of the industry standard techniques and a presentation of justified alternatives for

rigid spoolpiece metrology based on a theoretical analysis of the researched

information.

In essence however, the findings of the dissertation are that there are no major methods

of improving techniques. A major finding was that there is much opportunity for further

research and the development and application of new technologies.

DISCLAIMER PAGE

University of Southern Queensland


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ENG4111 & ENG4112 Research Project

Limitations of Use

The Council of the University of Southern Queensland, its Faculty of Engineering and

Surveying, and the staff of the University of Southern Queensland, do not accept any

responsibility for the truth, accuracy or completeness of material contained within or

associated with this dissertation.

Persons using all or any part of this material do so at their own risk, and not at the risk

of the Council of the University of Southern Queensland, its Faculty of Engineering and

Surveying or the staff of the University of Southern Queensland.

This dissertation reports an educational exercise and has no purpose or validity beyond

this exercise. The sole purpose of the course pair entitled "Research Project" is to

contribute to the overall education within the student’s chosen degree programme. This

document, the associated hardware, software, drawings, and other material set out in the

associated appendices should not be used for any other purpose: if they are so used, it is

entirely at the risk of the user.

Prof G Baker

Dean


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CANDIDATES CERTIFICATION

I certify that the ideas, designs and experimental work, results, analysis and conclusions

set out in this dissertation are entirely my own efforts, except where otherwise indicated

and acknowledged.

I further certify that the work is original and has not been previously submitted for

assessment in any other course or institution, except where specifically stated.

FALKEN, Jason Floyd

Student Number: 0031138002

27th October 2005

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ACKNOWLEDGEMENTS

This research was carried out under the principal supervision of:

A/Prof. Frank Young, USQ Staff member

Appreciation is also due to:

Mr Tim Farrow, Subsea7 Ltd Aberdeen

Special thanks for project sponsorship to:

Subsea7 Ltd, Aberdeen

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TABLE OF CONTENTS

Contents Page

ABSTRACT 3

DISCLAIMER 4

CANDIDATES CERTIFICATION 5

ACKNOWLEDGEMENTS 6

TABLE OF CONTENTS 7

LIST OF FIGURES 11

LIST OF TABLES 12

LIST OF APPENDICES 13

ABBREVIATIONS 14

CHAPTER 1 - INTRODUCTION

1.1 Outline of the study 15

1.2 Introduction 15

1.3 The problem 19

1.4 Research objectives 19

1.5 Methodology 19

1.5 Conclusion: chapter 1 21

CHAPTER 2 - LITERATURE REVIEW

2.1 Introduction 22

2.2 Review specifics 22

2.2.1 Introduction to metrology and the work environment 22

2.2.2 Metrology dataset requirements and mathematical procedure 23

2.2.3 Environmental and instrument sources of error 24

2.2.4 Typical tolerance requirements 25


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CHAPTER 3 – RESEARCHED INFORMATION REVIEW

3.1 Introduction 26

3.2 Resource analysis 26

3.3 Consequential effects 26

3.4 Minimum dataset 27

3.4.1 Horizontal angle between hub and spool 27

3.4.1.1 Structure heading 27

3.4.1.2 Spool heading 27

3.4.2 Hub-hub vertical difference 29

3.4.3 Hub inclination 29

3.4.4 Hub-hub baseline 31

3.4.5 Offsets 31

3.4.5.1 ROV sensor offsets 31

3.4.5.2 Compatt sensor and base-plate offsets 32

3.4.5.3 Structure dimensional control 33

3.5 Further mathematical considerations 35

3.6 Instrumentation: requirements, accuracies and precisions 35







3.6.5 Offsets 42

3.7 Sources of error: effects and solutions 43

3.7.1 Inclinometer to base-plate error 43

3.7.2 Acoustic errors 43

3.7.2.1 Acoustic range resolution 43

3.7.2.2 Timing accuracy 45

3.7.2.3 Speed of sound uncertainty 45

3.7.2.4 Ray refraction effects 48

3.7.3 Heading errors 48

3.7.4 Tidal variation 49

3.7.5 Instrument drift, alignment errors, rounding errors, etc 50

3.8 Metrology tolerances 50

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3.9 Data redundancy and error reduction techniques 50








CHAPTER 4 – ANALYSIS

4.1 Introduction 53

4.2 Critical analysis of metrology techniques 53







4.2.5 Offsets 56


CHAPTER 5 - CONCLUSIONS, DISCUSSIONS AND IMPLICATIONS

5.1 Introduction 57

5.2 Discussion 57

5.2.1 Limited technology options 57

5.2.2 Adequacy of current techniques 57

5.2.3 High level of skills offshore 58

5.3 Further research and recommendations 58

5.3.1 Dynamic GPS positioning of vessels at sea 58

5.3.2 Alternate offset measurement techniques 58

5.3.3 Alternate Compatt base-plate design 59

5.3.4 Gyrocompass end-cap for Compatts 59

5.3.5 Other new technologies 59


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APPENDICES 60

REFERENCES 62

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LIST OF FIGURES

Number Title Page

FIGURE 1: 3D Baobab field layout 13

FIGURE 2: Drill centre assembly 15

FIGURE 3: Compatt 16

FIGURE 4: Remotely Operated Vehicle 16

FIGURE 5: Top view of structures showing hub to spool alignment 25

FIGURE 6: Longitudinal profile between hubs 26

FIGURE 7: Hub inclinations in the vertical plane: pitch and roll 27

FIGURE 8: Typical ROV sensor offsets 28

FIGURE 9: Compatt, base-plate and hub vertical offsets 29

FIGURE 10: Booking sheet for DCS manifold 31

FIGURE 11: Trial fit of ROV dummy docking bar in DCN manifold 34

FIGURE 12: DCN and DCS array Compatts and baselines 35

FIGURE 13: Super (Ultra) Short Baseline method (SSBL) 37

FIGURE 14: Effects of Compatt transducer position on required baseline 39

FIGURE 15: Sound velocity profile 44

FIGURE 16: Error in heading due to latitude inaccuracy 46

FIGURE 17: Braced quadrilateral between manifold and x-tree 49

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LIST OF TABLES

Number Title Page

TABLE 1: Octans fibre-optic gyrocompass technical performances 33

TABLE 2: Compatt specification 36

TABLE 3: Surface positioning accuracy 38

TABLE 4: Sub-surface positioning accuracy 38

TABLE 5: Specifications for a 2000m rated digiquartz depth sensor 38

TABLE 6: Specifications for a 14,5° Schaevitz LSOP inclinometer 39

TABLE 7: Metrology tolerances 47

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LIST OF APPENDICES

Number Title Page

A Project Specification 61

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ABBREVIATIONS

The following abbreviations have been used throughout the text and bibliography:

C-O Calculated minus Observed

CNR Canadian Natural Resources

COMPATT Computing and Telemetry Transponder

CTDS Conductivity, Temperature, Density & Salinity

C/W Complete with

DCN Drill Centre North

DCS Drill Centre South

DGPS Differential Global Positioning System

EHF Extra High Frequency

FDP Field Development Plan

FPSO Floating Production Storage Offloading

FTA Flowline Termination Assembly

GMT Greenwich Mean Time

GPS Global Positioning System

LAT Lowest Astronomical Tide

LBL Long Baseline

PAN Programmable Acoustic Navigator

PC Personal Computer

PLEM Pipeline End Manifold

PLET Pipeline End Termination

QC Quality Control

SV Sound Velocity

TBC To Be Confirmed

TOUK Technip Offshore United Kingdom

USBL Ultra Short Baseline

WIN Water Injection North

WIS Water Injection South

WGS 84 World Geodetic System 1984

WROV Work-class Remotely Operated Vehicle

X-Tree Tree

CHAPTER 1

1 INTRODUCTION

1.1 Outline of the study The project investigates and quantifies the techniques and reduction methods currently

available and employed in the offshore oil and gas construction environment, for rigid

spoolpiece metrology. The research is based, for paradigmatic and illustrative purposes

only, on an actual project undertaken by the author for the sponsor-company.

As such, the project encompasses:

‘An analysis will be made to quantify sources of error, correct use and

application of measurements taken, effects of sensor accuracy and precision on

final reductions and the adequacy of existing techniques.’

1.2 Introduction The aim of this section is to provide some insight, through an in-depth background, into

the working environment, requirements, necessities and expectations of a metrology

project.

Figure 1: 3D Baobab field layout

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Following is an extract from the Survey and Positioning Procedure MV Maersk Winner

BAO-TEC-PP-PRO-32935 (08/12/2004):

‘CNR International (Côte d’Ivoire) S.A.R.L. have discovered oil in an

accumulation named Baobab in offshore Côte d’Ivoire Block CI-40 and lies

approximately 65km south-west of Abidjan. Baobab is located some 12km

south-west from CNR’s Espoir field in Block CI-26. CNR’s Partners in the

development are PETROCI Holdings and Svenska Petroleum Exploration A.B.

The Baobab Development is located in 1,000 – 1,700m of water offshore Côte

d’Ivoire some 9km from the continental shelf. The Baobab field is to be

developed using an FPSO moored in deep water (approximately 900m depth).

The seabed is continually sloping and features a steep central canyon in which

the exploration wells Baobab-1X and Baobab-2X are located. However, the

FDP does not include the two exploration wells for production.

Two drilling centres, DCN and DCS are to be located on the canyon west

shoulder in 1,100 to 1,300m water depth from which seven production wells will

be drilled and manifolded. These two drilling centres will be approximately

650m apart and both DCN and DCS will be provided with a six (6) slot

manifold. The current flow assurance work concludes that three 12inch

insulated production flowlines (with a pigging loop per pair of flowlines, located

on both DCN and DCS manifolds) should be employed to convey the produced

fluid from the manifolds to the FPSO, located some 4km from DCN.

All production wells will be gas lifted to maximise production. Testing of wells

is achieved by subsea multiphase meters located on each production manifold.

Water injection providing pressure support and water sweep is required to

improve hydrocarbon recovery by exploiting the reservoir effectively. Two

drilling centres, WIN and WIS are to be located on the easterly flank of the

reservoir in 1,100 to 1,300m water depth from which three (3) water injection

wells will be drilled and manifolded. These two drilling centres will be 3.5km

apart and each will be provided with a four (4) slot manifold. A single 12inch

water injection flowline will supply treated water to each manifold from the

FPSO, located some 4km from WIN.

Flowlines arriving at the FPSO will be terminated by a deep water riser system.

A multiplexed electro-hydraulic subsea control system will be installed to

control and operate the subsea facilities. A single riser umbilical will connect to

two separate umbilicals, one to the production facilities and one to the water

injection facilities.’

Metrology is the term used to describe the series of measurements and calculations

required to produce a 3D spatial relationship between 2 or more subsea structures

(EHXT Hub and Manifold Hub in drawing), used as dimensional offsets for the

production of a connecting structure (Well Jumper in drawing).

The need for subsea measurement exists because the manifold and x-tree (EHXT), as an

example, are installed independently on the seabed to a much poorer accuracy than is

needed for the jumper to fit. As a result, all required dimensions of distance, heights,

bearings and inclinations are affected. The metrology has to be done post installation of

the manifold and EHXT.

The basic requirements of a dual-bore rigid spool (jumper) metrology are horizontal

distance between hubs, height difference between hubs, inclination at each hub, heading

at each hub and bearing of the spool. Dual-bore refers to 2 bore-holes inside the hub,

therefore making relative headings between structures important to align the bores on

the hubs to the end-caps on the spool.

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Figure 2: Drill centre assembly
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Due to the accuracy and precision required, the measuring instruments used are high-

end subsea units capable of providing the necessary tolerances. The primary instrument

is the EHF Compatt with digiquartz and inclinometer end-cap. These acoustic

instruments are capable of fulfilling the distance, heighting and inclination requirements

of the dataset.

Figure 3: Compatt

The water depth is far beyond the workable range of divers, so ROV’s are used for all

subsea manipulations. These vehicles provide ideal platforms for mounting survey-rated

sensors for use in high precision metrology measurements. Fibre-optic gyroscopes

(FOG), digiquartz depth sensors, and acoustic transducers (ROVNav) are some of the

crucial metrology-specific sensors mounted on the ROV’s.

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Figure 4: Remotely Operated Vehicle

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Specific sensor uses, techniques and sources of error will be discussed in greater detail

in a later chapter.

1.3 The problem The abstract nature and adverse working environment of a metrology project lends itself

to frequent insitu technique adaptation and improvisation. As a specialist construction

project, it suffers the effects of an industry-driven solution, where time and financial

constraints do not allow for an academic study of the subject. The existing methodology

for such a task thus commonly comprises techniques adapted from conventional survey

and subsea measurement techniques. A scenario thus exists where professionals and

companies have devised their own preferred methods.

1.4 Research objectives This research will firstly quantify and explain the typical dataset and computational

requirements of a basic metrology project, followed by an in-depth analysis of the

techniques and equipment implications available. It is not the intent to find an ultimate

solution, but rather an in-depth analysis of one such project with the aim of improving

techniques or verifying the correct application thereof within the donor-project.

The research will thus present a best practise solution and the justification thereof, for

rigid spoolpiece metrology, based on a theoretical analysis of the researched

information.

The objectives of the dissertation will follow the program methodology as listed in the

Project Specification and is listed as 1-10 in the methodology below.

1.5 Methodology The project is a theoretical analysis of the assembled information and the isolation of a

best practice for rigid spoolpiece metrology. Hence, the method will be to review the

relevant literature, critically assess the options and find the best technique. The

objectives listed below are sequential and in a result-dependent order.

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The first phase will be to research and review the appropriate, available literature. The

sequential steps 1-6 below will achieve this.

1 Briefly review available literature and common practises, with reasoning towards

need for further investigation into techniques.

2 Investigate minimum dataset requirements and determine basic mathematical

procedures for data reduction.

3 Evaluate and review instrumentation requirements and sensor accuracies and

precisions.

4 Research and investigate sources of environmental and instrument error.

5 Research typical tolerance requirements for spoolpiece metrology and fabrication.

6 Critically examine and analyse surveying techniques used for collection of dataset.

The second phase will be to critically assess and analyse the alternatives and find and

justify the best technique. Steps 7-11 are thus the next phase of analysing and

interpreting the data.

7 Investigate and review alternate techniques for measurement of inclinations,

bearing, distance and heighting, with particular emphasis on the effect of network

design for 3D solutions in a least squared adjustment.

8 Investigate and review effects of sources of error on data quality and compliance to

tolerance requirements.

9 Investigate and review techniques for data redundancy and error reduction.

10 Design alternate procedure for collection and reduction of metrology data, if

deemed necessary.

11 Evaluate and analyse outcome of chosen procedure.

The effects of the above, allow the methodology to be simplified and aligned with the

objective list. The flow and progress to project completion will closely follow the

objective sequence. The techniques for generating results and completing each objective

will thus be largely analytical and will rely heavily on the quality and extent of the

literature review.

It can be seen that a major part of the project lies in the research and acquisition of

relevant information. This is the reason for the first objective and its aim of collating

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and reviewing as much information relative to the topic, as is deemed necessary.

Throughout the project, this objective will be revisited on-the-fly, to acquire more

information as the need arises.

A major aspect of the analysis is assurance and quality control. This will be maintained

by interaction, at significant levels within each objective, with institutional support (via

the project supervisor) and industry support (via the associate supervisor and sponsor-

company). This will minimise compound errors and maintain confidence throughout.

1.6 Conclusion: chapter 1 The dissertation aims to investigate current procedure and techniques used for subsea

rigid spoolpiece metrology. The research will use a case-study for comparison and the

procedure and techniques used therein will be analysed.

The research will produce a scientific review of the industry standard techniques and

offer justified alternatives based on the outcome of the research.

The dissertation is research-intensive and will draw heavily on the quality of the

available literature. The literature review is a major undertaking of the dissertation and

will be revised throughout the length of the project.

The outcome of the study will clarify and justify appropriate procedure and techniques

for rigid spoolpiece metrology.

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CHAPTER 2

2 LITERATURE REVIEW

2.1 Introduction Chapter 1 identified the need for an in-depth literature review to investigate current

procedure and techniques used for subsea rigid spoolpiece. The discovery and review of

this information is a major undertaking of the project. All analysis and conclusions will

draw heavily from the quality and availability of this information.

Each objective as discussed in chapter 1.4 and 1.5 requires specific research. The

literature review is thus a continuous task to supply this data and search for new and

relevant information.

This chapter outlines the findings and relevance of all literature reviewed and used in

this dissertation.

2.2 Review specifics The literary review is addressed according to specific requirements as set out in the

project objective list. These requirements are headed below with significant reviews

sequentially listed.

Due to the adverse nature of the topic, not much specific information was available

through libraries (USQ Library and others). As a result, the majority of the literary

resources used were sourced through the sponsor company and other offshore

construction service-provider companies.

2.2.1 Introduction to metrology and the work environment

Survey and Positioning Procedure MV Maersk Winner BAO-TEC-PP-PRO-

32935 (08/12/2004)

An extract can be viewed in 1.1 and serves as a layman’s introduction to

the marine environment and metrology. The passage is self explanatory.

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Introduction to Physical Oceanography (2002)

This textbook served as a brief introduction and reference to the

technical terms and environmental conditions of the marine environment.

It is not a major contributor to this dissertation, but is invaluable as a

reference text.

2.2.2 Metrology dataset requirements and mathematical procedure

Flexible and Rigid Spoolpiece Metrology Procedure BAO-TEC-PP-PRO-32932

(08/12/2004), sec. 2.4

The Scope of Work as defined in the procedure for the case-study

provides an overview of the procedure required and expected outcomes

of a typical metrology project. The dissertation will investigate the

adequacy and relevance of this procedure.


(08/12/2004), sec. 8

This section defines in detail the client-agreed procedure and

requirements for conducting the rigid spoolpiece metrology from start to

end. The dissertation will investigate the adequacy and relevance of this

procedure.

Elementary Linear Algebra (2000)

The textbook serves as an introduction to linear algebra and focuses

mainly on matrix techniques. Linear transformations are a major part of

metrology calculations, and as such, matrices represent the most

constructive means of addressing this problem. However, the text does

not offer in-depth and adequate information on linear transformations

relative to metrology. The text does offer good introduction to matrices

and will be used as referral for general matrix techniques.

DCS Production Manifold Dimensional Control Results ET0141E-I005/GO

(15/10/2004)

This report provided the specifics of the offset measurements for the

subsea structure for the DCS manifold. The document is an accurate

report of actual procedures and results used and obtained for dimensional

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control surveys. As such, it provided a solid basis for the analysis of the

techniques and expected outcomes.

2.2.3 Environmental and instrument sources of error


(08/12/2004), sec. 7

The section handles the error budget for the operation and outlines

expected sources of error for each critical measurement. The text will be

viewed in conjunction with manufacturer’s specification for the

instruments to be used.

Acoustic Positioning Training Course Lecture Notes (24/04/1997)

The lecture notes provide an extremely detailed, in-depth analysis of

sources of acoustic noise, interference and error of particular significance

to Sonardyne LBL positioning using Compatts. As this is the primary

means of metrology measurement offshore, the notes are adequately

suited and will provide an important information source for the

dissertation. Due to the length and depth of the document, the source will

be reviewed and analysed further throughout the length of the

dissertation preparation.

Octans User Guide MU/3453/EGF/003/D (08/2005)

This guide is a detailed presentation of the specifications for the iXSEA

Octans fibre-optic gyrocompass. It is also a very comprehensive

technical manual on fibre-optic gyroscope systems. It proved very useful

in detailing options for heading determination by gyrocompass

instruments.

Compatt Mk4 Product Brief (28/10/2003)

As the primary subsea measurement instrument, this brochure provided

the core specifications for the Mk4 Compatt.

Maintenance Manual EHF Compatt Mk4 (20/06/2001)

As a maintenance manual, this document provided in-depth information

on the workings and options available for EHF Mk4 Compatts. This

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document was used extensively in the research for information pertaining

to these sensors.

User’s Manual for Digiquartz Broadband Intelligent Instruments (10/2004)

The manual was used to investigate the requirements, accuracy

expectations and specification for digiquartz depth sensors. The text was

sufficient to provide a background into this type of sensor.

The Character of the Tide (06/11/2003)

The whole text provides an insight into the science of tides and was used

extensively in the dissertation. In particular, the book provided detail for

tidal prediction and the effect of tide on depth determination to the

accuracy required for rigid spoolpiece metrology.

2.2.4 Typical tolerance requirements


(08/12/2004), sec. 5, sec. 6

The Survey Tolerances and Dimensional Control requirements as set out

in the procedure are brief and will need further investigation. Questions

of attainability and suitability of suggested requirements and techniques

will be analysed.

2.3 Conclusions: chapter 2 It was found that relevant literature is scarce, due to the specialist and abstract nature of

the subject. However, information drawn and interpolated from the available literature

and the authors own experience is deemed sufficient to adequately address the questions

posed by the topic. This is expressed in subsequent chapters.

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CHAPTER 3

3 RESEARCHED INFORMATION REVIEW

3.1 Introduction Having been introduced to the subject matter, objectives and methodology of the

dissertation in chapter 1, chapter 2 then provided the backbone of the research via an

intensive literature review.

This chapter summarises the bulk of information exposed by the literature review in

chapter 2, collated for the first primary objective of the dissertation. Following is thus a

breakdown of the minimum requirements and outcomes for a typical metrology

undertaking. The information addresses all requirements of the case-study.

3.2 Resource analysis As a largely analytical research paper, resources are restricted to only 3 major variables.

1 Case-study data.

2 Literature.

3 Hardware and software requirements.

The case-study as described in the introduction has been completed and implemented in

the field. All data has been collated and made available for this dissertation and is

presented in this chapter.

3.3 Consequential effects Due to the analytical nature of the project, consequential effects are limited.

Consequences of safety, sustainability and ethics pertaining to this dissertation have

been considered and found to be largely insignificant.

However, it is worth noting that the possible effects of the result indicating a

significantly different procedure can highlight issues of previously inappropriate

technique or raise questions of time management of a metrology task.

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It is the intention of the dissertation to investigate and bring any such discrepancy to the

fore. The consequence is understood and the dissertation is justified by its perceived

contribution to the measurement sciences. These effects, if any, are discussed and

analysed in chapters 4 and 5.

3.4 Minimum dataset As was identified in chapter 1, there is a need to measure the dimensions required to

bridge the 2 hubs by fabricating a spoolpiece. Metrology is used to determine these

dimensions within the specified tolerances. A minimum dataset thus exists to provide

the basis of the 3 dimensional relationships required for the manufacture of a

spoolpiece. This chapter covers these minimum measurement requirements.

3.4.1 Horizontal angle between hub and spool

With respect to dual-bore hubs, this is critical as it accounts for the angular

alignment of the end-cap relative to the spool. In figure 5, this is accounted for by

angles α and β. This measurement is derived from the difference between 2

azimuths.

3.4.1.1 Structure heading

Azimuth is only required in a relative frame, but True North is used as

datum, due to the use of north-seeking gyrocompass instruments. The

structure heading is used along with measured dimensional offsets for the

structure, to determine the Hub heading. The Hub heading is indicated in

figure 5 as axis Y1 and Y2.

3.4.1.2 Spool heading

This is the bearing between the two Hubs and is determined by

mathematical polar after deriving coordinates for each Hub within an

acceptable north-oriented projection. Subsea, this is accomplished by a

least squares adjustment of range measurements between Compatts

installed at each Hub and a calibrated Compatt array pre-installed on the

seabed. Compatt transducer positions are translated to Hub centre

positions by a 7-parameter shift using measured shift parameters to be

fully described in a subsequent chapter.

Figure 5: Top view of structures showing hub to spool alignment

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3.4.2 Hub-hub vertical difference

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measurements. Lowest Astronomical Tide is used as the datum for the case study.

3.4.3 Hub inclination

itch and roll, relative to the vertical plane or Z-axis. As

cal gravitational direction at either end of

This value accounts for the Z-axis in the 3 dimensional relationships between the 2

hubs. As each structure is installed on differing seabed topography, it is expected

that this value is an unknown that needs to be accounted for within the required

tolerances. A tidal datum is essential, due to the possibility of tidal drift between

Figure 6: Longitudinal profile between hubs

Inclination refers to the p

each structure is not installed perfectly level, due to topography or subsidence, and

each Hub is not evenly aligned to the structure, it is evident that the inclination

must be known to allow for flush fitting of the spool end-cap to the Hub.

Inclinations are derived relative to the lo

the spool. The inclination is determined aligned to the structure heading for ease of

measurement, but is required in relation to the spool heading. It is translated from

inclination in plane of structure heading to plane of spool heading using the

previously determined horizontal angle between hub and spool.

Figure 7: Hub inclinations in the vertical plane: pitch and roll

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3.4.4 Hub-hub baseline

This is the primary horizontal-plane measurement. Using Compatts installed in

each Hub, ranging between the transducers produces the primary range

measurement. It is needed to translate the slant distance between transducers to a

horizontal distance between the two Hub centres using the acquired Hub headings,

spool heading and Hub inclinations in the plane of the spool. Figure 6 indicates the

horizontal and required slope distance between Hub centres.

3.4.5 Offsets

3.4.5.1 ROV sensor offsets

Figure 8: Typical ROV sensor offsets

The primary ROV sensors are a ROVNav Transducer, a Fibre-Optic

Gyrocompass, a Motion Reference Unit, a Paroscientific Digiquartz

Depth Sensor, a Direct Reading Sound Velocity Probe and a structure

docking-bar. The ROVNav is the primary source of ROV position, while

the MRU provides pitch, roll and heave values. The gyrocompass

provides rotation and the Digiquartz provides depth readings. By using

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all these sensors along with measured x, y and z offsets for each from a

reference point of the ROV, a 3d position can be generated for any other

point measured point of the ROV. The docking-bar is used to dock into

the manifold or x-tree structure, so as to transfer heading and position to

the structure. Position and heading can thus be calculated for the

docking-bar. The docking-bar is assumed flush to the structure docking-

receptacle.

3.4.5.2 Compatt sensor and base-plate offsets

The EHF Compatt comprises 3 sensors namely, the transducer head, the

inclinometer sensor and the digiquartz sensor. These are aligned

vertically with each other in the Compatt tube. The inclinometer sensor

readings do not require translation by offsets as they are angular

measurements of pitch and roll, however the transducer and digiquartz

sensors measure distances and depths respectively and thus require

offsets measured in the vertical plane of the Compatt to reduce them to

the Compatt base-plate plane. The Compatt base-plate is of a measured

thickness and thus the digiquartz depth can be translated to a top of Hub

depth by using these measured offsets. The Hub centre to top of Hub

offset is supplied from dimensional control surveys of the structure, so

that these depths and distances can be translated to the Hub centre.

Figure 9 below depicts the Compatt and Hub cross-section.

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0.905m

0.441m Ref

0.020m

Transducer C O M P A 1.366m T T Digiquartz

Base-plate H U B

Figure 9: Compatt, base-plate and hub vertical offsets

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3.4.5.3 Structure dimensional control

During the manufacture of the x-tree and manifold, alignment errors are

introduced in the hub and docking receptacle positions. In order to

translate bearings and positions from the docking receptacle to each hub,

the x, y and z offsets have to be known to within an acceptable accuracy.

These are determined in the fabrication yard using conventional land

survey techniques by Total Station. Figure 10 below shows the DCS

Manifold with all relevant targets such as the hubs and docking

receptacle used during the dimensional control survey.

Figure 10: Booking sheet for DCS manifold

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3.5 Further mathematical considerations The Compatt transducer positions (E, N, and Z) for each hub, derived from the least

squares adjustment for spool heading calculation, is reduced to hub centre positions (E,

N, and Z) by the addition of DE, DN and DZ. These changes in positions are computed

using the inclination in the plane of the hub heading, the hub heading and the horizontal

and vertical offsets (Compatt transducer to Hub centre). These are used in a 7-parameter

translation matrix which is reduced to 4-parameters by the absence of scale, x and y

offsets (transducer and hub centre are coincident in vertical plane).

Inclinations are translated from the plane of the hub heading to the plane of the spool

heading using the horizontal angle between the hub and the spool headings. This is a

simple rate of change calculation through 360°.

The direct range between transducers is used as the final range and so, the hub positions

need to reflect this. The manifold transducer position, direct range, bearing between

hubs and difference in height is used to determine a new x-tree transducer position. This

is easily accomplished using linear geometry. The DE, DN and DZ determined earlier

are then added to the x-tree transducer positions to determine a new x-tree hub position.

These calculations then leave us with the following:

1. Inclinations in the plane of spool heading

2. Hub centre positions (Easting, Northing and Depth) based on the direct range

between Compatts

From these it is then possible to easily derive all the required measurements.

3.6 Instrumentation: requirements, accuracies and precisions The typical sensors used for metrology will now be introduced. Due to the accuracy and

precision required, the measuring instruments used are high-end subsea units capable of

providing the necessary tolerances and surviving the adverse environmental conditions.

Each measurement requirement as highlighted in the previous chapter will be addressed

in terms of the requirement and adequacy of these sensors.



The structure (manifold or x-tree) heading is determined by using a ROV

mounted survey-grade subsea gyro for determining the structure

headings relative to true north. Gyros are prone to drift errors and

changes in speed and latitude. Fibre-optic gyros provide a significant

advantage in repeatability over a standard mechanical gyrocompass. For

the case-study, an Octans fibre-optic subsea gyrocompass by iXSEA was

used to provide manifold and x-tree headings relative to True North.

Listed below is the performance specification as supplied by the

manufacturer.

Dynamic Accuracy (whatever sea-state) ± 0.2° Secant Latitude (*)

or 0.1° Rms

Settle point error ± 0.1° Secant Latitude

or 0.05° Rms

Settling time (static conditions) 1 Minute

Settling time at sea 3 Minutes

Repeatability ± 0.025° Secant Latitude

Resolution 0.01°

No Latitude limitation

No speed limitation

Table 1: Octans fibre-optic gyrocompass technical performances

The structure heading is attained by physically docking the ROV onto

the structure and logging data for the FOG on the ROV. This is

accomplished by docking receptacles on the relevant structure and a

docking bar attached to the ROV bow. The ROV docking bar can be seen

in figure 8. Below is an illustration of a dummy docking bar slotted into

a docking receptacle of the DCN manifold.

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Figure 11: Trial fit of ROV dummy docking bar in DCN manifold


As stated previously, the bearing between the two hubs is determined by

mathematical polar after deriving coordinates for each hub within an

acceptable north-oriented projection. To arrive at the coordinates,

Compatts are installed in each hub, as well as in an array around the area.

The figure 12 below illustrates the array Compatts as installed around

DCN and DCS drilling clusters. With Compatts able to acoustically

range between each other, it is possible to solve for the absolute positions

of each hub Compatt transducer in a least squares adjustment, if the array

Compatt transducer positions are known within a suitable projection.

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Figure 12: DCN and DCS array Compatts and baselines

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Table 2: Compatt specifications

To ensure minimum transmission loss of the acoustic signal and highest

accuracy of range measurement, the highest available frequency is used.

Despite their limited range, EHF Compatts are deemed most suitable to

the task. The Sonardyne (manufacturer) supplied data for the available

Compatt frequencies are supplied in table 2 above.

It can be seen from this table, that the EHF frequency band offers the

best range accuracy, but has a disadvantage in the maximum effective

range of the acoustic signal. This will limit the placement of the array

Compatts to within 1km of each other. Some ranges may not be

attainable, thus possibly negatively influencing the balance of the least

squares adjustment.

Assuming that all inter-Compatt ranges can be adequately measured, the

array Compatts will also need to be coordinated within the reference

framework of the chosen projection. This is accomplished by

coordinating at least 2 Compatts into the projection, and solving the

remainder in a least squares adjustment. The technique used for this is to

coordinate the chosen Compatts by a suitably accurate GPS system

combined with an acoustic positioning system termed USBL (Ultra Short

Baseline).

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he Sonardyne Lecture Notes describe USBL positioning as follows:

‘In the … Ultra Short or USBL … technique, … a single, more

In effect, simul nt are taken to

or the case study, a Starfix HP high-accuracy DGPS system by Fugro

T

complex hydrophone which can measure the angle of arrival of

an acoustic signal in both horizontal and vertical planes, ... This

is achieved by phase comparison techniques. Thus a single

beacon may be "fixed" by measuring its range and bearing.’

Figure 13: Super (Ultra) Short Baseline method (SSBL)

taneous USBL and DGPS measureme

produce a position for the Compatt. A series of these measurements,

termed a box-in, is gathered and meaned to produce a most likely

position for the Compatt within the chosen projection (translated within

the software from GPS WGS84). The term box-in refers to the pattern of

data gathering to reduce errors in the acoustic segment, introduced by

transducer and ship alignment errors.

F

was used as well as a modern HiPAP 500 USBL system by Simrad. The

manufacturer supplied system accuracies are supplied below.

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3.6.2 Hub-hub vertical diff

rmining relative depths at each Hub, is the

hermal sensitivity: 0.0008% Full scale = 16mm

erence

The primary instrument used in dete

Digiquartz end-cap attached to the Compatt. It is a Compatt add-on module which

allows depth and temperature values to be measured and telemetered via the

acoustic signal through the water column to a ship or ROV mounted transceiver. A

set of readings for a 2000m rated sensor, will typically yield an error of 16mm in

the relative depths between the two hubs.

T

Resolution: 0.000001% Full scale = 0.02mm

3.6.3 Hub inclinat

rmined by using a Compatt with an inclinometer end-cap

Surface Positioning System Expected Accuracy

Primary DGPS: Starfix HP +/- 0.5m

Secondary DGPS: Starfix HP +/- 0.5m

Sub-surface Positioning System Expected Accuracy

HiPAP 500 USBL

<0.5% of slant range, with angular

resolution of 0.3° (slant range is

limited to twice water depth)

Table 3: Surface positioning accuracy

Table 4: Sub-surface positioning accuracy

Table 5: Specifications for a 2000m rated digiquartz depth sensor

ion

Hub inclinations are dete

installed instead of a digiquartz end-cap. The sensor, measures the pitch and roll

relative to an alignment mark on the Compatt body and the local gravitational

vertical. The Compatt is aligned in the hub with the structure heading as reference.

The Compatt acoustically telemeters the pitch and roll values expressed as a

voltage, to a surface transducer. A formula is provided by the manufacturer to

convert voltages into angles. The inclinometers are available in 3 different

resolutions characterised by the trade off between accuracy and effective range.

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The 14,5° maximum range inclinometer, is the most accurate available and as such

is the choice inclinometer for metrology operations.

Accuracy: ±0.1°

Resolution: ±0.01°


Compatts are capable of ranging between each other and telemetring the total turn-

around time back to the surface transceiver. If the speed of sound is then known,

this time can be converted to a slant distance between the 2 Compatt transducers.

The EHF bandwidth provides an achievable range accuracy of less than 0.025m as

already discussed under Spool Heading determination above.

To arrive at a hub-hub baseline from a transducer-transducer baseline, we have to

consider the inclination and horizontal angle between the hub and the spool as

illustrated in figure 14 below.

3.6.5 Offsets

The offset consideration is straight-forward, and has been adequately underlined in

section 3.2.5. The required offsets are either supplied by the manufacturer,

measured by tape or total station.

Figure 14: Effects of Compatt transducer position on required baseline

Table 6: Specifications for a 14,5° Schaevitz LSOP inclinometer

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3.7 Sources of error: effects and solutions

3.7.1 Inclinometer to base-plate error

A base-plate is required to slot and align the Compatt into the hub. However, an

error is introduced as the Compatt cannot be mounted perfectly perpendicular to the

base-plate. The inclinations as measured by the Compatt are thus not a true

reflection of the hub inclination due to this error. The solution employed is to rotate

the Compatt through 90° and acquire readings in all 4 quadrants. The mean of these

readings effectively nullifies the effects of this error. The base-plate has 4

alignment markings suitably named north, south, east and west. The Compatt is

attached, aligned to the north marking and the base-plate (and Compatt) can now be

rotated while slotted into the hub. The base-plate markings are now easily aligned

to the hub heading mark. Figure 3 illustrates a Compatt mounted on a base-plate,

with base-plate alignment marks shown.

3.7.2 Acoustic errors

Factors affecting the signal strength performance of an acoustic system will not be

considered, as these do not affect the accuracy of the LBL system. As such, only

the factors affecting the accuracy will be reviewed. The Sonardyne Acoustic

Positioning Training Course Lecture Notes (24/04/1997) define 4 major influencing

factors:

3.7.2.1 Acoustic range resolution

The resolution of the acoustic pulse detection system is the primary

limitation on short-range LBL accuracy. This is largely dependent on the

frequency and signal processing techniques.

EHF has a bandwidth of ±800Hz incorporating 12 separate channels and

represents the highest acoustic frequency available and thus also the

highest bandwidth due to direct proportionality. Ranging precision is

also proportional to bandwidth and the more channels in the band, the

lower the precision. With a wider bandwidth, pulse length can be

reduced, allowing for extended battery life and less pulse overlap

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between different channel replies. The Sonardyne Acoustic Positioning

Training Course Lecture Notes (24/04/1997) aptly states:

“The total number of channels determines the frequency

separation, which in turn determines the electrical bandwidth in

the receivers and thus the ranging precision. A larger number of

channels reduce the bandwidth, thus decreasing precision and

requiring longer pulses.”

Signal detection and validation are techniques used by the LBL system

for signal assurance. Detection and validation times increase as the

frequency decreases due to the lower bandwidth at low frequency, thus

requiring longer pulse lengths. The consequence of this is lowered

precision. With EHF this effect is largely reduced due to the high

frequency. Full wave processing allows the timing variation to be

effectively halved under low noise conditions (such as deep water

operations). This feature is incorporated as standard in EHF equipment

allowing for specialised uses such as in spool piece metrology surveys.

Multi-Path interference is defined as follows:

“Multi-Path interference occurs when multiple signals due to

refraction or reflection arrive at a receiver coincident with the

direct signal, causing pulse overlap.”

If the ghost signal arrives before the detection time, the direct signal may

not be detected as result of destructive interference. If the ghost signal

arrives during the validation process, the direct signal will most likely

not be validated. The effect of these signals will result in either no signal

being validated, or a later return (corresponding to reverberation) being

detected and validated giving a range delay. This can occur when either

the sea surface or subsea structures reverberate on similar amplitudes to

the direct signal. Lower frequency bandwidths are greater affected due

to the longer pulse lengths and as such, EHF is least affected. Multi-path

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is managed by data-redundancy techniques and the comparison against

expected outcomes.

3.7.2.2 Timing accuracy

The range between two Compatts is achieved by measuring the two-way

travel times between the interrogator and the other Compatt and then

multiplying the travel time by the speed of sound to obtain distance.

We have to consider the turn-around delay (the time between the

detection of the interrogation signal and the reply transmission) and the

transponder detection delay. The turn-around time can be user-set to

manage pulse-overlap problems and is controlled by a crystal timer

accurate to 2.5mm. Detection delay is compensated for in the firmware.

Both timing issues can thus be ignored as a source of influential error.

3.7.2.3 Speed of sound uncertainty

The speed of sound at the measurement depth acts as a scaling factor for

acoustic range measurement. The range accuracy depends on the

accuracy to which the speed of sound has been measured. The speed of

sound can be measured in 3 ways:

1 Using a velocimeter

2 Using known targets on the seabed

3 Computed

A velocimeter operates on the sing-around principle where a high

frequency pulse is timed over a fixed distance. These instruments are

calibrated under known propagation conditions in a laboratory and must

be supplied with current calibration certification. Accuracy expectations

are within ± 0.03 m/second (0.2 percent). These are the most accurate

means of determining the speed of sound and are thus the instrument of

choice for metrology.

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The second option is based on the same principle, but on a larger scale.

The difficulty in determining the distance between transducers limits the

accuracy to which the principle can be applied.

It is also possible to determine the speed of sound by formulae

requirement measurements of depth, salinity and temperature. As this

technique is easy to incorporate into a single sensor and provides

consistent results without frequent recalibration, it is not as accurate as a

velocimeter due to the propagation of errors through the individual

sensors. A temperature change of 0.2°C gives a speed error of 0.8m/s or

0.05%, assuming a speed of 1488.0m/s. This is equivalent to a range

error of 0.05m over a 100m baseline and is inadequate for metrology.

Due to changing conditions, continuous monitoring and update of the

speed of sound is essential to maintain quality assurance during the

duration of the range measurement.

VESSEL Maersk WinnerDATE 24-April-05 11:30

DIVE NO.LOCATION

INSTRUMENT Winson Tritech

MEAN VELOCITY 1493.09 m/sBOTTOM VELOCITY 1488.10 m/sT'DUCER VELOCITY 1535.70 m/s

AVERAGE DENSITY 1.0266OBSERVED JF/JCCHECKED MM/SB

DCS MetrologyROV 89 Dive 136

T/S DIP @ DCS MetrologyWCH 26-P6 Xtree & WCH 22-P8 Xtree

24-APR-05 11:30

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1480.00 1490.00 1500.00 1510.00 1520.00 1530.00 1540.00

Sound Velocity (m/s)

Dep

th (m

etre

s)

Figure 15: Sound velocity profile

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3.7.2.4 Ray refraction effects

The speed of sound varies with depth due to the changing temperature,

depth and possibly salinity (due to layers of fresh water mixing). Ray

bending is the phenomenon where these layers result in refraction of the

acoustic wave front as it passes through the water. These conditions

introduce an error into the measured range due to changing speed of

sound. A slant-range correction matrix will need to be applied using the

sound velocity profile. Due to the short range and minimal depth

difference for a typical deepwater metrology project, the ray bending

effect is minimal and a constant speed scaling factor is assumed. The

profile in figure 15 illustrates the effect of dropping temperature on

speed of sound between the surface and 800m where after, the effect of

increasing pressure takes over and increases the speed of sound.

3.7.3 Heading errors

The fibre optic gyrocompass is the primary heading measurement sensor and has a

few errors associated with it. All gyrocompasses are sensitive to the speed of travel

of the vessel and current latitude. The x, y, z offsets relative to the docking bar for

the gyro mounted on the ROV also needs to be considered.

The international standard (ISO 8728) defines that:

“Course error in degrees for a gyrocompass aligned north-south is

determined by the formula V/5π x the secant of the latitude, where V is the

North component of the speed in knots”

With a dynamic accuracy of 0.2 degree x secant of latitude for a Octans fiber-optic

gyroscope, the speed in knots at which an error greater than this appears is:

“V_MAX = 0.2 x 5π = 3,2knots”

It can be seen that since the heading readings are logged while the ROV is

stationary (while docked onto the structure), this error can be largely ignored as its

effect will be within the system accuracy. It is still necessary to ensure the speed

setting for the gyro is set to zero.

As the gyrocompass approaches the poles, the heading error due to the secant of the

latitude tends towards infinity. An inaccuracy in the known latitude introduces a

system inaccuracy which becomes more relevant as the gyro approaches the poles.

This error is however very small and is made further insignificant by the localized

nature of a metrology project and the availability of accurate latitude from GPS

units. The curve in the figure 16 shows the heading error in degrees multiplied by

the secant of the latitude versus the latitude of the current location, assuming that

the latitude entered in the Octans unit is incorrect by one degree.

Figure 16: Error in heading due to latitude inaccuracy

3.7.4 Tidal variation

The acquisition of the relative depth difference between hubs requires that the same

instrument is used to measure depths at each hub due to reasons discussed in

previous chapters. The time between readings allows for a tidal variation to occur,

thus introducing an error into the relative depth between the 2 hubs. Tidal

prediction software is used to determine tidal effect at each reading and this is used

to align each reading to a height datum such as Lowest Astronomic Tide. GPS time

is logged at each reading to accurately manage the timing of each measurement.

Tidal prediction is not an exact science and its accuracy is relative to the adequacy

of the tidal model for the area. As such, time between measurements at each hub is

kept within 30 minutes to reduce the total effect of tidal variation.

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3.7.5 Instrument drift, alignment errors, rounding errors, etc

The remaining errors are not specific to metrology and are managed by adopting

good survey practice. Data redundancy techniques, covered in a subsequent

chapter, also minimises some of these errors and provides quality assurance of the

integrity of the acquired measurements.

3.8 Metrology tolerances The tolerance defined in the table below accounts for the tolerance allowed between the

spoolpiece end-cap face and the hub face, where L, T, V accounts for X, Y, Z

respectively. The Z (or V) tolerance has subsequently been deemed unachievable and

increased to ±50mm.

Table 7: Metrology tolerances

It can be seen from the above requirements and the instrument accuracies discussed in

previous sections, that the ultimate tolerance is achievable using existing

instrumentation.

3.9 Data redundancy and error reduction techniques



The ROV will dock onto each structure docking receptacle a minimum

of 3 individual times during which a set of 10 heading readings is

logged. A closed loop circuit will be adopted with the ROV docking onto

the manifold, then x-tree and ending back at the manifold. Any

difference due to instrument drift will be apportioned throughout the

circuit by time. As a quality assurance check, the ROV position will also

be logged using the EHF LBL array whilst docked in and used in

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conjunction with the hub position (derived during calculation of the

spool heading) to calculate a structure heading. This value will be used

as a gross error check on the gyro heading only, due to its poorer

accuracy.


Positions for each hub are calculated using the ROV position whilst

docked into the structure and the x, y, z dimensional control values for

each structure. These alternate hub positions are then used to compute a

true bearing between hubs and compared against the previously derived

spool heading.


A closed loop level circuit is adopted, with the same Compatt starting and ending in

the same hub. A series of 10 depths and time are logged at each occupation of the

hub. Any difference due to instrument drift will be apportioned throughout the

circuit by time. Two separate complete circuits will be completed to determine a

mean depth at each hub location. Each circuit will be completed within 30 minutes

to minimise tidal variation. An ROV manipulator-held digiquartz depth sensor will

be employed using the same technique above for quality assurance of the relative

depth readings.


The Compatt will be installed in the first hub with the base-plate aligned to

structure north and a set of 5 readings taken. The Compatt will be rotated clockwise

through 90° and 5 readings logged at each quadrant, ending with return

observations at structure north. The return observation is used as quality control on

the 4 quadrant readings. All the readings will be meaned to derive a single

inclination in the plane of structure north.


The direct range measurement between the two hub Compatts is the primary range

measurement. However, 2 extra Compatts are also installed in frames on the seabed

to form a braced quadrilateral array. 10 baselines are measured each way between

all Compatts and used in a least squares adjustment by holding the hub-hub bearing

and 1 hub position fixed. The solved distance between the 2 hubs is used as a

quality control check against the direct measured range. All outliers greater than

0.05m from the mean range is discarded from any solution.

Figure 17: Braced quadrilateral between manifold and x-tree

3.10 Conclusion: chapter 3 This chapter represents the requirements and methods used for the case-study and for

deep water rigid spoolpiece metrology in general. An in-depth description of these

techniques has been given in this chapter, and the following chapters will analyse these

methods and provide discussion on the adequacy of these current techniques and

provide justification for any improvements found.

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CHAPTER 4

4 ANALYSIS

4.1 Introduction Until now, this paper has provided an in-depth breakdown of metrology techniques

employed during the case-study metrology project. All relevant information has been

reviewed and presented through detailing the nature, requirements and expectations of a

metrology project.

This chapter presents the analysis of these techniques and the examination of there

appropriateness to rigid spoolpiece metrology. It will also detail the improvements on

techniques and equipment found and highlights the need for further development.

4.2 Critical analysis of metrology techniques



The use of an ROV for docking onto each structure to attain its heading

is both cumbersome and time consuming, as is the nature of ROV work.

This ultimately translates into a cost increase as well as the introduction

of errors through instrument drift over time. Alternative options are

limited, but the problems associated with the current technique will be

overcome with the use of a standalone FOG or inertial gyrocompass that

can be inserted and aligned directly into the hub. This will also negate

the error introduced by the application of offsets to transfer the heading

from the docking receptacle to the hub centre. It is unlikely that a

manipulator-held gyrocompass will improve timing, as it will still

require ROV intervention. Acoustic telemetry of a gyrocompass reading

is a viable option, but there is currently no such Compatt end-cap or

other option commercially available.

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LBL positioning is the only available high accuracy option for subsea

positioning of the hubs. There are, however, considerations for

improving the technique. Firstly, the ultimate quality is dependent on the

accuracy of the ship-borne GPS used. GPS quality control and assurance

is the basis of separate studies and is beyond the scope of this

dissertation. For the purposes of this dissertation this is accepted as a

limitation of the product and is accounted for in procedure design.

Acoustic positioning can be improved by adopting an alternative to the

box-in procedure described in chapter 3.4.1.2. A more suitable LBL

global calibration technique exists in the clover-leaf method. This is an

iterative approach where simultaneous surface positions and ranges

(surface transducer to each Compatt) is gathered while the vessel sails

around the array in a clover leaf pattern. The dataset gathered is used in a

best fit criteria model to derive global positions for each seabed

transponder. If used in conjunction with direct ranging between

Compatts, the method represents a much tighter fit of global positions for

each transponder. This equates to a more accurate spool heading

computation.


The only option available for determining depth is by using pressure sensitive depth

sensors. The Digiquartz depth sensor is currently the pinnacle of commercially

available depth sensor accuracy and these have been employed in the case-study for

rigid spoolpiece metrology (refer to chapter 3.4.3 and 3.6.2 for further description).

Technique improvements thus exist only in the reduction of time between

successive measurements and the accuracy to which tide can be predicted. Due to

the remote location of places like the Ivory Coast, more accurate tidal data is not

available and is unlikely to be so in the near future. In the case-study, the time

between successive hub measurements is below 30 minutes and the tidal variation

over this period is generally within 10mm. There are few options available to

decrease this time, as it is ROV dependent. It is worth noting at this stage that the

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overall effect of tide is manageable within a 30 minute period in areas of low tidal

variation, even without accurate tidal data.


The 14,5° inclinometer Compatt end-cap is the prime choice for inclination

measurement due to it’s acceptable accuracy and it’s ease of use (refer to chapter

3.4.3 and 3.6.3 for further description). The main area of improvement thus lies in

improving the rotational alignment of the Compatt in the hub. The current

technique of meaning measurements attained from each of the 4 quadrants largely

reduces this error. A further improvement would require a redesign of the hub and

mounting base-plate such that the base-plate slots perfectly into the hub at each

quadrant. This requires engineering considerations for the structure fabrication and

is thus outside the intention and scope of this dissertation.


For deep water applications, Compatt ranging represents the only available option

for distance measurement. In shallower water, where divers are able to operate, it is

possible to employ a technique termed taut-wire metrology, which uses the

installation and tensioning of a wire cable between the 2 hubs to determine the

distance between hubs. This technique however, is not currently applicable to

deepwater metrology projects.

The direct range is favoured over the least squares adjustment of the braced

quadrilateral (see chapter 3.7.4) due to its simplicity and the absence of

compounded errors. It is however most vulnerable to scaling and large errors due to

these being spread out in the least squares adjustment. It is therefore imperative to

use the least squares adjustment as a check as well as having adequate quality

assurance of the speed of sound at the transducer level. These have been covered in

previous chapters and no improvements for the technique have been found.

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4.2.5 Offsets

Offset measurements of the manifold and x-tree structures (refer to chapter 3.4.5

for further description) are conducted in the fabrication yard using single second

total stations and conventional land surveying techniques. This method is thus by

far sufficient for metrology purposes, as the errors and inaccuracies introduced in

the subsea sector far outweigh the errors introduced by offset measurement. The

current techniques employed for offset measurement is sufficient and any further

analysis thereof is the scope of a separate study and outside the aims of this

dissertation.

4.3 Conclusion: chapter 4 It is of note that no major improvements for the techniques or equipment used have

been found. It was however found that spool heading accuracy can be improved by an

alternate LBL box-in technique termed the Clover Leaf Method. In addition, in order to

improve all the measurements beyond their current level of accuracy, new technology or

engineering considerations have to be taken into account. These considerations are

beyond the scope of this dissertation, but the need for further study has successfully

been accentuated. The following chapter will discuss consequences of these findings

and conclude the dissertation.

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CHAPTER 5

5 CONCLUSIONS, DISCUSSION AND IMPLICATIONS

5.1 Introduction The aims and objectives identified in chapter 1 have been addressed by the reviews and

analyses presented in the chapters 2 to 4. Specifically, the analysis quantified sources of

error and the accuracy, precision and relevance of the existing techniques for rigid

spoolpiece metrology.

This chapter will summarise the outcomes and implications emphasised by the research.

5.2 Discussion The findings of the analysis presented in chapter 4 allude to two important

considerations. Firstly, despite the preference of the clover leaf method over the box-in

method for LBL calibration, there are no major findings of technique improvement.

Secondly, there are limited alternatives for instruments and sensors. The primary

reasons for these are as follows.

5.2.1 Limited technology options

The adverse environmental conditions do not allow for a wide range of

technologies to be applicable thus sound remains the sole waveform for subsea

positioning. The depth of water specific to this type of metrology and the

case-study is also a limiting factor and the acoustic sensors used in the case-study

are already at the pinnacle of instruments used for deep water metrology. The depth

also restricts the subsea manipulation to ROV operations which is slow and

cumbersome in comparison to diver-intensive operations.

5.2.2 Adequacy of current techniques

The current metrology techniques employed are sufficient to meet the tolerance

requirements to ensure a ‘fit’ of the spoolpiece. The spoolpiece design is such as to

take into account the errors associated with metrology and allows for the tolerances

to be met by the metrology operations. This results in a lack of demand for more

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accurate metrology techniques and sensors. Industry responds in suit with a

decrease in rate of technological advancements.

5.2.3 High level of skills offshore

The intensive work environment offshore, driven by the high cost of operation,

ensures a high level of skills among personnel. This results in metrology operations

being managed by competent professionals. This is evident in the high levels of

data redundancy and error reduction techniques that have been devised specifically

for deep water rigid spoolpiece metrology.

Even though the existing techniques and instrumentation are adequate to fulfil the needs

of a metrology project, the dissertation has exposed numerous areas where further

investigation is needed. These areas are the topics of separate studies and thus beyond

the scope of this paper. These further studies are however listed below as outcomes of

this dissertation and are as follows.

5.3 Further research and recommendations The breadth of the topic is substantial and is supported by very little academic study of

the subject. As such, this paper opens the door to further research and development. The

following topics are examples of these needs as explained in chapter 4.

5.3.1 Dynamic GPS positioning of vessels at sea

GPS is the primary surface positioning system available on vessels today. Their use

in metrology is described in chapter 3.4.1.2. The accuracy required by metrology,

the adverse environmental conditions at sea and the large distances from land are

demanding conditions for high accuracy GPS positions. A separate study is needed

to investigate the options available.

5.3.2 Alternate offset measurement techniques

The derivation of x, y and z offsets of the subsea structures are attained in the

fabrication yard using total stations. There are however other options available and

these should be investigated in a separate study to determine the best technique

choice.

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5.3.3 Alternate Compatt base-plate design

The Compatt base-plate is simply a plug for slotting the Compatt into the hub.

From an engineering perspective, there is room for improving this mechanism so

that the Compatt can be aligned automatically with the hub alignment mark. This is

an engineering consideration and alternative designs need to be explored.

5.3.4 Gyrocompass end-cap for Compatts

This is a new implementation of existing technology. The possibility,

appropriateness and functionality need consideration in a separate study.

5.3.5 Other new technologies

There are few technology alternatives for metrology, as has been discussed in

chapter 4. The implementation of and further research in new techniques and

technologies should be an ongoing endeavour and the subject of future

investigation.

5.4 Conclusion: chapter 5 In conclusion, it is apparent that there is a need for further investigation of the options

available to rigid spoolpiece metrology. This dissertation has outlined the requirements,

expectations and problems associated with a deep water metrology project. It culminates

in an analysis of the assembled data and the adequacy of current techniques and

equipment. It was found that there is not much room for improvement due to reasons

mentioned previously in this chapter. There is however, much opportunity for further

study and the development and application of new technologies.

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APPENDIX A

PROJECT SPECIFICATION

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University of Southern Queensland Faculty of Engineering and Surveying

ENG 4111/2 Research Project

PROJECT SPECIFICATION FOR: Jason Floyd FALKEN (0031138002) TOPIC: Critical review of techniques for rigid spoolpiece

metrology observation and reduction. SUPERVISOR: A/Prof. Frank Young ASSOCIATE SUPERVISOR: Tim Farrow, Subsea7 Ltd SPONSORSHIP: Subsea7 Ltd, Aberdeen Scotland PROJECT AIM: An analysis will be made to quantify sources of

error, correct use and application of measurements taken, effects of sensor accuracy and precision on final reductions and the adequacy of the existing techniques.

PROGRAMME: Issue B, 20th October 2005 1. Briefly review available literature and common practises, with reasoning towards

need for further investigation into techniques. 2. Investigate minimum dataset requirements and determine basic mathematical

procedures for data reduction. 3. Evaluate and review instrumentation requirements and sensor accuracies and

precisions. 4. Research and investigate sources of environmental and instrument error. 5. Research typical tolerance requirements for spoolpiece metrology and fabrication. 6. Critically examine and analyse surveying techniques used for collection of dataset. 7. Investigate and review alternate techniques for measurement of inclinations,

bearing, distance and heighting, with particular emphasis on the effect of network design for 3D solutions in a least squared adjustment.

8. Investigate and review effects of sources of error on data quality and compliance to tolerance requirements.

9. Investigate and review techniques for data redundancy and error reduction. 10. Design alternate procedure for collection and reduction of metrology data, if deemed

necessary. 11. Evaluate and analyse outcome of chosen procedure. 12. Complete dissertation and submit. AGREED: ______Jason FALKEN_________________________ ___/___/___ ______Frank YOUNG__________________________ ___/___/___

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CNR BAOBAB FIELD DEVELOPMENT (08/12/2004): Survey and Positioning

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