POSITIONING METHOD STATEMENT

FOR

J-LAY DP VESSEL

(Copyright Advanced Subsea  sas  2001-2002)

 1. SCOPE AND PURPOSE OF DOCUMENT

This document describes the outline of the method to be adopted by Contractor during the execution of the work.  The detailed procedure for use offshore will be developed by Positioning & Survey Subcontractor after award of Positioning & Survey Subcontract.

J-LAY Vessel DP interfacing and LBL Acoustic Positioning method are detailled in dedicated documents which are given in Annex.

2. DEFINITIONS

PLET Pipeline end termination
CTD probe Conductivity, Temperature, Depth probe
UTM Universal Transverse Mercator
DGPS Differential Global Positioning System
GPS Global Positioning System
USBL Ultra Short Base Line system
L/USBL Long/ USBL system
LF, MF, EHF LBL Low Frequency, Medium Frequency, Extra High Frequency Long Base Line system
DP Dynamic Positioning
ROV Remote Operated Vehicle
ROV TMS ROV Tether Management System
LAT Lowest Astronomical Tide
MLWS Mean Low Water Spring
UPS Uninterruptible Power Supply

3. REFERENCE DOCUMENTS

TBA

4. POSITIONING & SURVEY PROCEDURES

4.1. Positioning Method Statement

Due to high solar activity, a particular care will be paid to DGPS positioning systems selection in order to minimize the positioning errors observed mainly  in Sub-Equatorial areas.

DGPS based on HF transmitted corrections and Dual frequency shall be used as primary and secondary system.
Current subsea positioning will be performed using Acoustic Systems providing combined LBL/USBL modes, specially tuned for the use in deep waters.

In case of  loss of DGPS, contingency methods, based on LBL and USBL modes, will be used.

LBL positioning shall be systematically used for operation requiring a high level of accuracy, typically at initiation and lay-down location, structure installation and for work in areas close to subsea structures.
A Position reference System based on the installation on the seabed of permanent Transponder stands will be implemented when required.

High accuracy EHF LBL systems shall be used for metrology purpose only.

Note: This section is to be read in conjonction with the following tables placed in annex: DP Reference Matrix .

4.1.1. Positioning Systems

Installation Operation shall be carried out using the following reference systems :

* Surface Positioning System : Minimum 2 DGPS systems shall be used for both Dynamic Positioning and Navigation:
* Primary DGPS with HF  transmitted multi-references corrections
* Secondary DGPS dual frequency, with satellite transmitted references corrections
* Relative position/ Range measurement (FANBEAM/ GPS/ etc…..)
* Subsea positioning : 2 L/USBL systems :
* Simrad " Superior " HiPAP 500 including LBL  mode option
* Alternatively Sonardyne MF L/USBL system
* Subsea positioning : LBL Acoustic Positioning Systems
* LF/MF  Sonardyne LBL Positioning Systems

* Jumpers Metrology (2 methods):
* U/w  gyrocompass, Dual axis inclinometers, EHF LBL based method
* Taut wire method.

4.1.2. General Method Statement

* High accuracy surface/ subsea positioning will be obtained by selecting high standard equipments and by trying to minimise any source of error .
* Improvement of DGPS performance
* use of high accuracy GPS receivers having submeter capability.
* selection of reference stations corrections transmitted by HF in order to minimise the delay of application of differential corrections.
* selection of  references stations located close to Work site
* systematic use of DGPS dual frequency receivers on both reference stations and mobiles in order to minimise the effect of  Solar Activity.
* Improvement of  USBL accuracy, stability and repeatability by means of::
* L/USBL acoustic systems working in dual USBL and LBL/ USBL modes
* keeping the mobiles inside 30° transducer cone
* using high power directional transponders (exceeding 200 db).
* Improvement of sound speed measurement accuracy using SV sensors and performing systematic collection of   SV profiles.
* Strict Application of Quality Assurance plan and Quality Control procedures.

* Reference wells will  be positioned  using MF LBL Acoustic Positioning System. Absolute positions will be obtained by boxing-in high power transponder set-up on top.
* Orientation and Attitude of structures will be obtained by using underwater gyrocompass coupled with dual axis inclinometer.
* Relative altitudes of structures will be derivated from high accuracy bathy-systems
* Initiation, Lay-down, SLEDs  laydown in targets  will be monitored using MF/EHF LBL system carefully calibrated and correlated to reference Wells.
* Cut-to-length  will be determined using LF/MF LBL acoustic Positioning systems carefully calibrated and correlated to existing references.
* Pipelines/ flowlines Touch-down point will be performed in USBL mode using ROV equipped with High power reponder/transponders.
* Flowline/umbilical abandonment loops will be monitored by measuring range to target structures using  transponders in LBL mode.
* Flowline/umbilical ranges to existing lines will be monitored using ROV high-definition sonar.
* Jumper measurements shall be performed using ROV operated metrology methods based on EHF or taut wire measurements.
* LBL positioning shall be preferably based on permanent Position reference System made of fixed frames and Existing Well-Heads

Note: Details concerning LBL acoustic positioning methods will be found in document: LBL Acoustic Positioning Method statement.

4.1.3. Contingency Method Statement

This section propose contingency positioning methods to be implemented in order to maintain a good quality of positioning for DP Operation in case of  loss of surface positioning references due to solar activity effects.

4.1.3.1. Principle

The principle of the methods is to keep DP System working in all circumstances by using alternative methods of positioning:
- Surface systems
- DGPS shall be based different mode of transmission (VHF/HF and  satellite)
- DGPS receivers shall use different technologies
- Alternative surface positioning systems may be used: Fanbeam, Artemis, Syledis,etc..
- Sub-surface systems
- Acoustic systems using different technlogies with comparable performances:Simrad HiPAP, Sonardyne Big Head LUSBL, Thales Marconi Posidonia (LF based USBL)
- Pre-installed Arrays of transponders working in LBL and/or USBL mode
- Pre-installed transponders deployed along the route working in USBL mode

see DP references choice Matrix table/ file in annex.

4.1.3.2. Contingency Modes of Operations

- L/USBL mode for limited areas: 3 to 4  transponders are deployed on the seabed building a square with side typically equal to half the water depth. Transponders are calibrated in absolute and relative modes using respectively USBL and LBL capabilities of acoustic system. When applicable, existing subsea structures are previously marked with beacons fixed on top.
This method allows to provide an high accuracy fixed  reference position to DP system.

-"Absolute multi-USBL" mode for Laying: Transponders are deployed on the seabed at a distance equal to half the water depth. After box-in or direct USBL positioning of transponders, this "array" provides a continuous absolute USBL reference system to DP System.

- "Slipping L/USBL" mode for Laying:  This case is based on the use of existing and calibrated LBL arrays  or stands with typical mesh of 600/800 m . Vessel is able to interrogate continuously a minimum of  4 transponders and thus to work in L/USBL mode.   When progressing the DP operator is able to select new transponders.
This method can be performed with a  minimum of transponders when a permanent reference position System based on transponders stands, has been implemented. In this case, transponders are re-positioned by ROV.

- "Pseudo Followsub" mode with ROV:  this mode is used when laying along an existing pipeline. ROV is successively positionned on pipeline joints by steps of 3 to 4 joints, vessel keeping station on depoyed winch transponders when ROV is re-positionned.

Definition:  LUSBL mode (Simrad/ Sonardyne) allows to use both LBL and USBL modes. (Calibration of transponders can be performed automatically when surface positioning is available).

Definition:   MLBL mode is a Hipap multi-user mode which allows several vessels to work in the same area using same LBL array - a syncro transponder is necessary (not considered here)

4.1.3.3. DP System Acceptability Criteria

These points has to be checked with DP Manufacturers.
Refer to  J-LAY Vessel  DP System Interfacing Procedure`

4.2. Tolerances and Measurement Accuracy

Tolerance tables (typical) will show the expected level of accuracy corresponding to proposed Means & Methods.

4.3. Positioning & Survey tasks

Positioning and Survey activities will be performed onboard a dedicated Vessel

4.3.1. CONSTRUCTION SUPPORT  VESSEL 

4.3.1.1. Construction Support  Tasks

The following tasks will be performed from the Construction Support Vessel

* Pre-survey activities including pipeline route survey with ROV, Existing Well-heads inspection, SLED’s lay-down survey
* Pipeline Touch down Monitoring (contingency)
* LBL Support tasks including Transponders stands deployements and positioning, LBL deployement and calibration, Cut-to-length calculation and "lay-down in target" monitoring.
* Additional Construction Support on request of the barge


4.3.1.2. Positioning and Survey  Equipment

Construction Support Vessel shall be equipped with the following :

* 2 complete GPS Systems including 1 LRK (kinematic DGPS) when possible
* 2 independent GPS Differential Correction Signals
* DGPS Quality control system
* Sonardyne L/USBL acoustic positioning with big head and transponders
* LBL MF Acoustic positioning system and transponders
* LBL LF Acoustic positioning System  and transponders
* LBL stands installation winch+cable
* Navigation and survey System
* USBL & LBL calibration and QC software package
* ROV sensors and systems interfacing, including video overlay unit and data acquisition & logging.
* Responder/transponders for ROV
* CTD/SV sensors

4.3.1.3. ROV System

A Work ROV fully equipped for LBL work (detailled in section 4.4) and survey, shall be operated from CSV in order to perform LBL beaconing of well-heads, installation of Transponders on stands and to assist FDS on request.

4.3.2. J-LAY VESSEL

J-LAY VESSEL scope includes installation of rigid and Flexible lines, Production and Injection Umbilicals, Manifolds, SLEDS, Well-heads jumpers and flying leads deployment, etc…

4.3.2.1. Positioning & Survey tasks

*  J-LAY Vessel  systems interfacing (2 DGPS, 2 USBL, 2 Gyrocompass, 2 MRU)
*  J-LAY Vessel  ROV sensors and systems interfacing, including video overlay units, LBL Rovnav (2 WORK ROV)
*  J-LAY Vessel  ROV Survey sensors interfacing
* DGPS Systems calibration check (Health check)
* Alternate Position reference System calibration
* HiPAP and L/USBL calibration
* 3 Gyrocompass calibration
* Positioning and Navigation during operations
* Touch-down monitoring
* Free-span detection and inspection
* As-laid survey while laying

4.3.2.2. Positioning and Survey Equipment

 J-LAY Vessel  shall be equipped with the following :

* 3 complete GPS Systems (including permanent GPS)
* 3  independent GPS Differential Correction Signals (HF/ Inmarsat/Spotbeam)
* DGPS Quality control system
* Secondary Positioning system (when applicable)
* 2 L/USBL HiPAP systems (CV one’s)
* L/USBL Sonardyne system (when required)
* Navigation and survey System
* LF/MF LBL Acoustic positioning system (when required)
* Data processing and Mapping System
* Land Survey equipment
* ROV mounted Survey Sensors as per section 4.9

4.3.3. Touch-Down Monitoring

For Umbilicals, Touch-down Point monitoring shall be performed with ROV operated from Construction Vessel in order to provide real time confirmation of off-track position, distance of the touch-down point.
For Sealines, Touch-down Point monitoring shall be performed with ROV operated from Construction Vessel with extended TMS tether or from Construction Support Vessel in order to provide real time monitoring of off-track position and touch-down point lay-back.
When applicable Positioning Data and Video will be transmitted to Construction Vessel by telemetry systems.
When laying parallel to another line, lateral distance between lines will be monitored by ROV using either direct HD Sonar monitoring or when buried, by performing regular offset check in tracking the existing line using pipe/ cable tracker and measuring Pipe position with HD sonar.

Vessel position, ROV position, pipeline route and field informations, will be continuously displayed at the DP operation desk, in ROV control rooms. In addition this information will be displayed on the CCTV system;

4.3.4. As-Laid Survey

For sealines, As-laid survey data will be typically obtained using ROV operated from the Construction Support Vessel. This ROV shall be equipped with central and Dual boom video-cameras, gyrocompass, HD sonar, bathy-system,  Dual head scanning Sonar and pipetracker.

For umbilicals, As-laid survey will be performed while laying by one of Construction Vessel ROV equipped with central video-cameras, gyrocompass, HD sonar, bathy-system,  Dual head scanning Sonar and pipetracker.

ROV will be positioned using USBL system.

The following information will be collected:
- position of the pipe using USBL system
- continuous video recording of the umbilical using central cameras
- line profile from bathy-system
- line cross-profile from DHSS
- obstruction position from HD sonar
- Free span position/ length and height from DHSS and dual boom video cameras
- Events position (anodes, joints, defaults,…)from real-time video

Flying leads and Jumpers shall be inspected and positioned directly upon completion of  their installation.

4.3.5. As-laid Report

As-laid survey processing and reporting  will be performed onboard  vessel in charge of As-Laid survey : alternatively Construction Vessel or CSV.
Preliminary Pre-lay reports will be available before operation is started and will include all necessary information.
Final reports will be completed according to section 4.6 of this document within COMPANY required delays.

4.4. Positioning and Survey Specifications

4.4.1. Geodetic Reference System (Angola)

All operations shall be based on the use of the following Datum:
* Clarke 1880 RGS  Spheroid
* Datum CAMACUPA
* Projection UTM Zone 33 South
* Central Meridian : 15 degrees East.
* UTM coordinates in meters.
* Vertical Reference shall be LAT derivated from a combination of  local and Luanda port previsions.

4.4.2. Geodetic Reference System (GOM)

All operations shall be based on the use of the following Datum:
* Clarke 1866  Ellipsoid
* Datum NAD 27
* Projection UTM Zone 16 North
* Central Meridian : 87 ° West.
* UTM coordinates in US Survey Feet.
* Vertical Reference shall be MSL

4.4.3. Primary & secundary positioning systems

Primary and secondary positioning system will be based on a mono/bi-frequency, Multi-reference DGPS able to provide an high standard in term accuracy/ reliability.
* Correction solution shall be taken as the result of a multi reference QA/QC system, analysing in real time HF and Inmarsat/Spotbeam transmitted Reference Station corrections.
* Priority will be given to HF corrections allowing high accuracy positioning. Accuracy being proportional to inverse of  range and age of the correction.
* GPS receivers will provide a sub-metric accuracy.
* Onboard Construction Vessels, Reference Correction will be directly interfaced to Dynamic Positioning System.
* All equipment will be provided with full back-up.
* Survey DGPS systems will be powered by their own UPS.

4.4.4. Optional additional positioning system

DGPS, Fanbeam, Artemis may be used depending on the field configuration .
* All equipment will be provided with full back-up.
* Positioning systems will be powered by their own UPS

4.4.5. Real-time QA/QC system

Multi-references QA/QC system will monitor in real-time the quality of DGPS corrections in order to allow the best reference station to be used for positioning.

4.4.6. USBL Acoustic positioning

* High accuracy USBL systems ( HiPAP, L/USBL ) will be used .
* DP USBL systems will be interfaced to Survey Navigation System.
* High energy Transponders will be used for L/USBL positioning.
* USBL systems will be properly calibrated using calibration/ quality control software  package following manufacturer guidelines.
* High accuracy CTD sensors or SV probes will be used in order to determine sound celerity and water density profile from surface to seabed  deployement winch or ROV will be used.
* All transponders will be equipped with depth sensors and acoustic release and provided with independent mooring line and floatation collar.  Necessary quantity of Sand bags will be used for Transponders mooring.
* Most of the time, Transponders recovery shall be performed by ROV or acoustically released.

Note: Contigency methods will be deployed in case of need, based on LBL/ USBL transponders deployed on the seabed along the flowlines/ umbilicals route.

4.4.7. Land Survey spread

  Land survey spread will be maintained onboard the Construction Vessels in order to perform DGPS and Gyrocompass calibrations/ calibration checks as per quality plan instructions . This spread will comprise as a minimum:
* Total Station with set of prisms, reflectors, tripods
* This station will be able to position a moving target.
* 3 Marine VHF hand held radios

4.4.8. ROV Survey Sensors

In order to perform Survey tasks, the following survey sensors will be included in the WorkROV spread.
* High definition Sonar (quality of the sonar similar to side Scan Sonar)
* Colour, B&W SIT  cameras mounted on Pan & Tilt including S-VHS recorders (PAL-M / NTSC standard)
* Dual boom cameras (when onboard Construction Support Vessel).
* USBL high power transponder/ responder
* Dual Head Scanning Sonar (DHSS)
* LBL ROVNav (when LBL positioning is required)
* North seeking gyrocompass
* High accuracy bathy-system.
* Pipe/cable tracker system.

4.4.9. ROV sensors data acquisition & logging

Onboard Construction Vessels,  ROV sensors will be interfaced to Navigation computer : it includes ROV gyrocompass, attitude , depth, altitude.
Processed information will be dispatched to ROV's video overlay units (typically 2x ROV onboard DP Construction Vessels, 1x  onboard Construction Support Vessel).

4.4.10. ROV Survey sensors data acquisition

  ROV Survey sensors and systems will be interfaced to Navigation Computer: it includes ROV gyrocompass, ROV attitude sensors, bathy-system, Dual Head Scaning Sonar, pipe-tracker, LBL ROV Nav.
Processed information will be dispatched to Video overlay units of  the survey ROV

4.4.11. Video overlay unit  interfacing

Navigation computer will be interfaced to ROV's video overlay units.
Current operations parameters shall be displayed on ROV videos:
* For Surveys: Project/Current task,Date/ Time, ROV Northings, Eastings, KP/ DoL, Gyro and Depth/ Altitude from ROV.
* For construction operations: Project/ Current task, Date/ Time, ROV Northings, Eastings, Heading, ROV pitch/ roll, Depth/ Altitude.

4.4.12. Navigation Data Display

Navigation displays will be installed in each ROV control rooms, at DP operator desk, in Operation Control room. Navigation information will include all mobiles involved in the current task.

4.4.13. Data and Video transmission to Construction Vessel

  A data and video link shall be maintained  between Construction Support Vessel and  J-LAY Vessel  in order to transmit current ROV position and to display onboard Construction Vessel video from ROV cameras.

4.4.14. Survey data processing and reporting

Preliminary Reporting and Mapping tasks will be performed onboard Construction/ Construction Support Vessel. It includes pre-survey/ as-laid survey reports and preliminary charts.
Reporting spread will be comprised of all necessary computers, softwares, printer, plotters ,…
Note: Compatibility with Autocad formats is mandatory.

4.5. Quality Control

4.5.1. Calibration certificates

All equipment used for calibration/ metrology purpose will be provided with recent bench calibration certificates: it includes North-seeking Gyrocompass, CTD probes, Inclinometers, LBL transponders, Bathy-systems, depth sensors, etc…

4.5.2. DGPS calibration check

DGPS calibration checks against a known reference will be performed at mobilisation. Comparison between DGPS systems will be performed systematically.

4.5.3. Secondary Surface Positoning calibration

Secondary positioning systems calibration will be performed using manufacturer calibration guidances and Company agreed procedures. Comparison with DGPS systems will be systematically performed.

4.5.4. USBL calibration

USBL systems calibration shall be performed using manufacturer calibration guidances and Company agreed procedures. Cross-comparison with LBL array transponders will be performed when possible.

4.5.5. Gyrocompass calibrations

Gyrocompass will be calibrated following agreed procedures at mobilisation stage. Calibration checks using sun shots method will be regularly performed.

4.5.6. LBL calibrations

LBL array will be calibrated following standard procedures.
CTD/ SV profiles will be systematically performed prior to calibrations tasks.
* Absolute calibrations will be performed by boxing-in transponders installed on permanent frames (LBL mode). Positions of the frames will be stored in the reference system database.
* Relative calibration will be performed before any installation work is performed.
Note : for complex arrays, specific methods of calibration shall be implemented.
* When arrays are already deployed, ping-around checks will be performed.
* Position of existing structures will be calculated in relation with the calibrated LBL arrays.
* At the begining of each task, all coordinates will be checked.

4.6. Report Specification

  Survey reports will be issued upon completion of each step of the installation. Processing team will be mobilised with all necessary equipment in order to provide fields preliminaries reports and charts. Final reports will be issued shortly upon completion of the operations.

4.6.1. Field Positioning database

  A Field Positioning Database shall be developped at the survey stage and maintained during the operations.
* Database will include all positioning information concerning platform, existing pipelines, sub-sea structures, wellheads, manifolds, wrecks, obstacles, etc...
* Updated database will be systematically transmitted to positioning team onboard vessels involved in the project.

Note: In the G.O.M. this work is direcly performed by Survey Subcontractor, in relation with MMS

4.6.2. As-laid Report

Upon completion of field operations, a final survey report will be issued as per COMPANY  specifications.
* CONTRACTOR's comments  will be included before final issue of  the  reports.
* This report will contain  the following informations:
* Introduction and Summary,
* Results Analysis and Conclusions,
* Quality control,Positioning parameters, Calibration results, Safety,
* Diary of Events, List of Personnel and equipment,
* As-laid charts including all informations collected during the survey and particularly information concerning lines position, longitudinal and cross profiles, anodes, defaults, obstruction, free spans,
* annotated & commentated video tapes of  surveyed lines, jumpers and structures inspections.
* In addition all information will be integrated in an electronique Survey Manager system base on a multimedia database allowing indexation of text, data, pictures, video records.
* Final Field Positioning Database file of all existing structures, obstacle,  pipeline positions verified and updated upon completion of the as-built survey.
* Report including LBL Positioning and quality control information, Position reference frames and LBL arrays deployed and calibrated  during the operations.