POSITIONING CHALLENGES ON FALLPIPE VESSELS

Gert Brouns – The Hague, 3rd February 2016

BRIEF OVERVIEW

1. Introduction to Fallpipe Vessels

2. Positioning Principles

1. Vessel

2. ROV

3. Prerequisites: Calibrations &

Position Check

4. Vessel Positioning: Offshore/Nearshore

5. FPROV Horizontal Positioning

6. FPROV Vertical Positioning: Absolute versus Relative

7. Survey performance during rock installation

8. Dealing with residual shifts

TIDEWAY | DPFPV “TIDEWAY ROLLINGSTONE”

TIDEWAY |SUBSEA ROCK BERM CONSTRUCTION

SURVEY | AIM

• Install rock• On the right spot• Demonstrate this to the client

SURVEY | POSITIONING SYSTEMS

1. Position VESSEL

Flexible connection = umbilicals

2. Position FPROV

xy z

xy z

SURVEY | VESSEL POSITION

MRU and GyroGPS

GPS and Gyro

SURVEY | FPROV POSITION

USBL, PSUSBL, SB

IMUBathyDoppler

Responders

Multibeam Echosounder

SURVEY | PREREQUISITES

• Calibrations• Vessel systems• ROV systems• Cascade: if one system is changed, it has a knock-on effect

• Position check• Check if total Survey System is within accuracy• Check Geodetic Parameters in online software• On known seabed feature

• E.g. existing berm• Client input

SURVEY | VESSEL POSITION

• Horizontal• Offshore: PPP• Nearshore: RTK or PPP

• Vertical• Not relevant for work with FPROV -> see FPROV• Only relevant for IFPS (inclined fallpipe system) work when multibeam

is mounted on a pole on the vessel• RTK height• Tidal data and draft

Vessel positioning accuracy is not a limiting factor

SURVEY | FPROV

• Fall Pipe Remotely Operated Vehicle

SURVEY | FPROV HORIZONTAL POSITION

• USBL: coarse• Doppler Velocity Log (DVL): fine• INS: finest

Motion & gyro included

Still: Shifts Happen!Main cause is USBL

No DVL applied:

SURVEY | FPROV VERTICAL POSITION

• Deep water:• Bathy

• Shallow water:• Umbilical length + RTK• Tidal data + average bathy

Still: Shifts Happen!Main causes are Tide & pressure readings

SURVEY | FPROV VERTICAL POSITION: DEEP

• Bathy: registers pressure• Good for deep water; directly absolute depth• Effect of long swell in shallow water

→ sometimes unusable→ use shallow water approach

• Deeper, less effect;• Shorter swell, less effect• Tide is usually predicted so

generates offset error

SURVEY | FPROV VERTICAL POSITION: SHALLOW

• RTK height + umbilical length• ROV is lifted when sailing• RTK is not always present

• Tidal data + average bathy• Keep FPROV at same height• Keep ROV speed constant to avoid lift effect• Apply heave• Tide is usually predicted so generates offset error• Add bathy value to this relative survey to make it absolute

16

• Normal acoustic conditions

Before survey – During survey – Shift happens

Accurate speed and course of ROV

Accurate position (X/Y) of ROV

SURVEY | DURING SURVEYING

Good Survey

17

• Disturbed acoustic conditions

Before survey – During survey – Shift happens

SURVEY | DURING ROCK INSTALLATION

noise

No speed and course indication

No constant accurate position (X/Y)

dust Bad Survey

SURVEY | DEALING WITH RESIDUAL SHIFTS

Multibeam survey Apply shift in Z Ref. = undisturbed seabed Apply shift in X,Y Ref. = features

Shift or wrong installation?

pre

post

SURVEY | SUMMARY

Take all necessary precautions to obtain the best possible positioning in X, Y and Z Depends on circumstances

Offshore/nearshore Shallow/deep water Metocean conditions

Is normal survey practice Improves rock installation

efficiency Residual shifts are unavoidable

Take them into account Match surveys to demonstrate

rock installation is accordingto specifications

SURVEY | DPFPV “FLINTSTONE”

THANK YOU!