Cathodic Protection and

Underwater Survey for an

Floating Storage Offloading

(FSO) System

Michael Surkein*, Omarou Faricou & Joseph Kenmoe

ExxonMobil

*Mike is currently retired from ExxonMobil and Principal at Surkein Corrosion

Email: Mike@surkeincorrosion.com

Web: WWW.Surkeincorrosion.com

Mobile: +1 281-989-4134

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Agenda

• Facilities

• FSO and Mooring Tower Jacket

• Cathodic Protection (CP) Design Basis

• Underwater Inspections

• Hull and Coatings

• Anodes

• Reference Cells

• Cathodic Protection Monitoring

• Conclusions

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FSO and Mooring Tower Jacket

• Converted FSO permanently

connected by the stern to a

Mooring Tower Jacket and a

submarine pipeline

connection to shore

• Moored in a depth of 35 m

• FSO is free to weathervane

around mooring

• FSO dimensions

• Length 350 m

• Breadth 60 m

• Depth 28 – 35 m

• Storage - 2.3 million barrels

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FSO and Mooring Tower Jacket

• In service late 2003

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Cathodic Protection Design

• Design life = 15 years

• Submerged surface area = 32,000 m2 (344,445 ft2)

• Hull coating multi layer with anti-foulant

• Design current density = 30 mA/m2 (2.79 mA/ft2)

• Vendor standard approach

• Anode capacity 1000 A

• Using DNV RP B401 (1993)

• Final current density = 90 mA/m2

• Coating breakdown factor = 0.245

• Calculated current requirement = 706 A

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• Used vendor standard approach

• Five - 200 A power sources for 1000 A capacity

• Six anodes

• Six alloy anodes - 3.65 m x 102 mm (12 ft x 4 in)

• Rated at 200 A each

• Epoxy dielectric shield - 3.8 m x 6.7 m (12.5 ft x 22 ft)

• Installed in cofferdam

• Two reference cells – high purity zinc

Cathodic Protection Design

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FSO and Tower Yoke

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Underwater Inspections

• Diver, video, pictures

• General visual inspection

• Splash zone, hull, riser, etc

• Hull survey

• Coatings*

• Metal loss

• Marine growth*

• Cathodic protection*

• Anode depletion*

• Cathodic protection levels*

• Coating*Cathodic protection meter

Ultrasonic thickness meter

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Anode Layout

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Anode Location

2000 m

m

(6.5

ft)

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Hull Fouling

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Hull Anode Inspection

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Hull Anode Inspection

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Anode Condition

Anode Prior to Cleaning

Anode Bolt Visible

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Zinc Reference Cell

• CP MEASUREMENT (-mV)

• ZINC TEST PRE DIVE -1046mV

• ZINC TEST POST DIVE -1048mV

• AROUND ANODE -786 / -784mV

• CP MEASUREMENT (-mV)

• ZINC TEST PRE DIVE -1046mV

• ZINC TEST POST DIVE -1048mV

• AROUND ANODE -786 / -784mV

Cathodic Protection

Measurement

Zinc

Pre Dive

-1046 mV

Zinc

Post Dive

-1048 mV

Around

Anode

-786 mV

-784 mV

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Location Potential,

mV vs.

Ag/AgCl

Location Potential,

mV vs.

Ag/AgCl

Zinc Test

Block

(Pre)

-1049 Zinc Test

Block

(Post)

-1051

C1 FWD -845 C1 AFT -846

C2 FWD -846 C2 AFT -849

C3 FWD -845 C3 AFT -846

C4 FWD -854 C4 AFT -853

C5 FWD -854 C5 AFT -854

C6 FWD -855 C6 AFT -855

C7 FWD -856 C7 AFT -858

C8 FWD -856 C8 AFT -858

CP Monitoring

CP system turned off during measurements

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CP Monitoring

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Dielectric Shield Coating

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Hull Coating Under Fouling

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• Anodes were half cleaned and inspected

• Anode assembly consists of alloy anodes mounted in a resin holder

• Anode assembly mounting area covered with an epoxy dielectric shield

• Marine growth was covering 80% of both anodes with a thickness around 50mm

• Anodes were still active and the hull was protected

• Different kinds of marine growths were adhered to and around the anodes

• In two locations, marine growth accumulation up to 500mm thick

• Marine growth on the anode average was between 5mm to 70mm

• After light / soft cleaning to avoid damage, anodes were found in good condition

• Resin holders were in good condition without any damage or visible cracks

• Anodes’ bolts were protected by polyurethane foam and well tightened

• Reference cells at port and starboard found in good working condition

• During inspection, CP current was off to allow divers to work in safe environment

FSO CP System Inspection

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Mooring Jacket CP Inspection

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Mooring Tower Jacket Fouling

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Anode Inspection

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CP Measurement on Riser

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• Mooring jacket inspected for type and thickness of marine growth

• Marine growth was cleaned jetting device on some locations

• Marine growth present on approx. 60% of surface and constituted by

hard and soft organisms

• Hard shells were principally barnacles, coral, oysters and tubeworms

• Soft marine growths were mostly sponges, white & brown algaes

• Coating not damaged during cleaning and only marine growth

removed

• Temperature measurements 22.2 C to 31 C

• All CP measurement in acceptable range

• Anode depletions estimated between 10% – 20 % (75 anodes

evaluated)

Mooring Tower Jacket Inspection

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Conclusions

• Underwater inspection successfully and safely conducted

• Cathodic protection system components in good condition

• Anodes & reference cells

• Significant fouling present on underwater FSO hull

• Fouling cleaned easily

• Anti-fouling coating working well

• FSO Hull coating in good condition

• FSO cathodic protection monitoring indicates system performing as intended

• FSO Dielectric anode coating showing slight degradation on one anode

• Mooring jacket CP system performing as intended

• Mooring jacket anode depletion within acceptable range