Cathodic Protection and Underwater Survey for an Floating Storage Offloading (FSO) System

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Cathodic Protection and Underwater Survey for an Floating Storage Offloading (FSO) System Michael Surkein*, Omarou Faricou & Joseph Kenmoe ExxonMobil *Mike

1 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 Web: Mobile:

2 Agenda Facilities FSO and Mooring Tower Jacket Cathodic Protection (CP) Design Basis Underwater Inspections Hull and Coatings Anodes Reference Cells Cathodic Protection Monitoring Conclusions 2

shore Moored in a depth of 35 m FSO is free to weathervane around mooring

3 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 m Storage million barrels 3

4 FSO and Mooring Tower Jacket In service late

5 Cathodic Protection Design Design life = 15 years Submerged surface area = 32,000 m 2 (344,445 ft 2 ) 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/m 2 Coating breakdown factor = Calculated current requirement = 706 A 5

6 Cathodic Protection Design Used vendor standard approach Five A power sources for 1000 A capacity Six anodes Six alloy anodes m x 102 mm (12 ft x 4 in) Rated at 200 A each Epoxy dielectric shield m x 6.7 m (12.5 ft x 22 ft) Installed in cofferdam Two reference cells high purity zinc 6

7 FSO and Tower Yoke 7

loss Ultrasonic thickness meter Marine growth* Cathodic protection*

8 Underwater Inspections Diver, video, pictures General visual inspection Splash zone, hull, riser, etc Hull survey Coatings* Metal loss Ultrasonic thickness meter Marine growth* Cathodic protection* Anode depletion* Cathodic protection levels* Coating* Cathodic protection meter 8

9 Anode Layout 9

10 Anode Location 2000 mm (6.5 ft) 10

11 Hull Fouling 11

12 Hull Anode Inspection 12

13 Hull Anode Inspection 13

14 Anode Condition Anode Bolt Visible Anode Prior to Cleaning 14

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

15 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 Zinc Post Dive Around Anode mv mv -786 mv -784 mv 15

16 CP Monitoring Location Potential, mv vs. Ag/AgCl Location Potential, mv vs. Ag/AgCl Zinc Test Block (Pre) Zinc Test Block (Post) 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 system turned off during measurements 16

17 CP Monitoring 17

18 Dielectric Shield Coating 18

19 Hull Coating Under Fouling 19

20 FSO CP System Inspection 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 20

21 Mooring Jacket CP Inspection 21

22 Mooring Tower Jacket Fouling 22

23 Anode Inspection 23

24 CP Measurement on Riser 24

25 Mooring Tower Jacket Inspection 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) 25

26 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 26

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