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
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
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 3
FSO and Mooring Tower Jacket In service late 2003 4
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 = 0.245 Calculated current requirement = 706 A 5
Cathodic Protection Design 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 6
FSO and Tower Yoke 7
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
Anode Layout 9
Anode Location 2000 mm (6.5 ft) 10
Hull Fouling 11
Hull Anode Inspection 12
Hull Anode Inspection 13
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 -1046mV ZINC TEST POST DIVE -1048mV AROUND ANODE -786 / -784mV Cathodic Protection Measurement Zinc Pre Dive Zinc Post Dive Around Anode -1046 mv -1048 mv -786 mv -784 mv 15
CP Monitoring 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 system turned off during measurements 16
CP Monitoring 17
Dielectric Shield Coating 18
Hull Coating Under Fouling 19
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
Mooring Jacket CP Inspection 21
Mooring Tower Jacket Fouling 22
Anode Inspection 23
CP Measurement on Riser 24
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
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