External corrosion of carbon steel process equipment on FPSO’s

• Process Piping & Vessels Corrosion Under Insulation-CUI

• Piping Corrosion under Pipe Supports-CUP

• Atmospheric Corrosion

• Summary

Process VesselsCUI

CUI• No reliable NDT method available• Inspect ”Hot Spot” areas, or full removalDiscovery• Programmed visual inspection revealed extensive pitting corrosion

Vessel Data• Carbon steel vessels, approximately fiveyears old

• Insulated with cellular glass and SScladding

• Cellular glass glued to vessel wall• Corrosion allowance 3 mmActions• All insulated separators – completeremoval of insulation

Extensive Corrosion Findings• Several hundred corrosion pits found on each vessel

• Corrosion areas ranging from surfacecorrosion to spot pits with depths up to 11mm

• Nominal wall ranging from 12,3 to 35 mm

Process VesselsCUI

Fitness for Service• Detailed mapping of corrosion• Calculation confirmed all vessels to

be within design code pending repair of one vessel

• Design pressure de-rating considered to establish corrosion allowance

• Patch weld repairs included to meet FFS crititeria

Specifications Vertical Vessels• Steel blast cleaned to Sa 2.• 1st layer: Zinc ethyl silicate primer at 75

µm NDFT• 2nd layer: Epoxy mastic at 150 µm • Foam glass glued to vesselsSpecifications Horizontal Vessels• Steel blast cleaned to Sa 2.• Two layers immersion grade epoxy phenol

coats, each 150 µ MDFT• Rockwool-fastened with s.s. BandsOperating environment• Ranging from 55°C to max 85°C, mean

approx. 65°C

Process VesselsCUI

Contributing factors• FPSO close proximity to the sea level causing

severe marine atmosphere in process areas –rapid corrosion progress

• Fire water deluge testing • Water influx through crevices in vessel

cladding and insulation • Water is trapped in insulation

• Insulation glued to steel (foam glass)• Water saturated Rockwool• Creating corrosive environment

• Coating with pores and cracks • Coating systems used, not well suited for

immersed conditions• Several corrosion mechanisms proposed,

exact mechanism not established• Hydrolysis acidification, formation of oxygen

concentration cells and MIC are probablecorrosion mechanisms

Process VesselsCUI

Contributing factors contd.Corrosion attacks• Typically circular spots varying in size with depth up to 10mm, also

uniform corrosion in larger areasAdhesion testing of coating• 7,6 MPa adhesion average, with 95% cohesive failure top layer,

visual inspection confirmed open porosity and fragile top coat surface• 8,2 Mpa adhesion with 90% adhesive failure between primer and

metal substrate, adhesion testing indicates porosity in both primer and top coat

• Visual inspection confirmed 0,1 . 0,2 mm porosity in zinc silicate primer• Appearance of coating brittlenessCladding and Insulation• Metal cladding and insulation systems ability to resist water penetration

is inadequateCoating• Organic top coats applied on inorganic primer forms pores in top coat• Water penetration caused zinc to corrode• Too thick primer coating, too high application temperatures or tool low

relative humidity may give rise to poresCoating Summary• Zinc ethyl silicate primer and epoxy top coat likely forms pores that

have initiated damages• NACE RP0198-2004 does not recommend inorganic zinc coating under

thermal insulation in the 50 – 150C range for long term or cyclic service• ”Immersion grade” coatings also failed• Norsok specifies TSA or TSZ for insulated systems

Process VesselsCUI

Corrosion MechanismsCorrosion RatesPit corrosion rates exceeding 3 mm/year observed, 1,5 mm/year average Corrosion Mechanisms Proposed• Galvanic corrosion: Reverse polarity zinc/steel is reported at temperatures

• > 70-80°C but not in the case of zinc silicate primers - no evidence of particular corrosion in weldHAZ’s - galvanic corrosion

• Likely not a major contributing mechanism• Oxygen concentration cell corrosion

• Areas with heavy corrosion products • Variable deposits/bacteria films • Areas with low O2 will act as anode, giving localized corrosion • Rates 5 - 8 times the rate of general corrosion

• MIC: Metal surfaces with corrosion products and/or marine growth• Anaerobic layer may be favorable for SRB activity with formation of iron sulfide films• MIC corrosion is typically smoothly rounded grooves with black corrosion product • Several of the corrosion areas, but not all appear to fit description• Corrosion rates reported 5 - 20 times general corrosion in sea water

• Hydrolysis: Creation of an aggressive acidic chloride environment in pits• Environment becoming rich in metal cat-ions (ions migrating to cathode) and anionic species (chloride)

Corrosion Mechanisms Summary• Corrosion developed through gradual breakdown of coating system in localized areas• Corrosion occurs mainly on top on horizontal vessels and under large nozzles on vertical vessels• Exact corrosion mechanism cannot be established• MIC, O2 concentration cells and hydrolysis are probable mechanisms

Process VesselsCUI

Repair Strategy VesselsVessel Repairs• Weld repair used where FFS not metVessel Coating Considerations• Damages to vessel coating dictated need forcomplete coating refurbishment

• Coating systems selected based onassumption of full separator re-insulation

Vessel Coating Selection• TSA: Not considered feasible to apply TSA offshore due hot work considerations

• Reapplication of original paint system: Not a feasible option based on experience

• Two systems considered: Carbon fiber system and immersion grade epoxy system

• Carbon wrapping system: originally considered pending need to reinforce vessels reinforcement, concluded not required

• Hot immersion grade epoxy systems selected - coatingdesigned for continuous immersion

• In cases for no re-insulation: Norsok - zinc primer with minimum three coats

First coat

Applying epoxy paste

Second coat

Piping CUIDiscovery• Programmed inspection revealed extensive pitting corrosion

• Previous ”spot” inspection failed to discover areas of corrosion

• Corroded areas occurred randomly, not where normally expected

• Most serious corrosion occurrs on piping diameters > 6 inches

Piping Data• Carbon steel piping, approximately fiveyears old

• Insulated with cellular glass and SScladding

• Corrosion allowance 3 mmActions• All insulated HC “critical” piping systems – complete removal of insulation

Extensive Corrosion Findings• Corrosion attacks ranging from surfacecorrosion to spot pits with depths up to 90% of nominal wall thickness

Piping CUI

SpecificationsPiping Material• Carbon steel A106 Gr. B SMLSNominal wall thicknesses• Ranging from 12,7 to 35 mmCoating Systems • Various 2-layer and 3-layer coating systems

• Common: Inorganic primerInsulation Material • Foam glassMantling• Stainless steel plate elements sealed with rivets and joints sealant paste

Operating environment• Ranging from 55°C to max 85°C, mean approx. 65°C

Piping CUI

Contributing factors• Root causes considered to bethe same as for process vessels

• Insulation mantling is susceptible to damage from maintenance activities

Fitness for Service• Detailed mapping of corrosion• Several areas did not meet FFS criteria

Repair• Temporary reinforcement ofpiping using carbon fiber wraps

• Patch welds considered but rejected

Piping Corrosion under Pipe Supports -CUP

Status• Piping CUP is an increasing challenge• Visual inspection assessing damage limited due to access

• Amount of corrosion products at pipe support usually a good indicator of severity of actual piping corrosion at support

• Full removal of supports considered to be the best method to uncover extent of piping corrosion

• NDT tools considered; ET systems, UT methods,RT – risk of overcalls or undercalls, ending up with full removal of support anyway

Piping Corrosion under Pipe Supports -CUP

Contributing factors• FPSO Environment accellerating problem

• Support design deficiencies• No doubling plates• Shoes not used

• General aging of coatings• Breakdown of coating due piping/support wear as a result of significant piping movement (Vessel flexing)

• Carbon steel piping/stainless steel supports – galvanic corrosion

Piping Corrosion under Pipe Supports -CUP

Support Repair• Support redesign - requires engineering calculations in most cases

• Cold cutting of existing supports• Assessment of piping corrosion damage

• Reinforcement of piping if necessary

• Recoating of piping and support• Reinstallment by bolting

General Atmospheric CorrosionFindings• Atmospheric corrosion is very fast on process systems on an FPSO

• General surface corrosion is not a concern – corrosion occurs as local attacks

• Local pits forming under paint initially• Combination of extreme marine conditions and hot surfaces have given corrosion rates up to 3-4 mm/year

• Not sufficient to assess corrrosion using general visual inspection – close visual with removal of corrosion products necessary

Repair Strategy• Spot repairs executed concurrent with programmed annual inspections to arrest corrosion attacks

• Abseilers used for this work • Area by area re-coating using normal paint crew

Carbon Steel Corrosion on FPSO’s -Summary

• Carbon steel external corrosion occurs much more rapidly on FPSO’s compared to fixed platforms

• Ensure that surface preparation, coatings and insulation materials are correctly selected and applied

• Only use offshore field proven systems• Repair paint and insulation damages from construction phase prior to offshore use of installation

• Do not insulate unless absolutely necessary• Ensure application of insulation mantling is correct• Prohibit use of insulated piping as ”walkways” and introduce maintenance program on insulation mantlings

• Use fresh water or temporary hoses for deluge system testing

• Attempt to improve surface treatment and insulation disciplines recognition in the project and operation management hierarchies

• Don’t compromise on specifications and quality