FPSO Research ForumForum Discussion
OTRC, College Station USA
April 2003
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Corrosion issues and challenges
• Corrosion protection– Corrosion margins
– Coating types
• Maintenance, repair & planning strategies– Repair or replace?
– Now or later?
– Owner/Operator vs. Class vs. Regulatory
– Balancing economic vs. safety vs. environment
• Different environmental & operating conditions
Optimal vs. minimal requirements
Corrosion protection
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Corrosion Protection
• Coating:– Critical but expensive:
• Application (fabrication schedule impact)
• Inspection (construction and in-service)
– Class focus during operation but not (normally) NB
– Note: Cannot coat steel with steel
• Anodes– Significant weight and cost. Can we optimize?
• Impressed current
Which provides the best economic return? CAPEX vs. OPEX
Maintenance, repair & planning strategies
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Corrosion impact
• Leaking:– Gaseous vapors explosion
– Pollution
• Fatigue:– Lives halved in corrosive environment
(corrosion is located at HS)
– General corrosion has less impact
• Yield strength:– typically small impact
Typically local impact only
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Cargo Tank 0.5tk = 0.5mm Ballast Tank 0.5tk = 1.0mm
Material 235MPa 315MPa 235MPa 315MPa
Long. Spacing tmin tmin tmin tmin
900mm 16.6mm 17.8mm 18.0mm 19.3mm
Typical VLCC 20 21
Δ 3.4mm (17%)
2.2mm (11%)
3.0mm
(14%)
1.7mm (8%)
2 – 3mm reduction in main deck plate will reduce
buckling capacity 10-15%
ttkk = 3.0mm (ballast tank) = 3.0mm (ballast tank)
= 14% of actual plate thickness= 14% of actual plate thickness
Corrosion impact - Buckling
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DNV’s acceptance criteria options
Thickness measurements
Repair to As-built- Easy to determine- Quick (WSD)
Pass/failNo action
Repair to WWT- Requires simple calc- Medium time (WSD)
Repair to Site- Rigorous calc- Time consuming (LRFD)
Which is the cost effective solution?
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Color Coded ModelThickness Measurements
Import Thickness Measurements
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“As-built” or “As Measured” Plot
Thickness Distribution Curves (S-Curves)(Tanker Structure
Cooperative Forum & DNV)
Prediction of Steel
Diminution
• Generate repair specifications • Establish volumes, weights, areas & staging• Store all collected information for analysis and later re-use
Compare against requirements
DIMINUTION 'S' CURVE FOR WING TANK 3 PORT M/T "SOKOLICA"
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
-5% 0% 5% 10% 15% 20% 25% 30%
Percentage Diminution
Per
cent
age
of R
eadi
ngs
31 2 4
Different environmental & operating conditions
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Different environmental & operating conditions
90% of future FPSOs are forecast to be in non-harsh environments but…
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Questions for discussion
• Should we focus on:– Corrosion protection systems? (Preventative), or– Monitoring, recording & decision making systems? (Reactive)
• What information do we need:– Corrosion margins– Corrosion rates
• How do we address and rank the variables?– Cargo type, temperature, steel type, etc.
• How do we incorporate into the Life Cycle:– Design– Fabrication inspection– Inspection and Maintenance Planning– Repair decisions– Feedback / Industry updates (ownership)
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Reference DNV Guidelines
• Classification Notes 72.1 Allowable Thickness Diminution for Hull Structure, November 2000(currently being updated)
• RP Corrosion Protection of Ships, 2000• Classification Notes 33.1, Corrosion Prevention
of Tanks and Holds, July 1999.