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.