Corrosion in condensate cargo tanks on an FSO
Corrosion in condensate cargo tanks on an FSO (Floating Storage and Offloading unit) is a persistent and high-risk issue, especially because condensate is more chemically aggressive than typical crude. Let’s break it down clearly and then look at how SPS cold work addresses it without shutting everything down.
Why corrosion happens in condensate cargo tanks
Condensate cargo creates a particularly harsh environment due to:
- High volatility and dissolved gases
Condensate often contains CO₂ and sometimes H₂S, which form acids when mixed with water → internal corrosion accelerates. - Water bottoms and phase separation
Even small amounts of water settle at the tank bottom, creating localized corrosion zones (pitting, MIC-microbiologically influenced corrosion). - Coating degradation
Over time, tank coatings fail due to:
- Thermal cycling
- Chemical attack
- Mechanical abrasion during loading/unloading
Once coatings fail, steel is directly exposed.
- Sloshing and structural stress
FSOs experience constant motion, which:
- Cracks coatings
- Exposes fresh steel surfaces
- Accelerates fatigue + corrosion interaction
- Limited access for maintenance
Cargo tanks are confined spaces with:
- Hazardous atmospheres
- Operational constraints
- Strict safety rules (especially around hot work)
Below picture showing location of internal spot corrosions typically occurred at the bottom plate of an FSO.
Traditional Repair Challenges
Typical steel renewal or repair involves hot work (welding), which brings major issues:
- Requires gas freeing and tank cleaning
- Full shutdown or offloading
- Explosion risk in hydrocarbon environments
- Long downtime and high cost
For an operating FSO, this is often impractical.
What is SPS Cold Work?
SPS (Steel Plate Strengthening) is a composite repair system using:
- Polyurethane (PU elastomer)
- Special adhesive
- Bonded directly between new top steel plate and onto existing steel
It creates a structural reinforcement layer without welding.
How SPS Solves Corrosion Issues (Without Hot Work)
- No hot work required
SPS is installed using surface prep + bonding:
- No welding sparks
- No ignition risk
- Can be done in hazardous zones (with proper controls)
This is the biggest advantage for FSOs!
- Minimal disruption to operations
- Can often be applied while the tank remains in service or during short maintenance windows
- No need for full tank decommissioning
- No dry docking required
This keeps production running!
- Structural reinforcement + corrosion barrier
SPS does two things at once:
a. Restores strength
- Takes over part of the load from thinned steel
- Compensates for corrosion loss
b. Provides sealing
- Isolates steel from water and corrosive agents
- Stops further corrosion progression
- Works well for localized damage
Ideal for:
- Pitting corrosion
- Bottom plate thinning
- Small perforations (with backing)
- Cracked or degraded areas
- Reduces need for steel replacement
Instead of cutting and welding new plates:
- Existing steel remains in place
- SPS acts as a “bonded doubler”
- Faster installation
Compared to steel renewal:
- Less manpower
- No heavy fabrication
- Shorter repair cycles
Typical SPS Application Process
- Surface preparation (slurry blasting or mechanical cleaning)
- Inspection and thickness verification
- Adhesive application
- New Top panel placement
- Curing (often ambient or mildly accelerated)
No heat input, no structural distortion.
Limitations (important To Be Realistic)
SPS is not a universal fix:
- Not suitable for severely wasted or buckled structures
- Requires good surface preparation
- Long-term performance depends on proper installation and QA/QC
- Class approval is required (e.g., ABS, DNV, Lloyd’s)
Where SPS Is Especially Valuable On An FSO
- Cargo tank bottom plates
- Splash zones inside tanks
- Bulkheads with localized thinning
- Areas difficult to access for welding
Bottom Line
Corrosion in condensate FSOs is driven by aggressive chemistry, water presence, and operational stress—and traditional repairs are disruptive and risky.
SPS cold work offers a practical alternative by:
- Eliminating hot work hazards
- Allowing repairs during operation
- Restoring structural integrity
- Slowing or stopping further corrosion
Scenario (Realistic Case)
- Asset: Condensate FSO
- Location of damage: Cargo tank bottom plating
- Extent: ~150 m² area with pitting + general corrosion (steel loss ~20–40%)
- Remaining life target: 5–10 years
- Constraint: Vessel must stay in operation (or minimize shutdown)
Option 1: Conventional Steel Replacement (Hot Work)
Execution steps
- Tank taken out of service
- Gas freeing + cleaning (critical and time-consuming)
- Scaffolding + access setup
- Crop & renew steel plates
- Welding, NDT, coating reinstatement
- Recommissioning
Downtime impact
- Affected including adjacent tanks outage: 5–8 weeks
- If it affects production/storage chain → can trigger:
- Reduced offtake capacity
- Shuttle tanker scheduling issues
- Potential production slowdown
Operational reality:
Even if only one tank is repaired, FSOs often lose flexibility → indirect production losses.
Cost Breakdown (Typical Offshore Range)
- Steel fabrication & installation using diving & cofferdam: $3,500–$7,000/m²
- Coating reinstatement: $300–$800/m²
- Tank cleaning & gas freeing: $0.5M–$1.5M
- Offshore logistics (vessel support, manpower): $1M–$3M
👉 Total direct cost:
~$3M – $8M
👉 Indirect cost (production impact):
Can exceed $5M – $20M+, depending on oil price and downtime
Risks
- 🔥 Hot work ignition risk (major hazard in condensate tanks)
- 🧪 Gas freeing may be incomplete → safety exposure
- ⏱ Schedule overruns (very common offshore)
- 🔧 Welding defects / rework
- 🎯 Scope creep (once steel is opened up, more damage appears)
Option 2: SPS Cold Work (Bonded Composite Repair)
Execution steps
- Local isolation / controlled entry
- Surface preparation (mechanical or slurry blasting)
- Thickness verification
- Adhesive + SPS panel installation
- Ambient cure
No cutting, no welding.
Downtime Impact
- Due to non-hot work presence, adjacent tanks can often remain:
- In service, OR
- Out for only a few days
Typical duration:
- Installation: 2–3 weeks
- No gas freeing required to the same extent as hot work
👉 Operational impact: minimal
Cost Breakdown (Typical Offshore Range)
- SPS system supply & install: $1,500–$3,500/m²
- Surface prep & access: $200–$600/m²
- Manpower, QA/QC + SPS injections: Less than $1M
👉 Total direct cost:
~$0.5M – $2.5M
👉 Indirect cost:
Near zero (no major production interruption)
Risks
- ⚠️ Surface prep quality is critical
- ⚠️ Requires trained applicators
- ⚠️ Not suitable for severely weakened structure
But:
- ❌ No ignition risk
- ❌ No welding defects
- ❌ Minimal scope creep
Side-by-Side Comparison
Factor | Steel Replacement | SPS Cold Work |
|---|---|---|
Hot work | Yes 🔥 | No ✅ |
Downtime | 5–8 weeks | 3-4 weeks |
Production impact | High | Minimal |
Direct cost | $3M–$8M | $0.5M–$2M |
Indirect cost | Very high | Very low |
Safety risk | High (explosion risk) | Low |
Installation complexity | High | Moderate |
Flexibility offshore | Low | High |
Side-by-Side Comparison
In actual FSO operations, the decision is rarely just about repair cost.
What really drives the choice:
- Production continuity
Losing even a few days of production can outweigh the entire SPS project cost.
- Safety case
Operators are increasingly avoiding hot work in hydrocarbon tanks unless absolutely necessary.
- Remaining life strategy
- Steel replacement: best for long-term renewal (15–25 years)
- SPS: ideal for life extension (5–15 years)
- Logistics reality offshore
Mobilizing welding teams, steel, and support vessels is:
- Expensive
- Weather-dependent
- Schedule-sensitive
SPS is far easier to deploy.
Bottom Line
For a condensate FSO cargo tank:
- If you need full structural renewal and long life → steel replacement
- If you need safe, fast, and cost-effective life extension → SPS cold work
In most real offshore cases today, operators choose SPS when:
- Corrosion is moderate (not catastrophic)
- Shutdown is unacceptable
- Safety constraints are strict
Below picture showing prefabricated SPS panels ready for transportation to install on board.
Below picture showing SPS panels installed at the affected bottom plate.
