Fitness For Service

When to Use

• Level 1 screening (conservative equations, minimal inspection data)
• Level 2 detailed assessment (refined analysis using thickness profiles)
• RSF and MAWP calculation from UT thickness data
• Remaining life projection with linear and power-law corrosion models
• Processing UT grid CSV data to CTP (Critical Thickness Profile) maps
• Run-repair-replace decision framework

Assessment Levels (API 579-1/ASME FFS-1)

Core Calculations

Remaining Strength Factor (RSF)

from digitalmodel.asset_integrity.rsf_calculations import calculate_rsf, check_ffs_level1

# Level 1 screening — General Metal Loss
result = calculate_rsf(
    t_measured=0.280,    # measured wall thickness (inches)
    t_required=0.150,    # required minimum wall thickness (inches)
    t_nominal=0.375,     # original design wall thickness (inches)
)
print(f"RSF: {result['rsf']:.4f}")        # e.g. 0.7467
print(f"FFS: {result['ffs_acceptable']}")  # True if RSF >= RSF_allowable (0.90)
print(f"MAWP ratio: {result['mawp_ratio']:.4f}")

# Level 1 acceptability check
check = check_ffs_level1(
    t_measured=0.280,
    t_required=0.150,
    t_nominal=0.375,
    design_pressure_psi=1200.0,
    smys_psi=65000,
    outer_diameter_in=12.75,
)
print(f"Level 1 acceptable: {check['acceptable']}")
print(f"MAWP remaining: {check['mawp_psi']:.1f} psi")

Remaining Life Projection

from digitalmodel.asset_integrity.rsf_calculations import remaining_life

# Linear corrosion model (most common)
life = remaining_life(
    t_current=0.280,           # measured wall (inches)
    t_minimum=0.150,           # required wall (code minimum)
    corrosion_rate_in_per_yr=0.008,  # measured corrosion rate
    model="linear",
)
print(f"Remaining life: {life['remaining_years']:.1f} years")
print(f"Next inspection by: {life['inspection_date']}")

# Power-law model for accelerating corrosion
life_pw = remaining_life(
    t_current=0.280,
    t_minimum=0.150,
    corrosion_rate_in_per_yr=0.008,
    model="power_law",
    power_law_exponent=1.3,
)

UT Grid — CTP Processing

from digitalmodel.asset_integrity.rsf_calculations import process_ut_grid

# Read UT measurement grid from CSV
# CSV format: rows=axial positions, cols=circumferential positions, values=thickness (inches)
ctp = process_ut_grid(
    csv_path="inspection_data/UT_grid_node_123.csv",
    t_nominal=0.375,
    t_required=0.150,
)
print(f"Minimum measured thickness: {ctp['t_min']:.4f} in")
print(f"Average thickness: {ctp['t_avg']:.4f} in")
print(f"Metal loss fraction: {ctp['metal_loss_pct']:.1f}%")
print(f"CTP grid shape: {ctp['grid_shape']}")
print(f"Points below t_required: {ctp['critical_points']}")

Run-Repair-Replace Decision Framework

Standards Reference

Related Skills

• cathodic-protection — prevents corrosion; FFS assesses once it occurs
• risk-assessment — probabilistic framework for inspection planning
• structural-analysis — structural integrity checks