1. Overview & Applications
Submerged pipelines experience upward buoyancy force equal to the weight of water displaced. Buoyancy control is essential for pipeline stability in rivers, lakes, marshes, offshore installations, and buried pipelines in high water tables.
River crossings
Trenchless installations
HDD and direct-pipe crossings under rivers require buoyancy control during construction.
Offshore pipelines
Subsea installations
Offshore oil and gas pipelines on seabed require negative buoyancy or anchoring.
Marsh/wetland routes
Shallow burial
Pipelines in marshlands and wetlands experience buoyant uplift from saturated soils.
High water table
Buried pipelines
Pipelines buried in high water table conditions require flotation analysis.
Key Concepts
- Archimedes principle: Buoyant force equals weight of fluid displaced by submerged object
- Submerged weight: Actual weight minus buoyant force (can be negative = net uplift)
- Specific gravity (SG): Density relative to water (SG = ρ/ρ_water)
- Weight coating: Dense material (concrete) applied to pipe exterior to increase submerged weight
- Negative buoyancy: Condition where submerged weight is negative (pipe sinks)
Why buoyancy matters: Empty or gas-filled pipelines can float to surface if not adequately weighted or anchored, causing operational failure, environmental damage, and safety hazards. Proper buoyancy analysis ensures pipeline remains in place.
IMAGE: Pipeline Buoyancy Force Diagram
Shows submerged pipeline with upward buoyancy force, downward weight components, and net force vectors
Failure Modes from Inadequate Buoyancy Control
- Flotation: Pipeline floats to surface, breaking out of trench or breaking connections
- Upheaval buckling: Buried pipeline bows upward from soil, creating high bending stress
- Lateral movement: Subsea pipeline displaced by currents when submerged weight too low
- Span formation: Pipeline bridges over seabed depressions, creating unsupported spans subject to VIV
- Construction flotation: Pipeline floats during flooding stages before anchoring or backfill
Design Codes and Standards
- ASME B31.8: Gas pipeline buoyancy control and weight coating requirements
- ASME B31.4: Liquid pipeline buoyancy requirements
- API RP 1102: Steel Pipeline Crossing Railroads and Highways (includes buoyancy for HDD)
- DNV-ST-F101: Submarine Pipeline Systems (offshore on-bottom stability)
- CFR Title 49 Part 192/195: Pipeline Safety Regulations (burial and protection requirements)
2. Buoyancy Force Calculations
Buoyancy force is determined by Archimedes principle: an object submerged in fluid experiences upward force equal to the weight of displaced fluid.
Archimedes Principle
Buoyant Force:
F_b = ρ_fluid × V_displaced × g
Where:
F_b = Buoyant force (lb or N)
ρ_fluid = Fluid density (lb/ft³ or kg/m³)
V_displaced = Volume of fluid displaced (ft³ or m³)
g = Gravitational acceleration (32.2 ft/s² or 9.81 m/s²)
For water (most common):
ρ_water = 62.4 lb/ft³ (fresh water)
ρ_seawater = 64.0 lb/ft³ (seawater, 3.5% salinity)
Simplified form (water, per unit length):
F_b = ρ_water × A_submerged × L
F_b (lb/ft) = 62.4 × A (ft²)
Where A = submerged cross-sectional area
Submerged Pipeline Buoyancy
For a cylindrical pipe with coating, calculate displaced volume:
IMAGE: Coated Pipeline Cross-Section
Shows concentric layers: steel pipe wall, FBE coating, concrete weight coating, with dimensions labeled
Displaced Volume (per foot of length):
A_displaced = π × (OD_coated)² / 4
Where:
OD_coated = Outer diameter including coating (in)
Convert to ft²:
A_displaced (ft²) = π × (OD_coated/12)² / 4
Buoyant force per foot:
F_b = 62.4 × π × (OD_coated/12)² / 4
Example - 20" pipe with 2" concrete coating:
Pipe OD: 20 inches
Coating thickness: 2 inches each side = 4 inches total
OD_coated = 20 + 4 = 24 inches = 2.0 ft
A_displaced = π × (2.0)² / 4 = 3.142 ft²
F_b = 62.4 × 3.142 = 196 lb/ft (fresh water)
Submerged Weight Calculation
Submerged Weight:
W_sub = W_air - F_b
Where:
W_sub = Submerged weight (lb/ft)
W_air = Weight in air (lb/ft) = pipe + coating + contents + appurtenances
F_b = Buoyant force (lb/ft)
Components of air weight:
W_air = W_pipe + W_coating + W_contents + W_misc
W_pipe = weight of steel pipe (lb/ft)
W_coating = weight of external coating (lb/ft)
W_contents = weight of product inside pipe (lb/ft)
W_misc = anodes, bracings, insulation (lb/ft)
Buoyancy sign convention:
W_sub > 0 → Pipe has net downward force (sinks)
W_sub < 0 → Pipe has net upward force (floats)
Design criterion (DNV-RP-F109 / ASME B31.8):
W_total ≥ SF × F_b (equivalently: W_sub ≥ (SF - 1) × F_b)
Where SF = safety factor (typically 1.1 per DNV-RP-F109)
Empty vs. Operating Condition
Most critical buoyancy condition is typically empty pipe during construction or maintenance:
| Condition | Contents Weight | Buoyancy Risk | Design Basis |
|---|---|---|---|
| Empty (air-filled) | ~0 lb/ft | Highest (worst case) | Design for this condition |
| Gas-filled (operating) | 1-5 lb/ft (low pressure gas) | High (still critical) | Check this condition |
| Liquid-filled (hydrostatic test) | 60-70 lb/ft (water) | Low (negative buoyancy) | Usually not governing |
| Liquid-filled (crude oil) | 45-55 lb/ft (API 30-40) | Medium | Check for light products |
| NGL/LPG | 30-35 lb/ft (propane, SG~0.5) | Medium-high | May govern for light products |
Worked Example: 24" Gas Pipeline Buoyancy
Given Data:
Pipe: 24" OD, 0.375" wall thickness (STD), Grade X65
Coating: 1.5" concrete weight coating (190 lb/ft³ density)
Anti-corrosion: 0.080" fusion-bonded epoxy (FBE) under concrete
Location: River crossing, fresh water
Condition: Empty pipe (air-filled)
Step 1: Calculate pipe weight
Steel pipe weight: 94.62 lb/ft (from ASME B36.10 table)
Step 2: Calculate FBE coating weight
FBE thickness: 0.080" = 0.00667 ft
OD_pipe = 24/12 = 2.0 ft
FBE volume = π × [(2.0 + 2×0.00667)² - 2.0²] / 4 = 0.0419 ft³/ft
FBE weight = 0.0419 × 90 lb/ft³ = 3.77 lb/ft
Step 3: Calculate concrete coating weight
Concrete thickness: 1.5" = 0.125 ft
OD_under_concrete = 24 + 2×0.080 = 24.16" = 2.013 ft
OD_over_concrete = 24.16 + 2×1.5 = 27.16" = 2.263 ft
Concrete volume = π × [(2.263)² - (2.013)²] / 4 = 0.8416 ft³/ft
Concrete weight = 0.8416 × 190 = 159.9 lb/ft
Step 4: Total air weight
W_air = 94.62 + 3.77 + 159.9 + 0 (empty) = 258.3 lb/ft
Step 5: Buoyant force
OD_coated = 27.16" = 2.263 ft
A_displaced = π × (2.263)² / 4 = 4.022 ft²
F_b = 62.4 × 4.022 = 251.0 lb/ft
Step 6: Net force (submerged weight)
W_sub = W_air - F_b = 258.3 - 251.0 = +7.3 lb/ft
(Positive = sinks; Negative = floats)
Step 7: Check DNV-RP-F109 (10% negative buoyancy)
Required: W_air ≥ 1.1 × F_b
Required: W_air ≥ 1.1 × 251.0 = 276.1 lb/ft
Actual: W_air = 258.3 lb/ft
Result: FAILS DNV check. Need additional 17.8 lb/ft
Solution: Increase concrete coating to 2.0" thickness
OD_coated = 24.16 + 2×2.0 = 28.16" → A_displaced = 4.325 ft²
F_b = 62.4 × 4.325 = 269.9 lb/ft
W_concrete = 1.1412 × 190 = 216.8 lb/ft
W_air = 94.62 + 3.77 + 216.8 = 315.2 lb/ft
Ratio = 315.2 / 269.9 = 1.17 ≥ 1.1 → PASSES DNV ✓
Design principle: Per DNV-RP-F109, total weight must exceed 110% of buoyant force for vertical stability. Even if pipe sinks (positive submerged weight), it may not meet the 10% negative buoyancy requirement.
Seawater vs. Freshwater Buoyancy
Seawater has ~2.6% higher density than freshwater, increasing buoyancy:
Buoyancy Difference:
Fresh water: ρ = 62.4 lb/ft³
Seawater: ρ = 64.0 lb/ft³ (3.5% salinity, 60°F)
For 24" coated pipe (OD = 27.16", A = 4.022 ft²):
F_b (fresh) = 62.4 × 4.022 = 251.0 lb/ft
F_b (seawater) = 64.0 × 4.022 = 257.4 lb/ft
Difference: 257.4 - 251.0 = 6.4 lb/ft (2.6% increase)
Design implication:
If designed for freshwater and installed in seawater, buoyancy increases.
Always design for actual installation environment.
3. Weight Coating Design
Weight coating (typically concrete) is applied to pipeline exterior to increase submerged weight and provide negative buoyancy. Coating thickness is calculated to achieve target submerged weight including safety factor.
Concrete Coating Properties
| Coating Type | Density (lb/ft³) | Specific Gravity | Application |
|---|---|---|---|
| Weight coating concrete | 190 | 3.05 | Standard weight coating for subsea/river crossings |
| High-density concrete | 220-240 | 3.5-3.8 | Iron ore or barite aggregate when thinner coating needed |
| Standard structural concrete | 140-150 | 2.2-2.4 | Not used for weight coating (insufficient density) |
| Fusion-bonded epoxy (FBE) | 85-95 | 1.4-1.5 | Corrosion protection layer under concrete |
Required Coating Thickness Calculation
Design Equation:
W_sub,target = W_pipe + W_FBE + W_concrete + W_contents - F_b
Set W_total,target = SF × F_b (where SF = 1.1 per DNV-RP-F109)
Solve for concrete thickness t_conc
Iterative solution (trial and error):
1. Assume concrete thickness t_conc
2. Calculate OD_coated = OD_pipe + 2×t_FBE + 2×t_conc
3. Calculate F_b = 62.4 × π × (OD_coated/12)² / 4
4. Calculate W_concrete = π × [(R_outer)² - (R_inner)²] × ρ_concrete
5. Calculate W_total = W_pipe + W_FBE + W_concrete + W_contents
6. Check if W_total ≥ SF × F_b
7. If not, increase t_conc and repeat
Rule of thumb (for quick estimates):
t_conc (inches) ≈ 0.08 × OD_pipe (inches) for empty gas pipelines
Example: 24" pipe → t_conc ≈ 0.08 × 24 = 1.9 inches (use 2.0")
Standard Coating Thicknesses
Typical concrete coating thickness ranges by pipe size and application:
| Pipe OD (in) | Minimum Thickness (in) | Typical Thickness (in) | Maximum Practical (in) |
|---|---|---|---|
| 4 - 8 | 1.0 | 1.5 - 2.0 | 3.0 |
| 10 - 16 | 1.25 | 1.5 - 2.5 | 4.0 |
| 20 - 30 | 1.5 | 2.0 - 3.0 | 4.5 |
| 36 - 48 | 2.0 | 2.5 - 4.0 | 5.0 |
| > 48 | 2.5 | 3.0 - 4.0 | 6.0 |
Note: Maximum thickness limited by handling weight, coating adhesion, and thermal stress. Very thick coatings may require reinforcement.
Coating Application Methods
- Plant-applied coating: Concrete applied at coating plant before shipping. Most common for long-line construction. Quality-controlled environment.
- Field-applied coating: Coating applied on-site (field joints, tie-ins). Requires portable equipment, weather-dependent. Used for connections and repairs.
- Impingement method: Concrete sprayed onto rotating pipe. Good adhesion, uniform thickness. Most common plant method.
- Wrap-on method: Pre-formed half-shells bolted or strapped to pipe. Used in difficult-to-coat situations. Less common.
Coating Design Considerations
- Impact resistance: Concrete must resist impact during handling, shipping, and installation
- Thermal compatibility: Coating and pipe thermal expansion must be compatible to prevent cracking
- Adhesion: FBE or other primer ensures concrete bonds to pipe, prevents slip
- Reinforcement: Wire mesh or fibers may be added to thick coatings (>3") for crack resistance
- Curing time: 28-day cure period required for full strength before installation
- Handling weight: Coated pipe may exceed crane capacity; affects logistics
Coating vs. anchoring trade-off: Thicker coating increases cost, handling difficulty, and shipping weight. For marginal buoyancy cases, mechanical anchoring may be more economical than very thick coating.
Coating Damage and Repair
Concrete coating can be damaged during handling and installation:
Allowable Damage Criteria (Industry Practice):
Minor damage (cosmetic cracks, small chips):
- Area < 100 cm² per location
- Depth < 25% of coating thickness
- No steel exposure
- Action: Document, no repair typically required
Major damage:
- Area > 100 cm²
- Depth > 25% thickness or steel exposed
- Cracks extending fully through thickness
- Action: Repair required
Repair methods:
1. Grind/clean damaged area
2. Apply epoxy primer
3. Rebuild with cement mortar or epoxy-based repair compound
4. Cure 24-48 hours before installation
Alternative Weight Systems
When concrete coating is impractical or insufficient:
| System | Description | Weight Range (lb/ft) | Application |
|---|---|---|---|
| Screw-on weights | Saddle-mounted weights bolted to pipe | 50-500 per module | Offshore, adjustable weight, post-installation addition |
| Grout bags | Sandbags or grout-filled bags over pipe | 100-1,000 per bag | Temporary or permanent, river crossings |
| Chain/cable weights | Heavy chain wrapped around pipe | 20-100 per wrap | Shallow water, temporary during construction |
| Articulated mattresses | Concrete blocks linked by cables | 500-5,000 | Offshore span control, post-lay stabilization |
4. Anchoring Systems
When weight coating alone is insufficient or impractical, mechanical anchoring systems secure pipelines against buoyancy and current forces. Anchors are spaced along pipeline to provide hold-down force.
IMAGE: Pipeline Anchoring Systems
Shows concrete deadman anchor with strap over pipeline, helical anchor, and saddle weight configurations
Anchor Types
| Anchor Type | Holding Capacity | Installation | Best Application |
|---|---|---|---|
| Concrete deadmen | 5,000-50,000 lb | Cast in-situ or precast blocks with straps | River beds, marshes, high water table burial |
| Helical anchors | 10,000-100,000 lb | Screwed into soil/rock with torque equipment | Stable soils, where drilling is feasible |
| Driven piles | 20,000-200,000 lb | Impact or vibration driving | Deep anchorage, high uplift loads, rock |
| Rock anchors (drilled) | 50,000-500,000 lb | Drill, grout threaded rod into rock | Rocky substrates, high-capacity anchoring |
| Suction anchors | 100,000-1,000,000 lb | Pump suction creates embedment in soft seabed | Offshore soft clay/silt seabeds |
| Plate anchors | 5,000-50,000 lb | Buried plates with straps to pipe | Shallow burial, lateral restraint |
Anchor Spacing Calculation
Required Anchor Force:
F_anchor,total = (F_b - W_sub) × L_span × SF
Where:
F_b = Buoyant force per unit length (lb/ft)
W_sub = Submerged weight per unit length (lb/ft)
L_span = Length of pipeline span between anchors (ft)
SF = Safety factor (1.5 to 2.0)
Anchor spacing:
L_span = F_anchor,capacity / [(F_b - W_sub) × SF]
Example:
24" pipe, submerged weight = +11.5 lb/ft (net uplift)
Buoyant force = 251.0 lb/ft
Net uplift per foot = 11.5 lb/ft
Anchor capacity = 10,000 lb (concrete deadman)
Safety factor = 1.5
L_span = 10,000 / (11.5 × 1.5) = 580 ft
Use anchor spacing: 500 ft (conservative)
Total anchors for 5,000 ft crossing: 5,000/500 = 10 anchors
Concrete Deadman Design
Most common anchor type for pipeline crossings:
Deadman Weight Required:
W_deadman = F_uplift / (μ - tan(θ))
For vertical uplift with no sliding (θ = 90°):
W_deadman = F_uplift (minimum, assumes no soil resistance)
With soil resistance (bearing capacity):
F_capacity = W_deadman × γ_soil × A_base × N_c
Where:
γ_soil = Soil unit weight (lb/ft³)
A_base = Deadman base area (ft²)
N_c = Bearing capacity factor (5-9 for soft to stiff clay)
Typical deadman dimensions:
For 10,000 lb capacity:
Block: 4 ft × 4 ft × 2 ft high
Concrete volume: 32 ft³
Concrete weight: 32 × 150 = 4,800 lb
Effective capacity = 4,800 + soil resistance
Strap: Stainless steel band or cable, 1-2" wide, wrapped over pipe
Embedment: Minimum 3 ft below scour depth
Helical Anchor Design
Helical anchors are screw-in foundations providing high capacity in competent soils:
Helical Anchor Capacity:
Q_ult = A_helix × q × N_q
Where:
A_helix = Total helix plate area (ft²)
q = Soil bearing pressure (psf)
N_q = Bearing capacity factor (depth and soil dependent)
Torque correlation (during installation):
Q_ult = K_t × T
Where:
T = Installation torque (ft-lb)
K_t = Empirical torque factor (3-10 ft⁻¹ depending on soil)
Typical installation:
- Helix diameter: 8-14 inches
- Shaft diameter: 1.5-3 inches
- Embedment depth: 10-30 ft
- Installation torque: 5,000-30,000 ft-lb
- Capacity: 10,000-100,000 lb
Lateral Stability and Current Forces
Offshore and river crossing pipelines also experience lateral forces from water current:
Drag Force (Lateral):
F_drag = 0.5 × ρ_water × V² × C_D × A_projected
Where:
ρ_water = Water density (lb/ft³ / g)
V = Current velocity (ft/s)
C_D = Drag coefficient (0.6-1.2 for cylinders)
A_projected = Projected area = OD × L
Lift Force (Vertical):
F_lift = 0.5 × ρ_water × V² × C_L × A_plan
Where:
C_L = Lift coefficient (0.4-0.8 for cylinders near seabed)
A_plan = Plan view area = OD × L
Total lateral stability requirement:
F_anchor ≥ √[(F_drag)² + (F_lift)²] + (F_buoyancy - W_sub)
Anchors must resist combined buoyancy and hydrodynamic forces.
Anchor placement strategy: Concentrate anchors at high points in pipeline profile where buoyancy is greatest. Reduce spacing in strong current areas. Allow for scour depth when specifying embedment.
Geotechnical Investigation for Anchoring
Soil conditions dictate anchor type and capacity:
| Soil Type | Preferred Anchor | Typical Capacity | Installation Challenge |
|---|---|---|---|
| Soft clay/silt | Suction anchor, deadman | Low to medium | Low shear strength, limited holding |
| Stiff clay | Helical anchor, driven pile | Medium to high | Good holding, may require pre-drilling |
| Dense sand | Helical anchor, deadman | Medium | Good drainage, scour potential |
| Gravel | Driven pile, deadman | Medium to high | High friction, difficult drilling |
| Rock | Drilled rock anchor | Very high | Expensive drilling, excellent holding |
| Organic/peat | Deep pile to bearing layer | Low (surface), high (depth) | Very soft near surface, anchor to competent soil |
5. On-Bottom Stability
Offshore subsea pipelines laid on the seabed must resist lateral movement from waves and currents. On-bottom stability analysis combines buoyancy, hydrodynamic forces, soil resistance, and pipe-soil friction.
DNV-ST-F101 Stability Criteria
DNV (Det Norske Veritas) standard for submarine pipelines requires:
Lateral Stability Criterion:
F_stabilizing ≥ F_destabilizing × SF
Stabilizing forces:
- Submerged weight (negative buoyancy)
- Soil friction (μ × W_sub)
- Passive soil resistance (for partially embedded pipe)
Destabilizing forces:
- Hydrodynamic drag (current)
- Hydrodynamic lift (current over pipe)
- Wave-induced forces (shallow water)
Safety factors:
SF = 1.1 for normal design
SF = 1.5 for shallow water (<30 m) or high-energy environment
Passive Soil Resistance
Pipeline embedment into seabed provides lateral resistance:
Passive Resistance Force:
F_passive = γ_soil × z² × N_p × L
Where:
γ_soil = Submerged soil unit weight (lb/ft³)
z = Embedment depth (ft)
N_p = Passive pressure coefficient (2-10 depending on soil)
L = Length of pipe segment (ft)
Pipe-soil friction:
F_friction = μ × (W_sub + F_lift)
Where:
μ = Friction coefficient (0.3-0.6 for pipe on clay/sand)
Total lateral resistance:
F_total = F_friction + F_passive
This resistance must exceed lateral hydrodynamic forces.
Trenching and Burial
Burying pipeline eliminates hydrodynamic forces and provides maximum stability:
| Installation Method | Typical Cover Depth | Advantages | Disadvantages |
|---|---|---|---|
| Surface lay | 0 (on seabed) | Low cost, simple, fast | Requires stability analysis, exposure to anchors/trawls |
| Post-lay trenching | 0.5-2 m | Protects pipe, reduces stability requirements | Additional cost, trenching equipment, backfill stability |
| Pre-lay trenching | 1-3 m | Pipe lowered into trench, immediate cover | Trench must remain open, higher cost |
| Jetting/plowing | 0.5-1.5 m | Simultaneous lay and bury, cost-effective | Limited to soft soils, shallow burial depth |
| HDD/Direct pipe | 3-30 m | Deep burial, no surface disturbance | Very high cost, limited to crossings |
Buoyancy During Construction
Critical phase: pipeline may float during flooding before burial or anchoring:
Construction Sequence (Offshore):
1. Pipelay: Pipeline laid on seabed by lay barge
- Initially filled with air (buoyant)
- Held down by lay tension and pipe stiffness
2. Flooding: After lay complete, flood with seawater
- Buoyancy eliminated
- Requires venting air through pig launchers/receivers
3. Trenching/Stabilization:
- Trench and bury OR
- Install rock dumping/mattresses OR
- Allow self-burial (soft seabeds)
Critical risk period: Between lay and flooding/stabilization
- Weather/current limits for lay operations
- Emergency anchoring plan if weather deteriorates
- Temporary weights or anchors for extended periods
Long-Term Stability Considerations
- Scour: Seabed erosion around pipe exposes more area to currents, increases hydrodynamic forces, reduces soil restraint
- Spanning: Pipe bridges over local depressions, creating free spans subject to vortex-induced vibration (VIV)
- Liquefaction: Seismic or storm-induced soil liquefaction eliminates soil support temporarily
- Coating degradation: Concrete coating can degrade over decades, reducing submerged weight
- Marine growth: Barnacles and biological growth increase drag coefficient and projected area
Monitoring and Inspection
Verify stability and detect changes over pipeline life:
| Method | Frequency | Detects |
|---|---|---|
| ROV survey (offshore) | Annual or post-storm | Spans, scour, coating damage, lateral movement |
| Multibeam sonar | Baseline + 5-year | Burial depth, seabed changes, large-scale scour |
| Aerial survey (river crossings) | After high-flow events | Exposure, bank erosion, anchor displacement |
| Cathodic protection monitoring | Annual | Coating damage (increased current demand indicates coating breach) |
Design philosophy: Combine weight coating for baseline negative buoyancy with mechanical anchoring at critical locations (high points, strong currents, scour-prone areas). Design for worst-case: empty pipe, maximum current, 100-year storm conditions.
Case Study: River Crossing Buoyancy Failure
Incident Description:
16" natural gas pipeline, HDD installation under river
Design: 1.5" concrete coating, calculated W_sub = -8 lb/ft (SF = 1.15)
Construction: Pipe pulled through 24" HDD bore
Failure Sequence:
Day 1: Successful pullback, pipe in bore
Day 3: Heavy rain, river flood stage +8 ft above normal
Day 5: Bore collapse, annular space opens to river
Day 7: Pipeline floats to surface in middle of river
Root Causes:
1. Bore annulus filled with river water (not drilling mud as designed)
2. Buoyancy increased due to higher external water level
3. Bore collapse allowed lateral movement
4. Insufficient concrete coating for full-submersion scenario
Lessons Learned:
- Design for full external water pressure even in HDD bores
- Increase SF to 1.3-1.5 for HDD under water bodies
- Post-installation verification (pressure monitoring, survey)
- Contingency plan for bore collapse: grout annulus, add rock berms
Corrective Action:
- Excavate exposed pipe
- Add 1" additional concrete coating (2.5" total)
- Re-install in stabilized bore with pressure grouting
- Monitor annular pressure permanently
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