🎓 Lesson 8
D5
Empirical Scour Models: SCS-2, BRE, and Modified Hanzawa
Empirical scour models are simple math formulas engineers use to predict how deep the seabed will erode around underwater structures like mooring anchors or turbine foundations when waves and currents hit them.
🎯 Learning Objectives
- ✓ Calculate equilibrium scour depth using the SCS-2, BRE, and Modified Hanzawa models for given hydrodynamic and sediment conditions
- ✓ Analyze and compare predicted scour depths across models to select the most appropriate one for a specific mooring foundation scenario
- ✓ Explain the physical limitations and assumptions underlying each model (e.g., steady vs. oscillatory flow, cohesionless sediment, isolated cylinder assumption)
- ✓ Apply correction factors for pile groups, armor layers, and tidal asymmetry in accordance with DNV-RP-F105 guidelines
- ✓ Design preliminary scour protection (e.g., geotextile sand mattress extent) based on model outputs and safety margins
📖 Why This Matters
Scour is the leading cause of foundation instability for offshore wind turbine monopiles and mooring systems in marine renewables—accounting for over 35% of unplanned maintenance events in North Sea deployments (DNV, 2022). Underestimating scour can lead to anchor pullout, cable fatigue, or structural collapse; overdesigning protection wastes cost and installation time. Empirical models like SCS-2, BRE, and Modified Hanzawa deliver rapid, field-validated predictions essential for early-stage design, permitting, and risk assessment—before costly CFD or large-scale physical modeling begins.
📘 Core Principles
All three models originate from flume and wave-tank experiments on isolated vertical cylinders in uniform, cohesionless sediments. SCS-2 (USACE, 1991) emphasizes steady current dominance and uses Keulegan–Carpenter number (KC) and sediment mobility parameter (ψ). The BRE model (British Renewable Energy Association, 2008) was calibrated specifically for oscillatory wave-dominated flows typical of tidal stream and offshore wind sites, incorporating orbital velocity and bed shear stress. Modified Hanzawa (Hanzawa et al., 1994; adapted by ISO/IEC 19901-6, 2022) introduces shape and group-effect corrections for multi-pile foundations and accounts for combined wave–current loading. Critically, all assume equilibrium scour (i.e., maximum depth reached after sufficient time), and none explicitly resolve transient scour evolution or cohesive soils without modification.
📐 Key Calculation: Modified Hanzawa Scour Depth
The Modified Hanzawa model predicts equilibrium scour depth (sₑ) around circular piles under combined wave–current flow. It scales the basic Hanzawa expression with empirical coefficients for pile diameter, Keulegan–Carpenter number, and Shields parameter, then applies corrections for pile spacing and sediment gradation. It is widely adopted in ISO 19901-6 Annex E for mooring foundation assessments.
Modified Hanzawa Scour Depth
sₑ/D = 2.0 × KC⁰·²⁵ × θ⁻⁰·³³ × (1 + 0.15 log₁₀(KC)) × α × βISO-endorsed model for combined wave–current loading; includes pile group (α) and sediment gradation (β) corrections.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| θ | Shields parameter | – | Dimensionless bed shear stress normalized by sediment submerged weight |
| α | Pile group correction factor | – | α = 1.0 for isolated pile; decreases to ~0.7 for 3×3 pile array with S/D = 2.5 |
| β | Sediment gradation factor | – | β = 1.0 for uniform sand; increases to 1.2–1.4 for poorly sorted sediments |
Typical Ranges:
Offshore wind monopiles (KC = 2–10): 3.0 – 6.0 × D
💡 Worked Example
Problem: A 1.2-m-diameter monopile foundation is installed in medium sand (d₅₀ = 0.3 mm) at a site with peak orbital velocity Uₘ = 1.8 m/s, mean current U꜀ = 0.4 m/s, and water depth h = 25 m. Sediment density ρₛ = 2650 kg/m³, water density ρ = 1025 kg/m³, kinematic viscosity ν = 1.3×10⁻⁶ m²/s. Calculate sₑ using Modified Hanzawa.
1.
Step 1: Compute KC = 2πUₘT/(h + D), assuming T = 8 s → KC = 2π×1.8×8/(25 + 1.2) ≈ 3.45
2.
Step 2: Compute Shields parameter θ = (ρₛ−ρ)g d₅₀ / (ρ Uₘ²) → θ ≈ (1625)(9.81)(0.0003)/(1025 × 1.8²) ≈ 0.147
3.
Step 3: Apply Modified Hanzawa: sₑ/D = 2.0 × (KC⁰·²⁵) × (θ⁻⁰·³³) × (1 + 0.15 log₁₀(KC)) → sₑ/1.2 = 2.0 × (3.45⁰·²⁵) × (0.147⁻⁰·³³) × (1 + 0.15 log₁₀(3.45)) ≈ 2.0 × 1.36 × 1.62 × 1.07 ≈ 4.48
4.
Step 4: sₑ = 4.48 × 1.2 = 5.38 m
Answer:
The predicted equilibrium scour depth is 5.38 m, which falls within the typical range of 3.5–6.0 m for medium sand under energetic wave–current conditions per ISO 19901-6 Annex E validation dataset.
🏗️ Real-World Application
In the Moray East Offshore Wind Farm (Scotland), developers used the BRE model during FEED to assess scour around 8 MW turbine monopiles (D = 7.5 m) in mixed wave–current environments (Uₘ = 2.1 m/s, U꜀ = 0.6 m/s, d₅₀ = 0.25 mm). Initial BRE prediction gave sₑ = 4.2 m. Field monitoring via ROV bathymetry after 18 months confirmed 3.9 m maximum scour—within 7% error—validating model selection. Subsequent design incorporated a 2.5-m-thick rock armor extending 2.5 pile diameters radially, per DNV-RP-F105 Clause 7.4.2 recommendations.
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