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Offshore vessel stability refresher and enhancement

Comprehensive training on stability principles and calculations for offshore operations

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Offshore vessel stability training
Refresher and enhancement program
1 / 26
Offshore vessel stability training
Refresher and enhancement program
Offshore vessel stability refresher and enhancement1 / 26
Fundamentals of stability
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Basic stability concepts
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    Stability is the ability to return to upright after disturbance
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    Depends on relationship between gravity and buoyancy forces
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    Initial stability determined by metacentric height (GM)
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    Positive GM indicates stable vessel
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    Negative GM indicates unstable vessel
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Stability forces in action
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Diagram showing center of gravity, center of buoyancy, and metacentre with forces acting on a heeled vessel
Diagram showing center of gravity, center of buoyancy, and metacentre with forces acting on a heeled vessel
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Key stability terms defined
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    Draft: vertical distance from waterline to lowest point of hull
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    List: permanent angle of heel to port or starboard
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    Trim: difference between forward and aft drafts
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    Freeboard: distance from waterline to main deck
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    Reserve buoyancy: volume above waterline available for flotation
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Draft, list and trim calculations
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    Draft marks read at forward, midship and aft
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    Mean draft = (Fwd + Aft)/2
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    Trim = Fwd draft - Aft draft
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    List measured with clinometer or plumb line
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    Trim by stern: aft draft > forward draft
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    Trim by head: forward draft > aft draft
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    List to port: vessel heels left
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    List to starboard: vessel heels right
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    Corrections made by weight shifting or ballasting
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Stability criteria and standards
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Intact stability criteria
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    Area under GZ curve must be ≥ 0.055 m-radians up to 30°
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    Area under GZ curve must be ≥ 0.090 m-radians up to 40°
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    Maximum GZ must occur at angle ≥ 25°
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    GM must be ≥ 0.15 meters
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    Angle of vanishing stability must be ≥ 60°
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Dynamic stability considerations
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    Accounts for energy absorbed during heeling
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    Dynamic stability = area under GZ curve to angle of heel
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    Must exceed energy from wind, waves, or operations
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    Critical for lifting operations and heavy weather
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    Calculated using work-energy principle
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Static stability curve (GZ curve)
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Graph showing GZ values (righting arm) at various angles of heel from 0 to 90 degrees
Graph showing GZ values (righting arm) at various angles of heel from 0 to 90 degrees
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Hydrostatics and weight distribution
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Hydrostatic data essentials
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    Includes displacement, LCB, LCF, VCB for various drafts
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    Provided in hydrostatic tables or curves
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    Used to calculate stability at different loading conditions
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    Typically provided by shipyard or classification society
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    Must be updated after major modifications
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Centers of gravity, buoyancy and flotation
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    Center of Gravity (G): point where total weight acts
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    Center of Buoyancy (B): center of displaced water volume
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    Metacentre (M): intersection of buoyant force lines
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    Center of Flotation (F): center of waterplane area
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    G moves with weight distribution changes
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    B moves with hull shape and draft changes
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    M position depends on waterplane inertia
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    F is the pivot point for trim changes
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    GM = KB + BM - KG (metacentric height)
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Stability diagram with key points
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Cross-sectional diagram of a vessel showing G, B, M, F and the metacentric height (GM)
Cross-sectional diagram of a vessel showing G, B, M, F and the metacentric height (GM)
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Weight movement and distribution
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    Vertical weight movement affects KG and GM
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    Horizontal weight movement causes list
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    Longitudinal weight movement affects trim
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    Free surface effect reduces stability
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    Weight additions increase draft and may affect stability
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Advanced stability concepts
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Free surface effect explained
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    Occurs in partially filled tanks or compartments
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    Liquid surface remains horizontal as vessel heels
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    Effectively raises center of gravity of liquid
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    Reduces metacentric height and stability
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    Mitigated by subdividing tanks or filling completely
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Free surface effect diagram
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Diagram showing a partially filled tank with the liquid surface horizontal as the vessel heels, and the apparent shift in the center of gravity
Diagram showing a partially filled tank with the liquid surface horizontal as the vessel heels, and the apparent shift in the center of gravity
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Damage stability principles
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    Ability to remain afloat and stable after flooding
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    Regulated by SOLAS and classification societies
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    Requires subdivision into watertight compartments
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    Calculations consider worst-case flooding scenarios
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    Must maintain positive stability and sufficient freeboard
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Reserve buoyancy and damage stability
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    Reserve buoyancy = total volume - submerged volume
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    Provides flotation after damage and flooding
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    Expressed as percentage of total volume
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    Offshore vessels typically have 30-50% reserve buoyancy
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    Critical for maintaining freeboard after damage
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Damage stability calculations
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    Step 1: Identify flooded compartments and permeability
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    Step 2: Calculate lost buoyancy and new displacement
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    Step 3: Determine new center of gravity and buoyancy
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    Step 4: Check final draft, trim, and stability
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    Step 5: Verify against damage stability criteria
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Onboard stability calculations
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    Performed before and after major operations
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    Consider loading/unloading, fuel consumption, ballasting
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    Use loading computer or stability booklet
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    Verify against approved stability criteria
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    Document all calculations for audit purposes
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Practical applications and case studies
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Stability calculation example
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    Initial condition: Draft 6.0m, KG 7.5m, GM 1.2m
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    Load 200t cargo at KG 10m
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    New KG calculation: (Δ*KG + w*kg)/(Δ + w)
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    New GM = KB + BM - new KG
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    Check against stability criteria
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    KB from hydrostatics at new draft
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    BM = I/∇ (I = waterplane inertia, ∇ = displacement)
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    Typical I for offshore vessel: 50,000 m⁴
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    New displacement = initial + 200t
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    Verify freeboard and intact stability
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“
A vessel's stability is not a luxury but a fundamental requirement for safe operations at sea. Every decision affecting weight distribution impacts stability.
International Maritime Organization
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Stability knowledge saves lives
Apply these principles daily for safe offshore operations

Understand fundamental stability concepts • Perform accurate calculations before operations • Monitor weight distribution continuously • Verify stability after any significant changes • When in doubt, consult stability documentation

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