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Ice Actions on Structures:
Best Engineering Practice
INTSOK
Arctic Offshore Operations Conference
Anchorage, Alaska, May 27th 2015
Arnor Jensen, Basile Bonnemaire, Ivar Eng - Multiconsult, Norway
multiconsult.no
• A multi-disciplinary consulting and
engineering company operating in 6
business areas
• 30 offices in Norway, HQ in Oslo
• International experienced
engineering partner in the Oil & Gas
and Energy (hydropower) area
• 100 years company history and 40
years of experience from the
Norwegian Continental Shelf
• 1700 professional employees in
total at home and abroad
• Listed at the Norwegian Stock
Exchange (Oslo Børs)
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Ice Actions on Structures - Best Engineering Practice
Contents:
• Introduction
• Design data
• Ice actions
• Model testing in ice basins
• Numerical simulations of ice actions and extremes
• Application examples
• Conclusion - best engineering practice
© http://www.shtokman.ru/en/project
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INTRODUCTION-Arctic offshore
• Environmental conditions in Arctic
© J P Hansen
=> One of the main drivers for field
development plans
• Ice actions
- High in magnitude if not managed (compared to open water)
- Significantly govern fixed and floating concept solutions
=> Concept driver (and cost driver)
• Ice presence impact on design
© B. Bonnemaire
=> More parameters and combinations arise
• Potential consequences (safety & costs)
=> High if uncertainties are not understood /
properly handled
• ISO 19906 (design)
- Provides guidelines for design of arctic offshore structures
© Multiconsult
including ice actions
=> Not a “stand alone” document for design
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Arctic physical (ice) environment
• Access to environmental data for
design in Arctic waters is challenging:
- Remote areas with little/no
measurements
- Lack of long term time series – “low
probability” actions are difficult to
assess
- Ice actions depend on a multiple of
parameters – often, complex
dependency
- ISO 19906 gives general guidelines,
but does not give any recipes on how
to combine parameters
© Wrigth - Perd/CHC
© Multiconsult
© E Hovland
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Ice actions and station-keeping in ice
• Full scale experience with station keeping in ice is somewhat limited
• Ice actions on structures have a complex nature. In particular:
- Vaning forces and drift reversals
- Subsurface impact, Ice transport and accumulation of ice
- Boundaries and far field conditions are important especially for broken ice
conditions (“tight ice”)
• Floating installations have a high degree of non-linearity (and complexity)
• Standards (ISO 19906):
- Good general guidance (normative part)
- No recipes, in particular for floating (informative part)
- There are analytical models in ISO 19906 that suffer from oversimplification
© Jensen
© JP Hansen
© Multiconsult
© Multiconsult
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Ice basin testing in ice engineering
• ISO 19906 points at ice basin testing as a good approach for assessing
structural response to ice actions
• Ice basin tests are often the most efficient way to simulate complex icestructure interactions,
• But challenging:
- Costs and time budget provides a limited number of test runs and
tested conditions
- Scaling and modeling of a moored structure: approximation are often
required
- Scaling of ice properties: Model ice does not manage to scale all ice
properties at the same time
- Monitoring of response is not ice actions
• Outcome of ice basin tests needs to be carefully interpreted
© A. Jensen
© B.Bonnemaire
multiconsult.no
Technology, Methods
- Physical environment and design data
• Design requires solutions that meets
following criteria:
- Sufficiently long data series for reliable
design
- Be able to include physical modelling of
environmental actions i.e ice drift
- Model statistical properties that meets
requirements to model
 correlation between parameters
 correlation in time (ice presence, wind, currents)
© Stephen E. Bruneau-icebergfinder.com
© B.Bonnemaire
© B.Bonnemaire
• Best practice available,
but no standards yet
© Jensen
© S. Løset
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Long synthetic time series
- used for reliability based design
• SATSS – Synthetic Arctic Time Series Software
• Purpose of synthetic time series:
- Give multiple stochastic realizations of a process (eg. MonteCarlo)
- Produce possible combinations of multiple physical environment
parameters (correlated)
- Include stochastic realization of processes with time dependent
correlation (eg. Markov)
- Possibility to include corresponding structural response
• Long multivariate time series can be used in many applications, in cold
climate such as:
 Time series for ice and ice drift: currents, wind, ice properties – e.g. curvature radius
 Time series for sea spray icing: waves, wind, sea surface, air temperature, spray
generation
• Main applications of the software:
- Calculation of multi variable extremes
- Study operability and characteristics of operations (downtime),
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Numerical simulation of station-keeping in ice
• A numerical model (simShipIce) simulating the response of a moored structures to
actions from variable drifting ice is developed (in-house development)
• Intact level ice and ridges – (limit stress method) based on continuum approach
• Equation of motion solved in time domain for the 6 degrees of freedom
accounting for:
- Inertia, hydrodynamic and damping forces,
- Non-linear mooring forces,
- Hydrostatic forces (non linear estimation),
- Ice actions
 The ice load is determined locally around the submerged hull – includes rubble
interactions (and accumulation)
 Updated at each time step, dependent on ice and ship situation.
 The variation in the waterline and sub-surface geometry is estimated at each time step
© Multiconsult
multiconsult.no
Numerical simulation of station-keeping in ice
• A numerical model (SIBIS) simulating the response of a moored structures to
actions from variable drifting ice is developed
• Discrete element/rigid body simulations of broken ice fragments and flows
• Simulation ice failure and ice breaking process
• Equation of motion solved in time domain for the 6 degrees of freedom
accounting for:
- Inertia, hydrodynamic and damping forces,
- Non-linear mooring forces, hydrostatic forces (non linear estimation),
- Ice interaction, ice field response, boundary effects
Broken ice cover
(floating ice pieces
with 6 DOF)
Ice tank
Moored floating
structure (6 DOF)
walls
Ice-ice
interactions
Ice-wall
interactions
© Multiconsult
Ice-structure
interactions
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SIBIS – broken ice
and variable ice drift
• Response of fixed and floating structure
• Variable ice drift and curvatures, included
ice drift reversals and accumulation of ice
• Operations in ice, including managed ice
• Accumulation of ice, sub surface ice
transport
• Can combine actions from ice, wind,
currents
Measurements
© BHSVA
© Multiconsult
Simulations
© HSVA
© Multiconsult
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The turret moored OIB - MS vs FS - correction
© Multiconsult
© A. Jensen
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Shtokman ice conditions – dynamic drift simulation
• Ice presence
- Statistical properties
- Correlation in time
• Ice concentration
- Statistical properties
- Correlation in time
• Morphological properties
- Statistical properties of flow size
- Statistical properties of Thickness
- Statistical properties of Ice ridges
• Metocean properties
- long time series (hindcast)
• Free drift ice model - physics
• =>SATSS realization of a long time
series
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Example Shtokman design
1. SATSS – «10 000» years time serie
- Physical environment
- Free ice drift model
• «True» Ice conditions :
2. «Forecasted» ice drift implemented
3. Ice mangement implemented
• ShimShipIce:
- Global respose and mooring loads
• Extreme design according to ISO 19906
• Operations and uptime
© Multiconsult
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Shtokman offshore development - extremes
Without ice management
With ice management
Ice Management is not always 100% efficient
=> Ice Management reduces the loads, but may not removes the extremes
=> Difficult to argue that disconnection is not required
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Best engineering practice for ice actions on structures
• Design based on ISO 19906 normative guidelines
• Integrated probability based design with synthetic data series with sufficient length
- Long data series of physical environment
- Realization of parameters based on statistical properties
- Multivariable physical processes incorporated (ie. ice drift)
- Performance of structure included response (ie. Structural response)
• Ice actions assessed by combination of numerical and physical ice basin testing
• Model testing in an ice basin:
- Test reference cases (can be design cases) and verification (according to ISO 19906)
- Vessel performance in ice (understand the “physics”)
• Numerical simulations:
- Compare measurement and simulations - validation
- Post-simulate the required range of design cases and load combinations
© J P Hansen
© http://www.shtokman.ru
© A.Jensen
© Wrigth - Perd/CHC
multiconsult.no
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furnished data or analysis.
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It takes long experience to handle
challenging conditions……
THANK YOU FOR YOUR ATTENTION
© B. Bonnemaire
multiconsult.no
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