HAZID hazard identification HPZ high pressure zone HSE health, safety, and environment HVAC heating, ventilating, and air conditioning IMO International Maritime Organization LAST lowest
Symbols
For the purposes of this document, the following symbols apply
F repetitive action f c concrete compressive strength f y characteristic yield strength
Q 2 short-duration variable action γs steel resistance factor γc concrete resistance factor η concrete efficiency factor
Abbreviated Terms
For the purposes of this document, the following abbreviated terms apply
ALARP as low as reasonably practicable
ALIE abnormal-level ice event
ALS abnormal (accidental) limit state
EER escape, evacuation, and rescue
ELIE extreme-level ice event
FPSO floating production, storage, and offloading
FSO floating storage and offloading
FY first-year, with reference to ice
HSE health, safety, and environment
HVAC heating, ventilating, and air conditioning
LAST lowest anticipated service temperature
MY multi-year, with reference to ice
SALM single anchor leg mooring
SLIE serviceability-level ice event
Fundamental Requirements
Offshore structures for use in arctic and cold regions shall be planned, designed, constructed, transported, installed, and decommissioned in accordance with ISO 19900 supplemented by this standard
Physical environmental parameters must be assessed following ISO 19901-1 and its additional requirements Seismic actions should adhere to API 2EQ, while foundation behavior is governed by API 2GEO The actions and resistances for steel, concrete, and floating structures are defined by ISO 19902, ISO 19903, and ISO 19904-1, respectively, along with the standard's supplementary requirements Additionally, the guidelines for marine operations, stationkeeping, and topsides in arctic and cold regions are included to enhance the API requirements.
The standards 2GEO, API 2SK, and 19901-3 outline essential requirements and recommendations regarding ice-related parameters for various structures, including man-made islands These documents should be utilized together to establish suitable physical environmental parameters, necessary actions, resistances, and design specifications.
Section 5 provides general requirements and conditions applicable to all types of offshore structures.
Design Methods
For designs performed in accordance with the design process and equations provided in this standard, levels of safety and performance are established in Section 7
An alternative rational design approach, grounded in theory, analysis, and established engineering practices, can replace the standard design process and equations, as long as it ensures safety and performance levels that meet or exceed those specified in Section 7.
To ensure the accuracy of new designs, it is essential to utilize data from full-scale measurements of ice actions whenever feasible Additionally, appropriately scaled physical and mathematical models can be employed to assess the structural response to ice actions, alongside current, wind, and wave influences Calibration of these models should be conducted using data derived from full-scale measurements whenever possible.
Site-specific Considerations
Identification, investigation and determination of site-specific conditions for sea ice and icebergs shall be made with due consideration to the phenomena and effects described in 6.5 and 8.2
The structure and its subsystems must be operated in an environmentally responsible way It is essential to identify conditions that may affect functional and operational requirements Additionally, it is important to establish and maintain proper procedures related to health, safety, and environmental (HSE) matters.
The assessment of potential hazard scenarios must encompass the likelihood of abnormal-level ice events (ALIE), the local and global repercussions of ice feature impacts, and geohazards specific to the Arctic region.
During the design service life of a structure, it is essential to account for potential changes in storm frequency and magnitude, ice conditions, ocean circulation, air temperatures, permafrost, wave heights, and water levels Incorporating these factors into the design is crucial for ensuring the structure's resilience and longevity.
The configuration of the structure should consider the following:
⎯ protection of risers and conductors;
⎯ concept for oil storage and export;
⎯ wet or dry storage system;
⎯ layout of facilities and separation distances from hazards;
⎯ potential for future expansion under phased development;
⎯ capability of local construction facilities and materials; and
⎯ available construction season for offshore operations
Different structural shapes, orientations, and profiles for the structure and the topsides should be considered for resisting sea ice or iceberg actions
When determining the structure's orientation at the site, it is essential to account for ice conditions, prevailing ice drift directions, and ice rubble accumulation The arrangement of the topsides must align with functional and operational needs, including resupply, offloading, flaring, and escape, evacuation, and rescue (EER), while also considering wind and ice encroachment.
To enhance the reliability of EER, platform supply, and offloading systems, several strategies can be implemented: effective ice management to avoid ice rubble buildup, the duplication of facilities on both sides of the platform, and the use of large crane booms designed to extend over accumulated rubble.
To ensure the safe operation of systems and equipment during winter, it is essential to implement winterization measures that prioritize personnel safety These measures should facilitate ergonomically sound task performance, taking into account factors such as temperature, wind, visibility, and the limitations of personal protective equipment (PPE) Additionally, it is crucial that the chosen options do not lead to negative environmental impacts.
Construction, Transportation, and Installation
The construction, transportation, and installation of offshore structures shall be planned to minimize the risks to personnel and equipment due to adverse environmental conditions associated with cold regions
When planning construction activities, it is essential to account for extreme low ambient temperatures Proper protection for materials and equipment must be ensured through heating or insulation measures.
Evaluation of minimal draft limitations shall be performed for all regions through which the structure can be transported in order to reach its final destination
When planning critical offshore activities, it is essential to specify all marine spread equipment and ensure personnel can safely conduct transportation and installation in challenging conditions, such as low ambient temperatures and low visibility Clear limitations on environmental conditions must be established, along with defined weather windows, to guide the planning and execution of these operations Additionally, comprehensive contingency planning is crucial to guarantee the safety of transportation and installation processes.
Marine operations associated with the transportation and installation of the structure shall be in accordance with API 2MOP.
Design Considerations
The combinations of actions and partial factors for determining action effects applicable to the different limit states shall be in accordance with the provisions of Section 7.
Environmental Protection
Arctic and cold region environments are particularly sensitive to pollution, necessitating careful design of structures to minimize environmental impact It is essential to limit structural systems that require active operations to prevent pollution Additionally, efforts should be made to reduce harmful environmental effects during the construction, transportation, installation, and decommissioning phases.
To prevent environmental pollution, fluids and materials must be stored in tanks equipped with double barriers It is essential to implement a thorough strategy for the inspection, maintenance, and repair of these tanks to ensure they remain secure and effective in containing potentially harmful substances.
Means for the containment and clean-up of polluting fluids shall be available at the site and be proven to operate under the expected physical environmental conditions
In the case of permafrost or freezing conditions, provision should be made to ensure the integrity of ballast tanks.
Serviceability Requirements Including Vibrations and Crew Comfort
To ensure crew comfort during normal operations, it is essential to address structural vibrations alongside the standard serviceability requirements for structural design outlined in section 7.2.2.2.
General
The owner is responsible for choosing suitable physical environmental design parameters and operating conditions, while also considering any applicable regulatory requirements These regulations may specify the minimum duration of site-specific data, the types of data required, and the definition of extreme design parameters.
General guidelines on metocean information are given in ISO 19900 and specific requirements in ISO 19901-1
A thorough evaluation of the physical environmental factors influencing the proposed offshore structure will be conducted This assessment will utilize all pertinent environmental data alongside relevant physical, statistical, mathematical, and numerical models to generate the necessary information.
To ensure safe operations and effective planning in construction and field activities, it is essential to identify normal environmental conditions for serviceability limit states (SLS) and to develop actions and effects for safe operations Additionally, long-term distributions of physical environmental parameters must be analyzed using cumulative statistics to define design situations and evaluate workability over time Understanding extreme and abnormal environmental parameters is crucial for developing actions for ultimate limit states (ULS) and abnormal limit states (ALS) Finally, it is important to consider regional environmental oscillations, cycles, and long-term trends related to these parameters.
The relevant physical environmental parameters can be dependent on the chosen structural form
To ensure accurate physical environmental parameters, it is essential to involve metocean and ice technology experts in data analysis and interpretation, which will aid in establishing suitable design situations and criteria.
6.1.2 Physical Environmental Data Monitoring Programs
Appropriate physical environmental parameters shall be monitored and forecast during the design service life of the structure to ensure safe operational procedures
6.1.3 Relationship with Remainder of this Standard
Physical environmental parameters described in 6.2 through 6.6 shall be obtained for use in the calculation of global and local action values, as described in the appropriate subsections of this standard
Data shall be analyzed in a format appropriate for use in the determination of relevant environmental actions
Daylight Hours
In arctic regions, the fluctuating daylight hours significantly impact initial data collection and the operations of offshore facilities It is essential to factor in this variation when planning operations to ensure effective management and safety.
Meteorology
Estimates of the probability distributions of air temperatures that a structure may experience throughout its design service life are essential The lowest anticipated service temperature (LAST) must be defined in accordance with section 3.48.
When selecting structural materials, machinery lubrication, sealants, and topsides winterization, it is crucial to consider the effects of temperature Additionally, the impact of thermal changes on structural behavior and human capability must be integrated into the design and operation of the structure.
Air temperature significantly influences the properties and conditions of ice Variations in air temperature, along with wind and snow depth, dictate thermal expansion and related behaviors in landfast ice Additionally, low air temperatures, when coupled with winds, waves, and sea spray, can result in marine icing events.
Wind related parameters shall be determined in accordance with ISO 19901-1
Wind chill is determined by the interplay of air temperature and wind speed, significantly impacting human comfort, machinery heat loss, and the winterization of topsides.
The importance of PPE, heated shelters, appropriate work procedures, and other actions should be considered when workers are exposed to outdoor conditions
During the design service life of a structure, it is essential to estimate the probability distributions of precipitation and snowfall it may experience Consideration must be given to the potential accumulation of snow and its impact on both the structure and machinery Additionally, when evaluating ice properties such as friction and strength, the influence of snow cover should be taken into account to accurately determine ice actions and assess vessel operations.
As snow accumulation can affect platform operations, considerations for snow removal should be made at the design stage
When designing structures, it is crucial to account for ice accretion from sources like sea spray, freezing rain, and fog, as it can significantly increase the diameter of structural components This accumulation of ice can lead to heightened forces from wind and self-weight, especially in long, slender structures such as flare towers Additionally, ice accretion poses risks to operational efficiency and personnel safety.
Visibility at the offshore structure site is influenced by various factors such as fog, blowing snow, and daylight hours It is essential to assess how these conditions impact operations and the monitoring of the physical environment.
Polar lows, which impact the subarctic and arctic regions, are challenging to observe and predict due to their small size and limited ground observation systems Although the associated waves and winds are typically not extreme, they can still influence operations Therefore, incorporating satellite remote sensing into standard meteorological forecasting is essential for identifying these features during activities.
Oceanography
The water depth at the site, along with its variations, must be assessed to understand its impact on ice action locations This evaluation will help identify the most detrimental effects on the foundation or structural components In shallower water depths, ice can accumulate around structures, leading to altered ice design scenarios, operational challenges, and potential complications in EER procedures.
Water depth is comprised of a stationary component relative to a reference datum, such as the lowest astronomical tide or mean sea level, and a variable component that reflects time-dependent changes These variations arise from factors like astronomical tides, storm surges influenced by wind and atmospheric pressure, long-term changes, seafloor subsidence, and tsunamis It is crucial to account for both positive and negative storm surges when specifying actions on structures, operating them, and planning vessel operations.
Wave parameters for open water shall be determined in accordance with ISO 19901-1
Ice coverage on the sea surface significantly influences the growth, propagation, and decay of waves It is crucial to account for the impact of ice cover when assessing extreme and operational wave parameters, particularly when utilizing numerical models for these evaluations.
The effect of increased speed and elevation of ice features due to waves shall be taken into account in determining the combined wave-ice actions upon the structure
Parameters related to currents, including tidal effects, shall be determined in accordance with ISO 19901-1
Marine growth shall be determined in accordance with ISO 19901-1
When assessing the impact of marine growth in ice-covered areas, it is essential to account for local conditions that influence the regular maintenance of structures affected by ice Special attention should be given to increased marine growth in proximity to warm water discharge zones.
The design implications of tsunamis shall be determined in accordance with ISO 19901-1
Consideration should be given to the effect of ice cover on tsunami wave parameters
Cold water regions often exhibit higher levels of dissolved oxygen, which can increase the risk of corrosion Therefore, it is essential to gather local data to evaluate this potential hazard concerning the structural materials employed.
The freezing point of water is influenced by its salinity, with normal ocean water salinity at 35 ‰ (parts per thousand) causing water to freeze at −1.9 °C This temperature represents the lowest water temperature typically observed in areas impacted by sea ice.
Sea Ice and Icebergs
To effectively define site-specific ice criteria, it is essential to gather information tailored to the specific location of the structure This data must encompass all stages of the structure's design service life.
Data can be gathered through direct observations, satellite imagery interpretation, or historical records of the installation's geographic area In cases where local ice data is lacking or not representative of severe conditions, information from nearby sites with similar ice environments may be utilized Additionally, numerical or statistical modeling can help extend these datasets while accounting for uncertainties It is essential to consider long-term trends and the interrelationships among various parameters when developing design criteria.
The expected types of ice include first-year (FY), second-year, multi-year (MY), shelf, and glacial ice Relevant statistics, including probability distributions and their parameters, will indicate the occurrence and concentration of these ice types.
Interannual and seasonal variations in the presence of ice types shall be considered
The occurrence and geometry of various ice features, including icebergs, ice islands, and pressure ridges, will be assessed through field measurements and historical data Key statistics, such as probability distributions and ice feature dimensions, along with ridge thickness, sail height, and keel depth, will be analyzed to provide a comprehensive understanding of these ice formations.
To assess the frequency, extent, size, potential gouge depth, and stability of grounded ice features like rubble piles and beach pile-ups, it is essential to gather data on these occurrences This information will inform the evaluation of ice actions, the design of flowlines and their burial depths, as well as the planning of access to facilities, logistics, and evacuation systems.
Interannual and seasonal variations in ice morphology shall be considered
Wind, waves, currents, and thermal expansion significantly influence the movement of ice and the pressure within pack ice To understand these dynamics, statistics like probability distributions, means, and extremes of movement rates for pack ice, ice floes, and distinct features such as icebergs and ice islands will be derived from field data The rates of ice movement impact the frequency of ice features encountered, as well as ice actions and operational activities Additionally, ice pressure plays a crucial role in affecting vessel traffic, ice management strategies, and evacuation procedures.
In the absence of field data for specific parameters, numerical modeling can be utilized to analyze the interactions between winds, waves, ocean currents, and ice, while carefully considering the uncertainties inherent in both the data and the modeling processes.
Interannual and seasonal variations in ice presence, polynyas, and physical parameters shall be considered
Appropriate mechanical and physical properties of ice shall be used for design and operational procedures The considerations of 8.2.8 shall apply
Monitoring ice conditions is essential for effective physical environmental data management Real-time ice information is crucial for operating ice management systems and implementing Emergency Evacuation and Rescue (EER) procedures during installation By integrating ice monitoring with a management plan for potential shutdowns and removals during the operational phase, it is possible to optimize the ice criteria for the structural design of both floating and bottom-founded mobile structures.
Seabed Considerations
The evaluation of seismic action and the design process for arctic offshore structures are influenced by the seismic risk category outlined in API 2EQ Seismic zones, defined by acceleration levels from API 2EQ maps, guide the selection of the appropriate seismic design procedure This selection is contingent upon the structure's exposure level and the intensity of ground motion.
Where seismic events are a design concern, appropriate analyses shall be carried out (see 9.10)
An investigation will assess the extent of permafrost near shorelines and at platform locations, as offshore structures built on permafrost necessitate specialized studies to evaluate soil performance under dynamic conditions such as earthquakes, waves, and sea ice Additionally, the design of all structure foundations must take into account the potential impacts of thaw consolidation in ice-rich permafrost due to the extraction of warm hydrocarbons.
Seabed gouging results from the interaction of ridges, icebergs, and stamukhi with the seabed, necessitating the collection of data on ice-induced gouging to assess the frequency, depth, width, length, and direction of gouges This gouge data is essential for designing and burying flowlines, umbilicals, subsea facilities, and platform tie-ins in seabed-utilized regions For detailed survey requirements, refer to section 9.2, and for information on the effects on subsea structures, see section 14.2.2.
Strudel scouring shall be investigated for structures in regions near the mouths of rivers See 9.2 for survey requirements, and 14.2.3 and 14.3.5 for effects on subsea structures
7 Reliability and Limit States Design
Design Philosophy
The design of the structure and its components must ensure reliable performance under various physical, accidental, and operational conditions throughout all phases of its service life, including construction, transportation, installation, and removal The necessary level of reliability is influenced by the exposure level, which is assessed based on the life-safety category and the environmental and economic consequences associated with the structure or its components.
Design must adhere to the limit states approach outlined in section 7.2, ensuring that the effects of factored actions do not surpass the factored resistances The determination of resistances and their corresponding partial factors should follow this standard and its referenced documents Actions considered include ice, seismic, oceanographic, meteorological, permanent, and variable factors Additionally, partial factors for action combinations related to Ultimate Limit State (ULS) and Accidental Limit State (ALS) must comply with section 7.2.4.
The life-safety category of a structure assesses its ability to protect personnel on the platform and ensure successful evacuation before a design environmental event occurs during its service life This categorization must align with the life-safety standards outlined in ISO 19902.
ISO 19903 and API 2FPS are essential standards for structural design As the life-safety category of a structure may evolve throughout its design service life, it is crucial that the structure is designed to comply with the relevant life-safety category at each phase Alternatively, the design should adhere to the most stringent life-safety category applicable during its entire service life.
For offshore structures, three life-safety categories are defined:
The consequence category of a structure and its components assesses the hazard potential related to personnel safety during incidents, environmental damage risks, and potential economic losses This categorization must align with the standards outlined in ISO 19902, ISO 19903, and API 2FPS.
For offshore structures, three consequence categories are defined:
The exposure level is determined by life-safety and consequence categories as outlined in Table 7-1 Manned, nonevacuated structures (S1) and high consequence structures (C1) are classified as L1, while unmanned (S3) and low consequence (C3) structures fall under L3 All other structures are categorized as L2.
Table 7-1—Determination of Exposure Level
The owner must determine the applicable exposure level for a structure or component before designing a new structure or assessing an existing one, with agreement from the regulator when necessary If multiple exposure levels are relevant, the most stringent one that meets the specifications should be applied Additionally, certain components or substructures may be categorized differently, allowing for varying exposure levels compared to the overall structure.
Categorization may be revised over the design service life of the structure as a result of changes in factors affecting life-safety or consequence category
The limit states design method ensures reliability based on exposure levels concerning Ultimate Limit States (ULS) and Accidental Limit States (ALS) These limit states are crucial, as their failure can lead to severe consequences, including loss of human life, environmental harm, and financial losses.
SLS are only assigned quantitative reliability targets under specific circumstances in this standard Such limit states shall be satisfied as appropriate
Alternative design methods can be employed when the reliability of the structure and its components meets or exceeds that of the limit state design approach outlined in section 7.2.
Limit States Design Method
Limit states are classified into four main categories: a) Ultimate Limit States (ULS), which relate to the structure's resistance against extreme loads; b) Serviceability Limit States (SLS), which address the criteria for normal operational use; c) Fatigue Limit States (FLS), which consider the cumulative impact of repeated actions; and d) Accidental Limit States (ALS), which pertain to unforeseen incidents and unusual environmental conditions.
The design of the structure must ensure strength and stiffness for Ultimate Limit State (ULS), provide sufficient reserve capacity and energy dissipation for Accidental Limit State (ALS), maintain adequate endurance against dynamic actions for Fatigue Limit State (FLS), and perform effectively under normal usage conditions for Serviceability Limit State (SLS).
The Ultimate Limit State (ULS) requirement guarantees that significant structural damage is avoided for events with a low probability of occurrence during the structure's design service life For ice-related design conditions, the extreme-level ice event (ELIE) must be taken into account, considering both local and global actions.
Exceedance of SLS results in the loss of capability of a structure to perform adequately under normal use
The specification of actions for SLS is generally the owner's responsibility, except for considerations that can lead to long-term structural degradation, such as corrosion of reinforcement in concrete
Exceedance of the fatigue limit state (FLS) in offshore structures occurs due to cumulative damage from repeated actions throughout their design service life, including the transportation phase to the installation site Ice actions typically involve cyclic variations linked to compressive and flexural ice failures.
The ALS requirement ensures that structures and foundations possess adequate reserve strength, displacement, or energy dissipation capacity to withstand significant actions and effects in the inelastic region while maintaining integrity Some structural damage is permissible under ALS For ice-related conditions, the ALS design criterion is the ALIE, which requires consideration of both local and global actions.
According to ISO 19900, actions affecting structures are categorized into permanent actions (G), variable actions (Q), environmental actions (E), repetitive actions (F) that can cause fatigue, and accidental actions (A) For more comprehensive specifications regarding these actions, refer to ISO 19902, ISO 19903, and API 2FPS.
For structures in arctic and cold regions, the design shall be based on both EL and abnormal-level (AL) events, which include ice actions arising from ELIE and ALIE
Each action will be assigned a representative value, with the primary one being the characteristic value This value is linked to a specific probability of being surpassed by adverse values over a reference period, typically one year.
In arctic and cold regions, there can be additional accidental events that should be evaluated, such as ice- driven ship impacts on a structure
7.2.2.2 Serviceability-level Ice Events (SLIE)
For SLS, structures must meet the standard's requirements when facing localized ice damage, which includes issues such as concrete cracking and shrinkage, as outlined in ISO 19903 Additionally, considerations must be made for the loss of concrete cover due to ice abrasion, the removal of paint, and the extraction of corroded steel caused by ice Structures should also address localized damage resulting from vibrations induced by ice and other environmental factors, while ensuring compliance with serviceability limits for vibrations and deflections.
Unless the owner has specified otherwise, the characteristic value of the SLIE used for SLS shall be determined based on an annual probability of exceedance not greater than 10 − 1
For Ultimate Limit State (ULS) design, structural analysis should primarily utilize linear elastic methods, although some localized inelastic behavior is permissible, as outlined in Section 11 for fixed steel structures.
The representative value for actions resulting from ELIE will be established using an annual exceedance probability of no more than 0.01 Additionally, ELIE does not apply to discrete events that have an annual occurrence probability lower than 0.01.
Nonlinear analysis methods are applicable for ALS, permitting structural components to exhibit plastic behavior while foundation piles can achieve axial capacity or develop plasticity The design approach integrates static reserve strength, ductility, and energy dissipation to effectively withstand ALIE conditions.
Dynamic ice actions and ice-induced vibrations must be assessed in relation to the Fatigue Limit State (FLS) and their impact on structures and foundations, particularly considering the risk of soil liquefaction.
Characteristic values for environmental actions must be determined at the extreme level (EL) with an annual exceedance probability of no more than 10^{-2}, and at the abnormal level (AL) with exceedance probabilities of 10^{-4} for Level 1 (L1) structures and 10^{-3} for Level 2 (L2) structures Level 3 (L3) structures do not require consideration of AL events For seismic actions, the probabilities for extreme-level earthquakes (ELE) and abnormal-level earthquakes (ALE) should follow the guidelines outlined in API 2EQ.
It is not necessary to consider actions from environmental events other than ice if the action arising has an annual probability of exceedance less than 10 − 4
Effects due to temperature differences within the structure or component are to be considered in all relevant limit states
Accidental situations shall be addressed in accordance with ISO 19902, ISO 19903, and API 2FPS depending on the structure type For man-made islands, the provisions of ISO 19902 shall be applied
7.2.3 Principal and Companion Environmental Actions
To derive action combinations for each environmental action, it is essential to consider each representative value of the Environmental Layer (EL) and Action Layer (AL) as the primary action These values will be supported by corresponding companion environmental actions.
Potential combinations of principal and companion environmental actions are given in Table 7-2
When specifying companion actions, it is essential to account for changes in water levels Storm surges must be treated as stochastically dependent on wind when the primary action is influenced by wind Similarly, tidal elevations should be regarded as stochastically dependent when the main action relies on tidal currents.
General
The design of a structure must take into account the actions and action effects influenced by its physical environment and the expected reliability Relevant guidance can be found in standards such as ISO 19900, ISO 19901-1, API 2EQ, ISO 19902, ISO 19903, and API 2FPS.
ISO 19901-1, API 2EQ, ISO 19902, ISO 19903, and API 2FPS address various environmental actions, including wave, wind, current, ice, and seismic activities Specifically, Section 7.2.4 outlines the provisions for managing ice actions and their interaction with other action types, supplemented by additional requirements in Section 8.
The design of arctic offshore structures must account for global actions that ensure the overall integrity of the structure, foundation, and stationkeeping system, as well as local ice actions affecting specific components or sections of the structure.
Design actions must account for all phases of the service life, including construction, transportation, installation, and removal It is essential to consider potential weight increases and shifts in the center of gravity due to ice accretion during fitting out in cold regions The design should reflect seasonal and geographical variations Additionally, differential strains or lock-in stresses resulting from temperature changes between construction and permanent locations must be incorporated into the design.
Structures exposed to ice interaction events with an annual probability greater than the specified value in section 7.2.2.4 must be designed to withstand ice actions If data to quantify potential ice events is lacking, ice actions should still be evaluated if there is a reasonable likelihood of interaction It is essential to consider all realistic interaction scenarios that could lead to adverse consequences Additionally, refer to section 8.3.1.4 for information on wave-induced ice motion.
Ice Actions
8.2.1 General Principles for Calculating Ice Actions
When assessing the impact of ice on structures, it is essential to consider both global and local scales of direct ice actions and their interactions These actions encompass a range of effects, including static, quasi-static, cyclic, and dynamic forces (EL and AL) Additionally, cyclic and dynamic actions can lead to structural fatigue, liquefaction, and discomfort for personnel Furthermore, spatial actions such as rubbling, pile-up, and ride-up can obstruct operations and must be taken into account.
The assessment of global ice actions and their specific points of action will follow the guidelines outlined in section 8.2.4 to ensure the structural integrity is maintained This evaluation encompasses factors such as resistance to sliding and overturning, foundation capacity, fatigue, and the potential for foundation liquefaction Additionally, local actions will be addressed in accordance with section 8.2.5.
Designing ice actions on offshore structures should rely on full-scale action and response data from instrumented structures, considering the applicability and uncertainties of the data and interpretation methods In the absence of local data, measurements from other regions can be extrapolated by understanding the ice regimes, metocean conditions, climate, brine volume, and strength parameters Additionally, small-scale ice strength data collected locally, ideally in situ, can aid in this extrapolation To enhance the reliability of the findings, physically based models and scale model tests may also be employed, while accounting for uncertainties in their application.
Further requirements relating to ice actions are found in 10.3 for man-made islands, in 13.4 for floating structures, and in 14.3 for subsea structures
8.2.2 Representative Values of Ice Actions
The design shall be carried out for ELIE and ALIE, as defined in 7.2.2.3 and 7.2.2.4
Representative values for ice actions shall be calculated using probabilistic methods or deterministic methods for ELIE and ALIE
Design ice actions shall reflect
⎯ the relevant ice scenario, limiting mechanisms and ice failure modes for the geographical location of the structure, with reference to the provisions of 8.2.4, 8.2.5, 8.2.6, and 8.2.8; and
⎯ the structural configuration and the relevant operational scenarios, including seasonal operation, ice detection, physical ice management, maneuvering of the structure, and disconnection, with reference to the provisions of 8.2.7
The ELIE and ALIE must be established for each applicable ice loading scenario In cases where multiple ice and operational scenarios are pertinent to a specific structure, the scenarios that produce the maximum ice actions for each limit state should be prioritized in the design process.
When evaluating scenarios, it is essential to consider structures interacting with various ice features, including first-year (FY) ice such as level ice, rafted ice, landfast ice, floes, ridges, rubble fields, and refloated stamukhi Additionally, multi-year (MY) ice features like level ice, floes, ridges, rubble, and hummock fields should be taken into account, along with icebergs and fragments of ice shelves known as ice islands.
Annex B provides the foundation for identifying different ice regimes in offshore waters, which is essential for determining potential scenarios The significance of these scenarios will be assessed based on the structural configuration.
Subsidiary conditions that can act in combination with the ice features or can influence the nature of the interaction include:
⎯ ocean currents including tidal effects;
The assessment of global ice actions must utilize methods that integrate full-scale measurements, reliable model experiments, or calibrated theoretical approaches Key conditions to evaluate include: a) quasi-static actions from level ice, where inertial effects are negligible; b) dynamic actions from level ice, necessitating dynamic analysis due to significant inertial effects; c) quasi-static actions from ice rubble and ridges, again with negligible inertial effects; d) impacts from discrete features like icebergs and large ice formations; e) quasi-static actions from features pressed against the structure by surrounding ice or metocean forces; f) adfreeze action effects, including frozen-in conditions; and g) thermal action effects.
The following limiting mechanisms shall be considered
Limit stress refers to the forces that arise when there is enough energy to impact a structure, leading to ice actions across its entire width These actions can manifest as direct ice failure against the structure, ice failure within rubble that is lodged against it, as well as floe buckling or splitting.
Limit energy refers to the mechanism where interactions are constrained by the kinetic energy of ice features, typically marked by a lack of surrounding ice This phenomenon often occurs due to the impacts of icebergs, refloated stamukhi, MY floes, or ice islands.
Limit force refers to the mechanism that occurs when metocean actions drive interacting features against a structure, yet these actions are not strong enough to cause local ice failure and envelop the structure This phenomenon typically arises when large ice features interact with a structure due to the influences of wind, current, pack ice pressures, or a combination of these factors.
When calculating global actions for various ice failure modes, such as ice crushing, shear, flexure, splitting, and buckling, it is essential to consider the relevant ice conditions and limiting mechanisms Key factors must be evaluated to accurately determine the ice actions in each scenario.
⎯ geometry of the ice or ice features;
⎯ mass and added mass of the ice feature;
⎯ mechanical properties of the ice or ice feature;
⎯ adfreeze bond between ice and the structure;
⎯ inertial effects (and added inertia) for both the ice and the structure;
⎯ velocity and direction of movement of the ice features;
⎯ ice rubble buildup, and implications for encroachment, structure freeboard requirements, and actions transmitted to the structure;
⎯ clearing of ice around the structure;
⎯ ice jamming between the members of a multi-leg structure;
⎯ compliance and damping of the structure and stationkeeping system;
⎯ degree of contact between the ice and the structure;
⎯ friction between the ice and the structure;
⎯ thermal effects in the ice;
⎯ environmental actions of wind, current, and pack ice pressure available to drive the ice and their persistence;
⎯ surface morphology and the presence of snow on the ice; and
⎯ influence of shoals and other barriers
Local ice actions shall be based on relevant full-scale measurements or established theoretical methods
Due account shall be taken of geographical differences and water level changes in their specification
Local ice actions must be evaluated for all structural components that affect overall integrity and stability, as outlined in sections 7.2.2.3 and 7.2.2.4 In the case of steel structures, these local actions are crucial for the design of elements such as sheet piling, plates, stiffeners, frames, and bulkheads.
When designing concrete structures, it is essential to account for local actions affecting wall thickness, steel reinforcement, and abrasion This consideration applies universally to all structural materials, particularly at corners, structural discontinuities, and appendages, as these factors significantly impact the overall integrity of the structure.
When designing contact areas, it is essential to take into account the local structural configuration, which includes factors such as frame spacing, plate thickness, and appendage dimensions The selection of size and placement for these local contact areas must prioritize addressing the most critical scenarios effectively.
Local ice actions shall be considered in the context of background actions on adjacent panels or areas of the structure
When designing structures, it is essential to account for the time-varying characteristics of ice actions and the resulting ice-induced vibrations An assessment of the potential dynamic amplification of action effects, caused by the lock-in of ice failure and natural frequencies, is crucial Special focus should be directed towards dynamic actions impacting narrow structures, flexible designs, and vertical surfaces that are exposed to ice forces.
Structural fatigue and foundation failure as a consequence of dynamic ice actions shall be considered
8.2.7 Operational Procedures to Reduce Ice Actions
Metocean Related Actions
ISO 19901-1 addresses metocean actions on structures, with specific additional requirements outlined in section 8.3 for structures situated in arctic and cold regions These requirements are categorized into three main areas: the impact of ice accumulation on the structure and its components above the waterline, the unique structural forms utilized in these environments, and the local ice loading induced by waves.
8.3.1.2 Ice Accumulation on the Structure
Icing effects must be considered in the ELIE and ALIE analyses of structures, as it can occur due to fog, freezing rain, trapped green water, seawater spray, or tidal changes This icing alters the aerodynamic and hydrodynamic properties, as well as the static stability and dynamic responses of the structure, impacting the buoyancy and stability of floating structures.
Icing is quantified by the thickness, volume, or mass of ice that accumulates on structures Estimates can be derived from observations of icing on comparable structures in the vicinity or through theoretical models It is essential to calibrate these theoretical models with actual observations for accuracy.
Icing has a substantial impact on wave and current actions, as it increases the projected area, volume, and surface roughness of structures This elevation in icing levels can lead to heightened forces exerted by waves and currents on these structures.
The accumulation of ice on structures significantly increases the projected area and volume above the mean waterline, leading to roughness on circular cylinders These factors must be considered in the design calculations for areas, volumes, and drag coefficients to accurately assess environmental actions, as they can substantially elevate the forces acting on a member compared to scenarios without ice Guidance on selecting drag coefficients for the Morison equation is provided in ISO 19902, while ISO 19903 and API 2FPS offer recommendations for diffraction analysis in large volume structures Additionally, when utilizing computational fluid dynamics, it is essential to validate the method against high-quality model tests.
Wind actions must account for icing effects The impact of icing on wind-induced drag should be analyzed similarly to how Morison drag actions from waves and currents are addressed in section 8.3.1.2.2.
Icing significantly impacts the dynamic and static responses of structural members and systems Key considerations include modifying actions that excite dynamic responses as outlined in sections 8.3.1.2.1 and 8.3.1.2.3, adjusting Strouhal numbers for vortex-induced vibrations, and recalibrating the masses of members and systems in relation to icing Additionally, it is crucial to account for the effects on vertical actions, stability, static positioning, reductions in structure freeboard, and the righting moments of floaters.
8.3.1.3 Particular Considerations for Large Volume Structures
Nonlinear effects influencing water surface elevation beneath structures supported by large columns are challenging to analyze theoretically For fixed arctic offshore structures, wave focusing caused by large caissons and earth mounds complicates wave-structure interactions Therefore, model testing is essential to evaluate the necessary air gap beneath a platform's deck.
Wave actions shall be assessed for ice resisting structures with sloping faces and conical forms
Deck elevations, structure freeboard, and the height of bow walls or ice deflectors must consider wave-induced vertical motion and ice features It is essential to account for ice accumulations on the structure and the underside of the topsides deck or substructure Additionally, the routing of risers, caissons, J-tubes, and other appurtenances should be designed to reduce the risk of impact from ice features For more detailed guidance, refer to section 15.1.1.3.
Nonlinear wave-structure interaction raises water surface elevations near the structure, leading to ice exerting forces on the structure that exceed predictions from most numerical models Therefore, the design of ice strengthening must consider these elevated impact levels.
8.3.2 Wave-induced Velocity of Ice Features
The velocities of ice features affecting a structure are influenced by the incident wave and current, making their consideration essential Estimating these velocities relies on the ratio of the incident wave's wavelength to the dimensions of both the ice and the structure.
Ice pieces with dimensions smaller than 1/15 of the incident wavelength are assumed to follow the water particle velocity For larger ice pieces, their velocities can be determined by solving the equation of motion, considering the effects of wind, waves, and currents When ice dimensions are less than 1/5 of the incident wavelength, the Morison equation's relative velocity formulation can be used to derive actions For larger ice pieces, diffraction analysis is applicable for calculating wave actions Additionally, computational fluid dynamics or model testing can be utilized to estimate the induced actions and velocities.
Slopes and variations in water depth can enhance and sharpen incoming waves due to refraction It is important to consider the impact of underwater mounds or caissons when their tops are situated less than half a wavelength beneath the water's surface.
When the dimensions of the ice or structure are 1/15 or more of the wavelength of the incident wave, the previously described method becomes less precise and tends to overestimate the velocity of the ice induced by the incident wave field.
Small ice features can significantly affect structures by increasing impact velocity, particularly when influenced by breaking or near-breaking waves This phenomenon is also observed in shallow waters near shorelines, where shoaling waves are present Additionally, gently sloping structures, such as man-made islands and extended berms, are susceptible to these conditions.
Seismic Actions
Offshore structures in Arctic and cold regions must be designed to meet the requirements for Extreme Limit State (ELE) and Accidental Limit State (ALE) as outlined in API 2EQ Furthermore, steel structures are required to adhere to ISO 19902 standards, while concrete structures must comply with ISO 19903 regulations.
This standard outlines detailed requirements for seismic actions and their effects on various structures, including foundations (Section 9.4.4), man-made islands (Section 10.4), steel structures (Section 11.7), concrete structures (Section 12.3.6), subsea structures (Section 14.4), and topsides (Section 15.3).
Ice and seismic events can be viewed as stochastically independent To accurately assess their relationship, it is essential to evaluate their joint probability, taking into account the duration of each event.
The probabilities of simultaneous short-duration EL and AL ice actions occurring alongside seismic actions are often very low Therefore, it is unnecessary to account for accompanying seismic actions in the evaluation of ELIE and ALIE.
When offshore structures in arctic and cold regions are designed for ELE and ALE, appropriate companion ice actions shall be included
The likelihood of simultaneous EL or AL seismic events occurring alongside short-duration ice events is extremely low Therefore, the specification of companion ice actions will primarily focus on long-duration ice conditions When the joint probability of seismic and ice events is determined, the companion actions listed in Table 7-2 can be considered irrelevant.
8.4.4 Factors Influencing Seismic Actions and Effects
The relative speed of the structure and the ice shall be considered in the specification of companion ice actions
When calculating the mass of a structure for seismic analyses, it is essential to consider ice accretion and landfast ice, as well as grounded ice rubble that adheres to the structure Local restraint from these factors can alter the natural modes of the system and may lead to significant local and global forces.
The presence of a surrounding ice sheet shall be considered in the calculation of hydrodynamic added mass
8.4.5 Seismic Isolation and Damping Systems
Seismic isolation and damping systems can be effectively utilized in arctic offshore structures to comply with established standards, ensuring they are engineered for reliable performance in anticipated environmental conditions.
Further requirements for seismic isolation and damping systems are provided in 15.3.5
General
Foundations for structures in arctic and cold regions must account for unique factors that affect their reliability, differing from standard offshore geotechnical practices The ice loading environment can generate significant horizontal forces, which may be eccentric and cyclic, with varying magnitudes, frequencies, and durations compared to other environmental influences like waves and earthquakes Consequently, these ice action conditions can lead to structural designs that differ from those intended for temperate waters, resulting in a limited experience base for the performance of arctic offshore structure foundations.
Climate conditions, both past and present, have led to the widespread presence of permafrost in Arctic and cold regions, including deep water areas Identifying and characterizing permafrost is a crucial aspect of soil investigations, necessitating specialized sampling techniques and expertise It is essential to consider the stability of frozen soils throughout the design service life of structures Activities related to hydrocarbon exploration and production can induce permanent or cyclic thermal changes in permafrost Additionally, lower ground temperatures heighten the likelihood of encountering frozen gas hydrates at relatively shallow depths in cold climates.
Soils in arctic and cold regions have been significantly influenced by recent glacial episodes, leading to a higher probability of encountering boulders and gravels in specific geological settings Additionally, ongoing geological processes, such as seabed gouging by moving ice keels, can alter seafloor topography and impact the properties of soils, as well as the loading conditions for buried structural components and flowlines.
Quick clays, also known as highly sensitive clays, are found in specific nearshore Arctic regions These clays have a natural moisture content that significantly exceeds the liquid limit, leading to very low remolded shear strength.
Site Investigation
9.2.1 Purpose and Scope of the Investigations
Site investigations are essential for all offshore structures located in arctic and cold regions These investigations aim to gather crucial bathymetric, geophysical, and geotechnical data, which are necessary for accurately characterizing site conditions and determining the material properties of the area.
Site investigations shall take the following into consideration:
⎯ type of structure and foundation actions;
⎯ near-field and far-field conditions;
⎯ data available from previous investigations in the area;
⎯ available performance data from existing structures in the area;
The investigation will be conducted by experts in geology, geophysics, ice engineering, seismology, and geotechnics, ensuring a comprehensive assessment of all pertinent existing data.
When assessing potential geohazards, it is essential to investigate various factors, including ice gouges, sand waves, boulder beds, submarine slope instability, faults, shallow gas, subsidence, mud volcanoes, gas hydrates, permafrost, and overpressurized geological strata.
Soil research in arctic and cold regions necessitates comprehensive data collection to effectively design foundation structures, particularly concerning dynamic soil properties This involves specialized geophysical and geotechnical investigations, following the guidelines outlined in A.9.2.4.
Investigations shall consider both far-field and near-field seabed conditions
The far-field investigations shall determine the impact of the following on the design of the structure:
⎯ slope stability and the potential for mass movements;
⎯ present sedimentary environment and erosional processes
The near-field investigations shall address the local issues related to
⎯ a detailed soil/rock seabed stratigraphy;
⎯ the foundation stability and displacement;
⎯ sediment movements adjacent to the structure;
⎯ the presence and influence of ice gouges, boulders, permafrost, and gas hydrates and shallow faults
The near-field conditions must be assessed to gather comprehensive quantitative and qualitative information on bathymetric and geomorphological features, geological processes, and geotechnical parameters that influence structural design The scope of the near-field investigation should align with the dimensions, zones of influence, and placement tolerances of the proposed structure, as well as the complexity of the site conditions.
The near-field investigation shall include as a minimum a) a bathymetry survey; b) a geophysical survey, including ice gouge delineation where applicable; c) a geotechnical investigation
Bathymetric data is essential for understanding the seafloor's relief and elevations, which aids in assessing the potential draught of ice features that may impact the structure.
Bathymetry must be assessed using techniques that consider vessel heave, tidal changes, and other factors affecting local and relative datums The methods' precision and survey accuracy should align with the structural design requirements.
The survey lines must be appropriately spaced, oriented, and extensive to align with the investigation's objectives and the specific type of study, whether near-field or far-field, ensuring a comprehensive definition of bathymetry and morphology.
Geophysical data will be collected to aid in identifying and outlining significant geological features as specified in section 9.2.1 This data will be analyzed alongside existing bathymetric and geotechnical information to enable cross-verification and validation.
When selecting high-resolution geophysical survey methods and equipment, it is essential to consider site conditions and the specific objectives of the survey Additionally, the equipment's capabilities in terms of penetration depth and resolution must be taken into account.
The geophysical survey lines must be designed to align with the investigation's purpose and type, whether near-field or far-field, while also considering the characteristics of the seafloor and seabed features.
Key features relevant to arctic and cold regions include the depth, frequency, and distribution of ice gouges, as well as the presence of shallow permafrost and gas hydrates In regions with shallow permafrost, it is essential to utilize specialized geophysical investigation techniques, such as shorter receiver arrays and resistivity methods.
The geotechnical investigation program will be developed using geological and geophysical data, incorporating borehole drilling, sampling, in situ testing, and laboratory testing.
The investigation will focus on the relevant soil and rock layers that impact the structure, emphasizing the assessment of shallow soils and rocks Thin layers of weak materials near the seabed can significantly affect the stability of gravity structures Additionally, the study will address the zone of anticipated thermal disturbance in areas with permafrost, prioritizing the reduction of both thermal and mechanical disturbances during permafrost sampling.