Section
3 Environmental conditions
3.1 General
3.1.1 The Owner/Operator or designer is to specify the environmental criteria
for which the unit is to be considered. The extreme environmental conditions
applicable to the location, or operating areas are to be specified, together with
all operating environmental limits. Detailed specialist environmental reports are to
be submitted, with sufficient supporting information to demonstrate the validity of
the limiting criteria, see
Pt 3, Ch 10, 3.3 Metocean data.
NOTE: For information on typical industry requirements on specialist
environmental reports, “ISO 19901-1, Specific Requirements for Offshore Structures -
Part 1 Metocean design and operating considerations” may be consulted. For the
purposes of these Rules, the definitions of environmental terms, e.g. squall and
cyclone, given in ISO 19901-1 apply. The Class requirements remain those found in
the Rules for Offshore Units, especially this Section.
3.1.2 A comprehensive set of operating and extreme environmental limiting
conditions is to be submitted. This is to cover the following cases, as applicable,
and any other conditions relevant to the system under consideration:
- Extreme environmental conditions.
- Limiting environmental conditions in which the unit and/or ship
may remain moored.
- Limiting environmental conditions in which the unit and/or
ship’s main operating functions may be carried out (e.g., production and/or
transfer of product).
- Limiting environmental conditions in which the unit and/or ship
may (re)connect.
3.2 Environmental factors
3.2.1 The following environmental factors are to be considered in the design
of the positional mooring system:
- Water depth range, local bathymetry, tidal variations and storm
surges.
- Wind, (including gust spectral characteristics and squall
characteristics as applicable).
- Waves (both wind waves and swell) with characteristic heights,
periods, spectra and associated parameters.
NOTE: Where applicable, concomitant multiple swell regimes with various
frequency and directional characteristics need to be reported
- Current (inclusive of all components, as well as vertical
profile).
- Relative angles between wind, wind driven waves and current (and
where applicable swell or squall).
- Marine growth.
- Air and sea temperatures.
and in addition for floating offshore installations at a fixed
location:
- Sea bed conditions.
- Soil conditions.
3.2.2 In certain locations the following factors may need to be considered in
the design of the positional mooring system:
- Sea ice or icebergs.
- Seismic characteristics and events, such as earthquakes.
- Sea water density (especially in the vicinity of
estuaries)
- Snow or ice accretion.
3.3 Metocean data
3.3.1 As part of the environmental data, the following metocean data will
normally be required to be submitted:
- 100 (or 50 for mobile offshore units), 10 and 1-year return
period values for wind-speed, significant wave height and current.
- Directional data for extreme values of wind, waves and
current.
- Wave height/period joint frequency distribution (wave scatter
diagram).
- Wave spectral parameters.
- Wind/wave/current angular separation data.
- Current speed and/or directional variation over the water
depth.
- Long-term wave statistics by direction.
- Squall time series data where relevant.
3.3.2 Data from a calibrated hindcast model covering the service life of the
Offshore Unit and providing for each sea state (usually described as 3 hours
stationary sea conditions) the data as follows:
- wind sea significant wave height, direction, peak period(s) and
other parameters;
- swell sea significant wave height, direction(s), peak period(s)
and other parameters;
- wind speed and direction; and
- current speed and direction.
NOTE: The data set should also report spectral formulation and
parameters, as necessary. Where applicable, concomitant multiple swell regimes with
various frequency and directional characteristics need to be included.
3.4 Environmental parameters
3.4.1
Water depth. Minimum and maximum still water levels are to be determined,
taking full account of the tidal range, sea bed subsidence, wind and pressure surge
effects. For floating offshore installations at a fixed location, data is to be
submitted to show the variation in water depth in way of the installation. This data
is to be referenced to a consistent datum and is to include, where relevant, the
water depth in way of each anchor or pile, gravity base or foundation, pipeline
manifold, and in way of the radius swept by an attached ship. The likelihood of sand
waves or variation in sea-bed re-settlement at the site shall be documented (See
also
Pt 3, Ch 10, 3.4 Environmental parameters 3.4.10 on sea-bed re-settlement).
3.4.2
Wind. The one-hour wind speed, plus wind gust spectrum, will normally require
to be applied in design. The following wind gust spectra formulations can be adopted
for the time varying component:
The site specific environmental data report shall indicate whether the
site is subject or not to squalls. In areas where squalls are prevalent, a
specialist report is to provide a representative set of squall time series data. The
data should be based on a number of recorded events and extrapolation or scaling
techniques are to be documented as well as confidence intervals. Environmental
parameters (current and waves) associated with the design squall event (see
Pt 3, Ch 10, 3.3 Metocean data and Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters) are to be documented. The report shall address
such aspects as directionality, typical development and travel speed. Scaling
techniques should be documented and special attention should be paid to the
determination of rising slope and decay time in proposed scaled design squall time
histories.
3.4.3
Waves:
- A site specific specialist report on meteorology, atmospheric
and oceanic conditions is required to provide sea state characteristics and
data for the location of operation. The sea state characterisation and data
is to differentiate, as applicable to the location, between: local wind
waves, swell and their combination.
- Sea state characteristics are to include as a minimum, spectral
formulation and associated parameters, significant and maximum wave heights
with associated range of peak and zero up-crossing periods.
- The data should include contours of equal probability of
occurrence of significant wave height and peak period. Appropriate method of
developing such wave contours is to be used, see Pt 12, Ch 1, 9.1 Recognised codes and standards for concrete structures. The source data, any extrapolation technique and the
detailed derivation of the contours shall be fully documented.
- For certain locations, the sea conditions
may be governed by a combination of local wind-driven waves and remotely
generated swell, the specialist report shall provide information on the
joint occurrence of wind driven waves and swell. The angular separation
between directions of propagation of these two components shall also be
informed.
- Where the metocean specialist report states
that sufficient and adequate wave height /period joint distribution data are
not available for the location, the report shall highlight what data is
missing to enable such contours to be derived, and indicate alternative
source for the missing data. The specialist report shall also propose a
conservative range of wave heights and periods combinations for the location
and design under consideration.
3.4.4
Current. A specialist report should document current data including velocity
and direction and their vertical variation through the water depth, taking into
account all relevant components including the following:
- Tidal currents.
- Circulation currents.
- Wind driven current.
- Storm surge generated current.
- loop and eddy currents
- soliton currents.
3.4.5
Marine growth. A specialist report is to document the characteristic data on
typical local marine growth, such as growth rate, thickness and mass density.
3.4.6
Air and sea temperature. A specialist report is to provide pertinent air and
sea temperatures data to substantiate the minimum and maximum air and sea design
temperatures criteria for the location of operation in accordance with Pt 3, Ch 1, 4.4 Minimum design temperature.
3.4.7
Sea bed conditions. For floating offshore installations at a fixed location,
the sea bed conditions at the proposed locations of the anchor points and along the
anchor line corridors are to be determined to provide data for the design of the
anchoring system. Requirements for site investigation are contained in Pt 3, Ch 14 Foundations.
3.4.8
Soil conditions. For floating offshore installations at a fixed location, the
soil conditions at the proposed locations of the anchor points are to be determined
to provide data for the design of the anchoring system. Requirements for site
investigation are contained in Pt 3, Ch 14 Foundations.
3.4.9
Sea ice and icebergs. A specialist report (taking into consideration the
recommendations and guidance from ISO 19906 as applicable) is to indicate whether
the offshore location is prone to sea ice conditions or icebergs drifting. In such
areas and where subfreezing temperatures can prevail for a major portion of the
year, causing the formation of sea-ice data should be collected to assess the
feasibility and establish relevant design criteria.
The data should at least include:
- the seasonal distribution of sea ice,
- the distribution and probability of ice floes, pressure ridges
and/or icebergs,
- the effect of ice-gouges on the seabed from icebergs or ice
ridges,
- the type, thickness and representative features of sea ice,
- drift speed, direction, shape and mass of ice floes, pressure
ridges and/or icebergs, and
- strength and other mechanical properties of the ice.
3.4.10
Seismic. For areas that are determined to be seismically active, a specialist
report shall document the characteristic seismic activity of the region (for further
requirements see
Pt 3, Ch 14, 1.9 Earthquake). Potential for soil
liquefaction or seabed resettlement need to be reported. In shallow water depths,
like coastal areas, specialist report shall also consider the seismicity of
surrounding regions and indicate whether these could cause tsunamis at the site.
3.4.11
Sea water density and salinity. A specialist report is to document the local
water salinity and density variations, (especially in vicinity of estuaries) and
their impact on current, corrosion rate etc.
3.4.12
Snow or ice accretion. A specialist report (taking into consideration the
recommendations and guidance from ISO 19906 as applicable) is to indicate whether
the offshore location is prone to snow or subfreezing temperatures during parts of
the year and provide data to substantiate and estimate the extent to which snow can
accumulate on the structures and topsides and of its possible effect on the
structure.
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