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. 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:

API RP 2A.
NPD/ISO
Other published spectra formulations may be accepted, see Pt 3, Ch 17, 1.2 Recognised Codes and Standards 1.2.15 .

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 3.3.1 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 3, Ch 17, 1.2 Recognised Codes and Standards 1.2.16. 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.