Section 2 ENVIRONMENTAL CONDITIONS

201. General

1. General

The environmental conditions to which an offshore installation may be exposed during its life are to be described using adequate data for the area in which the structure is to be transported and installed. All data used are to be fully documented with the sources and estimated reliability of da- ta noted.

202. Environmental Factors

1. Environment factors

(1) In general, the design of an offshore installation will require investigation of the following envi- ronmental factors.

(A) Waves

(B) Wind

(C) Currents

(D) Tides and storm surges

(E) Air and sea temperatures

(F) Ice and snow

(G) Marine growth

(H) Seismicity

(I) Sea ice

(2) Other phenomena, such as tsunamis, submarine slides, seiche, abnormal composition of air and water, air humidity, salinity, ice drift, icebergs, ice scouring, etc. may require investigation de-

pending upon the specific installation site. The required investigation of seabed and soil con-

ditions is described in Sec 7.

203. Environmental Design Criteria

1. General

The combination and severity of environmental conditions for use in design are to be appropriate to the installation being considered and consistent with the probability of simultaneous occurrence of the environmental phenomena. It is to be assumed that environmental phenomena may approach the installation from any direction unless reliable site-specific data indicate otherwise. The direction, or combination of directions, which produces the most unfavorable effects on the installation is to be accounted for in the design.

2. Design Environmental Condition

(1) Design environmental condition is to be described by a set of parameters representing an envi- ronmental condition which has a high probability of not being exceeded during the life of the structures and will normally be composed of the followings.

(A) The maximum wave height corresponding to the selected recurrence period together with the associated wind, current and limits of water depth, and appropriate ice and snow effects.

(B) The extreme air and sea temperatures

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(C) The maximum and minimum water level due to tide and storm surge

(2) However, depending upon site-specific conditions, consideration should be given to the combina- tions of events contained in item (A) above.

(3) The recurrence period chosen for events (1), (A) to (C) above is normally not to be less than one hundred years, unless justification for a reduction can be provided. For platforms that are unmaned, or can be easily evacuated during the design event, or platforms with shorter design life than the typical 20 years may use a recurrence interval less that 100 years for events (1),

(A) to (C) above. However, the recurrence interval is not to be less than 50 years.

(4) For installation sites located in seismically active areas(see 204. 8), the magnitudes of the pa- rameters characterizing these earthquakes having recurrence periods appropriate to the design life of the structure are to be determined. The effects of the earthquakes are to be accounted for in design but, generally, need not be taken in combination with other environmental factors.

(5) For installations located in areas susceptible to tsunami waves, submarine slides, seiche or other phenomena, the effects of such phenomena are to be based on the most reliable estimates avail- able and, as practicable, the expected effects are to be accounted for in design.

3. Operating Environmental Conditions

For each intended major function or operation of the installation, a set of characteristic parameters for the environmental factors which act as a limit on the safe performance of an operation or func- tion is to be determined. Such operations may include, as appropriate, transportation, offloading and installation of the structure, drilling or producing operations, evacuation of the platform, etc. These sets of conditions are herein referred to as Operating Environmental Conditions.

204. Specific Environmental Condition

1. Waves

(1) General

Statistical

wave data from which design parameters are determined are normally to include the

frequency of occurrence of various wave height groups, associated wave periods and directions.

Published

data and previously established design criteria for particular areas may be used where

such exist. Analytical wave spectra employed to augment available data are to reflect the shape and width of the data, and they are to be appropriate to the general site conditions.

(2) Long-Term Predictions

All long-term and extreme-value predictions employed for the determination of design wave con- ditions are to be fully described and based on recognized techniques. Design wave conditions may be formulated for use in either deterministic or probabilistic methods of analysis, but the

method of analysis is to be appropriate to the specific topic being considered.

(3) Data

(A) The development of wave data to be used in required analysis is to reflect conditions at the installation site and the type of structure. As required, wave data may have to be developed to determine the followings.

(a)

Provision for air gap

(b) Maximum mud line shear force and overturning moment

(c)

Dynamic response of the structure

(d) Maximum stress, fatigue, impact of local structure

(B) Breaking wave criteria are to be appropriate to the installation site and based on recognized

techniques. Waves which cause the most unfavorable effects on the overall structure may differ from waves having the most severe effects on individual structural components. In general, more frequent waves of lesser heights, in addition to the most severe wave con- ditions, are to be investigated when fatigue and dynamic analyses are required.

2. Wind

(1) General

Statistical wind data is normally to include information on the frequency of occurrence, duration and direction of various wind speeds. Published data and data from nearby land and sea stations may be used if available. If on-site measurements are taken, the duration of individual measure- ments and the height above sea-level of measuring devices is to be stated. Sustained winds are to be considered as those having durations equal to or greater than one minute, while gust winds are winds of less than one minute duration.

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(2) Long-Term and Extreme-Value Predictions

Long-term and extreme-value predictions for sustained and gust winds are to be based on recog- nized techniques and clearly described. Preferably, the statistical data used for the long-term dis- tributions of wind speed should be based on the same averaging periods of wind speeds as are used for the determination of loads. Vertical profiles of horizontal wind are to be determined on the basis of recognized statistical or mathematical models.

(3) Vertical Profiles of Horizontal Wind

Vertical profiles of horizontal wind for use in design can be determined using the following equation.

Ë

Ļ Ż Ł Ÿ

ÃŻ G ÃĂ ĿĿ Ă M

where,

ÃŻ :

ÃĂ : 1/Ÿ :

Ÿ :

wind speed at height Ż above a reference water depth(m/sec)

wind speed at reference height Ă, usually 10 m above a reference water depth(m/sec) exponent dependent upon the time-averaging period of the measured wind speed ÃĂ

the value of Ÿ typically ranges from 7 for sustained winds to 13 for gust winds of

brief duration. For sustained winds of 1 minute duration, Ÿ equal to 7 may be used; for gust winds of 3 second duration, Ÿ equal to 12 may be used.

(4) Time-averaging Data

In the event that wind speed data is not available for the time-averaging period desired for use in design, conversions to the desired time-averaging periods may be made on the basis of Table 3.1.1.

Table 3.1.1 Wind Speed for Time-Averaging Period Ź

Linear interpolation may be used with the Table 3.1.1 to determine the factor to be applied to the time-averaging period wind speed relative to the 1 hour wind speed. For wind speeds given

in terms of the "fastest meter of wind", ÃX , the corresponding time-averaging is given by Table 3.1.1.

period Ź in second

Ã

Ź G ĒŊÈĖ

X

where,

ÃX : The fastest meter of wind at a reference height of 10 m (m/sec)

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3. Currents

(1) General

Data for currents are generally to include information on current speed, direction and variation with depth. The extent of information needed is to be commensurate with the expected severity of current conditions at the site in relation to other load causing phenomena, past experience in adjacent or analogous area and the type of structure and foundation to be installed. On-site data collection may be indicated for previously unstudied area and/or area expected to have unusual or severe conditions. Consideration is to be given to the following types of currents, as appro- priate to the installation site: tidal, wind-generated, density, circulation and river-outflow.

(2) Velocity Profiles

Current velocity profiles are to be based on site-specific data or recognized empirical relationships. Unusual profiles due to bottom currents and stratified effects due to river outflow currents are to be accounted for.

4. Tides

(1) General

Tides, when relevant, are to be considered in the design of an offshore fixed structure. Tides may be classified as lunar or astronomical tides, wind tides, and pressure differential tides. The combination of the latter two is commonly called the storm surge.

(2) The water depth at any location consists of the mean depth, defined as the vertical distance be-

tween the sea bed and an appropriate near-surface datum, and a astronomical tides and storm surges.

(3) Astronomical tide variations are bounded by highest astronomical nomical tide, LAT, still water level(SWL) should be taken as the

ical level plus the storm surge.

fluctuating component due to

tide, HAT, and lowest astro- sum of the highest astronom-

(4) Storm surge is to be estimated from available statistics or by mathematical storm surge modeling.

(5) Design Environmental Wave Crest

For design purposes, the design environmental wave crest elevation is to be superimposed on the SWL. Variations in the elevation of the daily tide may be used in determining the ele- vations of boat landings, barge fenders and the corrosion prevention treatment of structure in the splash zone. Water depth assumed for various topics of analysis are to be clearly stated.

5. Temperature

Extreme values of air, sea and seabed temperatures are to be expressed in terms of recurrence Periods and associated highest and lowest values. Temperature data is to be used to evaluate se- lection of structural materials, ambient ranges and conditions for machinery and equipment design, and for determination of thermal stresses, as relevant to the installation.

6. Ice and snow

(1) For structures intended to be installed in areas where ice and snow may sea ice hazards may develop, estimates are to be made of the extent to may accumulate on the structure.

(2) Data may be derived from actual field measurements, laboratory data or areas.

7. Marine Growth

accumulate or where which ice and snow

data from analogous

(1) Marine growth is to be considered in the design of an offshore installation. Estimates of the rate and extent of marine growth may be based on past experience and available field data.

(2) Particular attention is to be paid to increases in hydrodynamic loading due to increased diame-

ters and surface roughness of members caused by marine fouling as well as to the added weight and increased inertial mass of submerged structural members.

(3) Consideration should be given to the types of fouling likely to occur and their possible effects

on corrosion protection coatings.

8. Seismicity and Earthquake Related Phenomena

(1) Effects on Structures

The effects of earthquakes on structures located in areas known to be seismically active are to

be taken into account. The anticipated seismicity of an area is, established on the basis of regional and site specific data

to the extent practicable, to be including, as appropriate, the

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following.

(A) Magnitudes and recurrence intervals of seismic events

(B) Proximity to active faults

(C) Type of faulting

(D) Attenuation of ground motion between the faults and the site

(E) Subsurface soil conditions

(F) Records from past seismic events at the site where available, or from analogous sites

(2) Ground Motion

The seismic data are to be used to establish a quantitative strength level and ductility level earthquake criteria describing the earthquake induced ground motion expected during the life of the structure. In addition to ground motion, and as applicable to the site in question, the follow-

ing earthquake related phenomena should be taken into account.

(A) Liquefaction of subsurface soils

(B) Submarine slide

(C) Tsunamis

(D) Acoustic overpressure shock waves

9. Sea Ice

(1) The effects of sea ice on structures must consider the frozen-in condition(winter), break-out in the spring, and summer pack ice invasion as applicable.

(2) Impact, both centric and eccentric, must be considered where moving ice may impact a

structure. Impact should consider both that of large masses(multi-year floes and icebergs) moving under the action of current, wind, and Coriolis effect, and that of smaller ice masses which are accelerated by storm waves.

(3) The interaction between ice and the structure produces responses both in the ice and the struc- ture-soil system, and this compliance should be taken into account as applicable.

(4) The mode of ice failure(tension, compression, shear, etc.)depends on the shape and roughness of the surface and the presence of adfrozen ice, as well as the ice character, crystallization, tem-

perature, salinity, strain rate and contact area. The force exerted by the broken or crushed ice in

moving past the structure must be considered. Limiting force concepts may be employed if thor- oughly justified by calculation.