Wave Steepness

Wave Steepness

Definition(s)

Wave Steepness

Characteristic of individual waves calculated as wave height divided by wave length. NOTE For periodic waves, the concept is straightforward as H/λ. For random waves, the definition is used with the significant wave height (Hs) and the wave length that corresponds with the peak period (Tp) of the wave spectrum in deep water. The significant wave steepness is then defined as Hs/λp = Hs / [(g/2π) Tp2] and is typically in the range of 1/16 to 1/20 for severe sea states. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Wave Spectrum

Wave Spectrum

Definition(s)

Wave Spectrum

measure of the amount of energy associated with the fluctuation of the sea surface elevation per unit frequency band and per unit directional sector. NOTE 1 The wave frequency spectrum (integrated over all directions) is often described by use of some parametric form such as the Pierson-Moskowitz or JONSWAP wave spectrum. NOTE 2 The area under the wave spectrum is the zeroth spectral moment m0, which is a measure of the total energy in the sea state; m0 is used in contemporary definitions of the significant wave height Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Tropical Cyclone

Tropical Cyclone

Definition(s)

Tropical Cyclone

Closed atmospheric or oceanic circulation around a zone of low pressure that originates over the tropical oceans NOTE 1 The circulation is counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. NOTE 2 At maturity, the tropical cyclone can be one of the most intense storms in the world, with wind speeds exceeding 90 m/s and accompanied by torrential rain. NOTE 3 In some areas, local terms for tropical cyclones are used. For example, tropical cyclones are typically referred to as hurricanes in the Gulf of Mexico and North Atlantic, while in the South China Sea and NW Pacific they are called typhoons. In the South Pacific and South Indian Ocean, however, they are commonly referred to as cyclones. NOTE 4 The term cyclone is also used to refer to a tropical storm with sustained wind speeds in excess of 32 m/s (Beaufort Force 12). Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Swell

Swell

Definition(s)

Swell

Sea state in which waves generated by winds remote from the site have travelled to the site, rather than being locally generated. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Storm Surge

Storm Surge

Definition(s)

Storm Surge

Change in sea level (either positive or negative) that is due to meteorological (rather than tidal) forcing. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Still Water Level

Still Water Level

Definition(s)

Still Water Level

Abstract water level typically used for the calculation of wave kinematics for global actions and wave crest elevation for minimum deck elevations NOTE Still water level is an engineering abstract calculated by adding the effects of tides and storm surge to the water depth but excluding variations due to waves (see Figure 1). It can be above or below mean sea level. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Squall

Squall

Definition(s)

Squall

strong wind event characterized by a sudden onset, a duration of the order of minutes and a rather sudden decrease in speed NOTE 1 A squall is often accompanied by a change in wind direction, a drop in air temperature and by heavy precipitation. NOTE 2 To be classed as a squall the wind speed would typically be greater than about 8 m/s and last for longer than 2 min (thereby distinguishing it from a gust). Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
nth spectral moment

nth spectral moment

Definition(s)

nth spectral moment

Screen Shot 2015-02-05 at 9.52.51 AM Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Spectral Moment

Spectral Moment

Definition(s)

Spectral Moment

Screen Shot 2015-01-29 at 4.11.41 PM Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Short-term Distribution

Short-term Distribution

Definition(s)

Short-term Distribution

probability distribution of a variable within a short interval of time during which conditions are assumed to be statistically constant NOTE The interval chosen is most often the duration of a sea state. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Seiche

Seiche

Definition(s)

Seiche

oscillation of a body of water at its natural period. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Residual Current

Residual Current

Definition(s)

Residual Current

part of the total current that is not constituted from harmonic tidal components (i.e. the tidal stream) NOTE Residual currents are caused by a variety of physical mechanisms and comprise a large range of natural frequencies and magnitudes in different parts of the world. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Polar Low

Polar Low

Definition(s)

Polar Low

depression that forms in polar air, often near a boundary between ice and sea. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Most Probable Maximum

Most Probable Maximum

Definition(s)

Most Probable Maximum

Response where the probability density function constructed for the extreme response is at a maximum. Source: API Standard 2RD, Dynamic Risers for Floating Production Systems, Second Edition, September 2013. Global Standards

Most Probable Maximum

Value of the maximum of a variable with the highest probability of occurring NOTE The most probable maximum is the value for which the probability density function of the maxima of the variable has its peak. It is also called the mode or modus of the statistical distribution. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Monsoon

Monsoon

Definition(s)

Monsoon

wind which blows for several months approximately from one direction NOTE The term was first applied to the winds over the Arabian Sea which blow for six months from north-east and for six months from south-west, but it has been extended to similar winds in other parts of the world. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Mean Wind Speed

Mean Wind Speed

Definition(s)

Mean Wind Speed

time-averaged wind speed, averaged over a specified time interval NOTE The mean wind speed varies with elevation above mean sea level and the averaging time interval; a standard reference elevation is 10 m and a standard time interval is 1 h. See also sustained wind speed (3.37) and gust wind speed (3.8). Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Mean Lower Low Water

Mean Lower Low Water

Definition(s)

Mean Lower Low Water

for regions with semi-diurnal or mixed semi-diurnal tides, the mean of the lower of the two low tides that occur each tidal day, determined over the period of water level observations Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
MLLW

MLLW

Definition(s)

MLLW

For regions with semi-diurnal or mixed semi-diurnal tides, the mean of the lower of the two low tides that occur each tidal day, determined over the period of water level observations. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards

MLLW

Mean lower low water. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Mean Low Water

Mean Low Water

Definition(s)

Mean Low Water

The mean of all low tides, determined over the period of water level observations. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
MLW

MLW

Definition(s)

MLW

The mean of all low tides, determined over the period of water level observations. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards

MLW

Mean low water. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards  

MLW

Mean lower low water Source: API RP 2SIM, Structural Integrity Management of Fixed Offshore Structures, First Edition, November 2014. Global Standards
MHW

MHW

Definition(s)

MHW

the mean of all high tides, determined over the period of water level observations . Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards

MHW

mean high water. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Mean Higher High Water

Mean Higher High Water

Definition(s)

Mean Higher High Water

for regions with semi-diurnal or mixed semi-diurnal tides, the mean of the higher of the two high tides that occur each tidal day, determined over the period of water level observations. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
MHHW

MHHW

Definition(s)

MHHW

For regions with semi-diurnal or mixed semi-diurnal tides, the mean of the higher of the two high tides that occur each tidal day, determined over the period of water level observations. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards

MHHW

Mean higher high water. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Marine Growth

Marine Growth

Definition(s)

Marine Growth

living organisms attached to an offshore structure. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Marginal Probability

Marginal Probability

Definition(s)

Marginal Probability

Statistical distribution (probability) of the occurrence of a variable A that is obtained by integrating over all values of the other variables B, C, … NOTE The marginal probability of A for all values of B, C, … is written as P(A). The concept is applicable to metocean parameters, as well as to actions and action effects. EXAMPLE When considering wave conditions, A can be the individual crest elevation for all mean zero-crossing periods B and all significant wave heights C, occurring at a particular site. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards  

Marginal Probability

Marginal probability is the unconditional probability of event A, P(A). It is the probability of A regardless of whether event B did or did not occur. If B can be thought of as the event of a random variable X having a given outcome, then the marginal probability of A can be obtained by summing (or integrating, more generally) the joint probabilities over all outcomes for X. Sample Usage: Analysts estimated the marginal probability of a water system contamination, irrespective of the source or type of contaminate. Source: DHS Risk Lexicon, U.S. Department of Homeland Security, 2010 Edition. September 2010 Regulatory Guidance
Marginal Distribution

Marginal Distribution

Definition(s)

Marginal Distribution

statistical distribution (probability) of the occurrence of a variable A that is obtained by integrating over all values of the other variables B, C, … NOTE The marginal probability of A for all values of B, C, … is written as P(A). The concept is applicable to metocean parameters, as well as to actions and action effects. EXAMPLE When considering wave conditions, A can be the individual crest elevation for all mean zero-crossing periods B and all significant wave heights C, occurring at a particular site. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Long-term Distribution

Long-term Distribution

Definition(s)

Long-term Distribution

probability distribution of a variable over a long time scale NOTE The time scale exceeds the duration of a sea state, in which the statistics are assumed constant (see short-term distribution in 3.29). The time scale is hence comparable to a season or to the design service life of a structure. EXAMPLE Long-term distributions of ⎯ significant wave height, ⎯ significant wave height in the months May to September, ⎯ individual wave heights, ⎯ current speeds (such as for the vortex induced vibrations of drilling risers), ⎯ scatter diagrams with the joint distribution of significant wave height and wave period (such as for a fatigue analysis), or ⎯ a particular action effect. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Highest Astronomical Tide

Highest Astronomical Tide

Definition(s)

Highest Astronomical Tide

Level of high tide when all harmonic components causing the tides are in phase NOTE The harmonic components are in phase approximately once every 19 years, but these conditions are approached several times each year. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Gust Wind Speed

Gust Wind Speed

Definition(s)

Gust Wind Speed

Maximum value of the wind speed of a gust averaged over a short (3 s to 60 s) specified duration within a longer (1 min to 1 h) specified duration NOTE 1 For design purposes, the specified duration depends on the dimensions and natural period of the (part of the) structure being designed such that the structure is designed for the most onerous conditions; thus, a small part of a structure is designed for a shorter gust wind speed duration (and hence a higher gust wind speed) than a larger (part of a) structure. NOTE 2 In practice, for design purposes, the gust wind speeds for different durations (e.g. 3 s, 5 s, 15 s, 60 s) are derived from the wind spectrum. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards
Gust

Gust

Definition(s)

Gust

Brief rise and fall in wind speed lasting less than 1 min NOTE In some countries, gusts are reported in meteorological observations if the maximum wind speed exceeds approximately 8 m/s. Source: API RP 2MET, Derivation of Metocean Design and Operating Conditions, First Edition, November 2014. Global Standards