Metocean Ext
47
Part A | 3
3. Metocean Extreme and Operating Conditions
George Z. Forristall, Cortis K. Cooper
Metocean stands for meteorology and oceanography, an acronym that is commonly used in
the offshore oil industry to encompass almost all
topics involving the quantitative description of the
ocean and atmosphere needed to design and operate man-made structures, facilities, and vessels
in the ocean or on the coast. The metocean environment controls many aspects of facility design
and operation, so errors in quantifying metocean
conditions can cascade through the design and
operational decisions. Errors can result in damage
and lost lives. Conversely, if the variables are overestimated, costs will be overestimated perhaps to
the point that the project becomes uneconomic
and is never built.
Metocean criteria are typically broken into two
categories: operating and extreme. The former involves quantification of metocean conditions in
which the facility or vessel should be capable of
achieving the routine functions of its primary purpose. Extreme conditions are typically associated
with storms.
The approach taken by this chapter is to outline the methods commonly used in industry to
quantify the most important metocean variables
that impact offshore facilities. These methods are
drawn largely from the offshore oil and gas industry but they are also readily applicable to other
engineering applications involving the design and
operation of vessels, coastal structures, offshore
wind farms, navigational aids, coastal geomorphology, and pollution studies.
Of course there are a multitude of metocean
variables which could be covered but this chapter
focuses on winds, waves, and currents (WWC), since
these are the variables that most often control extreme loads or operating conditions on man-made
facilities. Because of space constraints it is necessary to only briefly cover some of the topics and
to provide the reader with references for further
reading.
After the introductory section, the next section reviews key physical processes: WWC spectra,
wind and current profiles, wave growth and wave
breaking. It is followed by two sections that briefly
address some of the more important issues that
can arise regarding measurements and models. Section 3.5 examines ways to calculate the
marginal probability of WWC processes. Section 3.6
describes some of the analysis products that are
typically used to quantify operating conditions.
Finally, the last section covers the topic of extreme
criteria.
3.1 Quantifying the Metocean Environment ... 48
3.2 Overview of WWC Processes...................... 49
3.2.1 Winds .......................................... 49
3.2.2 Waves .......................................... 51
3.2.3 Currents ....................................... 53
3.3 Measurements ........................................ 55
3.3.1 Historical Storm Databases ............ 55
3.3.2 Satellite Databases........................ 56
3.3.3 In Situ Measurements ................... 57
3.3.4 Mobile Measurements ................... 58
3.4 Modeling ................................................ 58
3.4.1 Winds .......................................... 59
3.4.2 Waves .......................................... 59
3.4.3 Currents, Surge, and Tides ............. 60
3.5 Joint Events ............................................ 61
3.5.1 Response-Based Analysis .............. 61
3.5.2 Load Cases ................................... 62
3.5.3 Environmental Contours ................ 63
3.5.4 Inverse FORM................................ 63
3.6 Operational Criteria ................................. 64
3.6.1 Probability Distributions................ 64
3.6.2 Persistence................................... 65
3.6.3 Currents ....................................... 65
3.7 Extreme Criteria ...................................... 66
3.7.1
Risk and Reliability ....................... 66
3.7.2
The Historical Method ................... 67
3.7.3
Synthetic Storm Modeling .............. 68
3.7.4 Modeling Versus Measurements ..... 68
3.7.5 Accounting for Physical Limits........ 69
3.7.6 Seasonality .................................. 69
3.7.7
Directionality ............................... 69
3.7.8 Combining Long and Short-Term
Distributions ................................ 69
3.7.9 Rogue Waves ................................ 70
47
Part A | 3
3. Metocean Extreme and Operating Conditions
George Z. Forristall, Cortis K. Cooper
Metocean stands for meteorology and oceanography, an acronym that is commonly used in
the offshore oil industry to encompass almost all
topics involving the quantitative description of the
ocean and atmosphere needed to design and operate man-made structures, facilities, and vessels
in the ocean or on the coast. The metocean environment controls many aspects of facility design
and operation, so errors in quantifying metocean
conditions can cascade through the design and
operational decisions. Errors can result in damage
and lost lives. Conversely, if the variables are overestimated, costs will be overestimated perhaps to
the point that the project becomes uneconomic
and is never built.
Metocean criteria are typically broken into two
categories: operating and extreme. The former involves quantification of metocean conditions in
which the facility or vessel should be capable of
achieving the routine functions of its primary purpose. Extreme conditions are typically associated
with storms.
The approach taken by this chapter is to outline the methods commonly used in industry to
quantify the most important metocean variables
that impact offshore facilities. These methods are
drawn largely from the offshore oil and gas industry but they are also readily applicable to other
engineering applications involving the design and
operation of vessels, coastal structures, offshore
wind farms, navigational aids, coastal geomorphology, and pollution studies.
Of course there are a multitude of metocean
variables which could be covered but this chapter
focuses on winds, waves, and currents (WWC), since
these are the variables that most often control extreme loads or operating conditions on man-made
facilities. Because of space constraints it is necessary to only briefly cover some of the topics and
to provide the reader with references for further
reading.
After the introductory section, the next section reviews key physical processes: WWC spectra,
wind and current profiles, wave growth and wave
breaking. It is followed by two sections that briefly
address some of the more important issues that
can arise regarding measurements and models. Section 3.5 examines ways to calculate the
marginal probability of WWC processes. Section 3.6
describes some of the analysis products that are
typically used to quantify operating conditions.
Finally, the last section covers the topic of extreme
criteria.
3.1 Quantifying the Metocean Environment ... 48
3.2 Overview of WWC Processes...................... 49
3.2.1 Winds .......................................... 49
3.2.2 Waves .......................................... 51
3.2.3 Currents ....................................... 53
3.3 Measurements ........................................ 55
3.3.1 Historical Storm Databases ............ 55
3.3.2 Satellite Databases........................ 56
3.3.3 In Situ Measurements ................... 57
3.3.4 Mobile Measurements ................... 58
3.4 Modeling ................................................ 58
3.4.1 Winds .......................................... 59
3.4.2 Waves .......................................... 59
3.4.3 Currents, Surge, and Tides ............. 60
3.5 Joint Events ............................................ 61
3.5.1 Response-Based Analysis .............. 61
3.5.2 Load Cases ................................... 62
3.5.3 Environmental Contours ................ 63
3.5.4 Inverse FORM................................ 63
3.6 Operational Criteria ................................. 64
3.6.1 Probability Distributions................ 64
3.6.2 Persistence................................... 65
3.6.3 Currents ....................................... 65
3.7 Extreme Criteria ...................................... 66
3.7.1
Risk and Reliability ....................... 66
3.7.2
The Historical Method ................... 67
3.7.3
Synthetic Storm Modeling .............. 68
3.7.4 Modeling Versus Measurements ..... 68
3.7.5 Accounting for Physical Limits........ 69
3.7.6 Seasonality .................................. 69
3.7.7
Directionality ............................... 69
3.7.8 Combining Long and Short-Term
Distributions ................................ 69
3.7.9 Rogue Waves ................................ 70