Part A | 3.1
48 Part A Fundamentals
3.7.10 Extremely Rare Events ................... 71
3.7.11 Quantifying Uncertainty ................ 71
3.7.12 Stationarity .................................. 71
3.8 Conclusions ............................................ 72
References..................................................... 72
3.1 Quantifying the Metocean Environment
Metocean is an acronym from meteorology and
oceanography and 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. When
engineers design a major facility or vessel to operate
and survive in the sea, they must consider the loads and
other constraints that may affect the structure. If those
loads and constraints are underestimated, then damage
can result and lives may be lost. Conversely, if loads
and constraints are overestimated, then the costs will be
overestimated perhaps to the point that the project becomes uneconomic and is never built.
The metocean environment controls so many aspects of facility design and operation that errors in
quantifying metocean conditions can cascade though
the design and operational decisions. For instance, overestimating a design wave height for a deepwater floating
production platform could result in adding too many
mooring lines. Since these additional lines would add
tons of static load, a larger facility would be needed to
provide the necessary buoyancy, thus generating additional capital cost well beyond the cost of the excess
mooring lines. In short, the accurate quantification of
metocean criteria can have far-reaching effects on the
safety and profitability of offshore facilities. For this
reason, metocean criteria are usually specified and described in a separate chapter or stand-alone document
in a project’s design documents. In 2005, the American
Petroleum Institute (API) recognized the influence of
metocean criteria and began publishing a stand-alone
set of recommended practices for the offshore industry [3.1].
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. Examples of
routine functions include pumping oil, drilling, receiving or pumping out natural gas, and generating wind
energy. Typical products used to quantify operational
conditions include a cumulative probability distribution
of wave height and a table of wind speed persistence.
These products are used in estimating the fatigue lives
for components. In contrast, extreme conditions occur
rarely and are often generated by episodic events (e.g.,
storms). During extreme conditions, normal operations
are usually suspended – the vessel is slowed, oil or gas
production is stopped, wind turbines are feathered, etc.
A common example of an often used extreme condition
parameter is the 100-year maximum wave height – the
largest wave expected over a three-hour period once in
100 years.
With this background in mind, the goal of this chapter can now be stated: it 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
to some extent, pollution studies. While we attempt
to provide some physical insights into the underlying metocean processes, this chapter focuses on the
methodology for deriving the key variables, and the nuances of their correct application.
Of course there are a multitude of metocean variables that could be covered in this chapter. Potential
topics include water temperature, tides, and salinity.
While these variables can be important for some engineering applications such as acoustics, this chapter
will focus on winds, waves, and currents (WWC), since
these are the variables that most often control extreme
loads or operating conditions on man-made facilities.
However, even this narrowing leaves countless aspects
of WWC that could be covered with far too little space
to do them justice. Thus we again have chosen to
narrow the frame further by specifically focusing on aspects of WWC that tend to drive capital or operating
decisions in large offshore facilities. For those interested in coastal features where shallow-water effects are
important, the Coastal Engineering Manual [3.2] serves
as an excellent reference.
Much as we the authors have had to narrow the topics, a metocean design basis for a major project must
narrow the variables that are covered. This is because
the sea and atmosphere are filled with complicated
processes, many of which are site specific and poorly
understood. If aggressive filtering is not undertaken,
then too much time can be spent quantifying variables
48 Part A Fundamentals
3.7.10 Extremely Rare Events ................... 71
3.7.11 Quantifying Uncertainty ................ 71
3.7.12 Stationarity .................................. 71
3.8 Conclusions ............................................ 72
References..................................................... 72
3.1 Quantifying the Metocean Environment
Metocean is an acronym from meteorology and
oceanography and 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. When
engineers design a major facility or vessel to operate
and survive in the sea, they must consider the loads and
other constraints that may affect the structure. If those
loads and constraints are underestimated, then damage
can result and lives may be lost. Conversely, if loads
and constraints are overestimated, then the costs will be
overestimated perhaps to the point that the project becomes uneconomic and is never built.
The metocean environment controls so many aspects of facility design and operation that errors in
quantifying metocean conditions can cascade though
the design and operational decisions. For instance, overestimating a design wave height for a deepwater floating
production platform could result in adding too many
mooring lines. Since these additional lines would add
tons of static load, a larger facility would be needed to
provide the necessary buoyancy, thus generating additional capital cost well beyond the cost of the excess
mooring lines. In short, the accurate quantification of
metocean criteria can have far-reaching effects on the
safety and profitability of offshore facilities. For this
reason, metocean criteria are usually specified and described in a separate chapter or stand-alone document
in a project’s design documents. In 2005, the American
Petroleum Institute (API) recognized the influence of
metocean criteria and began publishing a stand-alone
set of recommended practices for the offshore industry [3.1].
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. Examples of
routine functions include pumping oil, drilling, receiving or pumping out natural gas, and generating wind
energy. Typical products used to quantify operational
conditions include a cumulative probability distribution
of wave height and a table of wind speed persistence.
These products are used in estimating the fatigue lives
for components. In contrast, extreme conditions occur
rarely and are often generated by episodic events (e.g.,
storms). During extreme conditions, normal operations
are usually suspended – the vessel is slowed, oil or gas
production is stopped, wind turbines are feathered, etc.
A common example of an often used extreme condition
parameter is the 100-year maximum wave height – the
largest wave expected over a three-hour period once in
100 years.
With this background in mind, the goal of this chapter can now be stated: it 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
to some extent, pollution studies. While we attempt
to provide some physical insights into the underlying metocean processes, this chapter focuses on the
methodology for deriving the key variables, and the nuances of their correct application.
Of course there are a multitude of metocean variables that could be covered in this chapter. Potential
topics include water temperature, tides, and salinity.
While these variables can be important for some engineering applications such as acoustics, this chapter
will focus on winds, waves, and currents (WWC), since
these are the variables that most often control extreme
loads or operating conditions on man-made facilities.
However, even this narrowing leaves countless aspects
of WWC that could be covered with far too little space
to do them justice. Thus we again have chosen to
narrow the frame further by specifically focusing on aspects of WWC that tend to drive capital or operating
decisions in large offshore facilities. For those interested in coastal features where shallow-water effects are
important, the Coastal Engineering Manual [3.2] serves
as an excellent reference.
Much as we the authors have had to narrow the topics, a metocean design basis for a major project must
narrow the variables that are covered. This is because
the sea and atmosphere are filled with complicated
processes, many of which are site specific and poorly
understood. If aggressive filtering is not undertaken,
then too much time can be spent quantifying variables