Offshore Structures - Design Aspects
One of the key issues in the design of offshore structures is to define the environmental
conditions for the transportation route and installation site, and to determine the environmental
loads acting on the structure for conditions such as transit, installation, operational extreme
and survival. The parameters to be defined in the environmental conditions may be found from
design codes such as API Rp 2T, among of several other codes.
The prediction of extreme values is required for the structural strength evaluation. Various
methods have been proposed for determining the extreme values. Both long- and short-term (surviving storm) wave data approaches are detailed.
The aim here is to give an overall picture of the environmental conditions and loads
for offshore structural design, and to detail the recent developments in the prediction of
extreme response. A systematic method for structural analysis of offshore structures has been
developed to predict extreme response and fatigue assessment under wave loads.
Vibrations and the associated dynamic effects are also an important factor in structural design
and vibration control.
Wind
Wind is a significant design factor. The wind conditions used in a design should be
appropriately determined from collected wind data and should be consistent with other
associated environmental parameters. Two methods are generally used to assess the effects of
wind in design:
Wind forces are treated as constant and calculated based on the 1-minute average velocity.
Fluctuating wind forces are calculated based on a steady component, the 1-hour average
velocity plus a time-varying component calculated from an empirical wind gust spectrum.
The choice of methods depends on the system’s parameters and goals of the analysis. Either
approach may give more severe load than the other, depending on the system‘s mooring and
the wind spectrum used. The design wind speed should refer to an elevation of 10 meters
above the still water level. Rapid changes of wind direction and resulting dynamic loads
should be considered in the design of offshore structures.
Waves
Wind-driven waves are a major component of environmental forces affecting offshore
structures. Such waves are random, varying in wave height/length, and may approach an
offshore structure from more than one direction simultaneously. Due to the random nature, the
sea-state is usually described in terms of a few statistical wave parameters such as significant
wave height, spectral peak period, spectral shape, and directionality, etc.
The calculation of extreme wave loads and their load effects may be based on selected shortterm
sea-states. The overall objective of this approach is to estimate loads and load effects
corresponding to a prescribed annual exceedance probability, e.g. lo-’ or IO4, without having
to carry out a full long-term response analysis. This is the so-called design storm concept.
An appropriate formulation of the design storm concept is to use combinations of significant
wave height and peak period along a contour line in the HM and TP plane. Such a contour line
can be established in different ways. The simplest way to establish the contour line at a
probability level of 10” is to first estimate the value of HM along with the conditional
mean value of Tp. The contour line is then estimated from the joint probability model of Hmo
and Tp with constant probability density. The estimation of the load effect at the probability level of is then obtained by
determining a proper extreme value for all seastates along the contour line and taking the
maximum of these values.
conditions for the transportation route and installation site, and to determine the environmental
loads acting on the structure for conditions such as transit, installation, operational extreme
and survival. The parameters to be defined in the environmental conditions may be found from
design codes such as API Rp 2T, among of several other codes.
The prediction of extreme values is required for the structural strength evaluation. Various
methods have been proposed for determining the extreme values. Both long- and short-term (surviving storm) wave data approaches are detailed.
The aim here is to give an overall picture of the environmental conditions and loads
for offshore structural design, and to detail the recent developments in the prediction of
extreme response. A systematic method for structural analysis of offshore structures has been
developed to predict extreme response and fatigue assessment under wave loads.
Vibrations and the associated dynamic effects are also an important factor in structural design
and vibration control.
Wind
Wind is a significant design factor. The wind conditions used in a design should be
appropriately determined from collected wind data and should be consistent with other
associated environmental parameters. Two methods are generally used to assess the effects of
wind in design:
Wind forces are treated as constant and calculated based on the 1-minute average velocity.
Fluctuating wind forces are calculated based on a steady component, the 1-hour average
velocity plus a time-varying component calculated from an empirical wind gust spectrum.
The choice of methods depends on the system’s parameters and goals of the analysis. Either
approach may give more severe load than the other, depending on the system‘s mooring and
the wind spectrum used. The design wind speed should refer to an elevation of 10 meters
above the still water level. Rapid changes of wind direction and resulting dynamic loads
should be considered in the design of offshore structures.
Waves
Wind-driven waves are a major component of environmental forces affecting offshore
structures. Such waves are random, varying in wave height/length, and may approach an
offshore structure from more than one direction simultaneously. Due to the random nature, the
sea-state is usually described in terms of a few statistical wave parameters such as significant
wave height, spectral peak period, spectral shape, and directionality, etc.
The calculation of extreme wave loads and their load effects may be based on selected shortterm
sea-states. The overall objective of this approach is to estimate loads and load effects
corresponding to a prescribed annual exceedance probability, e.g. lo-’ or IO4, without having
to carry out a full long-term response analysis. This is the so-called design storm concept.
An appropriate formulation of the design storm concept is to use combinations of significant
wave height and peak period along a contour line in the HM and TP plane. Such a contour line
can be established in different ways. The simplest way to establish the contour line at a
probability level of 10” is to first estimate the value of HM along with the conditional
mean value of Tp. The contour line is then estimated from the joint probability model of Hmo
and Tp with constant probability density. The estimation of the load effect at the probability level of is then obtained by
determining a proper extreme value for all seastates along the contour line and taking the
maximum of these values.
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