Deterministic Descriptions
of Offshore Waves
Bruce J. Muga
To evaluate the fluid-induced forces acting on a structure or on any of its components, we need to know its surrounding hydrodynamic flow field. For offshore
structures, this flow field arises from time-varying natural processes: winds,
currents, and surface gravity waves. Although these processes nearly always
occur in various combinations, the scope of this chapter is limited to descriptions of surface gravity waves, which are usually wind-generated. It has been
observed that these time-dependent waves occur on two different scales. The
shorter time scale, measured in minutes or seconds, is useful for describing detailed features such as wind gusts and surface wave periods. This shorter time
scale corresponds most closely to the response time of fixed offshore structures.
The longer time scale, measured in terms of hours, days, or even years, is useful
for describing variations in the wave intensity and in its statistics. The longer
time scale is important too because structures may fail in low cycle fatigue fracture after months or years of service, a topic discussed by Etube (2001). In
this présent chapter, we give a brief synopsis and critique of the classical wave
théories used most often by engineers for preliminary calculations of forces on
offshore structures. Here, the focus is on the shorter time scale. For compréhensive bibliographies of wave théories, see Sarpkaya and Isaacson (1981) and
Young (1999).
Illustrated in Figure 3.1 are the two fundamentally different descriptions of
surface gravity waves: deterministic and probabilistic. Deterministic descriptions, analytical or numerical, are used to characterize the short time scale
features of waves. Deterministic analytic descriptions encompass classical wave
théories, which in turn are subdivided into linear and nonlinear types. Probabilistic descriptions are used to characterize the long time scale features of
offshore waves. In both the deterministic and probabiüstic descriptions, linear
wave theory is important to engineers for two reasons: it is simple to apply
when estimating forces on offshore structures during the preliminary phases of
design; and it affords a simple basis for estimating the probability of failure of
a given structure. Examples in future chapters illustrate these ideas.
61
of Offshore Waves
Bruce J. Muga
To evaluate the fluid-induced forces acting on a structure or on any of its components, we need to know its surrounding hydrodynamic flow field. For offshore
structures, this flow field arises from time-varying natural processes: winds,
currents, and surface gravity waves. Although these processes nearly always
occur in various combinations, the scope of this chapter is limited to descriptions of surface gravity waves, which are usually wind-generated. It has been
observed that these time-dependent waves occur on two different scales. The
shorter time scale, measured in minutes or seconds, is useful for describing detailed features such as wind gusts and surface wave periods. This shorter time
scale corresponds most closely to the response time of fixed offshore structures.
The longer time scale, measured in terms of hours, days, or even years, is useful
for describing variations in the wave intensity and in its statistics. The longer
time scale is important too because structures may fail in low cycle fatigue fracture after months or years of service, a topic discussed by Etube (2001). In
this présent chapter, we give a brief synopsis and critique of the classical wave
théories used most often by engineers for preliminary calculations of forces on
offshore structures. Here, the focus is on the shorter time scale. For compréhensive bibliographies of wave théories, see Sarpkaya and Isaacson (1981) and
Young (1999).
Illustrated in Figure 3.1 are the two fundamentally different descriptions of
surface gravity waves: deterministic and probabilistic. Deterministic descriptions, analytical or numerical, are used to characterize the short time scale
features of waves. Deterministic analytic descriptions encompass classical wave
théories, which in turn are subdivided into linear and nonlinear types. Probabilistic descriptions are used to characterize the long time scale features of
offshore waves. In both the deterministic and probabiüstic descriptions, linear
wave theory is important to engineers for two reasons: it is simple to apply
when estimating forces on offshore structures during the preliminary phases of
design; and it affords a simple basis for estimating the probability of failure of
a given structure. Examples in future chapters illustrate these ideas.
61