Transient design waves in random sea are optimized by a Sequential Quadratic Programming method. The procedure is based on the linear wave superposition model which allows an exact representation of the wave train in frequency domain. For a given design variance spectrum, the desired characteristics of the design wave regime regarding wave height and crest structure are generated by optimizing an initially random phase spectrum. The solution depends on the initial phase values resulting in different wave regimes for the same problem definition. The analysis of the linear wave evolution is expanded to a fully nonlinear simulation using the finite element method based on potential flow theory. Results are shown for a high transient design wave within a tailored group of three successive waves embedded in random sea which is described by the finite depth TMA spectrum. The maximum wave height of the transient design wave is twice the significant wave height.
Reliable data on the extreme wave environment is an essential prerequisite tbr designing safe and economic offshore structures. Air gap, green water, slamming impacts and dynamic instability of ships highlight some engineering problems related to high wave crests. TLP-ringing is believed to be caused by steep transient waves resulting in high stresses and other undesirable effects. Extremely high single waves or wave groups will typically arise within an irregular sea state and are of transient nature. Deterministic design wave regimes for nmdel tests or numerical simulations need to be described by global parameters as the variance spectrum which is defined by significant wave height and peak period. In addition, local characteristics of the wave pattern and extreme waves are of great interest. The generation of these complex design wave regimes is highly complicated due to the nonlinearity of the free surface.
