Jan Oberhagemann

Application of CFD in Long-Term Extreme Value Analyses of Wave Loads

Abstract This paper discusses ways to embed time-domain field methods in extreme value predictions. Approaches are suggested that appear to give most reliable results. They rely on Monte-Carlo simulations, a reduction of parameter variations and extrapolation of exceedance rates over significant wave height. The computational effort is large, yet it can be handled with modern cluster computers.

Prediction of Ship Response Statistics in Severe Sea Conditions Using RANS

The International Association of Classification Societies (IACS) promotes the paradigm shift in structural design rules for ships towards risk based approaches. This requires improvements in the assessment of structural design loads and new methods for estimation of wave loads and responses, amongst others with respect to extreme value distributions. In this paper we present a numerical method based on the solution of RANS equations to deal with large wave-induced ship motions and corresponding loads for different ship types. Nonlinearities of wave excitation and ship response are included. Short-term ship response distributions from time domain simulations are compared with model test data. Significant deviations from Rayleigh distribution of amplitudes are observed, especially for hull girder loads including effects of structural elasticity.Copyright

Green Water Loads on a Cruise Ship

A linear boundary-element method and a Reynolds-averaged Navier-Stokes (RANS) equations solver were combined to predict maximum green water loads on a typical cruise ship of medium size. For structural analysis, a one-way coupling mapped the hydrodynamic pressure from the finite-volume grid onto the computational structural dynamics finite element mesh. First, linear long-term maximum ship responses were determined by a boundary element method combined with long-term statistics based on spectral methods; transfer functions of these responses were used to define response-conditioned wave trains inducing the linear long-term maximum ship response. The investigated wave sequences were correlated to a dedicated probability level for a lifecycle time of 20 years in the North Atlantic environmental wave conditions and for a ship speed of six knots.Critical impact locations were found to include the weather deck in the foreship, the front wall of the superstructure and the overhanging bridge deck. Predicted loads were compared to experimental data obtained in conditioned wave trains and in extreme irregular sea states. Numerical and experimental results revealed significantly higher loads than design loads specified by classification society rules. Pressure peaks on the weather deck and the superstructure front wall were comparable to rule-based design pressures for breakwaters on containerships and exceeded pressure peaks on the bridge deck.Copyright

First Order Reliability Analogies of Nonlinear Bending Moments in Ships

Hydroelasticity codes based on the solution of the Navier-Stokes equations are beneficial for extreme value predictions of hull girder loads. Since direct long-term analyses using these numerical methods are prohibitive due to excessively large required computation times, strategies are sought to reduce the computational effort. An extrapolation approach allows reducing the required simulation duration significantly. Application to hogging bending moments of flexible containerships agrees with Monte-Carlo simulations in random sea state realisations.Copyright

Hydro-Elastic Simulation of Stern Slamming and Whipping

Rational assessment of stern slamming of a large twin-screw LNG carrier comprised prediction of hydrodynamic impact loads and their effects on the dynamic global structural behaviour of the hull girder. Linear theory obtained regular equivalent waves that caused maximum relative normal velocities at critical locations underneath the ships stern. Reynolds-averaged Navier-Stokes equation (RANSE) computations based on the volume of fluid (VOF) method yielded transient (nonlinear) hydrodynamic impact (slamming) loads that were one-way coupled to a nonlinear motion analysis of the ship in waves. Hydrodynamic loads acting on the hull were converted to nodal forces for a finite element model of the ship structure. Shape and duration of computed slamming pressures agreed well with full-scale measurements carried out on other ships, indicating that computed results captured all essential physical phenomena. Maximum slamming pressures were close to, but did not exceed classification society rule values. Hull girder whipping was analyzed to investigate dynamic amplification of structural stresses. The analyses indicated a significant amplification (up to 25%) of bending moments due to whipping.

Stern Slamming of a LNG Carrier

Rational assessment of stern slamming of a large twin screw liquefied natural gas (LNG) carrier comprised prediction of hydrodynamic impact loads and their effects on the dynamic global structural behavior of the hull girder. Linear theory obtained regular equivalent waves that caused maximum relative normal velocities at critical locations underneath the ship’s stern. Reynolds-averaged Navier‐Stokes computations based on the volume of fluid method yielded transient (nonlinear) hydrodynamic impact (slamming) loads that were coupled to a nonlinear motion analysis of the ship in waves. At every time step of the transient computation, the finite volume grid was translated and rotated, simulating the actual position of the ship. Hydrodynamic loads acting on the hull were converted to nodal forces for a finite element model of the ship structure. Slamminginduced pressure peaks, typically lasting for about 0.5 s, were characterized by a steep increase and decrease before and after the peak values. Shape and duration agreed favorably with full-scale measurements and model tests carried out on other ships, indicating the plausibility of our numerical predictions. Hull girder whipping was analyzed to investigate dynamic amplification of structural stresses. Short-duration impact-related slamming loads excited the ship structure to vibrations in a wide range of frequencies. Excitation of the lowest fundamental eigenmode contributed most to additional stresses caused by hull girder whipping. Although, for the cases investigated, longitudinal stresses and shear stresses caused by quasisteady wave bending were uncritical, we obtained a significant amplification (up to 25%) due to the dynamic structural response. DOI: 10.1115/1.3124131