DNV GL ANALYSIS TECHNIQUES V-1-3 PDF

Introduction

High-frequency hull girder response is characterized by hull girder vibrations known as whipping and springing. Whipping, induced by wave impacts under the ship’s flared bow, the overhanging stern, or the bottom, leads to transient, decaying hull girder vibrations which typically occur in moderate or harsh seaways. Springing is caused by regular, periodic wave trains that excite resonant hull girder vibrations occurring in low to moderate seaways.

High-frequency hull girder excitations have not yet been explicitly incorporated in strength related design rules, because the amount of high-frequency excitation varies for different ship types and sizes, ship speeds, loading conditions, and operating areas, and present design rules cannot be changed until the associated uncertainties are clarified.

Two long-term full-scale measurement campaigns were recently conducted onboard a Panamax and a post-Panamax container ship to consolidate design rules for currently operating container ships and to contribute to rule development for container ships. A further measurement campaign will be launched on a Post-Panamax containership operating world-wide. In addition, a first-principle numerical CFD method is under development to assess high-frequency hull girder response. It accounts for all relevant nonlinear flow features, such as those related to forward speed effects and instantaneous wave elevations. However, systematic validation is not completed yet, and practical application is still limited because computation times are excessive.

The critical part of high-frequency hull girder response was found to be attributed to vertical bending vibration modes. Although torsional vibrations exist, especially for modern container ships, full-scale measurements showed that their influence is relatively small, particularly regarding fatigue strength. Recent investigations indicated that ship structures designed for torsion based on GL rules lead to reassuring safety levels.

This guideline was established to aid the designer in assessing effects of hull girder fatigue loads caused by the high-frequency response of the ship’s structure. It comprises a semi-empirical approach based on the results from the measurement campaigns as well as on collected damage data from the fleet of container ships, currently classed by Germanischer Lloyd.

The method is based on high-frequency loads extracted from full-scale strain measurements obtained from the above mentioned measurement campaigns, and they comprise all kinds of hull girder vibrations, including whipping and springing. These high-frequency loads are transferred to any considered container ship as a function of averaged rule slamming pressures which are considered to comprise parameters important for high frequency ship response.

The procedure described in this guideline reflects current knowledge and is subject to further developments.