1 W AVE L AND B ASIC C ONCEPTS W AVE L AND is a specialised computer - PDF document

� � � � � � � 1.1 1 W AVE L AND B ASIC C ONCEPTS W AVE L AND is a specialised computer tool for assessment of wave impact on bow and deck structures of ships and FPSOs. The system simulates bow slam and green sea effects on ships and FPSO; later extensions will include loads from waves and run-up around columns of floating offshore platforms. The formulation combines state-of-the-art description of random, steep non-linear wave events in a stochastic sea description with slamming force models and structural response analysis. A semi- empirical approach is taken, in which analytical formulations are calibrated against measured data . Main results from the program are • probability of bow slamming or water on deck for FPSOs • design loads for bow slam, water impact and green sea loading on deck-mounted equipment • assessment of structural integrity Target applications for W AVE L AND are illustrated in Figure 1-1. � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � Figure 1-1 Target applications

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � 1.2 1.1 SYSTEM OVERVIEW W AVE L AND comprises several modules that handle separate parts of the problem. Figure 1-2 illustrates the different analyses involved, and the interface between the different parts of the analyses. All modules are integrated within a common hydrodynamic analysis workbench, called ShipX � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � Figure 1-2Calculation procedure for water on deck / green sea loading 1.2 WATER KINEMATICS AND RELATIVE MOTIONS The system simulates incoming waves by a second-order random wave model, which describes water elevation as well as kinematics. Vessel motion transfer functions are taken as input from other systems, e.g. calculated by 3D diffraction or strip theory. The total wave field is calculated by combining the second-order wave with linear diffraction, either taken from full 3D diffraction analysis, from empirical correction or from experiments. 1.3 WATER PROPAGATION The water propagation across the deck is modelled by a shallow-water approach with time- varying boundary conditions from the wave kinematics analyses (surface elevation h i (t) and inflow velocity v i (t) ). Dam breaking can be treated as a special case, but M ARINTEK s experience indicates that the initial horizontal velocity of the water entering the deck has significant impact on the resulting impact loads.

1.3 Figure 1-3 Simulation of water propagation for Figure 1-4 Simulation of water propagation for case vessel according to a dam breaking model. case vessel with boundary conditions from wave Deck movement neglected kinematics analysis. Deck movement neglected

1.4 1.4 LOCAL LOADS FROM GREEN SEA Loads on deckhouses and other large objects are calculated with a similarity solution. This present formulation gives good agreement with respect to maximum impact forces, but does not simulate the decay in forces since the similarity solution does not include effects of gravity. Present method 1400 Dam breaking 1200 1000 Force [kN] 800 600 400 200 0 4735 4740 4745 Time [s] Figure 1-5 Measured impact forces Figure 1-6 Simulated impact forces 1.5 BOW SLAMMING FORCES Bow slamming forces are calculated by the program Slam2D. The program predicts forces, moments and the pressure distribution on ship sections due to slamming. The formulation follows a generalised Wagner’s solution, which allows for sections of arbitrary vertical, horizontal and rotational velocity (the horizontal and rotational velocities are assumed to be small compared to the vertical velocity). The body velocities are extracted from the wave kinematics analysis and relative motion analysis described above. 1.6 EXPORT OF IMPACT PRESSURES TO FINITE ELEMENT MODELS Calculated impact forces can be exported to industry standard FE analysis programs by the VESHIP program (VEres Structure-Hydrodynamics Interface Program). This applies to impact forces on deckhouses or other large deck mounted equipment, as well as bow slamming forces. The present version of VESHIP supports the following formats: � NISA II � SESAM (only load application and mass generation) � ASAS (only load application)

1.5 600000 500000 400000 Pressure [Pa] 300000 200000 100000 0 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 Transverse position [m] Figure 1-7 Export of forces to FE analyses

1.6 1.7 INPUT W AVE L AND is implemented as a ‘plug-in’ in ShipX, M ARINTEK s common platform for ship design analyses. The basic ShipX user interface is shown in Error! Reference source not found. . ShipX with the W AVE L AND plug-in activated is shown in Figure 1-8 Figure 1-8 ShipX with the WaveLand ‘plug-in’ loaded

1.7 1.8 OUTPUT W AVE L AND produces output in different formats: • Animation files ( GLview format) for relative motions and water propagation on deck • Plot tables ( ASCII time series files and binary time series files) • Printed tables ( ASCII files) Vessel motions are animated using the third-party system GLview, a scientific visualization program, especially designed as a tool for post processing of engineering data. Figure 1-9 GLview animation of vessel motions and wave kinematics Available time series information from the wave kinematics analysis is listed below. Error! Reference source not found. shows an example of wave statistics generated by KinemA. Error! Reference source not found. shows typical code check output from IntegA (one page per element). Green sea indicator Zero value indicates no green sea. A positive value indicates water above the bulwark height at one or more positions around the bulwark. The actual value of the indicator is the average value of water above the bulwarks at any given time. Bow slam indicator Based on monitoring rapid changes in the relative acceleration between the bow flare and the disturbed wave elevation. If the value is large there is a greater chance of having a serious bow-slamming event. Original incident wave at vessel origin Vessel motions in all six degrees of freedom Surface elevation based on linear wave, linear diffraction and second order correction. This is calculated for any wave point defined in the wave kinematics input. Point ID refers to the ID given in the wave kinematics input.

1.8 Relative surface elevation, i.e. surface elevation referred to a point on the vessel that moves, typically points at the bulwarks. Wave particle velocities the x- and y- directions. Since the coordinate system has an x-axis pointing towards the bow, a positive V X -value signifies water moving from the bow towards the stern.