Green Water on FPSO Predicted by a Practical Engineering Method and Validated Against Model Test Data for Irregular Waves

Abstract

This paper presents a series of numerical analyses performed with the potential theory-based Green Water engineer tool KINEMA3. KINEMA3 was designed to predict wave-induced impact loads on FPSOs in steep irregular waves, and for use in design load analysis. The purpose of the study presented herein is to validate KINEMA3 green water (deck overtopping) predictions in nonlinear irregular waves with results from model tests performed at the TPN (Tanque de Provas Numérico) laboratory at the University of São Paulo, Brazil. Comparisons are made for a selection of irregular wave cases, for two choices of anchoring conditions (free floating vessel and fixed vessel) and for three wave headings (180°, 225° and 270°: head, quartering and beam seas, respectively). KINEMA3 statistical green water predictions present a general good agreement with observations from the TPN model tests for all wave cases, headings and mooring conditions. Overall, observed trends for occurrence of green water and standard deviation/maximum of relative wave height are successfully reproduced by KINEMA3. In agreement with model test results, it is predicted that green water occurs more frequently for a free floating vessel and for beam seas. Additional comparisons between KINEMA3 predictions using different FPSO panel models (low-order and high-order models) present negligible differences with respect to green water estimates. The results presented herein demonstrate the robustness of the tool towards the prediction of green water for variable wave headings and sea states, and highlight the capability of KINEMA3 to be employed as an engineering-like tool for fast and multiple estimates of green water in early design studies. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.

Extreme runup events around a ship-shaped floating production, storage and offloading vessel in transient wave groups

“…Ship motion must be included but a linear representation of this appears to be adequate. This is consistent with the observations in Stansberg & Berget (2009) and Schiller et al (2014).…”

“…For a freely floating structure, Stansberg & Karlsen (2001) found that the largest relative wave-vessel motions in their dataset are close to Rayleigh distributed because the pitch motion is overpredicted, which cancels out the underprediction of the free surface elevation. Stansberg & Berget (2009) and Schiller et al (2014) concluded that the nonlinearities in the relative wave kinematics mainly result from the nonlinear asymmetry of the incident wave elevation and the nonlinearity associated with local waves diffracted/radiated from a floating FPSO. The vertical vessel motion (mainly due to the pitch motion), however, is found to be well predicted by linear analysis, except for waves that are clearly longer than the vessel.…”

“…It is noted that such an engineering-type method is attempted by Stansberg et al (2004) and Schiller et al (2014) in which an empirical 10 % increase (determined from their model test results) is added to linear diffraction/radiation solutions. The nonlinearity in the incoming wave is, however, simulated by a second-order random wave model.…”

“…A reasonably good agreement to the experimental measurements is obtained for the statistics. Unlike Stansberg et al (2004) and Schiller et al (2014), among others, a wave-by-wave analysis, rather than a statistical analysis on nonlinear random waves, is carried out in this work by using transient wave groups as input waves to excite nonlinear wave scattering from a simplified FPSO hull with a vertical bow. The FPSO is either fixed or freely floating.…”

On the Evolution of Different Types of Green Water Events

“…Interaction with other phenomena: The propagation of green water over the deck may also be affected by the interaction of the incident waves with environmental phenomenon, such as wind and currents. Although such interaction is possible, and studies regarding green water in irregular sea conditions exist (e.g., [20,65,66]), to the authors knowledge, there are no detailed studies regarding the combined effect of wind-current-waves on green water evolution in marine structures. Perhaps this could be considered in further research.…”