Robert Seah

Validation of Wave Run-Up Calculation Methods for a Gravity Based Structure

During the design of a Gravity-Base Structure (GBS) for harsh environments, it is essential to account for the maximum wave run-up in operational and extreme weather conditions. Linear diffraction theory and empirical correction factors are typically used in the early design phase of a project in which wave run-up is a concern. As the project nears final design, model tests are usually used to assess wave run-up and air gap requirements. This paper addresses the use of alternative methods for prediction of run-up around a GBS in approximately 100 m water depth. Results from a second-order diffraction code (WAMIT) and a fully nonlinear CFD program (ComFLOW) are compared to assess the importance of nonlinearities, which are shown to depend on incident wave steepness and wavelength. Extending diffraction theory to second-order significantly improves linear predictions and produces more realistic spatial patterns of maximum run-up. However CFD simulations are required to accurately predict run-up associated with very steep incident waves and highly nonlinear characteristics. In addition to regular wave computations, linear and second-order potential flow calculations are also compared against model test results for an irregular sea.Copyright

The Effect of Sampling Rate on the Statistics of Impact Pressure

Sloshing model tests have been performed to estimate the sloshing loads for design of LNG containment systems. The experiments have revealed that the sloshing phenomenon is highly stochastic and impact pressure varies significantly even for a simple harmonic excitation in one direction. It is important to select an appropriate sampling rate and duration to capture the true pressure peaks in order to obtain a reasonable statistical estimation. In this pursuit, experiments have been performed on a 2D model scale tank with sway motion for duration of 30 minutes at different sampling rates of 20, 40, 60, 80 and 100 kHz. Comparison of statistical quantities like maximum pressure, rise time, decay time and impulse for various sampling frequencies are presented. Exceedence probability is also evaluated for each case and reported. The high sampling rate runs are down sampled to see the effect on the magnitude of the pressure peaks. Also the 30 minutes runs are split into a set of three 10 minute runs to see how the statistics change for each segment. The paper makes recommendations on required sampling rate and test duration for model scale to capture the various local effects such as breaking waves and spray, pronounced during the liquid sloshing impact.Copyright

Analysis of Roll Motion due to Asymmetric Bilge Keels

Traditional frequency domain based vessel motion analyses operate under the assumption that the roll damping contribution from the port and starboard bilge keels are equivalent. In this work, we examine the roll motion of a vessel with bilge keels of unequal length using a novel methodology. Experiments conducted during the FPSO Roll JIP suggest that waves approaching from port versus starboard will induce different motion amplitudes due to the unequal bilge keel length. We examine the results from different approaches, comparing the computed response from a frequency domain analysis against those provided by a time domain model using Orcaflex with bilge keel represented by drag elements.Copyright

Liquefied Natural Gas Carriers

Global energy demand is rising as the world’s population is growing. Various forecasts predict that by 2050, there could be 2.5 billion more humans than today, who would use twice the energy consumed today. Energy companies are examining all possible sources of energy as part of their widening portfolio.

A Comparison of Time Domain Methods for Asymmetric Roll Predictions

Unlike ocean going vessels, FPSOs often have appendages, such as bilge keels or riser porches, at or below the waterline in an asymmetric configuration. In addition, the riser and mooring systems impose asymmetric loads on the hull. As a result, the expected roll motion response to a wave environment is asymmetric and traditional methodologies cannot be used to predict it. Morison drag elements can be incorporated to represent the asymmetric condition and are easily implemented in time domain simulations. The limitation to this engineering approach is that the drag coefficient can only be calibrated to produce accurate motions or accurate appendage loads but not both. In this paper we compare the response using two time domain approaches, the first being adapted from a commercial marine dynamics analysis tool [9] and the other being a specialized hydrodynamics motion prediction tool [4]. Here, the commercial tool utilizes constant coefficient drag elements in conjunction with traditional linear equivalent roll damping to model the effect of unequal port and starboard bilge keels as is typical when a riser balcony are present. In contrast, the newly developed hydrodynamic model relies solely on a Keulegan-Carpenter (KC) number dependent drag relation to represent the asymmetric drag contributions. The different calibration procedures will be discussed and a comparison for a design environmental condition between the two methodologies will be presented.Copyright

Prediction of Asymmetric FPSO Roll Using a Frequency Domain Method

FPSO roll prediction has traditionally been performed assuming symmetric roll damping resulting in identical roll responses from portside and starboard waves. Recent interest in the industry to predict asymmetric roll response, either due to asymmetric mooring and riser configurations or damping devices, has led to the development of time domain models utilizing asymmetric Morison drag elements. Here, a frequency domain methodology has been developed to account for asymmetric bilge keels leading to differing port versus starboard wave roll response. A nonlinear bilge keel drag formulation, that includes the effects of radiation velocity, is used, coupled with linearization techniques, to predict the difference in roll RAO from port versus starboard waves. The drag formulation is initially calibrated against FPSO decay tests before the model is validated against measured model test motions. Thus we show that the methodology proposed is capable of predicting the motions from an asymmetric configuration efficiently, such that it can be utilized in design projects requiring FPSO motions analysis.Copyright

Evaluation of Forces on Bilge Keels in Oscillating Flow Using CFD

The roll motion of a ship or a FPSO can be effectively mitigated by attaching keels along the bilges. Proper structural design of the bilge keels requires good estimates of the hydrodynamic loads acting on them. However, in simplified applications, Morison’s equation may be adopted with constant hydrodynamic coefficients for the force evaluation. Reported bilge keel failures suggest that the understanding of the total loading on the bilge keel is incomplete. In this paper, direct CFD simulations are carried out for bilge keels with added structural members in regular oscillating, prismatic flows. The hydrodynamic loads from CFD are shown to be significantly larger than those used in conventional design approaches. Sensitivity studies are also conducted here to investigate the dependence of the hydrodynamic loads on different factors including bilge keel configurations and FPSO roll velocities.Copyright