Wave Run-Up and Air Gap Prediction for a Large-Volume Semi-Submersible Platform

Abstract

This paper addresses the problem of estimating the air gap for a large semi-submersible production platform. Although it has a great impact on the design of the floating unit, many times the minimum deck height is still defined from simplified methods that incorporate relatively large safety margins. The reason for this is the intrinsic complexity of the associated hydrodynamic problem. Nonlinear effects on the incoming and scattered waves are usually relevant and sometimes non-linear effects on the motions of the floating hull may also play an important role. This discussion is illustrated by means of wave basin tests performed with the model of a large semi-submersible designed to operate in Campos Basin. Significant run-up effects on its squared-section columns were observed for the steepest waves in several design conditions. Also, the unit presented relatively large low-frequency motions in heave, roll and pitch, which also affected the dynamic air gap measurements. In order to evaluate the difficulties involved in modeling such phenomena, simplified tests were also performed with the model fixed and moored in regular waves of varying steepness. Wave elevation in different points was measured in these tests and compared to the predictions obtained from two different numerical methods: a BEM code that incorporates 2nd order diffraction effects (WAMIT) and a VOF CFD code (ComFLOW), the latter employed for fixed model tests only. Results show that a standard linear analysis may lead to significant errors concerning the air gap evaluation. Extending the BEM model to 2nd order clearly improve the results as the wave-steepness increases. Although the VOF analysis is considerably time-consuming, simulations presented very good agreement to the experimental results, even for the steepest waves tested.

Probability Analysis of Wave Run-Ups and Air Gap Response of a Deepwater Semisubmersible Platform Using LH-Moments Estimation Method

“…Extending the linear method to the second order clearly improved the prediction for steep waves compared to the measured data; however, wave amplifications near columns could still not be predicted exactly (Winterstein and Sweetman 2001;Stansberg et al 2005). To deal with highly nonlinear phenomena such as wave splashes or wave breaking along with wave run-up near columns in extreme waves, a CFD simulation tool based on the Navier-Stokes equations was developed and the fully free-surface dynamics were described using the volume of fluid (VOF) method (Kleefsman et al 2005;Veldman et al 2011;Matsumoto et al 2013). The time-consuming simulations showed good agreement with the experimental results even for the steepest wave cases in design conditions.…”

“…Shan et al (2011) investigated the wave run-up characteristics along square columns and revealed the relationships between air gap distributions and incident wave parameters through a wide range of monochromatic wave tests of a semi-submersible platform. Matsumoto et al (2013) reported significant run-up effects on square-section columns of a large-volume semi-submersible platform for the steepest wave tests in design conditions and evaluated the numerical predictions by using regular wave test results for different steepness values.…”

Analysis of Relative Wave Elevation Around Semi-submersible Platform Through Model Test: Focusing on Comparison of Wave Probe Characteristics

“…These analyses are particularly necessary around the columns of structures, because these areas exhibit strong nonlinearity. As the studies using CFD simulations, Iwanowski et al (2009) andMatsumto et al (2010) analyze the relative wave elevation around semi-submersible platform using the commercial software ComFLOW under regular waves and compared their results with experimental results. And Lee et al (2014) analyzed the relative wave elevation under irregular waves with 100-year return period.…”

A higher order time domain panel method for linear and weakly non linear seakeeping problems.

“…However, even when limiting the hydrodynamic model to the second-order, mean and slow drifts are not the only effects of interest. For example, resonant motions induced by second-order wave effects on the roll and pitch motions of a platform may have an impact on the definition of its minimum air gap and a second-order correction of the wave profile may provide better estimates of the wave run-up upon its columns (see, for example,Matsumoto et al [2013]).…”