Comparison of Various Methods for the Assessment of Wind and Current Loads on a Semi-Submersible Platform

Abstract

Abstract The wind and current loads on a large semi-submersible platform were assessed using various methods by several participants. All participants treated the current and wind analyses separately. The current analyses were performed in a uniform flow. The wind analyses were performed in an atmospheric boundary layer flow. Flow angles ranging from 0 to 360 degrees were considered. Participants 1 and 2 performed wind tunnel tests on the same 1:250 scale model. Participant 3 performed steady-state full scale CFD simulations. Participant 4 performed towing tests (for the current loads only) on a 1:60 scale model. This paper presents, compares and discusses the results obtained with the different methods. The objective of this paper is to evaluate the variability which can be obtained in the results when using different validated and trusted approaches. The comparison shows significant discrepancies between the obtained wind and current loads. These discrepancies may be due to various aspects of the method setups such as the modeling of the atmospheric boundary layer flow, the scaling effects and the experimental uncertainty regarding the measurements. Introduction Wind and current loads contribute significantly to the total environmental loads on offshore structures. An accurate determination of those loads plays an important role in the assessment of e.g. stability and design of positioning systems. While underestimating the loads may lead to unacceptable safety margins, overestimation yields an inefficient design of offshore structures, e.g. a reduced deck load and costly design of positioning systems. The flow patterns around the offshore platform are very complex. For accurate predictions of the wind and current loads, it is important that the assessment method includes a complete modeling of all the relevant physics. The traditional and most trusted method is based on wind tunnel tests on a scaled model. Towing tests also fulfill this requirement for current loads assessment. CFD simulations have also the potential to assess wind and current loads and are now recognized as a valid alternative to wind tunnel tests for analyzing flows around offshore structures. Independently of the method used, the loads must be determined in a carefully modeled environment. In this study, four participants were asked to evaluate wind and current loads on a floating semi-submersible platform. The hull comprises a ring pontoon with six columns to support the topside. Participants 1 and 2 performed wind tunnel tests on the same 1:250 scale model. Participant 3 performed CFD simulations. Participant 4 performed towing tests (for the current loads only) on a 1:60 scale model. This paper presents, compares and discusses the results obtained by the four participants. The variability which can be obtained in the results when using different validated and trusted approaches is evaluated in this paper.