ABSTRACT

Tests with a semi-submersible model were performed with and without risers connected to it, in order to gauge the influence on the floating unit dynamics, taking into account the damping effects caused by the lines. Moreover, these two different dynamics were imposed in the top of a numerical model of the riser used in the tests. With this methodology it was possible to evaluate changes in the riser top tension behavior, using the platform damped dynamics in its sizing activities. The results showed a considerable influence of the platform dynamics in the riser top tension, for the specific case of this study. Therefore, the main purpose of the paper is to present the results obtained, giving subsidies to discuss some aspects of the actual riser calculation/simulation procedure.

INTRODUCTION

In the actual world oil production scenario, it has been very important to study the viability of floating systems for ultra-depth water that reaches 3000 meters. Taking this into account, one of the aspects that is not so much explored, and may have a relevant impact in the development of offshore systems projects, is the damping influence of the risers and the mooring lines in the dynamic behavior of the floating systems. Nowadays, the consideration of this effect lacks further and appropriate study for large water depths. In this context, the intension is to analyze experimental results in towing tank, trying to quantify, in this specific case, the effect of lines in the dynamics of the floating unit. One example of the actual procedure adopted in some oil companies in design and sizing of risers, is the method proposed by the American Petroleum Institute - API, reference API RP 2T, for the calculation of tension, stress, fatigue and curvature radius. Figure 2 presents the basic flowchart of the riser designs activities.

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