Abstract

Fatigue analysis of mooring systems shall include line dynamics due to their significant contributions on the mooring tension variations. Full dynamic analysis including line dynamics in time domain needs excessive computational efforts. To manage fatigue design within the available time and budget, alternative analysis approach such as quasidynamic analysis has been widely used. This paper presents the line dynamic effects on the mooring fatigue response by comparing full dynamic and quasi-dynamic time history results for a sample deep water moored FPSO. Ratios of fatigue damage results between dynamic and quasi-dynamic results as well as the relation between fatigue damage and statistical characteristics of the line dynamic responses for different line configurations and load cases are studied in detail.

Introduction

Fatigue design, as an important engineering requirement for structural systems under cyclic loads, is one of the most challenging steps in analysis and design of mooring systems for offshore floating facilities. Mooring line fatigue analysis for offshore floating systems such as turret moored FPSO units requires analysis of large number of environmental load cases to cover long term seastate distribution in many directional combinations of the wave, wind and current loads. Stress range, due to time-varying internal loads, plays an important role in fatigue induced damage for mooring lines under cyclic loads. From structural points of view, tension range in the mooring lines is influenced by dynamic motions of the vessel as well as dynamic response of the lines.

Structural analysis of mooring systems shall be based on a dynamic approach on which line dynamics are taken into account (DNVGL, 2015). Line dynamic analysis include additional loads from the mooring system other than restoring forces, specifically the time varying effects resulted from high frequency oscillation of the upper end and the mooring line fluid interaction (i.e. hydrodynamic damping effects caused by relative motion between the line and fluid and inertial effects between the line and fluid).

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