This paper presents calculations of different types of wind induced vibrations. Results are viewed in light of current design rules such as the DIN 4133 (1991) and the Statoil design spec N-SD-001 (1990). The rules mainly considers stand-alone circular cylinders that are suceptible to direct vortex induced vibrations, while the structures under consideration are offshore flare booms, i.e. welded latticed structures. The results suggests that the design rules are too conservative for vortex shedding on these type of structures, and also do not consider other types of vibrations. This is based on the fact that surprising cracks have appeared in existing structures while no cracks were predicted at these places. Similarly, for structures that have undergone a re-evaluation, no cracks are found in places where cracks were expected. The criteria for evaluation are critical wind speeds and fatigue calculations, based on fixity conditions, eigenvalues, stress ranges and assumed number of vibrations.

1.1 Introduction

A study on wind induced vibrations has been performed on existing and planned flare booms. All flare booms are welded latticed structures consisting of main vertical or inclined tubulars, with secondary tubular bracings in between. The following platforms and their flare booms are considered:

  • Statfjord "8"

  • Gullfaks "R"

  • Heimdal

  • Sleipner "A" For some of the platforms, observations of vibrating members with wind speed and directions were available.

All platforms have been examined in view of the following phenomena:

  1. Vortex Shedding on exposed single members

  2. Wake Interference on shielded single members

  3. Wake Interference on shielded frame bracings Wake interference is the case when a larger upstream cylinder is causing oscillations in a wake behind it that may affect ‘shielded’ tubes positioned in that wake.

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