Troll Oil Pipeline: Calibration of safety factors for cross-flow vibrations of spans on very uneven seabeds
- DNVI, Oslo (Norway)
- Statoil, Trondheim (Norway)
- Snamprogetti, Fano (Italy)
Within Fensfjorden on the Norwegian coast the proposed Troll Oil Pipeline passes through an area of deep water with an extremely rough and rocky sea bed characterized by rather high near bottom current velocities. In this area the pipeline will be ``multi-spanning``, i.e. it will have many spans closely spaced which interact statically and dynamically and may be subjected to vortex shedding induced oscillations. The present paper describes structural reliability analysis conducted to determine rational safety factors to be used in the design of the Troll Oil Pipeline free spans in the above rough sea bed area. These safety factors are related to a design equation expressing the limit for onset of cross-flow vibrations caused by current induced vortex shedding (in-line vibrations are not considered in this paper). The probabilistic modelling employed is described. The probability distributions used to describe the natural frequency and the distributions for the extreme current component normal to the pipeline are discussed. The estimation of natural frequency is based on an analytical equation accounting for initial deflection, axial force and boundary conditions. The boundary conditions for the equation are obtained through a finite element model of a continuous length of pipeline covering the area of interest. The extreme current distribution is based on measurement data accounting for the results of numerical and physical modelling of the current driving mechanism. Target safety levels and calibrated safety factors to be used in the design equation for the as-laid and operation conditions are proposed. Application and extension of the methodology and results to more general situations is discussed.
- OSTI ID:
- 205491
- Report Number(s):
- CONF-950695--; ISBN 0-7918-1311-8
- Country of Publication:
- United States
- Language:
- English
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