Our work

Flexibles, risers, flowlines, prevention, pulsations

TNO provides an acoustic assessment to avoid the lock-in behaviour within the normal operating conditions. The on-set flow rate is being calculated.

With the recent increase in demand for offshore gas, a new phenomenon has been observed on floating production platforms that export or inject gas through flexibles (risers). As the gas flow is increased, high levels of distinctive tonal noise and vibration occur in the associated piping. The problem can be attributed to flow induced pulsations (FIP) that are generated on the inner corrugated layer of the flexible. TNO provides an acoustic assessment to avoid the lock-in behaviour within the normal operating conditions. The on-set flow rate is being calculated. This is the minimum flow rate at which the unwanted "singing" behaviour occurs.When the vortex shedding frequency is able to excite acoustic natural frequencies of topside piping or subsea manifolds, a resonance phenomenon known as lock-in occurs. This lock-in can cause excessive pulsation, vibration and noise levels in the topside and subsea piping. With the problem evident for gas flow velocities as low as 3 m/s, production capacity may be severely limited.

Approach of the acoustic assessment

The acoustic assessment makes use of our so-called FIP-lock-in source model. This model is based on the knowledge generated within the "Flexible Riser" JIP (see inlay) and field measurements on Statoil's Åsgard B, one of the major gas exporting platforms in the Norwegian sea (see photo). The FIP-Lockin model predicts onset-flow rates based on carcass geometry, operating conditions and the acoustic properties of the topside piping and subsea manifold. Field measurements on an export gas system have proven that the interaction between these three components play an important role in the determination of the actual onset flow rate. The model is presently integrated within our PULSIM (PULsation SIMulation) software.

Deliverables of the acoustic assesment

Ideally the analysis is carried out during the conceptual design phase. In that case the analysis provides recommendations in respect to:

  • selection of a suitable riser (carcass geometry)
  • installation direction of the flexibles
  • layout of the topside piping and subsea manifold

In case of existing flexibles, solutions to the problem can either be:

  • Installation of acoustic block(s) that shield the topside and subsea piping from the source or
  • Increasing on-set flow rates by lowering reflected energy.

The reflected energy from the topside piping and subsea manifold(s) can significantly be reduced by:

  • relocation of (control) valves, reducers, and header tie-in points
  • adjustment of side branches
  • omission of dead-legs.

The last stage of the analysis is execution of a mechanical scan. The objective of this scan is to identify areas that are most vulnerable to dynamic loads (and subsequent high-cycle fatigue). The scan can lead to recommendations regarding the supporting and bracing of these lines.

Contact

Harry Korst

  • Pulsations
  • vibrations
  • fatigue
  • failures
  • compressors
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