Accounting for VIV in Wake Induced Motion of Risers in Tandem at High Reynolds Number
- Jean-Francois Saint-marcoux (Subsea 7) | Robert D. Blevins
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference, 30 April-3 May, Houston, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2012. Offshore Technology Conference
- 4.1.2 Separation and Treating, 4.2 Pipelines, Flowlines and Risers, 4.2.4 Risers
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As production of oil and gas from deepwater and ultra deepwater increases,riser interference becomes critical for engineering design and analysis. Theuse of seafloor space must be optimized and the size of the floating unitsreduced. For example SCR's are closely spaced at the FPU and lay in close rangeat the seafloor. Similarly for decoupled riser systems, jumpers are located atshort distances at the FPU level.
Blevins provided the seminal paper on the forces and stability of a cylinder ina wake. One of the major conclusions of his paper was that accounting for dragas per Huse should be refined to be able to assess the potential instability ofa downstream cylinder in a wake.
To be complete it was necessary to expand the theory to the case of an upstreamcylinder in cross vibration about its steady position. This took more timebecause the wake behind a vibrating cylinder does not follow the classical bellshape curve. Obtaining reliable experimental data to be able to describe it wasdifficult, especially at high Reynolds number (105).
The paper presents experimental data for a wake behind a vibrating (and - forbenchmarking - a fixed) cylinder from 2D downstream to 50D downstream. Arelatively simple extension of the Blevins original theory allows accountingfor the modified shape of the wake.
Role of Interferences in the Design of Deepwater Riser Systems
Deepwater and ultra deepwater developments require a close spacing of therisers and flowlines in order to optimize the seafloor. Examples of suchpractical configurations are:
- Decoupled risers (Bundle-HRT and SHR);
+ interference between the BHRT or SHR risers themselves as they are locatedclose to each other
+ Turret-moored FPSO's with Bundle Hybrid Riser Towers (Bundle-HRT);interference between the top jumpers that run from the top of the Bundle HRT tothe turret-FPSO
+ For submerged buoy such as the BSR (Boia de Suporte de Riser - risers supportbuoy), interference between the risers as the spacing governs the length of thebuoy (this applies to SCR's as well as Flexibles)
- Coupled risers (SCR's, Flexibles):
+ For spread-moored FPSO's; interference between the risers at the FPSO side,as the spacing of the risers is critical. This is because the further away fromthe center of the vessel, the higher is the excitation provided by the pitchand roll of the FPSO
This is also referred to as Wake Induced Motion (WIM). As a generalguideline DNV (2009), recommends analyzing riser interference in the range ofup to 25 diameters downstream. For a typical riser of 0.3m (outer diameter),this corresponds to a 7.5m range, whereas this corresponds to about 60m for aBundle-HRT. Clearly those are ranges of actual engineering applications.
The general case of interference of arrays of risers is simplified in practiceby using strip theory and considering the two closest risers at any given time.Also the time-average hydrodynamic excitation forces are considered.
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