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Fresh water transport in the deep water

10 Jul 2009  


CYPRUS WATER SUPPLY PROJECT
The Turkish Directorate of State Hydraulics Works (DSI) is considering the installation of a fresh water pipeline between the Turkish mainland and the northern coast of Cyprus. The pipeline diameter will be 1600 mm and the length of the pipeline will be approximately 80 km. The pipeline material will be High Density Polyethylene (HDPE) as commonly applied for water transportation systems.

The challenge
A significant technical challenge for the Cyprus Water Supply Project is the offshore crossing, which must traverse water depths up to 1500 m. A suspended, floating pipe arrangement has been proposed for the offshore crossing. In this arrangement, the HDPE pipe is suspended at a water depth of approximately 280 m in spans of approximately 500 m length each. Adjacent spans are mechanically joined, and the joint is connected to the seabed by means of a tether and anchor system.


 

DHI with Arti Proje as subconsultant was contracted by Alarko Contracting Group to perform hydrodynamic experimental investigations for the Cyprus Water Supply Project as well as design review of the preliminary and detailed engineering of the offshore section. The purpose of the experimental investigations was to support the design of the water supply pipeline, particularly with regard to the hydrodynamic stability of the suspended pipeline.


The suggested suspended floating pipe arrangement. Adjacent spans are mechanically joined, and the joints are connected to the seabed by means of a tether and anchor system.


In DHI's 3D physical model test facility an extensive experimental program was organized and performed according to the main objectives of the model tests:

  • Hydrodynamic Loads
  • Vortex Induced Vibration
  • Stability of the On-bottom to Free Span Transition


Hydrodynamics   
Hydrodynamic loads are required for calculating the configuration of the suspended pipeline (e.g. equilibrium position) under various steady currents since the drag force moves the suspended pipeline away from vertical.



The hydrodynamic load moves the suspended pipeline away from vertical
  

The primary cause for the hydrodynamic loads (drag force) are vortex shedding, which occurs on cylindrical members in steady current. For certain flow velocity range, vortex shedding may lock to the motion of the cylinder and amplified dynamic responce occurs. This is called vortex induced vibration. For the present system this dynamic response will increase the drag force from the steady current and unsteady sway motions of the pipeline may be introduced.

Finally the transition from on-bottom to the suspended floating pipeline is critical because at one side there is a free spanning pipeline section susceptible to long period sway motions and vortex shedding and at the other side there is a fixed structure.

Eventually all test results were analysed, interpreted and condensed to input data for use in the detailed design of the suspended pipeline.



Moive: the camera was located downstream of the suspended pipeline and is showing the overall behavior (cross-flow) at the transition from on-bottom to suspended floating pipeline where there on one side is a free spanning pipeline section susceptible to long period sway motions and vortex shedding and at the other side there is a fixed structure. The time (play back) is running eight times faster than in the nature and the vertical scale has been exaggerated by a factor of two to enhance the amplified motion.)

Want to know more ?

Stefan Carstensen

sca@dhigroup.com