32.


The predicted scour depth around the suction caisson is influenced by the assumed height of the caisson. The true structural height of the caisson above the seabed is equal to the elevation of the top of the main 30m diameter cylinder, plus an allowance for the transition piece connecting the main cylinder to the surface- piercing vertical column framing into it. Inspection will reveal that the height of the transition piece is likely to be quite considerable and this will affect the intrusion of the structure into the water column, and hence the amount of scour that it can develop. The effective height of the caisson that has been adopted in this study has been inferred from photographs and drawings of model and prototype suction caisson designs and physical model test pieces.


33. When the dimensional characteristics of the suction caisson design become better established, it may be necessary to revise the predictions of the depth and volume of the scour hole accordingly.


7.3.3.3.2 5m Diameter Suction Caissons for Jackets or Tripods under Granular Sea Bed Conditions


34.


If the column diameter is large compared to that of the caisson, then the scour should be checked by idealising the column as a monopile and ignoring the caisson. This situation applies to the jacket and the tripod solutions where 5m diameter suction caissons are possible foundations for each leg.


35.


In practice, however, it is likely that the top of the caisson will partially suppress the downward-descending vortex that develops around a monopile, but the extent to which that will occur with a 2.5m diameter column framing into a 5m caisson is uncertain. Yeow and Cheng (2003), for example, found that when the ratio of column diameter to that of the caisson was increased to more than around 0.25, then the scour tended in the limit towards that exhibited by the column acting alone as a monopile, with little influence from the suction caisson.


36.


Consequently, the scour case for the 2.5m diameter column framing into a 5m diameter suction caisson has been idealised as a process of scour around a 2.5m diameter monopile alone, due to waves and currents combined and currents acting alone.


37.


This assessment is described further in sections 7.3.3.4 and 7.3.3.5 for tripods and jackets respectively, with results summarised below.


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm


Appendix 7.3 Page 15


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