Suction Zone |
 
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Suction Zone |
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The suction or restraining force experienced by a riser element in a “suction zone” just above the mudline on an elastic seabed is modelled with a linear spring resistance, similar to that provided against downward vertical motion by the elastic seabed itself. The suction force is of course directed downwards whereas the seabed resistance is vertically upwards. The inputs for this facility are suction “spring stiffness” per unit length, and the “suction zone extent” which is defined in terms of the elevation above the mudline at which suction forces disappear. The suction force varies linearly between zero at the mudline and the maximum force experienced at the top of the suction zone. Once an element clears the suction zone the suction force returns to zero. One important point about this facility is that suction forces are applied only to elements moving from the mudline into the suction zone. An element moving into the suction zone from above does not experience any force, nor does an element moving upwards through the suction zone unless it has impacted the mudline before ascending. Finally, the specification of suction forces is optional, and by default no suction effects are included in an analysis.
•*SEABED PROPERTIES is used to specify properties such as seabed type (i.e. rigid or elastic) and friction parameters. Specifically, the SUCTION= input is used to specify suction zone parameters such as suction stiffness and suction zone extent.