Axial vibrations are the main problem affecting the life time of a deep ocean mining riser. General geometrically non-linear analyses show that the vertical vibration of the riser is, in practice, not coupled with the lateral deflection and vibrations of the riser. Thus axial vibration can be analysed separately, which makes analyses much cheaper.
Rauma-Repola has developed an efficient micro computer program RDA (Riser Dynamic Analysis) to study the static and dynamic behaviour of the deep ocean mining risers. The program has been verified with comparison of the result8 with ANSYS FEM program results. RDA has proved to be reliable for design purposes and it is much more efficient than general purpose FEM programs.
In the design process of a deep ocean mining riser a lot of environmental and structural factors must be considered. The dynamic behaviour of a 6,000 m riser cannot be predicted analytically at sufficient accuracy. An efficient numerical design tool is necessary to study the effect of different sea states and compare different riser set ups.
A 6,000 m riser pipe must with stand heavy loads due to its own weight and dynamic stresses caused by waves and currents.
The static stress of a 6,000 m steel riser due to it?s own weight is up to 500 MPa. Unfortunately the longest resonance period of the 6,000 m riser system (about 6 seconds) is near to the natural heave period of a monohull vessel. Thus the axial stresses due longitudinal vibrations can be the most critical factor affecting the riser system design.
High bending stresses may occure locally if the rotations of the riser pipe are constrained. For example if the derrick is inclined 2 degrees it means a forced rotation of about 1 degree of the upper end of the riser pipe. This causes a bending stres8 of about 270 MPa if the upper end of the pipe is not locally strengthened.
Changes in horizontal movements happen quite slowly, i.e. it takes several minutes before the lower end of the riser system begins to move, when the vessel is started. In the same way the lower end continues its movement after the vessel stops or starts reversing.
Environment also makes the design process difficult in other ways: fatigue limit in seawater is only a fraction of that in an inert environment.
Ordinary loads and environmental aspects make it impossible to design a riser system able to withstand all weather conditions, so the riser system must be designed to include an emergency facility.
A huge research program is needed to assure the endurance of the 6,000 m riser system. This program can be divided into three parts:
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the development and verification of computer programs for the static and dynamic behaviour of the 6,000 m riser system
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the comparison procedure of different riser set ups
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material, coating endurance, added mass and drag force coefficient test
