The present invention relates to a deep sea mining method, the method comprising the steps of: generating an upward flow of material excavated from the bottom of the water body to a material processing platform in a riser line, wherein the processing platform is divided into a useful material part and a non-useful material part. Treating the material, generating a return flow in the return pipe to move the mixture of seawater and non-useful parts of the material from the treatment platform to the bottom of the water body, preventing collapse of the return pipe and/or cavitation of the return pipe. So as to, in particular, control the pressure in the return pipe to prevent collapse or cavitation over the return pipe.
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