<p>Building the technologies to commercialize the mining of seafloor polymetallic sulfides which is promising mineral resource is necessary. Based on the both the Basic Plan on Ocean Policy (approved by the cabinet on April 26, 2013) and the Plan for the Development of Marine Energy and Mineral Resources (formulated by Ministry of Economy, Trade and Industry (METI)), METI and JOGMEC (Japan Oil, Gas and Metals National Corporation) started the national seafloor polymetallic sulfides developing project and planed to demonstrate the pilot test in actual sea to validate the technologies for especially excavating and ore lifting, which is the first test in the world. Usually testing in actual sea is not safety and economical because the marine condition is severe for the operation. Therefore three years studies to ensure both safe and availability were conducted which led the pilot test to succeed. This paper shows our procedure of the availability analysis for the pilot test with the results of numerical simulation, model experiments and operational window analysis. To investigate the accuracy of the numerical simulation and confirm the effectiveness of the operational window analysis, the comparison with the measurements in the actual sea is also shown.</p>