ABSTRACT The paper deals with model tests as a means for research and development of deep sea mining vessels. Important data can also be collected for operation manuals concerned with precise specification of operations of ocean surface units. The paper reviews deep ocean mining vessels and operations and outlines the aspects which can be successfully dealt with in model tests in conventional wave basins. A range of special techniques are used to obtain the necessary information for the main set-up of the mining vessel in operation and the ore carrier in approach, attach, load, detach, and depart phases of the combined operations. It is discussed how operational conditions and survival conditions can be studied for the above objectives. The model test set-up for the purpose is complex since the many quantities and phenomena covered do not scale similarly. It is shown how servo-assisted free-model tests is the answer to the problems encountered in terms of model testing. The necessary servo units are discussed and dealt with in the appendices. Finally, the paper outlines a test programme and the ranges of test parameters and variables. INTRODUCTION Deep sea mining is a very young technology and very expensive to develop. Basic tests and pilot plants have been operated for some years to investigate possibilities and prepare the full-scale process when the law of the sea legislation and the mineral market conditions are right. The cost of an ocean mining installation for manganese nodules is about 700 million US $ and seems, under certain conditions of high nodule abundance and relatively easy recovery, to be economically feasible with a 20 years' operation in the mining sites. The nodule density of about 9 kg/m2 over a high priority area of 6,000,000 km2, will yield about 40 billion tons of dry nodules for recovery. The operation cost of such a mining system can be estimated to the order of 75 million $ per year exclusive of processing cost which might amount to 125 million per year, and exclusive of capital cost. The direct mining cost is thus 250,000 $ per day covering all operations at sea. Any attempt to improve efficiency and service operation quality or any attempt to prolong the possibility to operate in heavier weather is worth-while. Model tests are a highly qualified means of developing ocean structures, and many aspects of ocean mining can be solved in model tests. The vessels' motions and behaviour under environmental loads, steering and manoeuvring properties at the low speed of advance, carrying a 6 to 8 km long marine riser under the ship bottom, can be measured and analysed for design, development, and preparation of operation manuals. Films and video tapes can be prepared for instructions, marketing, and training purposes. Structural loads for design purposes concerned with mutual operation of the mining vessel and the ore carrier can be studied. Transfer of the recovered ore in dry or slurry form can be dealt with.