ABSTRACT If massive machinery and equipment constituting a plant could be prefabricated and assembled aboard a barge and towed on a river and/or sea to the plant site, the plant construction involved under various difficult conditions would become easy and the whole construction cost and period would be minimized. Under this concept, Ishikawajima-Harima Heavy Industries Co., Ltd. (IHI) developed the so-called IP System (Industrial Platform System), under which an industrial plant would be functionally built on a barge, or sometimes on several barges. In constructing a 750-T/D bleached kraft pulp plant for Jari Florestal e Agropecuaria, Ltd., Brazil, the IP System was practically and successfully employed and the plant went into operation in January, 1979, showing valuable technical achievements. From these technical achievements, the results of full scale measurements of the seakeeping performance of a plant barge in a seaway are introduced as they will be helpful to other plant construction projects to be carried out under a similar method. A 220 m long, 45 m wide and 14.5 m deep floating box-type plant barge was towed for about three months, covering a distance of 13;000 sea miles, from Japan to Brazil by way of the Cape of Good Hope. There was no precedent for such a large floating barge. During the towing, several things concerning motions and strength of the floating plant barge and wave heights were measured with full satisfaction. The authors conducted spectral and frequency distribution analyses, and obtained an auto power spectrum and a cumulative distribution on motions and strength. Based on these analytical methods, the measured values were compared with the calculated ones confirming the validity of their predictive calculation techniques currently in practice. For this purpose, the frequency response function in waves, the features of the short-term and the long-term probability distributions on motions and strength of the plant barge were investigated. All of the above results are continuously fed back for use in plant construction under the IP System. INTRODUCTION A unique construction system for industrial plants has recently been put to practice with great success. This concept has been realized in developing the Industrial Platform System under which a plant would be functionally built on a barge, or sometimes on several barges. In constructing the plant under this system, the plant barge was required to serve not only as a plant, but also as a barge with enough strength, stability and all other essential qualities ensuring its seakeeping performance during the period of towing to the plant site. In addition, the plants aboard the barge consisted of various machinery and equipment which are very sensitive to acceleration and inclination caused by the motions of the barges. Generally, the plant barge was a simple floating-box type and both its top view and profile were rectangular in shape. Their length-to-breadth ratio was small, and the draft was extremely small compared with ordinary vessels. For the purpose of formulating design standards for the plant barges, theoretical studies and experiments in the ship model basin were conducted scrupulously.