Abstract Environmental loads on offshore platforms may induce significant distortion of the deck structure. This distortion of the deck structure, resulting in differential vertical deflections of the module support points, causes module reactions to be redistributed. Wave loads will cause the module reactions to fluctuate. These effects have significance for both the module structures and the deck structure. The increase in module reactions may cause local overstressing (these increases will normally only be quantified when the design is well advanced) and the fluctuating reactions caused by wave loading will shorten the fatigue life of the deck structure and the modules. The problems created by this effect on Shell Expro's Brent C platform are outlined and a method of analysis is presented for calculating the redistribution of module reactions caused by environmental loads. Introduction Distribution of topside loading is dependent upon the relative stiffness of the modules and the deck structure. To quantify exactly the module reactions it is necessary to consider the module/deck structure interaction. The exact nature of the module/deck structure interaction, being dependent on the framing and strength of these structures, is not known until the design is well advanced. In addition to this incompatibility the problem is made more complex by the fact that the problem is made more complex by the fact that the modules and the deck structure are usually designed by different consultants. Some assumption must be made as to the distribution of the static vertical loads for design to commence. A convenient assumption is that the module supports are coplanar. The modules and the deck structure can then be designed for module reactions corresponding to this assumption. When design of the deck structure has advanced, deflections of the deck structure at the module support points may be computed. From these deflections, the vertical deflections of the module supports are found. With this information and with data obtained by on-site surveying, the module support points may be shimmed so that each module is supported on coplanar supports, this being the initial assumption. This approach to interaction solves the problem of distribution of vertical static loads and the procedure outlined is a convenient one for management of design. The interaction of modules and deck structures for environmental loading, however, is considerably more complex. This problem was encountered by the authors during design of Shell Expro's Brent C platform. platform. When design and fabrication of both the modules and the module support frame were well advanced, the authors were instructed by Shell Expro to quantify the effects of environmental loading on the module reactions taking into account interaction. At that time, the authors could find no literature on this subject. (Subsequently this subject has received attention.) The method of analysis which is presented below was developed in the course of the work. As a result of the work changes were made to the modules and the module support frame so as to reduce the fluctuating reactions to an acceptable magnitude. Shell Expro's experience on this matter has influenced their specifications regarding the support of modules by deck structures. It is the authors' opinion that interactive forces between modules and the deck structure should be considered during the conceptual design phase. METHOD OF ANALYSIS The following basic data must be developed for execution of the analysis:flexibility matrix of the deck structure;stiffness matrices of the modules;deck structure deflection data for the loadcase(s) under consideration. P. 207