This thesis is a collection of articles with an introduction which motivates the present work and relates the articles to each other. The main body of this thesis consist of six journal papers, which at the time of writing,

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In many future subsea projects, there will be a requirement to cool various fluid streams, either multi-phase or single phase. To meet this need, FMC Kongsberg Subsea AS (FMC) has undertaken a project to develop a practical and robust subsea cooler. The cooler is passive in that heat is transferred to the surrounding sea water by natural convection only. Because of the subsea application, the cooler must have a special geometry to meet requirements for modularization and easy installation/removal. The passive nature of the cooler means that the flow rate of the seawater coolant is not an independent variable, but is directly linked to the cooler geometry. Developing a design method for such coolers requires detailed knowledge of the important heat transfer parameters, to an accuracy far in excess of that normally required for industrial cooler design. This problem has been approached on several levels, including an extensive literature search, theoretical studies, and model testing. One of the first observations was that little research had been done previously on this type of cooler. Much information is available for various pieces of the problem, but it became clear that designing the cooler would require significant development work. Based on the knowledge gained during the initial theoretical studies, a theory for calculating cooler performance presuming one dimensional external coolant flow has been developed. While it is clear that the actual external flow is three-dimensional, the simplified theory gives important insights into how the various design parameters affect cooler performance. To fill in the gaps in theoretical knowledge, a series of model tests designed to quantify internal and external heat transfer coefficients for the special geometry is being proposed. The testing program covers several technical areas and has required the utilization of a number of advanced measurement techniques. For the next phase of the testing program, a complete new test facility has been constructed capable of testing coolers with cross-flows typical of ocean bottom currents. The cooler development program has provided new technology which will be used to construct robust and compact subsea coolers. Because of the emphasis on basic research, fundamental knowledge and insight of the heat transfer mechanisms governing the performance of this type of cooler are acquired. This knowledge gives FMC the capability to design and manufacture subsea coolers which are custom-made to match the exact requirements of a given application.