The title of the thesis - "Offshore Rankine Cycles" - is very general and cover a large range of engineering fields, e.g. thermodynamic cycles (Rankine, ORC, Brayton, Kalina, etc.), mechanical equipment (gas/steam turbine, heat exchangers and additional equipment) and safety concerns (flammable and/or toxic fluids, high temperature and pressures), to name the most important.The thesis try to give a brief overview of all critical points and alternatives, concerning employment of a waste heat recovery machine on offshore facilities, although focus has been on three more specified cases, namely:1. Comparison of a steam cycle vs. an organic Rankine cycle for high temperature operating conditions.2. Study of heat exchanger parameters on total cycle performance.3. Investigation of a modular expander setup versus a single expander.To compare a steam cycle to an organic cycle, a choice of working fluid for the organic cycle had to be made. After some investigation, toluene was chosen as it is a "common" fluid with known properties and was found to be a viable option for high temperature heat sources, both for subcritical and supercritical operation. Due to water being constricted to subcritical operation a CO2 cycle was implemented as a comparison to the supercritical toluene cycle. The main focus of the comparison was exergy losses during heat transfer and power output.The heat exchanger parameter study was conducted with a printed circuit heat exchanger as an example. The study of overall cycle performance has close connections to the heat exchanger size, since it is an important parameter concerning offshore employment due to costly "footprint". The cycle's dependency on the heat exchanger is mainly by the heat transfer rate, or heat load, which the heat exchanger applies to the cycle. The heat transfer rate is given by the heat exchanger`s ability to reduce the temperature of the exhaust gases. This ability depends on the two fluids involved and the geometry of the heat exchanger. While the choice in working fluid and pinch points sets the amount of heat transferred, the remaining analysis rest on the overall heat transfer coefficient (UA) to balance the heat load. When fluid properties are determined, the UA - value is again dependent on heat exchanger geometry and further variation of these parameters will in turn reveal the size of the heat exchanger. When imposing a working fluid to the cold side of the heat exchanger an optimization in heat exchanger volume could be found at specified heat load.A VBA macro has been made where expander parameters (rated power and efficiency vs. volumetric flow rate values) could be used as inputs to calculate the power output of two expanders in a modular setup relative to a single expander as reference.