Vertical slender cylindrical structural members such as risers or tendons are commonly clustered on deep water offshore platforms. Current analysis methods treat each individual riser or tendon as being single and assume these structural elements to be transparent to the incident wave and current flow. This study utilizes both numerical and experimental methods to investigate the basic interaction phenomena with the intent of quantifying this behavior in order to eventually provide some guidance for design of realistic configurations. The limits of current design methods are evaluated to determine the significance of interaction. A single cylindrical structural member is subjected to regular and random wave loading and is used in both the numerical and experimental studies to provide a reference for multiple cylinder configurations. The experiments are performed to systematically analyze the response of multiple cylinder configurations and to analyze several TLP-like configurations. A probabilistic finite element approach is developed for the purpose of characterizing the uncertainty in the formulated methods.