Field development in deep water often require floaters to be moored in close proximity, e.g. tender or offset drilling from spars, Dry Tree Units operating in conjunction with FPSOs. Mooring line interference is a key issue which needs to be considered in the design of the systems. This paper provides a study of this for a cell spar and semi-submersible operating in close proximity. Several nominal separation distances between the vessels were considered in the study. This paper presents an example for an initial separation distance of 530 ft (10% of the water depth). A time domain analysis of multi-vessel coupled responses and mooring line motion was performed to determine the potential of mooring line interference for a 100 year Hurricane and Loop Current condition. Results show that the 100 year Hurricane case would dominate for the mooring line interference for this case, and the interference for very close coupling might be avoided by pre-planning. In the worst case buoyancy might be added to the semi-submersible mooring to eliminate interference.

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