Floating wind-energy projects could open up vast areas of the world’s oceans to produce carbon-free power. But developers must first solve two key technical problems, according to France’s electric-grid operator.

Sea swell can cause vibrations that harm floating-substation equipment, while cables can be damaged by a build-up of shells and seaweed, Reseau de Transport d’Electricite (RTE) said. These issues must be tackled before floating wind can succeed on a large scale and RTE is among operators trying to fix them.

This nascent segment of the clean-power industry would significantly expand the potential areas of the sea where energy producers could install turbines. In many places, the best wind resources are in waters that are too deep to build a structure from the seabed. But the new floating technology is expensive and at risk of damage from waves and sealife...