Based on the relay protection design of “Blue Whale” ultra-deep water semi-submersible drilling platform, the design proposal of the relay protection for short circuit fault is put forward in order to achieve the safety level of class requirement in closed bus-ties mode of operation. The design of main and back-up relay protection is considered for short circuit fault and the upstream and downstream over-current protection of the power system needs to be selective to realize the sequence of over-current protection tripping from lower to superior circuit breaker.