A UK ship travelling from a UK port to an installation in the UK sector (of the North Sea) would not be travelling to a port, so this is not an international voyage. Even if the installation was in a foreign sector of the North Sea, it would still not be a port. The word port is not defined in SOLAS Chapter 1, however it is considered that it would be stretching the natural meaning too far to make it include an offshore installation.
Yacht management
Ensure that your vessel is taken care of from A-Z