Process water is all water that is used in oil processing on a platform
Crude oil can contain salt that must be removed before the oil is sent to refinery. Fresh water can be used to extract the salt from the oil.
Salt in crude oil is dissolved in the water in the crude oil, not in the crude oil itself. The desalting the oil is made by mixing fresh water in the crude oil. The fresh water will dilute the salt concentration in the oil. The water will then be separated using e.g. a gravimetric separator.
The salts that are most frequently present in crude oil are calcium, sodium and magnesium chlorides. If these compounds are not removed from the oil several problems arise in the refining process.