Corrosion and formation of heavy deposit has been observed in firewater piping in carbon steel with stagnant seawater on several offshore platforms. This has resulted in plugging and constrictions in the pipes. Many offshore installations that have been operated for several decades in the North Sea are currently being sent for decommissioning. DNV has received pipe segment from some of these 20 to 40 years old installations for examination of the internal condition. The aim of this investigation is to gain knowledge based on operational experience in the North Sea. Two carbon steel pipes taken from the firewater ringmain on two offshore platforms from the Norwegian Continental Shelf (NCS) were examined. The objective of the investigation was to establish the internal condition, extent and nature of the deposits, determine mass of the deposit related to the observed metal loss and the dominating corrosion mechanism.