A METHOD FOR HULL SCANTLINGS CALCULATIONS FOR SHIPS SAILING IN LOW SALINITY WATERS

摘要

In IACS UR I "Polar Class Requirements", which has been introduced in the Rules of all Classification Societies participating in IACS, there is no special consideration for the navigation in low salinity waters where the strength of ice is increased. In general the salinity of the sea is (31-39‰) but there are areas with lower salinity, like the Caspian Sea (salinity 0-14 ‰) and rivers (around 0‰). The Caspian Sea is an area rich in oil and Bureau Veritas has recently classified several Offshore Service Vessels for operation in this area. In this paper we investigate the influence of low salinity sea water on ice strength and consequently on the hull scantlings of icebreaking ships. We consider that the sea water salinity in low-salinity areas, which we call brackish waters, is the % of the normal salinity of the sea. The ice which is formed in low salinity areas is a low salinity ice. This makes the ice stronger and affects the force applied on icebreaking ships and consequently their hull scantlings. In IACS UR I, two ways of ice failure are adopted: flexural failure and crushing failure. The low salinity of the ice increases its flexural strength while the crushing strength is almost not affected. The influence of the low salinity on the flexural strength of the ice is calculated by the introduction of a new flexural failure coefficient C_f. The new increased values of this coefficient affect the hull scantlings, but only at the bow area, since according to IACS URI, only at bow we have flexural failure of the ice, while in non-bow areas we have crushing failure. This new coefficient Cf has been introduced in Bureau Veritas Rules and can be used for the hull scantlings calculations of ships sailing in low salinity waters.