Fatigue Life Prediction of Mooring Line on Indonesian Tsunami Early Warning Systems (Ina-TEWS) Buoy

Abstract

Ina-Buoy Gen 3.1 is one of the application of the program that has been developed by BPPT (Agency for the Assessment and Application of Technology) in order to reduce the risk of a tsunami disaster in Indonesia. This Indonesian Buoy for Tsunami Early Warning System (INATEWS - Buoy) consists of a tsunami buoy, an ocean bottom unit (OBU), a satellite and a ground station. This paper discuss about calculation of fatigue life due to wave loads for the mooring line that used to hold the buoy position in the sea. A taut mooring system is applied to the buoy until sinker at seabed. Calculation of operational tension at mooring line obtained using numerical simulation fully licensed software Orcaflex ver 11 owned by Centre Technology for Maritime Industrial Engineering (PTRIM), BPPT with wave input of Hs=2.5m and Tp=10.5sec. The depth of sea is varied at 2088m (Sunda Strait), 3563m (Cilacap Sea) and 4082m (Bali Strait). By knowing those tension forces, stress at time history on the mooring line can be obtained. This stress time history is then applied to Palmgren - Miner formula that based on SN-Curve of the mooring line material to predict the life of the mooring line. By applying a safety factor of 2.5 it is found that the fatigue life of the mooring line at water depth of 2088m in Sunda Strait is 6,9 years, then, at water depth of 3563 m in Cilacap Sea is 15,8 years, finally at water depth of 4082 m in Bali Strait is 10 years.