Estimation the remaining service-lifetime of wooden structure of geothermal cooling tower

•Wood has degraded until 2.5mm depth after used as cooling tower structural component for 17 years.•The density and allowable stress of used wood have been evaluated and compared to the NDS requirement.•Creep testing was conducted to measure the effect of long time loading.•The rate of degradation and longtime loading effect was used for estimating the wood remaining service-lifetime. Similar with other construction materials, wood strength is decreasing when applied by long term loading. Wooden cooling tower structure at Star Energy Geothermal (Wayang Windu) Ltd was built in 1998 and it should be evaluated to avoid sudden structural failure. Evaluation conducted through several steps: wood species identification, the physical and mechanical properties testing, and estimation for remaining service-lifetime by generating mathematical models derived from creep test and reduction of cross sectional area of the wood. Identification result that the wood are redwood (Sequoia sempervirens) and Douglas fir (Pseudotsuga menziesii). The wood density value has degraded from the surface until 0.25cm depth. Strength characteristics of the wood have considerably decreased, but the allowable stress for bending, tension parallel to grain, and shear were still higher than NDS2005 requirements. The allowable stress for compression parallel to grain was slightly lower than NDS, while compression perpendicular to grain was much lower. Average modulus of elasticity reduces become lower than the value stated by the code, but the minimum value of modulus of elasticity (Emin) of redwood was still higher than the code value, while Emin of Douglas fir is slightly lower. Then, in accordance with those findings, the construction would not failure yet but the deformation and vibration will occur in higher rate than design planning. This research develops mathematical models for estimating the remaining service-lifetime of the wooden cooling tower structure in geothermal power plant based on the wood performance in resisting long term loading and its deterioration rate. The deterioration rate of wood member of cooling tower structure at Star Energy Geothermal (Wayang Windu) Ltd is 0.0147cm depth per year, so equation for the residual service life estimation is σlaterσtoday=bh2(b−0.0147T)(h−0.0147T)2, and σlater must be lower than allowable stress.