|Ketut Suastika||Department of Naval Architecture, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Sahlan||Indonesian Hydrodynamics Laboratory (IHL), BPPT, Kompleks Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Wibowo H. Nugroho||Indonesian Hydrodynamics Laboratory (IHL), BPPT, Kompleks Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Achmad Zubaydi||Department of Naval Architecture, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Mohammad N. Misbah||Department of Naval Architecture, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
|Murdjito||Department of Ocean Engineering, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember (ITS), Kampus ITS Sukolilo, Surabaya 60111, Indonesia|
Waste steel from used ship propeller shafts is reused for the keel structures of InaTEWS buoys. Because of the application of waste material, fatigue life assessment is critical. The purpose of this study is to assess the mechanical and fatigue properties of the waste material and to estimate the fatigue life of the keel structure as a result of sea wave loading. Material tests, fatigue tests and model tests were performed to obtain the parameters required for the estimation of the fatigue life, together with application of the spectral analysis method, including the effects of spectral band width. Chemical and tensile tests identified the material as low-carbon steel, with mechanical properties comparable to AISI 1035 steel. The fatigue tests resulted in an S-N curve (NSm = K) with m = 7.7 and K = 3.2×1024, showing a lower fatigue strength than AISI 1035 steel. The observed reduction in fatigue strength is ascribed to the previous use of the shaft. The calculated fatigue life based on the experimental S-N curve and the observed in-situ wave data is approximately 9.5 years, with a safety factor of 5.
Fatigue life; Keel structure; Spectral analysis; Tsunami buoy; Waste steel
The Indonesia tsunami early warning system (InaTEWS) utilizes an array of surface buoys, some of which are installed and operated off the west coast of Sumatra and others off the south coast of Java (Nugroho & Sahlan, 2008; Yustiawan et al., 2013; BMKG, 2019). These stretches of Indonesian water are prone to tsunami events. For example, the Aceh tsunami of 2004 hit the west coast of northern Sumatra; the Mentawai tsunamis of 2004 and 2010 affected the Mentawai Islands off the west coast of West Sumatra; while the Banten and Lampung tsunami of 2018 hit the coast of South Lampung in southern Sumatra and the coast of Serang, Banten, in western Java (the Sunda Strait).
Such tsunami events result in loss of infrastructure, goods and lives. For example, in the 2004 Aceh tsunami, 300,000 lives were lost (Couldrey & Morris, 2005), while in the 2010 Mentawai tsunami, more than 20 villages were hit, displacing more than 20,000 people and affecting about 4,000 households (BBC News Online, 2010). The above examples underscore the importance of such a tsunami early warning system, particularly for saving lives.
The full concept of the InaTEWS is described in BMKG (2019), while in this study only the buoy system is considered. An ocean bottom unit (OBU) records the changes in water pressure due to seismic movements prior to a tsunami event and sends the readings to the surface buoy via an acoustic signal. The recorded signal is then transmitted to a ground station by a satellite for early warning and/or further analyses.
BTI-G2 buoys are some of the surface buoys utilized in the InaTEWS. They were developed through a series of development stages and designed for application in deep water. In contrast, the earlier BTI-G1 type buoys, which were modified versions of the so-called ex-seawatch BPPT buoys, were designed and applied for environmental data collection in relatively shallow water (the Java Sea). The principal dimensions of the BTI-G2 buoys are as follows: diameter D = 2.50 m, height H = 1.68 m and volume of displacement V = 2.45 m3 at draught T = 0.50 m. Figure 1 shows a launch test of a BTI-G2 buoy from an Indonesian Baruna Jaya BPPT research vessel.
Each surface buoy is held in position by the use of a single-taut mooring line, which is connected to the buoy hull using a keel structure (see Figure 1). Different configurations for this structure have been designed and manufactured during the development of the buoys. The one investigated in this study is shown in Figure 2 (Nugroho & Sahlan, 2008; Sahlan, 2011). The keel structure was manufactured from a used ship propeller shaft, which had been in service for approximately 20 years, and was considered as waste material.
Figure 1 Launch test of a BTI-G2 buoy from an Indonesian Baruna Jaya BPPT research vessel
Reuse and minimization of waste (Kusrini et al., 2018) through effective collaboration between stakeholders (Nursin et al., 2018) are key issues in sustainable development. Furthermore, waste materials such as used propeller shafts are traded freely in Indonesia and can be obtained easily at a relatively low price. There are therefore two main reasons for reusing such waste steel, namely: environmental sustainability and cost saving.
The use of waste material makes the assessment of the fatigue life of the buoy keel structure critical. Furthermore, fatigue life assessment is important for marine structures due to the cyclic nature of wave loading (ABS, 2005; DNV, 2016). The purpose of this study is to assess the mechanical and fatigue properties of the waste material and to estimate the fatigue life of the keel structure due to sea wave loading.
Figure 2 Front and side views of the keel structure investigated in this study (unit in mm) (Nugroho & Sahlan, 2008; Sahlan, 2011)
Waste steel from a used ship propeller shaft was reused as the keel structure for InaTEWS buoys. Material and fatigue tests show that the static mechanical properties of the material are comparable to AISI 1035 steel, but that its cyclic fatigue strength is lower. Its fatigue strength lies between those for AISI 1006 and 1020 steels. The observed reduction in fatigue strength is ascribed to the previous use of the shaft. The fatigue life of the keel structure, calculated using a spectral analysis method, including effects of the spectral band width, and based on the experimental S-N curve together with the observed in-situ wave data, is approximately 47 years without a safety factor. Including a safety factor of 5, as recommended by ABS (2003), results in a fatigue life of roughly 9.5 years. The spectral analysis method is straightforward and much faster than a typical time-domain fatigue life calculation based on the stress time series. The uncertainty of the result obtained from the method depends on the uncertainties in the experimentally determined S-N curve, the assumed spectra representing the sea states, and the calculation models used. A reliability study (eg. Khan & Ahmad, 2014) is recommended to quantify the above-mentioned uncertainties.
The authors thank Baharuddin Ali, M.Eng. at the Indonesian Hydrodynamics Laboratory (IHL), BPPT, Surabaya, Indonesia for the useful discussions on the analysis methods of model test data and interpretations of the results. This research project was partly supported by the Ministry of Research, Technology and Higher Education of the Republic of Indonesia under grants no. 016452.28/IT2.11/PN.08/2014 and 003246.171/IT2.11/PN.08/2015.
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