Fatigue Life Assessment of Waste Steel Reused as Tsunami Buoy Keel Structures: A Case Study
Corresponding email: ksuastika69@gmail.com
Published at : 29 Jul 2019
Volume : IJtech
Vol 10, No 4 (2019)
DOI : https://doi.org/10.14716/ijtech.v10i4.501
Suastika, K., Sahlan., Nugroho, W.H., Zubaydi, A., Misbah, M.N., Murdjito., 2019. Fatigue Life Assessment of Waste Steel Reused as Tsunami Buoy Keel Structures: A Case Study. International Journal of Technology. Volume 10(4), pp. 700-709
| 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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