Use of a scrapped ship as a floating breakwater for shore protection

Use of a scrapped ship as a floating breakwater for shore protection

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Use of a scrapped ship as a floating breakwater for shore protection A. Fernández Lázaro†, R.M. Gutiérrez Serret‡, V. Negro∞ and J.S. López-Gutiérrez§ † Laboratorio de Experimentación Marítima Centro de Estudios de Puertos y Costas CEDEX Antonio López, 81. Madrid, 28026, Spain [email protected]

‡ Laboratorio de Experimentación Marítima Centro de Estudios de Puertos y Costas CEDEX Antonio López, 81. Madrid, 28026, Spain [email protected]

∞ Research Group on Marine, Coastal and Port Environment and other Sensitive Areas Universidad Politécnica de Madrid, Profesor Aranguren, s/n. 28040, Spain

www.cerf-jcr.org

[email protected]

§ Research Group on Marine, Coastal and Port Environment and other Sensitive Areas Universidad Politécnica de Madrid, Profesor Aranguren, s/n. 28040, Spain [email protected]

ABSTRACT

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Fernández Lázaro, A., Gutiérrez Serret R.M., Negro, V. and López-Gutiérrez, J.S., 2012. Use of a scrapped ship as a floating breakwater for shore protection. Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 225-230, ISSN 0749-0208. The purpose of this research is to assess the effectiveness of a ship used as a detached floating breakwater for coastal protection and forming salients of sand or tombolos. Floating breakwaters have been extensively used as port or coastal protection structures and display advantages in terms of construction and ecology, amongst others. However, the greatest problem these structures present is the limited range of wave heights and periods for which they are really effective. Furthermore, ships may be considered as floating structures which, used as breakwaters, would keep the advantages of floating breakwaters and would increase their range of applicability. The possibility of using ships at the conclusion of their useful life for this purpose would also involve greater economic and environmental advantages. Tests were carried out to assess the ship’s effectiveness as a detached floating breakwater using a scaled down physical model to determine the vessel’s transmission coefficient (Kt) as to regular waves with significant periods of 5 sec to 12 sec and significant wave heights of 1.5 m to 4 m at depths from 20 m to 35 m. The ship proves effective for waves up to 4 m significant height and significant periods up to 9 sec. Hanson and Kraus and Pilarzyk’s analytical models, which take transmission coefficients into account, were used to analyse the shore’s response to the breakwater protection. The results obtained show that salients form for waves with periods between 6 sec and 9 sec. It is also concluded that the depths tested are far different from the more usual shallow water involved in constructing detached breakwaters and the shore’s response is therefore scarce. ADDITIONAL INDEX WORDS: Physical model tests, transmission coefficient, salient, shore’s response, scrapped ship.

INTRODUCTION The use of floating breakwaters as protection structures in marine engineering involves economic, ecological, construction, functional and safety advantages compared with other conventional solutions secured to the sea bed. However, this type of structure also displays problems which may be summarised in that most of them offer less resistance to wave height and period than sloping or vertical solutions. In addition, the types accepting greater waves such as floating concrete caisson breakwaters, constitute a greater challenge in technical, construction, transport and price terms. Furthermore, the floating structure par excellence is the ship. Ships are bodies designed to float and sail, with centuries of development and improvement in construction and operation behind them which, initially, may provide certain advantages over the usual floating structures. Their different types and dimensions ____________________ DOI: 10.2112/SI65-039.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013

and their mobility make this option a versatile solution, easy to install and dismantle for temporary breakwaters. Ships used for this function may be temporarily used vessels in good order or ships that have reached the end of their useful life, are recycled and altered to serve this purpose. Both possibilities would involve environmental and economic advantages compared with building new infrastructures. A possible application of a ship as a floating breakwater structure is its use as a detached floating breakwater for protecting a stretch of shoreline. A detached breakwater is an off-shore marine construction, isolated and noticeably parallel to the coast constructed a certain distance from the shore to protect a certain area of shoreline by reducing the amount of energy penetrating therein. The reduction in wave action the detached breakwater achieves causes materials in the area sheltered to sediment giving rise to salients forming. If a salient develops until it reaches the detached breakwater it is called a tombolo. The purpose of the investigation described in this paper is to assess how a ship performs as a detached breakwater for coastal`

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protection and the forming of salients or tombolos. The work was undertaken in the following phases: 1. First, a review was carried out on the most usual types of floating breakwaters and their effectiveness, expressed in terms of their transmission coefficients (Kt). 2. Tests were then performed on a reduced scale physical model in order to ascertain the vessel’s effectiveness working as a floating breakwater. The transmission coefficients and range of waves were compared with the most usual floating breakwaters. 3. Two analytical models were then used for designing detached breakwaters in order to determine the shoreline’s response to this alternative for its protection and to assess the possible forming of salients or tombolos. The models used were Hanson and Kraus (1990) and Pylarzyk (2003), due to their both considering the influence of the structure’s transmission coefficient. 4. Finally, the discussion on the obtained results, the proposal as to future lines of work and a list of references cited are included.

FLOATING BREAKWATERS: MOST USUAL TYPES AND EFFECTIVENESS The main function of a floating breakwater is to attenuate wave action. However, the structure cannot completely dissipate the incident wave which is partially transmitted, reflected and dissipated. In general, investigations concentrate on the structure’s effectiveness against wave action, studying its capability of dissipating their energy by defining the transmission coefficient by the quotient between the height of the wave transmitted and the height of the incident wave: Kt=Ht/Hi. Other concepts under investigation are the reflection coefficient, stresses in the anchoring and mooring lines and stresses in the elements joining the different modules in the case of modular solutions. The following main types of floating breakwaters have been synthesised following Hales (1981), McCartney (1985) and Mani (1991):  Box or pontoon type floating breakwater  Sloping pontoon type floating breakwater  Floating type breakwater  Floating frames  Anchored float breakwaters  Porous wall floating breakwaters  Floating concrete caisson breakwaters. The most relevant investigations in this field are summarised

hereafter and a range of satisfactory operation is established for each one. The general criterion for setting this range was the fixing of maximum thresholds for a 0.4 transmission coefficient, i.e., the wave heights and periods for which the structures tested dissipated at least 60% of wave energy. Ofuya (1968), Carver (1979) and, more recently, Koutandos (2005), Martinelli (2008), Dong (2008) and Peña (2011) have developed various experiments to test different pontoon type floating breakwaters. The general conclusions drawn of this type is that it works satisfactorily with wave heights around 1.5 m and periods around 5 sec. The sloping pontoon floating breakwater and some of its variations have been studied by Raichlen (1978), Bayram (1998) and Heng (2006), all with tests on a physical model. It works well with wave heights up to 2 m and periods of 5 sec. Another type uses old tyres, as studied by Kamel and Davidson (1968), Giles and Sorensen (1978) and Harms (1978) on different physical models, valid for waves 1 m in height and 5 sec period. Floating frames were studied on a physical model by Jackson (1964) and Ofuya (1968) and satisfactory results were obtained for waves of 0.7 m and 4 sec. Floating breakwaters formed by sets of floats anchored to the sea bottom were tested by Seymour and Isaacs (1974) and obtained effective results for waves of 0.5 m and 4 sec period. Floating porous wall breakwaters were tested by Richey and Sollit (1969) and, more recently, by Wang (2010), and good results were obtained for waves up to 2 m in height and periods of 7 sec. Finally, the Monaco breakwater’s construction (Peset et al., 2002), a floating breakwater formed by a concrete caisson of 352 m overall length secured to land by a hinge at one end and to the sea bed by chains and piles at the other is to be mentioned. The design wave was 6.2 m significant height and 11 sec peak period. The test method was heterogeneous, when not lacking information, amongst the investigations reviewed, making it difficult to draw homogeneous criteria from which to draw reliable conclusions. Thus, for example, Koutandos (2005) and Martinelli (2008) use significant wave heights and peak periods; Wang (2010) and Peña (2011) only specify that they work with regular waves and, in others of the investigations mentioned, it was not possible to ascertain what type of wave height and period were used in undertaking them. Despite these difficulties the conclusion may be drawn that, except for the case of the Monaco caisson, the types of floating breakwater investigated are effective for attenuating waves with a low height, around 2 m, and short periods, around 5 sec. The next step in this investigation is, therefore, to assess the transmission

Figure 1. Diagram of the physical model test set up for assessing the ship’s behaviour as a floating breakwater



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coefficient of a ship working as a floating breakwater to compare it with the types tested up to now and determine its usefulness as coastal protection.

TEST ON A PHYSICAL MODEL OF THE PERFORMANCE OF A SHIP AS A FLOATING BREAKWATER Description of the test The test was carried out in the facilities of the Centro de Estudios de Puertos y Costas (CEPYC) – Centre for Ports and Coasts Studies - of the CEDEX in Madrid, Spain, in a wave flume 30 m long and 3 m wide, on a 1/150 scale. Froude’s laws of similarity were used to establish the equivalence between model and prototype. Figure 1 shows a diagram of the test set up. At one of its ends, the flume has a hydraulically driven translational motion wave generating paddle. At the other end, a gravel beach was built as a wave anti-reflection device inside the flume. The flume’s bottom is flat over the first 10.5 m. As from this point, it slightly slopes to take the gradual closeness of the coast into account. Five probes were positioned to measure the waves. They were located approximately 5 m from the generating paddle. The fourth, located in the ramp area, was 1.67 m from the ship. These four probes measured the incident waves. The fifth, for measuring the waves transmitted, was located 1.33 m behind the ship. Ships with a shorter useful lifetime and, therefore, most liable to be recycled for this use, are bulk carriers and containers (Environment and sustainable development group of the COIN and the AINE, 2008). This is why the ship chosen was a container carrier of 2400 TEUs, 275.5 m overall length, 32.26 m beam, 23.90 m depth and 10.97 m draught under full load condition, all prototype dimensions. Therefore, model dimensions are 1.84 m length, 0.21 m beam, 0.16 m depth and 0.07 m draught, resulting in a 61% waterline flume blockage and a relative depth of 55% for 20 m depth and 31% for 35 m depth. The distance from the coast is about 500 m for 20 m depth and about 600 m for 35 m depth, prototype dimensions, hence about 3.5 m and 4 m model dimensions. The ship was subjected to waves of 1.5 m to 4 m significant wave heights and significant periods from 5 sec to 12 sec. Two different depths were tested, 20 m and 35 m, both of which will situate the ship foreseeably in transition waters, and an approach in shallower waters was left to a second phase of the investigation.

Figure 3. Ratio between transmission coefficient and significant period at 20 m depth An anchoring system was reproduced by means of a chain and anchor with four chains, two on the exposed side of the ship, 3 m long in the model and two on the land side, 1.5 m long in the model. The chain lengths and weights were established following the recommendations of Bureau Veritas (Bureau Veritas, 2000). The signals picked up by the probes were processed by conditioners that amplify and filter the signal and were transformed from an analogical to a digital format by a converter card. The data so collected were statistically processed with CEPYC developed software. Data were taken during 120 sec at the end of which the ship’s transmission coefficient (Kt) was evaluated. This coefficient is obtained by dividing the significant wave height measured by probe no.5 by the significant incident wave height, which is obtained by calibration prior to the test at probes nos. 1, 2 and 3. Figure 2 shows a photograph taken whilst the test was running.

Results obtained Figure 3 shows the results of the transmission coefficient recorded in terms of the significant incident wave height for the different significant wave heights studied at a depth of 20 m. The good manner in which the structure performed can be observed up to wave heights of 4 m and periods of 8 sec. and, therefore, the graph has been extended between the 4 sec and 9 sec periods. Transmission coefficient figures of up to 0.10 are seen (i.e., 90% of the energy is dissipated by the ship) for waves 2 m in height and with a 5 sec period and transmission coefficient figures of 0.30 for waves 4 m in height with a 8 sec period, i.e., reductions of up to 70% of the incident waves. Results ostensibly worsen as from the period of 9 sec. The worst result is obtained for the 11 sec period for which figure the ship’s movement is coupled to the waves and generates a wave of even greater amplitude than the Figure 2. Photograph during the test run


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incident wave. With a 9 sec period, only 30% of waves is dissipated whilst with 10 sec and 12 sec only 20% is dissipated. Figure 4 shows the results of the transmission coefficient recorded in terms of the significant incident wave period for the different significant wave heights studied at a depth of 35 m. In this case, the ship worked well up to 8 sec periods although the percentage of energy it could dissipate was slightly lower: from 90% in the best case with 5 sec periods, up to 60% with 8 sec periods. At 35 m depth, the ship’s movement coupled to regular waves in periods of 9 sec, and transmitted waves higher than incident waves were recorded whilst as from 10 sec period the energy transmitted was practically 100%. The conclusion may be drawn that this solution is effective for waves up to 4 m significant height and 8 sec significant period with between 90% and 60% effectiveness at a depth of 20 m and between 90% and 55% at a depth of 35 m. The ship’s effectiveness reduces at 20 m depth to around 20% above the 8 sec of significant period and at a depth of 35 m wave energy does not practically reduce above the 8 sec of significant period. The results represent significant improvement in the range of waves for which conventional floating breakwaters prove effective.

RESPONSE OF THE COAST PROTECTED BY A SHIP AS A DETACHED BREAKWATER A detached breakwater is a structure built some distance from the shore and generally parallel thereto in order to protect it from wave action by reducing its energy in the sheltered area. This reduction in wave energy causes an alteration in sediment transport and sediment accumulates and deposits behind the breakwater. If sufficient material is deposited, a sand salient may form and develop until reaching the detached breakwater itself

giving rise to a formation which is called tombolo. Numerous analytical models related to detached breakwater design are in existence and are used to either predict and define the response induced on the coast by a breakwater or system of detached breakwaters, or to calculate the geometrical characteristics of the works to be designed starting from the effect it is desired to achieve on the shoreline. In the case in question, two analytical models which use, amongst other parameters, the detached structure’s transmission coefficient (Kt) were chosen.

Hanson and Kraus (1990) Based on results of simulations of the shoreline’s evolution with a numerical model and on data from some detached breakwaters, Hanson and Kraus proposed the following model in 1990 to classify the shore’s response behind a detached breakwater:

H X  48  (1  K t )  0  salient L D H X  11  (1  K t )  0  tombolo L D where Kt is the transmission coefficient, X the structure’s length, L the incident wave length, H0 the significant wave height in deep water and D the depth at which the breakwater is located. By introducing the values each parameter takes in the tests carried out into these expressions, the shoreline’s behaviour as to the protection provided by the ship can be predicted. Table 1 gives the results obtained in the tests at a depth of 20 m.

Pilarzyk (2003) In 2003, in order to take the effect of wave transmission into account, Pilarzyk proposed that the factor (1-Kt) be considered in the elemental geometric formulas proposed by other authors to classify the type of shoreline response in terms of the length of the structure and distance from the shore:

Ls 1.0 a 1.5   tombolo X (1  K t ) Ls 1.0   salient X (1  K t ) where Ls is the length of the detached breakwater, X the distance to the shore and Kt the transmission coefficient. Pilarzyk’s proposed expressions allow the distance to the shore to be left as an unknown whilst setting all the other values so that the maximum distance at which the ship can be situated for a tombolo to be formed or the minimum distance to be kept for a salient to form can be calculated. The results obtained in tests at a depth of 20 m are shown in table 1.

Discussion of results

Figure 4. Ratio between transmission coefficient and significant period at a depth of 35 m

Table 1 shows the results obtained from the tests carried out at a depth of 20 m. In the light of the physical model’s results, the significant 5 sec to 9 sec period tests were considered to be of interest since, above 9 sec, the ship allows almost all the energy through. The results obtained for the 35 m depth turned out to be non significant because the long distance of the vessel to the shoreline makes its shadow effect practically nil. The conclusion may be drawn from the shoreline’s response resulting from applying the Hanson and Kraus model,that protection as provided by a ship acting as a detached floating breakwater will give as a result the forming of salients for incident waves from 6 sec to 9 sec significant period and significant wave heights up to 4 m. The most determining parameters for the breakwater’s effectiveness are the period, which appears to be more influential than the wave height, and the depth, which will,



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Table 1. Response of the shore protected by a ship acting as a detached breakwater according to Hanson and Kraus (column 6) and the breakwater’s maximum and minimum distance to the shore for forming tombolos or salients according to Pilarzyk (columns 7 and 8)

Significant period Ts [sec]

Significant height H0 [m]

Height transmitted Ht [m]

Wave length L[m]

Transmission Coefficient Kt [-]

Hanson and Kraus shore response

5

1.50

0.24

38.91

0.160

Nil

231.42

154.28

231.42

5

2.00

0.19

38.91

0.095

Nil

249.33

166.22

249.33

5

2.50

0.53

38.91

0.212

Nil

217.09

144.73

217.09

5

3.00

0.85

38.91

0.283

Nil

197.44

131.63

197.44

6

2.10

0.59

55.05

0.281

Nil

198.10

132.07

198.10

6

3.00

0.94

55.05

0.313

Nil

189.18

126.12

189.18

6

4.00

1.15

55.05

0.288

Salient

196.29

130.86

196.29

7

2.63

0.39

71.98

0.148

Salient

234.65

156.43

234.65

7

2.97

0.58

71.98

0.195

Salient

221.70

147.80

221.70

7

4.06

0.90

71.98

0.222

Salient

214.43

142.95

214.43

8

2.32

0.64

88.79

0.276

Salient

199.50

133.00

199.50

8

2.92

0.90

88.79

0.308

Salient

190.59

127.06

190.59

8

3.91

1.30

88.79

0.332

Salient

183.90

122.60

183.90

9

2.28

1.39

105.21

0.610

Nil

107.54

71.69

107.54

9

3.55

2.45

105.21

0.690

Salient

85.37

56.91

85.37

9

4.45

3.30

105.21

0.742

Salient

71.20

47.46

71.20

in most cases, be related to the distance from the coast. The ship’s influence on the coastline is nil with periods less than 6 s. This does not mean that the ship acting as a breakwater does not dissipate the waves, since the transmission coefficients are excellent, but that the wave lengths for those periods at that depth are very short. The transmission coefficient for periods over 9 sec proves very high and the ship’s resistance to wave action is no longer effective. The forming of tombolos is not foreseen in any of the tested cases. The results obtained from applying the Pilarzyk model to determine the critical distance to the shoreline for tombolos or salients to form show a range of distances from approximately 50 m to 250 m. Specifically, for 9 sec significant period and 4.45 m significant height, the range of distances between ship and shore for which a tombolo would form range from 47.46 m to 71.20 m, as from which distance a salient would form up to a maximum distance yet to be determined and as from which the ship would have no effect on the shore. Moreover, the distance as from which a salient would form for a significant 5 sec period and significant wave height of 2 m is 249.33 m. In the light of these results, it can be reasoned that these critical ship to shore distances are typical of water shallower than that corresponding to the depth at the site where the ship was located for the tests. Most detached breakwaters are located at depths below 10 m, which are less than the 20 m and 35 m chosen for this test. Therefore, the ship in the configuration tested is far from the shallow water which most suitable for the working of detached breakwaters.

Pilarzyk X max. for tombolo [m]

Pilarzyk X min. for salient [m]

With respect to the forming of tombolos or salients, the method employed in this investigation enables the coast’s response to the protection provided by the ship and the critical distance for creating one or the other formation to be determined. However, it does not indicate anything as regards the volume of accretion or about the stability or instability of the salient formed. Another possible line of investigation would therefore be to turn to other formulations or models (whether physical or analytical) in order to predict the shore’s response to this effect. Further lines of investigation also open in relation to the ship acting as a floating breakwater. In our opinion, the evaluation of tensile stresses in the anchor chains, the different anchoring and the configuration of lines, the use of different types of ships and the testing of different draught conditions for the ship may be highlighted for their interest amongst the different configurations which can be tested. There is also a limitation due to the experiment being only applied to one and the same ship. Different sized ships must be tested in order to improve the reliability of the experiment. Finally, mention must be made of the difficulties which arise, both in this test and those proposed, from considering the different factors of scale and the geometrical similarities coming into play in work of this type. For example, the flume’s width is a limitation to be borne in mind when choosing the ship’s overall length. There is also a limitation in the aspects of Froude’s law when working with 1/150 scale, which may not be the best for processing factors such as wave action, sea bed or volume of sand. On the other hand, working with scales with a lower denominator makes it impossible to use the model of ship chosen.

Proposals for future lines of work A first line of investigation related to the shoreline’s response will therefore consist of bringing the ship into shallower water. Obviously, to this effect, there will be a draught condition determined by the ship to be used.

CONCLUSIONS This article gives the results of the investigation carried out on the effectiveness of a ship acting as a detached breakwater for shore protection and the forming of salients or tombolos.


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From studying the most frequent types of floating breakwaters as carried out in the first phase of the investigation, the conclusion may be drawn, that, in general, the different investigations are heterogeneous as to determining physical model testing (nature of the flume, paddle reflection absorption, factor of scale, criteria of similarity, type of wave action employed, etc.), and it is therefore difficult to obtain homogeneous results on the basis of preceding investigations. Even so, the general criterion has been established that the usual floating breakwaters are effective for wave heights around 2 m and periods around 5 sec. A range of validity for container ships tested as floating breakwaters for significant wave heights between 1.5 m and 4 m and significant periods between 5 sec and 9 sec was established from the physical model tests performed in the second phase. This result extends the range for using the most usual floating breakwaters to laminate the effects of incident waves. The third phase of the investigation consisted of employing analytical models which use the transmission coefficient to assess the shore’s response to the protection provided by the ship acting as a detached floating breakwater. The models chosen were those of Hanson and Kraus and Pilarzyk. In the light of the results obtained, the conclusion may be drawn that under test conditions, the coast is protected by the ship, giving rise to salients forming for periods between 6 sec and 9 sec. Below 6 sec, the shore is well protected in the light of the transmission coefficient figures but neither salients nor tombolos are formed. Above 9 sec, the ship does not suffice to dissipate waves in order to give rise to either of these formations. In addition, the conclusion drawn is that the depths tested are located in intermediate water, a little distant from the shallower water more appropriate for building detached breakwaters. It is assumed that by bringing the ship closer to the shore, it will work better although the ship’s draught should be borne in mind as an important conditioning factor. Finally, results were discussed and new lines of work identified, amongst which may be highlighted the reproduction of physical model tests in shallower depths, study of the stability of salients formed and estimation of the volume of accretion and variations in the anchoring system and measuring tensile stress in anchor lines. Mention should be made of the important condition of draught the ship chosen will impose when testing depths lower than studied here, as well as the difficulties in factors of scale relating to the various elements present in the test such as the ship, the sea bottom, the waves and sedimentary material. The physical model’s limitations as to both the flume and the ship are therefore known. Tests were performed with the limiting factor being the standard ship rather than the system’s hydrodynamics both in the moving bottom and in generation conditions and wave-bed interaction. In the same way, there is a limitation in the use of the Hanson & Kraus and Pilarzyk formulas (typical for wave transmission in a porous environment) for studying transmission in a floating element. In this respect, the test constitutes a first approach for determining the structural and functional response and a modification of certain test conditions enabling coastal models to be used will be necessary.

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