Floating boom performance under waves and currents

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Abstract

Floating booms constitute a fundamental tool for the protection of marine and coastal ecosystems against accidental oil spills. Their containment performances in exposed areas are often impaired by the action of waves, currents and winds in a manner which is dependent on the boom's response as a floating body, and which is not fully understood at present. In this work the relationship between the design parameters of a floating boom section and its efficiency against the mode of failure by drainage under a variety of wave and current combinations is investigated by means of physical modelling. Seven boom models with different geometries and buoyancy–weight ratios are tested with an experimental setup that allows them to heave and rotate freely. The model displacements under waves (both regular and irregular) and currents, as well as those of the free surface adjacent to the model, are measured with a Computer Vision system developed ad hoc. Two efficiency parameters are defined—the significant and minimum effective boom drafts—and applied to the results of an experimental campaign involving 315 laboratory tests. Thus, the manner in which the design parameters influence the boom's efficiency under different wave and current conditions is established.

Introduction

Oil spills resulting from maritime disasters have the potential to cause extensive environmental damage to the marine and coastal ecosystems. In spite of the safety enhancement measures recently implemented in oil transport by sea and the observed decrease both in the number of accidents and volume of oil spilled worldwide since 1970 [1], [2], oil spills still constitute a significant risk. It is therefore not surprising that oil spill modelling is the subject of intensive research efforts (e.g. [3]). In order to reduce this risk it is important, first, to develop the ability to predict the path of an oil slick under the prevailing wind, wave and current conditions. A second issue of importance concerns the in situ response methods that may be used to deal with the oil slick on its approaching the coastline so that its environmental impact is as low as possible [4]. Mechanical methods, based on the use of floating booms to contain the oil slick and skimmers to recover the pollutant either from ships or the shoreline, are the most common countermeasure [5], among other reasons owing to a significant advantage over other methods (chemical dispersants, in situ burning, etc.)—the absence of adverse environmental effects. The success of mechanical methods hinges on the containment efficiency of floating booms. Though usually sufficient in sheltered waters, such as port basins or estuaries, it is often insufficient in open waters under the action of currents, waves and winds.

Among the various modes of failure of a containment boom, drainage failure—in which the contaminant escapes underneath the boom (Fig. 1)—is one of the most important. A crucial concept in this respect is the effective boom draft, or the draft available at a given moment considering the displacements of the free surface in the vicinity of the boom and those of the boom itself under the action of waves, currents and winds. Drainage failure occurs when the effective boom draft becomes lower than the oil slick thickness. The oil slick thickness depends on the characteristics of the particular oil spill (volume involved, hydrocarbon properties, etc.) and its evolution as determined by currents, winds, and waves (e.g. [6]). For this reason, unless focusing on a particular point in space and time during a particular oil spill, the oil slick thickness must be regarded as an unknown. Fortunately the other aspect of the problem (the effective boom draft) can be analysed in more general terms—and therein lies the motivation of this work.

The effective boom draft is a function of the geometry of the boom section and its motion, itself dependent on the hydrodynamic agents and the boom characteristics (geometry, buoyancy/weight ratio, etc.) In spite of the importance of boom motions, many previous works investigated drainage failure by means of fixed models, in most cases with highly simplified geometries (typically a vertical plate) and considering only the action of currents [7], [8], [9], [10]. In these works, the boom draft was constant—undoubtedly a great simplification, though not a very realistic one. The draft reduction due to boom motions caused by wave action was investigated in a few works by means of numerical models based on the potential-flow assumption [11], [12], [13], generally taking into account the vertical displacement of the boom relative to the free surface, and also by means of laboratory experiments [13].

In this work the efficiency of seven boom section designs in relation to the mode of failure by drainage is investigated by means of realistic physical models that move vertically and rotate freely under the action of both waves and currents. For the reasons outlined above, allowing for these motions is crucial to a good assessment of the boom's performance. The motions are measured with a Computer Vision system based on the analysis of digital images. This system, designed ad hoc for this research [14], not only records the displacements of the floating model throughout each test, but also those of the free surface in its vicinity, which are necessary to determine the effective boom draft. The performance of the seven model booms is characterised by means of efficiency parameters based on the evolution of the effective boom draft during each test. These parameters, applied to an experimental campaign comprising, in total, 315 laboratory tests, enable to compare the performance of the different boom designs under various wave and current conditions and to analyse the influence of each design parameter on the boom's performance.

Section snippets

Physical models

Seven physical models with different design sections were built at a 1:10 scale. The models represent a floating boom module with a length of 6.4 m, a buoyancy cylinder with a diameter of 0.80 m, and a vertical skirt with two possible heights: 0.80 m (models M1 to M4) and 1.20 m (models M5 to M7) (Fig. 2). The buoyancy cylinder was made of polystyrene and the vertical skirt was a PVC sheet. To achieve different values of the buoyancy–weight ratio (B/W), ballast was added in the form of stainless

Results and discussion

The experimental results of significant and minimum effective draft obtained from the tests will be analysed separately for regular and irregular wave conditions. First, the general trends observed in the behaviour of the physical models as a function of the hydrodynamic conditions (current velocity, wave height and period) are commented. Next, the behaviour of the different models as a function of their respective design parameters (buoyancy–weight ratio and initial draft) is compared.

Conclusions

Floating booms constitute a fundamental tool for the protection of marine and coastal ecosystems against accidental oil spills. Their containment efficiency in open water can be significantly impaired under currents, winds and waves. In spite of the great practical importance of floating booms, our understanding of their response in the face of these hydrodynamic agents is still incomplete. In this work the performances of floating booms against the mode of failure by drainage under currents

Acknowledgements

This research has been funded by the European Union, within the framework of the Interreg IIIC programme (Ref. IIIA-PROLIT-SP1E194/03). The authors are grateful to two anonymous reviewers, whose suggestions and comments have contributed to improving the manuscript.

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