FIELD INVESTIGATION ON WAKE WASH GENERATED BY HSC IN THE BAY OF NAPLES

Transcription

1 FIELD INVESTIGATION ON WAKE WASH GENERATED BY HSC IN THE BAY OF NAPLES Ermina Begovic 1, Guido Benassai 2, Erica Nocerino 2, Antonio Scamardella University of Naples Federico II, Department of Naval Architecture, Naples, Italy 2 - University of Naples Parthenope, Department of Applied Sciences, Naples, Italy ABSTRACT In this paper the first field measurements of wake wash produced by High Speed Craft (HSC) operating in the Bay of Naples are presented. The fleet in service in the Bay is analysed and the main characteristics of these units are given. Their routes are monitored and three critical points, where the distance from the coast is less than 700 m, for wake wash investigation were identified. In two of them a pressure gauge was installed and the field measurements of wake wash were performed. While wake wash was recorded by the pressure gauge, contemporaneously the route was monitored aboard the crossing HSC and exact speed, distance from the monitored site, and water depth were determined. The recorded wake wash measurements are analysed and maximum wave height and corresponding period are identified for each registration. The obtained values are compared with the International Rules and at last some comments and conclusions are given. 1. INTRODUCTION In the Bay of Naples shipping companies offer a daily service to connect the city with the Islands of Capri, Ischia and Procida. This service is mainly used by commuters living at islands and by the great number of tourists, which increases transportation demand by 60% in the summer season. In order to assess the related risk and to minimize the undesirable effects of wave production, the Province of Naples has commissioned the Department of Applied Sciences of University Parthenope to perform a study on the wake wash generated by HSC operating in the Bay of Naples. The aim of the study is to assess guidelines for safe navigation and to minimize environmental interference with the shoreline. The study was developed in three phases: the first one consisted of the analysis of vessel fleet operating in the Bay, the second one was route monitoring and determination of critical points along the routes, and finally assessment of the wake wash through field measurements in the critical points. To have a better overview of potential environmental risks, the obtained data have been revised through different International Rules. The less restrictive one adopted by Danish Maritime Authorities (DMA) prescribes some benchmark values for wave height and period. The Washington State Ferries (WSF) Rule prescribes the maximum value of 0.28 m together with the maximum energy contained in wave front. Therefore the analysis of the recorded wake wash measurements consists of identification of the maximum wave height and corresponding period for each registration, and calculation of energy according to WSF Rule. The obtained values are compared with the prescribed values and at last some comments and conclusions are given. 2. FLEET OPERATING IN THE BAY OF NAPLES There are currently forty-five high speed craft operating in the Bay, on the routes in the Bay itself or from Naples to Salerno and to the Aeolian Islands. Complete fleet is composed from twenty monohulls, sixteen catamarans, and nine hydrofoils. In the Table 1, only the high speed craft in service in the Bay of Naples, on daily connection of the city with the three islands and their main characteristics are given. It can be observed that all the vessels are ranging from to meters in length between perpendiculars. Overall beam is limited to m for entering in narrow harbours. The average number of passengers is about 348, ranging from a minimum of 153 for the hydrofoils up to 450 for the monohulls. The average speed is about 30 knots, the fastest craft is the catamaran Giunone. Most vehicles have water jet propulsion. It can be noted that the great part of the fleet has been built in nineties. The recently built vessels are two catamarans (Maria Celeste and Maria Sole Lauro) and two monohulls (Annamaria Lauro and Snav Orion). It can be highlighted that both monohulls and catamarans have increased length, indicating the new trend to increase the passengers number. It should be commented that the last two ships from the Table 1 are not classified as HSC by Maritime Authorities but, as they were engaged on days of field measurements, also their wake wash generation was included in the analysis. Session A 243

2 Table 1. Main characteristics of HSC operating in the Bay on Naples Type Name L OA L BP BBOA D V S GT NT DWT Pax Prop Date Main (kn) (tonn) (tonn) (tonn) No Build Route CAT J 1989 M - I MH Annamaria Lauro P 2000 B I MH Celestina J 1988 M I MH Europa J 1988 S C CAT Giove J 1985 B I CAT Giunone J 1988 S C CAT Maria Celeste J 2004 CAT Maria Sole Lauro J 2005 B I CAT Nettuno J 1988 B I MH Rosaria Lauro J 1988 B I MH Super Flyte P 1993 S C CAT Achernar J 1992 B I HY Aldebaran P 1987 B I HY Alnilam P 1986 B I MH Isola di San Pietro J 1992 B C HY Panarea P 1990 N - C HY Salina P 1990 N - C CAT Snav Alcione J 1991 N - C CAT Snav Alfa J 1990 Metrò CAT Snav Altair J 1990 N - C CAT Snav Andromeda J 1988 Metrò CAT Snav Antares J 1990 N - C CAT Snav Aries J 1988 M I (CM) CAT Snav Auriga , J 1990 Metrò CAT Snav Aurora J 1990 Metrò MH Snav Orion J 2003 N C MH Amalfi J 1991 B C MH Capri J 1988 B C MH Ischia J 1989 B I MH J 1989 B S MH Città di Forio J 1987 S C MH Città di J 1994 S C * - the bold indicates the HSC in service on days of field campaign 3. ROUTE MONITORING Nowadays, traffic in the Bay of Naples consists mainly of few routes served by different operators: Alilauro, Caremar, Navigazione Libera del Golfo and SNAV. The two main departure points are the Beverello Pier, in the Port of Naples, and the Mergellina Pier, about two kilometres to the north. In Table 1, the main route of each HSC is given in the last column, even if it is sometimes possible to change route. The letters used in Table 1 have the following meanings: M I - Mergellina-Ischia (15 Nautical Miles) M I (CM) Mergellina Ischia-Casamicciola (17.5 NM) N C - Naples-Capri (15 NM from Mergellina) B I - Beverello-Ischia (17 NM) B P I - Beverello-Procida-Ischia (17.7 NM) B S - Beverello- (13 NM) B C - Beverello-Capri (17.6 NM) S C - -Capri (8.7 NM). Session A 244

3 - in post processing, in order to display the HSC track on navigational charts. Starting from the coordinates of the pressure gauge, positioned firstly in CP1 and then in CP2, the vessel distance from the measurement point was calculated. The accuracy of the distance measurements was metric, which is adequate for this study ROUTE MERGELLINA ISCHIA Figure 1. Routes in the Bay of Naples With the cooperation of the ships owners, the actual courses served by the different fast ferries have been monitored by means of GPS instrumentation with a fair degree of precision. Route monitoring has been conducted in two phases. The first step has involved several trips on board of HSC, in order to visually identify some critical points for investigation of wake wash generation and propagation. Such sites (indicated with magenta circles in Figure 1) are those ones the vessels are passing close to shoreline, generally used for recreational and bathing activities. The route monitoring by GPS aboard different HSC reported exact vessel speed, distance from the coastline and water depth, as the most important parameters for further wake wash analysis. The results concerning Route Mergellina Ischia are given in the Table 2. Table 2. Parameters for CP1 measured aboard the HSC From M to I From B to I Distance Speed (kn) Water depth In Figure 1 the monitored routes are displayed: - the green track is related to route Beverello Mergellina Ischia; - the red one to route Beverello Procida Ischia; - the yellow one to route Beverello - Capri; - the orange one to route Beverello Capri The Figure 1 evidences three critical points: CP1 Posillipo, on the route Mergellina Ischia; CP2 Marina di Puolo, on the route Capri; CP3 Punta Molino, close to Ischia port. Highlighted these crucial points, the pressure gauge has been positioned, and at the same time the second phase of the route assessment is started. Now, route monitoring on board of the selected HSC consisted in determination of exact position of ship (in terms of distance from the selected point and followed course), water depth and speed along the track. Up to now, the wake wash measurements were performed in critical points 1 and 2, as identified in Figure 1 and these results are presented. Two GPS 12-channel L1 Code handheld receivers have been employed in order to determine the route followed by the monitored craft. The GPS data was collected at the rate of 1 Hz. The receivers have been used in two different modalities: - in real time, in order to check continuously the ships path (the control panel showed the user s azimuth and speed); Figure 2. Route Mergellina - Ischia It can be seen from the Table 2 that the route Mergellina Ischia is closer to the shoreline (indicated with the green line). Route Beverello Ischia (shown with red line in Figure 1) is rather distant from the CP 1 and therefore not examined in this phase ROUTE SORRENTO CAPRI The route Capri, shown in Figure 3, is operated by Alilauro during the whole year, while in summertime this trade is served by Metrò del Mare and Linee Marittime Partenopee, a consortium formed by Alilauro and Navigazione Libera del Golfo. It can be noted from the Figure 1 that the route Beverello Capri (orange line in Figure 1 ) is too distant from the shoreline, as reported in the Table 3. Therefore only the wake wash generated by the HSC operating on the yellow route was investigated. The monitored route is shown in Figure 3 and the results are reported in Table 3. Session A 245

4 Wave heights and periods of vessel wash were measured using a submerged pressure measuring/recording instrument package, shown in Figure 5. Figure 3. Route - Capri Table 3. Parameters for CP 2 measured aboard the HSC From S to C From S to C Distance Speed (kn) Water depth WAKE WASH MEASUREMENTS For wake wash assessment, the possible options are: CFD simulations, model scale testing, and field measurements. A detailed numerical analysis of flow requires the hull form geometry and very detailed route reproduction with exact water depths description. Very often, the hull forms and loading conditions are considered confidential by shipyards and owners, so numerical approach will fall a priori. Wake wash analysis by testing models in towing tank presents both problems of model construction and geometry of towing tank, in order to have shallow water condition and possibility to measure the transversal waves sufficiently faraway from the navigation track. Even if towing tank is specific for wake wash analysis, the problem of interaction of generated waves with bathymetry remains and this part can not be resolved by towing tank experiments. Therefore the most realistic and cost-effective way to determine the wake wash generated by HSC is to perform field measurements, facing all the relative problems as influence of climate conditions (wind generated waves) during the measurements, local currents, etc. Figure 5. Pressure wave gauge within steel frame The pressure wave gauge was installed in a steel frame which ensures stability and keeps the gauge oriented to vertical. The instrument and steel frame weights are about 15 kg and 20 kg, respectively, so the weight of all system ensures self-anchoring. The height of the pressure head above sea bed was approximately 0.60m. The depth at the measurement site offshore Posillipo (CP 1) was 4.70 m with up to 0.50 m excursion in tidal variation, while in Marina di Puolo (CP 2) was 4.20 m, with the same tidal excursion. The measurements were made on a three-day period for critical point 1, and on a four-day period for critical point 2. The weather during the field measurements was calm, with essentially no wind. The pressure gauge was configured to collect pressure data at a rate of 2 Hz. Sampling time was 15 minutes every hour thus creating 1800 data for each record. 5. DATA ANALYSIS After the data was gathered, custom software converted the pressure readings to wave heights as a function of time, enabling measurements and plots of wave patterns passing over the buoy. All the consistent and reasonable data were analyzed in order to determine the highest wave and the corresponding period generated by crossing vessels WAKE WASH RECORDS FROM CRITICAL POINT 1 Figure 4. A monitored HSC in navigation The record of wave data started at 17th of April 2007 and lasted as long as the battery life permitted (19th of April). During the acquisition period, the morning service from Mergellina to Ischia was provided by two HS catamarans ( and Giove ) while the evening service was provided by two sister ship monohulls (Celestina and Rosaria Lauro). Session A 246

5 In the Diagrams 1-10 the part of wake wash registration (i.e s), where the waves are occurring, is presented for the selected registrations. ACAPULCO JET _ 17/04/2007 AT GIOVE JET _ 18/04/2007 AT Diagram 1. CAT wake wash record Diagram 5. CAT Giove wake wash record 4.6 ACAPULCO JET _ 17/04/07 AT ,1 GIOVE JET _ 18/04/07 AT , , , , , Diagram 2. CAT wake wash record 4,75 4, Diagram 6. CAT Giove wake wash record ACAPULCO JET _ 17/04/2007 AT ROSARIA LAURO _ 18/04/2007 AT Diagram 3. CAT wake wash record Diagram 7. MH Rosaria Lauro wake wash record 4,8 CELESTINA _ 17/04/2007 AT ACAPULCO JET _ 19/04/2007 AT , , , , Diagram 4. MH Celestina wake wash record Diagram 8. CAT wake wash record Session A 247

6 ACAPULCO JET _ 19/04/2007 AT CITTA' DI SORRENTO _ 11/05/2007 AT ,1 5,05 5 4,95 5 4,9 4,85 4,8 4,75 4, Diagram 9. CAT wake wash record Diagram 13. MH Città di wake wash record CELESTINA 19/04/2007 AT CITTA' DI SORRENTO _ 11/05/2007 AT Diagram 10. MH Celestina wake wash record Diagram 14. MH Città di wake wash record 5.1. WAKE WASH RECORDS FROM CRITICAL POINT 2 EUROPA JET _ 12/05/2007 AT 8.15 The record of wave data started at 11 th May and finished at 14 th of May During the acquisition period, the service from to Capri was provided by different high speed monohulls: Superflyte,, Europa, Città di and Città di Forio. Same as in Diagrams 1-10, also in the following Diagrams the 300 s registration is presented Diagram 15. MH Superflyte wake wash record SUPERFLYTE _ 11/05/2007 AT SORRENTO JET _ 12/05/2007 AT ,95 4 3,95 3, Diagram 12. MH Superflyte wake wash record 3,9 3,85 3,8 3,75 Diagram 16. MH wake wash record Session A 248

7 CITTA' DI FORIO _ 12/05/2007 AT SUPERFLYTE _ 14/05/2007 AT ,7 4,65 5 4, Diagram 17. MH Città di Forio wake wash record Diagram 21. MH Superflyte wake wash record SUPERFLYTE _ 13/05/2007 AT CITTA' DI SORRENTO _ 14/05/2007 AT Diagram 18. MH Superflyte wake wash record Diagram 22. MH Città di wake wash record CITTA' DI SORRENTO _ 13/05/2007 AT 8.15 CITTA' DI SORRENTO _ 14/05/2007 AT , ,9 3, Diagram 19. MH Città di wake wash record Diagram 23. MH Città di wake wash record 4,7 4,65 4,6 5 EUROPA JET _ 13/05/2007 AT In Tables 4 and 5 the maximum wave height and associated period from time history diagrams are reported for each trip and each vessel. Although the wave gauge acquisition was programmed to start measurements at departure time, in some cases the recorded waves are influenced also by external disturbances, such as passing vessels or return trip, and therefore were excluded from the analysis. 5 Diagram 20. MH Europa wake wash record Session A 249

8 Table 4. Maximum wave heights and periods for CP 1 Departure HSC H WW T WW date time (h) (s) 17/04/ ,5 17/04/ /04/ ,0 17/04/ Celestina ,0 18/04/ Giove ,5 18/04/ Giove ,5 18/04/ Rosaria Lauro ,0 19/04/ ,5 19/04/ ,0 19/04/ Celestina ,0 Table 5. Maximum wave heights and periods for CP 2 Departure HSC H WW T WW date time (h) (s) 11/05/ Superflyte /05/ C di /05/ C di /05/ Superflyte /05/ /05/ Citta di Forio /05/ Superflyte /05/ C di /05/ Europa /05/ Superflyte /05/ C di /05/ C di From Table 4, it can be noted that the wake wash heights and periods for catamarans and monohulls are very similar. The measured wave heights are in the range from 0.17 m up to 0.34 m. The measured period values of all HSV range from 6 to 8 seconds regardless hull configuration. The value of 4 second is surprisingly low in comparison with other measured values. As regard the results from the CP 2, given in Table 5, it can be highlighted again that all the vessels engaged during the field measurements were monohulls. In these measurements can be noted that the values of the wave heights are from 0.20 m up to 0.34 m, while the periods are generally about 4.5 seconds. It should be commented that Macfarlane and Renilson (2000) have concluded that although monohulls generate larger wave heights than catamarans for given speed, displacement and waterline length, the overall difference is not so great, because the wave periods produced by monohulls are lower than equivalent multihulls. Whittaker (2002) reported the results of series of experiments with generic monohulls and catamarans. The wave records at F NH 1 are very similar for both configurations, with regard to both maximum wave height and maximum wave energy. At F NH 1.8 the difference between wash patterns is much more noticeable. The here presented results are confirming both of these conclusions, because in CP 1 the F NH value is about 0.92 (water depth is 20 m) while in the CP 2, the F NH value is about 0.5 so it is still not a critical regime. Finally these results should be commented also with regard to the measured distance from the navigation track. The results presented in Table 4 are the measured values at distance of 400 m from navigation track with the wave gauge at 4.7 m depth. The results presented in Table 5 are those measured at distance of about 700 m from navigation track with the wave gauge at 4.5 m depth. Generally the field measurements performed by Danish Hydraulic Institute (DHI) are reported to the values at 700 m from the navigation track, according to Hansen (1999), Hannen and Varayani (1999). Usually American authors Stumbo et al. (2002), Bruno et al. (2002) have presented measured data at 300 m distance. 6. INTERNATIONAL RULES In order to minimize the undesirable effects of wave wash, several Maritime Authorities adopted Rules in which wave height or wave height and energy values have to be within prescribed limits. The Wash Rule based on Danish Maritime Authority (DMA Order No 307 dated May 1997, Approval of the Safe Navigation of High Speed Ferries) is given by Equation (1) where H and T are: 4.5 H 0.5 (1) T H maximum wave height in meters of the generated long period waves, measured in still water depth of 3 meters; T the average generated wave period of the long period waves, measured in seconds. Defined in this way, the Wash Rule becomes an empirical relationship ensuring that the effects for any combination of H and T are similar to those ones for benchmark combination of wave height and wave period: 0.5 and 4.5 namely in (1). Washington State Ferries (WSF) compares wave height and wave energy in deep water at 300 m from the ship track requesting that the maximum wave height do not exceed 28 cm and energy density 2450 (J/m). In this formulation energy density is defined as: E 2 2 = 1961 H T (Joules/meter of wave front) (2) where H and T are: H the height in meters from peak to trough of the highest wave in the series of waves produced by the crossing of measured vessel; Session A 250

9 T the period of the highest wave in the series of waves produced by the crossing measured vessel, in seconds. Wake wash is measured or mathematically normalised to a distance of 300 meters perpendicular to centreline of vessel. 300 meters is chosen to provide a basis for comparison between various vessels measured under similar circumstances. In United Kingdom the operator has to perform a qualitative risk assessment. The assessment includes identification of hazardous areas and assessment of hazard severity (i.e. severity of the wake wash impact) and the likelihood of occurrence. 7. COMPLIANCE WITH THE RULES AND COMMENTS As regard the DMA Rule, two cases were considered, the first for monohulls and the second for catamarans. In the first case the period value was taken from the measurements at CP 2, and the most frequent value of 4.5 s is applied in formula (1). In this way, the limiting value of wake height becomes 0.5 m. The second case is done with the longer period, more appropriate for catamaran waves and the maximum measured period of 8 s was chosen as it will give the lowest wave height value. H, calculated from formula (1) with 8 s period, is equal to m. In order to be able to review the measured values according to WSF Rule, all the field data are normalised to 300 m distance according to the formula (3) for wave height decay proposed by Stumbo (1998) where: H 2 d = 1 3 (3) H 1 d 2 H 1, H 2 wave heights; d 1, d 2 distances from the navigation track. These results are reported in Table 6. It can be noted that both hull configurations, monohull and catamaran, are complying with the DMA wake wash Rule. The two points close to the limiting values ( m and Città di m ) have not been repeated in other measurements of the same ships. It should again be highlighted that the maximum value has been taken in analysis, not the mean or significant wave height value. As regard WSF Rule, the maximum height of 0.28 m is satisfied in some cases for catamaran in CP 1 but never for monohulls measured in CP 2. The energy criteria is satisfied just in one case. Table 6. Maximum wave heights and periods at 300 m from navigation track HSC V h F NL F NH H WW H 300 E (kn) (J/m) Celestina Giove Giove Rosaria Lauro Celestina Superflyte C di C di Superflyte Citta di Forio Superflyte C di Europa Superflyte C di C di CONCLUSIONS In this paper the first results of a study on wake wash generation in the Bay of Naples are presented. In the first part of study, the HSC operating in the Bay are analysed and their routes are identified. Three critical points were identified where the route of vessels seemed to be very close to the shoreline. In two of these points the field measurements are performed with the pressure gauge, monitoring simultaneously the speed and distance of crossing vessel. The recorded values of wake wash produced by high speed craft in critical points 1 and 2 can be summarized in following: Wake wash measured data are complying with the Danish Maritime Authority wake wash Rule, prescribing the maximum wave height for monohulls and for catamarans; Wake wash measured data are not always complying with the Washington State Ferries Rule, imposing 0.28 m fro wave height; Session A 251

10 Values for energy density calculated according to the WSF Rule are higher than value prescribed by WSF, except in one case. 9. ACKNOWLEDGEMENTS We are grateful to the ship owners companies (Alilauro, Caremar, Navigazione Libera del Golfo, SNAV) and to the crews for their collaboration in gathering the data. Many thanks to Massimo De Stefano and Gennaro Bianco for their cooperation during the field measurements. We would very much to thank Doctor Researcher Giuseppina Prezioso of the Department of Applied Sciences for her significant technical support on board of monitored HSC and for route data processing. Stumbo Associates Analysis of AMHS Fast Ferry Wake Wash Predictions - Phase 2, Report FHWA- AK-RD Stumbo, S. & Fox, K. & Elliot, L Hull Form Consideration in the Design of Low Wake Wash Catamarans, Proc. Int. Conference FAST 99, Seattle, WA Whittaker T., Bell A, Shaw M., Patterson K (1999). An Investigation of Fast Ferry Wash in Confined Waters, RINA Int. Conference Hydrodynamics of High Speed Craft, London, UK, paper No.13 Whittaker, T A Physical Study of Fast Ferry Wash Characteristics in Shallow Water, MCA Research Project 457, UK 10. REFERENCES 1. Begovic, E., Benassai, G Wake Wash Analysis of HSC Catamarans, Proc. Int. Conf. ISOPE, Lisboa, 1-6 July 2007 Bruno, M, Fullerton, B, Datla, R (2002). Ferry Wake Wash in NY Harbor, Technical report SIT-DL Coccoli, D. & Scamardella A The Passengers Point Of View Onboard High Speed Craft, Proc. Int. Conf. IAME, Izmir 30 une 02 July 2004 Coccoli, D. & Scamardella A What Do the Crew Think About the Operability of High Speed Craft?, Proc. Int. Conf. ICMRT, Ischia, Italy28-30 September 2005 Hannon, MA, Varyani, KS (1999). The Wash Effect of High Speed Ferries in Coastal and Inland Waterways, RINA Int. Conference Coastal Ships and Inland Waterways, London, UK, paper No.9 Kofoed-Hansen, H, Jensen, T, Kirkegaard, J, Fuchs, J (1999). Prediction of Wake Wash from HSC in Coastal Areas, RINA Int. Conference Hydrodynamics of High Speed Craft, London, UK, paper No.12 Macfarlane, GJ, Renilson, MR, (2004). The development of vessel wave wake criteria for the Noosa and Brisbone Rivers in Southest Queensland, Coastal Environment V, incorporating Oill Spill Studies, WIT Press, ISBN , pp Macfarlane, GJ, Renilson, MR, (2000). When is low wash? An investigation using a wave wake database, RINA Int. Conference Hydrodynamics of High Speed Craft, London, UK, paper 9 Stumbo S., Fox K., Dvorak F. Elliot L. (1998). The Prediction, Measurement and Analysis of Wake Wash from Marine Vessels, SNAME Pacific Northwest Section Stumbo Associates Analysis of AMHS Fast Ferry Wake Wash Predictions - Phase 1, Report FHWA- AK-RD Session A 252