Tuesday, May 28, 2013

Cruise Ship Surge Study

Port Canaveral’s central location on Florida’s Atlantic coast offers access to major markets for products such as petroleum, cement, citrus fruit and juices, gypsum board and newsprint, as well as autos, trucks and heavy equipment. The port’s proximity to Orlando makes for a bustling tourism and cruise industry. With the historical presence of the US Coast Guard, military and commercial space launch organizations with ship-borne aerospace traffic, the US and Royal Navy missile test programs, and the Military Sealift Command’s ocean transportation services, Port Canaveral hosts a wide range of large vessels.

As the size of cruise ships has grown, outbound transits, in particular during stronger winds, generate surge effects within the harbor that have proven problematic for vessels at berth. As part of the Canaveral Harbor Integrated Section 203 Navigation Study sponsored by the Canaveral Port Authority (CPA), vessel-induced surge modeling was performed to provide an assessment of passing vessel hydrodynamic effects and forces on berthed vessels within the port, and potential changes following the proposed channel widening and deepening project. The analysis evaluated potential hydrodynamic changes on a port-wide scale and changes in loads on berthed vessels.

Vessel-induced surge modeling was performed according to scenarios developed collaboratively and coordinated among the CPA, their project engineering consultants, the Canaveral Pilots, the local US Coast Guard and the “Mission Partners,” which collectively refers to the Cape Canaveral Air Force Station (CCAFS), Naval Ordinance Test Unit (NOTU), and the Military Sealift Command (MSC). Modeling scenarios consist of different port configurations, berthed vessel configurations, and passing ship scenarios. Two port configurations were considered: Existing Conditions and Recommended Plan (channel deepening and widening). The scenarios included 16 berths of interest and five berthed vessel configurations (A-E) among Mission Partner and commercial wharf and pier facilities. Passing ship scenarios consisted of unique maneuvering schemes for each of the two port configurations, as well south or north wind conditions.

Port Configurations
Canaveral Harbor is a complex water body, with three main basins located off the main channel. Berths operated by the Mission Partners and the CPA are located within the basins and along the north and south sides of the main channel. Waterfront structures primarily include open-piled piers, wharves, steel sheetpile bulkheads, submerged sheetpile toewalls, ro/ro ramps, and pre-cast concrete pile or steel monopile mooring and breasting dolphins. CPA commercial terminals largely serve cargo, bulk and break-bulk carrier, tanker, and cruise operations.

Individual berths were described using known dimensions and locations of surface-piercing bulkheads, submerged toe walls, under-pier and berth box back slopes, and other features provided in the design drawings. The Recommended Plan modeling domain was developed based upon the widened and deepened navigation project features, adjacent slopes and new berths, as well as bulkheads in the case of West Basin future improvements.

Design Vessels
Design vessel data and hull geometry were provided, including type, class, name and particulars of all passing and berthed vessels. The Mission Partners supplied hull data in various formats, including vessel lines drawings, section and frame geometric descriptions and three-dimensional CAD drawings. Some CAD drawings were complete with superstructure geometry, whereas some commercial vessel hull models were generated using scaled versions of hull forms from existing databases.

Passing Vessel Configurations
Three design passing vessels and their maneuvering characteristics were selected prior to the analysis. The three cruise ships included Carnival’s Fantasy Class (Fantasy), Royal Caribbean’s Freedom Class (Freedom), and Disney’s Dream Class (Dream). On specific days of the week, cruise ship outbound transits occur successively within a relatively short period of time, and on some occasions have proven problematic for ships at berth. Each ship’s conning position, distance north or south of the existing or recommended project channel centerline, speed and drift angle were defined for either northerly or southerly wind conditions. These variables were all prescribed at six locations along the main channel. The modeling simulated three cruise ships maneuvered in succession, beginning with Fantasy, Freedom and Dream at 15-minute departure intervals. This is a typical weekend afternoon succession of vessel maneuvering activity at Port Canaveral with events as described in detail by the local pilots.
Existing Conditions

Surge Modeling
Passing vessel hydrodynamics were calculated using the Vessel Hydrodynamics Longwave Unsteady (VH-LU) model (Fenical et al., 2006). The VH-LU model predicts water level and velocity fluctuations surrounding the passing ships and in all areas of the harbor. The model has been thoroughly validated using a wide array of field and laboratory hydrodynamic and berthed vessel loading measurements (Fenical et al., 2011) for a number of previous marine engineering projects. Field validations have included successful comparisons with measured water levels and velocities, and validations with laboratory measurements have included successful comparisons of water levels, velocities and forces on berthed vessels.

Qualitative Model Validation
The quantitative validation of the numerical model via other project locations and field and laboratory measurements was accepted by the CPA project team as applicable for the Port Canaveral modeling in order to demonstrate the relative difference in large-scale, port-wide, passing ship effects between the existing and recommended port plans. Therefore no field measurements were collected on the project for model validation. Qualitative validation was performed using Canaveral Pilot site observations for several terminals and personal experience while conducting vessel movements in the harbor.
Four primary site observations were reported and reproduced in the model.

1) South Cargo Pier 4 (SCP4) - some of the strongest surge effects have been experienced here - a typical parallel-passing situation. The navigation area becomes more confined moving east of SCP4, starting with the solid bulkhead at SCP3, resulting in an increased pressure field intensity and surge (longitudinal) force at SCP4 in the model. This is observed in the modeling results.

2) Trident Wharf - experiences surge effects, passing speeds 7.5kts or greater generally cause issues. Since Trident Wharf is far from the channel, surge amplification must be demonstrated in the modeling to reproduce known impacts. The modeling results clearly reproducethe surge amplification. The pressure field wave reflects against the north end of Trident Basin, and produces a draw-down at that location with pressure field intensity similar to that immediately adjacent to the passing vessel.

3) North Cargo Pier 2 (NCP2) - Surge issues are not significant when passing vessels go by, but berthed ship motions occur 10-15 minutes later (time lag depending on passing vessel speed), even for a single passing ship. To illustrate this, additional simulation for Existing Conditions was performed using the Dream as a single passing vessel. The single passing ship simulation predicted that a pressure field wave is generated at the stern of Dream as it leaves the confined water of the Inner Reach. This effect is generally observed where passing vessel pressure fields interact with the transition between a narrow waterway and open water. Then, the pressure field wave arrives at NCP2, approximately 11.5 minutes after Dream originally passed the berth, which is within the 10-15 minute observation. Observation 3 is reproduced in the modeling results.

4) North Jetty Area - just inside the north jetty is a wide, shallow sloping shelf, which is an observed area of water level retreat and pressure field wave breaking. The model predicts that as the pressure field wave approaches the shallows, the water retreats away from the shoreline. As the pressure field wave moves into the shallows, it grows in height and breaks, and moves north until impacting the jetty.
The accurate reproduction of these complex physical processes reported from site experience provides a high level of confidence in the modeling results and conclusions regarding the potential mooring impacts associated with the proposed deepening/widening project.

Surge Changes from Channel Improvements
Passing vessel hydrodynamics were generated for the recommended plan conditions in a manner consistent with modeling for existing conditions. In general, pressure field intensity is reduced surrounding the passing cruise ship due to the wider and deeper channel. In some areas, the deepening alters the timing of the fluctuations. Water level fluctuations inside the harbor are highly variable and strongly depend on the location of interest.

Time histories were taken at all Mission Partner and commercial berths, and water level fluctuations are generally reduced at all berths of interest following channel improvements. In order to characterize the surge changes on a spatially complete port-wide scale, exhibits were generated for two parameters: maximum water level and maximum surge wave height.

Following harbor improvements, particularly in the narrow channel areas, and near Poseidon Wharf and the north end of Trident basin, modeling showed a reduction in both the maximum water surface elevations and the maximum surge height throughout the harbor, with a similar magnitude of the reduction for the two different wind and sailing conditions evaluated.

Loads on Berthed Vessels
The VH-LU modeling system also computed the hydrodynamic loads on the berthed vessels. The results of the hydrodynamic analysis include vessel specific time histories of surge force, sway force, and yaw moment for each of the modeling scenarios. Loads were calculated for those berthed vessels of interest as specified within the Surge Modeling Plan and at the time frame spanning the passing of the third and final outbound ship, the Dream. Changes in loads on the ships were largely analogous to changes in water level fluctuations introduced by the channel improvements. The vast majority of the berthed ships experienced reduced peak surge and sway forces for Recommended Plan modeling, with the timing of the forces altered as compared to the Existing Conditions modeling.

Vessel hydrodynamic analyses were performed on a port-wide scale, to evaluate future harbor surge and mooring conditions at 16 distinct terminals within Port Canaveral, Florida, for a series of three cruise ships departing in succession. The hydrodynamic analyses utilized passing vessel hydrodynamics and berthed vessel forces calculated using the VH-LU vessel hydrodynamic modeling system. A total of 24 different hull models were constructed, including 10 unique Mission Partner hull models and 14 commercial vessel hull models.

The Recommended Plan channel improvements result in a reduction in the intensity of pressure fields generated by the passing cruise ships. Water level fluctuations are reduced throughout the harbor, both in terms of maximum water level and maximum surge wave height. The Recommended Plan channel improvements result in a general reduction in the forces and moments on the berthed vessels, ranging from negligible to approximately 35 percent as compared to Existing Conditions. Immediate practical recommendations offered included an increase in the time between successive cruise ship departures .