Siting and design considerations for marine terminals
Shelden, Jeffrey G
The Data Collection and Study Requirements for an Optimal Solution
The planning, designing, permitting and constructing of new marine terminals require the collection of significant amounts of site-specific data and other information. Detailed analyses of the data and engineering studies must be performed to determine the most feasible location for a new terminal, and to optimize its design both from a financial and an operational point of view.
Required Data and Information
Metocean Data. To site a marine terminal, we must first understand the existing environmental conditions, including normal and extreme wind velocities and directions, wave heights, periods and directions by month and year, and water levels. In addition, the bathymetry in the vicinity of the proposed terminal must be defined. Data sources from the public domain are often useful, but in the event that sufficient data do not exist, it is often necessary to perform site– specific hindcast studies to develop historical conditions and/or install instrumentation to collect the required data.
In many locations though, only very limited data are available and time and/or budget constraints do not allow for collection of additional data during the feasibility or conceptual design stage of a project. In these cases, engineering judgement must be used, based on past experience at similar sites and any local knowledge that can be discovered. It will often be necessary to assume a range of potential conditions to use during the various studies discussed below in order to “bracket” possible results and determine the sensitivity that each of the parameters may have on the design of the project. This type of sensitivity analysis also provides insight into where limited data collection budgets should be focused and which parameters are of lower importance.
Geotechnical/Geophysical Data. The next requirement is the collection of pertinent geotechnical and geophysical data. These data will fall into two categories: those needed for the design of the marine terminal foundation structures and those needed to evaluate sedimentation/erosion in the vicinity of the terminal facility.
Typical geotechnical data required for design include classification and strength parameters of the soil materials. This information is typically used to make initial decisions on pile type, pile capacity, pile length, drivability, etc. Geophysical data is used to determine locations of rock and other subsurface features that may impact decisions regarding pile installation methods and dredging.
In order to properly evaluate the potential for any sedimentation or erosion, an entirely different group of soil parameters is required. These include suspended sediment concentrations in the water column, material type and grain size, erodability, settling velocities and critical shear stresses. As with the metocean data, in many situations the necessary data is not available during the feasibility or schematic design stage of a project and an assumed range of conditions must be analyzed to bracket the potential results.
Required Studies
Numerous studies/analyses are required to locate and determine the feasibility of a new marine terminal. Several are discussed below.
Wave Transformations. As waves propagate toward a project site, they are transformed both in direction and in height, due to the effects of the shallowing seafloor. It is important to quantify these effects so that a deep– water wave climate may be adapted to describe the nearshore climate. The nearshore climate may then be utilized for designing nearshore marine facilities and determining operational criteria for these facilities. One of the most important decisions may include the requirement (or not) for a breakwater to reduce wave-induced ship motions.
Sophisticated modeling techniques have been developed in recent years that use finite-difference or finite-element techniques to calculate the wave conditions (height, period and direction) at nodes (normally several hundred or thousand) defined within the boundaries of a model grid. These models take into account the effects of refraction and shoaling due to varying depth, local wind generation, energy dissipation due to bottom friction and wave breaking, and the effects of wave-current interaction. Additionally, wave diffraction around a breakwater or other feature may be a concern. A wind field can also be included in the model to account for any localized effects of wind-generated waves, as opposed to swell, which may originate some distance from the project site.
Hydrodynamic Analyses. In areas where current velocities or water levels are expected to be important design parameters, a hydrodynamic numerical model is often needed to assist in predicting the magnitudes of these items. The models used are typically 2D in nature. That is, they assume depth-averaged currents but can describe current velocities in both directions in the horizontal plane. The 3D models that can also describe the current velocities through the water column require significantly more data and time to provide reliable results and, thus, are much more expensive and time consuming. This expense is often difficult to justify (but not always) given the limited additional information such a model provides to the planning and design process. Conversely, 1D models, while suitable for unidirectional flows in rivers, cannot accurately describe flow patterns in tidal estuaries where most marine terminals are situated.
Sediment Transport Analyses. It is often necessary to determine potential sedimentation/erosion rates in the vicinity of a marine terminal. This is especially the case when dredged channels are proposed and estimates of maintenance dredging requirements are necessary to determine the feasibility of such a channel. Furthermore, if significant structures are proposed that could impact the water flows or wave climate, then these structures may also result in detrimental sedimentation or erosion patterns. Thus, it is often necessary to perform sediment transport analyses to determine what the effects may be.
Typically, there are three concerns with regard to sediment transport. The first is whether the marine facilities will have detrimental impacts to the adjacent shorelines. The second is the need, magnitude and frequency of maintenance dredging of channels, turning basins and/or harbors. The third concern is whether the new structures will cause siltation and/or erosion (scour) at other new or existing structures.
Mooring and Berthing Analysis. Berthing and mooring studies for the range of design vessels proposed for the new marine facility are required to determine appropriate loading on the marine structures. These studies are also required to determine operational and survival limits for the facility. The berthing analysis must consider such items as the availability and size of tugs to assist the ship into the berth, and the resulting velocity with which the ship will impact the structure. Additionally, the type of fender system proposed is an important consideration.
A mooring analysis first requires a review of the environmental conditions at the marine terminal. A decision must be made whether currents and/or waves are significant enough to be included in the analysis. If either waves or gusting winds are important, then a dynamic analysis will be required rather than a simple static analysis. Additional information required for such a mooring analysis includes the location of chocks, bitts and winches on the ship and their capacities, and the number, type and strength of mooring lines used, as well as the line arrangement. Once these analyses are completed, the resulting loads can be used to properly size the structural components of the marine terminal.
Berth Downtime Analyses. Once the environmental conditions are known at the proposed project site, a berth downtime analysis can be performed.
This analysis can be done based on monthly statistics of the conditions at the proposed site. Alternatively, the analysis can be performed on an actual time series of conditions at the site if sufficient data is available either at the site or at a nearby location such that it can be transformed to the project site.
Threshold values are established for each of the design parameters, such as wind velocity, wave height, current velocity, water level or ship motion. These values are based on the structural capacity of the marine terminal and any operational requirements of the facility. The analyses are performed to determine the frequency and duration of time that these threshold values may be exceeded. It is important to recognize that some of the parameters being evaluated are not independent of each other (i.e., wave heights and ship motions or high winds and water levels during a tropical storm), and thus they cannot simply be analyzed separately and added together. Rather, the analyses must consider the time when more than one parameter is exceeded concurrently.
These downtime analyses allow for the evaluation of the feasibility of alternative sites during the planning stage and can provide critical information in determining the extent of shore facilities such as storage tanks that may be required at any given location. Additionally, the need for any new protection structures, such as breakwaters, can be determined and a trade-off cost analysis performed between these new structures and the additional shore facilities required.
Vessel Maneuvering and Navigation Study. Oftentimes, marine terminals are located in areas where navigation is difficult. This can be due to constricted navigational channels, severe environmental conditions (currents, winds or waves), location of grounding hazards or congestion of the waterway due to other vessel traffic. In these situations, a vessel maneuvering study is necessary to determine whether a ship can safely access a new marine terminal or whether adjustments must be made to the navigation channels, navigation aids, terminal location or number and size of assisting tugs.
Preliminary evaluation tools include using a fast-time simulation computer program to simulate the maneuvering behavior of vessels. Often, at this early stage, advice from experienced pilots and/or vessel masters on a particular maneuver is obtained to validate the computer models. At later stages of the project design development, a full mission bridge simulation can be done which will allow mariners to “drive” the ship into the proposed berth using a 3D simulator and fully operational bridge.
Structural Considerations. Once a structural concept is developed for the marine terminal, a structural analysis is necessary to determine preliminary member sizes and the associated costs of constructing the facility. Local availability of materials, labor force skill level and experience should be taken into consideration when evaluating different structural concepts.
The seismicity of an area should also be considered when evaluating structural concepts and marine terminal sites. Seismic design criteria are usually governed by local building codes; however, in remote areas, specialist advice must be sought to determine the site-specific response spectra and maximum probable ground motions and accelerations.
Conclusion
The siting of a new marine facility can require a significant amount of data and study in order to determine the most feasible location. These items encompass a wide range of topics, ignoring any of them may result in unnecessary costs and could result in a facility having a fatal flaw that hinders its operations. Thus, undertaking these studies at the initial stage of a project can result in significant savings during the engineering, construction and operational stages.
References
“Site selection and design for LNG ports and jetties (Information Paper No. 14),” Society of International Gas Tankers and Terminal Operators, January, 1997. /st/
By Jeffrey G. Shelden
Senior Coastal/Hydraulic Engineer
Moffatt & Nichol Engineers
Raleigh, North Carolina
and
Scott C. Butler
Senior Project Manager
Moffatt & Nichol Engineers
San Francisco, California
Jeff Shelden is a senior coastal/hydraulic engineer with Moffatt & Nichol Engineers. He received a B.S. in civil engineering from the University of Virginia in 1984 and an M.S. degree from North Carolina State University in 1985. He is experienced in the analysis of coastal/riverine processes and hydraulics, as well as mooring analysis for
the berthing of ships, design offender systems and loading analysis of piers and wharves. His numerical modeling capabilities include the analysis of shoreline processes, tidal flows and pollutant transport for bays, estuaries, river mouths and harbors.
Scott Butler is a senior project manager with Moffatt & Nichol Engineers. He received a B.S. in ocean engineering from California State University, Long Beach, in 1986. He has over 19 years experience in the management, planning, design and construction of marine terminal facilities throughout the world. Through this experience, he has
acquired knowledge in the planning and design of ports and harbors including vessel maneuvering/berthing, mooring systems, vessel traffic studies, development of metocean design criteria, channel sizing, dredging and related coastal engineering issues such as wave protection for vessel berthing areas.
Copyright Compass Publications, Inc. Mar 2003
Provided by ProQuest Information and Learning Company. All rights Reserved
