Introduction
With Liquefied Natural Gas (LNG) as one of the key solutions to the rapid increase in energy demand, there is a need to employ integrated planning to deal with the marine transport and terminal design issues for new LNG facilities.
At present, LNG vessels range from 65,000 cubic metres to over 185,000 cubic metres in capacity, but much larger vessels are under development. Once an LNG vessel reaches a receiving terminal, the LNG is unloaded into large tanks and stored until it is re-vaporised and piped into the natural gas distribution network. As the capital cost of the tanks is significant, there is a need to optimise the storage capacity.
Further, the marine structures at an LNG terminal may have to be designed to serve a variety of vessel sizes and configurations and the appropriate storage capacity to handle their respective cargos. Sandwell has the tools to take an integrated approach to the design of a total marine system including: The simulation of the marine transportation, the investigation of the appropriate storage capacity at the berth, simulation of vessel operations and the optimisation of the berth itself. This paper summarises Sandwell’s approach to LNG terminal planning.
Transportation simulation
The planning of an LNG terminal is complex and its design needs an integrated approach to satisfy various ship/shore functional requirements adequately, as well as to minimise delays due to weather conditions.
A key Sandwell tool for terminal development is simulation of the LNG transportation system from the loading of vessels at the source, through the ocean transit conditions, to the receiving terminal and its storage system. Simulation can clearly identify cycle time variation throughout the year for each step of the LNG logistics chain and be used to optimise vessel characteristics, terminal operations, and storage requirements.
Accommodating the entire fleet of LNG vessels is desirable from a flexibility point of view; however, due to widely varying characteristics of individual vessels, this approach presents a major challenge to the terminal designer.
Simulation modeling can greatly reduce the number and type of variables that need to be considered for a proposed LNG operation and the subsequent detailed design of its marine terminal. It is the starting point for many of the projects Sandwell is asked to evaluate.
Initial terminal planning
Initial planning and site selection of a LNG marine terminal is very often governed by factors not related to terminal operations per se; e.g., land ownership, proximity of the supply or demand source, or other socio-economic factors. Thus, in most cases, the terminal planning exercise is carried out within a confined predetermined area that may not have all of the desirable elements to develop an effective LNG terminal.
Site-specific climatic data and bathymetric survey are essential for the initial terminal planning. The climatic data, if sourced from previously collected data, are best when confirmed by sitespecific measurements, even if only of limited duration. With the site-specific bathymetric and climatic data, terminal planning can proceed with two focus areas:
• Ship motion analysis
• Full bridge navigational simulation
Ship motion analysis
The ship motion analysis is generally carried out using a computer programme for hydrodynamic analysis of floating structures and their mooring systems for the site specific wind, wave and current conditions (see Figure 1 illustrating a vessel model using the AQWA suite of programmes).
A ship motion analysis permits the LNG terminal planner to optimise the berth and mooring layout and orientation, as well as to assess the anticipated ship motion due to wind, wave and current for all six degrees of freedom. This assessment then becomes the tool to determine the expected weather downtime and also to determine the limiting operating climatic conditions for a safe operation.
Full bridge simulation
Full bridge navigational simulation is done using a computer-aided real-time simulation facility to ensure that approaches to, and departure from, the proposed terminal would be safe, as well as to develop weather threshold and safety parameters for the terminal operations. The simulation also determines the tug requirements and checks the viability of the mooring system. There are a number of facilities that can be used for navigational simulation. A properly planned and executed ship motion analysis andn navigational simulation programme generally will lead to an optimised terminal layout, which can be further developed to meet the operational requirements.
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