The size and type of vessel being used in liquefied natural gas (LNG) transportation has implications on jetty and terminal design. LNG vessels are classified according to their cubic metre carrying capacity. Among these vessels, two types of large LNG tankers are Q-Flex and Q-Max. Broadly, LNG vessels can be of two types – the spherical moss tank type and the membrane type.
While selecting a site for an LNG terminal, several factors need to be considered. These include proximity to the gas grid, pipelines and end-users; lowest possible sea transportation costs; the least possible operational risk, capital cost and cost per unit of LNG. From a mariner’s point of view, marine risks while positioning the jetty need to be minimised.
Most LNG terminals are of the conventional type in which LNG regasification is done on land and away from population centres to ensure safety in the eventuality of a gas leak. The choice of floating storage and regasification units (FSRUs) versus conventional terminals is primarily governed by commercial factors and agreements. Offshore FSRUs may not be all-weather depending on the wave climate, while near-shore FSRUs may require shelter and mooring arrangements. Land availability is also an important criterion in planning an LNG terminal.
Planning the terminal
LNG terminals are often a part of multi-purpose ports. It is essential to plan for an LNG terminal at the onset of port development, as it is difficult to retrofit an LNG terminal subsequently. Land-side and marine-side safety clearances are also important. Land requirements for conventional terminals can be substantial. FSRUs too have specific land requirements in terms of safety zones and buildings. Also, while planning a terminal, it is important to plan for the long term and for large ship sizes.
The local wave climate too has to be kept in mind while designing an LNG terminal. This is important for both FSRU and conventional terminals. The type of breakwater constructed depends on wave energy, depth, presence of rocks, cost, etc. Rubble mound breakwaters, with or without proprietary armouring, are the most common in India. Island breakwaters are not common as they are difficult and expensive to construct. Offshore LNG terminal operations can be impacted by metocean conditions. Tranquillity is required at the pilot boarding location and within the harbour. Modelling studies are needed to establish tranquillity. A spectral wave model, SWAN (developed by Delft Hydraulics), dedicated to coastal and port applications is widely used to model waves.
LNG jetty design
The number and size of berths in a terminal are determined by the quantity of LNG to be delivered, the size of ships, time interval between two calling ships and specific site conditions. The berths may be installed either parallel or perpendicular to the bank at the end of the jetty. A docking aid and piloting system helps the pilot and vessel master monitor speed and distance accurately so as to manoeuvre safely alongside the jetty.
The berth usually includes breasting and mooring dolphins, and an unloading platform with unloading arms and other facilities such as fire and gas detection systems, firefighting systems, etc. Land access to moored ships is provided. Based on model studies and downtime analysis, a breakwater is provided if deemed necessary. No uncontrolled ignition source is permitted within a predetermined safe area. All mooring points are equipped with quick release hooks. The design of the approach trestle should cover deflection for piles, placement of different pipes, and the gap between the approach road and process piping.
The types of vessels calling at a terminal also play a role in determining jetty design. For a five-compartment spherical vessel, the vessel manifold is off-centre and this governs the spacing of the mooring dolphins. The greater the variation in vessel size, the higher the number of mooring dolphins and the more expensive the jetty.
Planning an LNG harbour is similar to planning any other type of harbour. Particular care is needed at the harbour entrance where a part of the ship is subject to wave action while the rest is within the tranquil harbour. Once a harbour is created, the result is a quiescent area where sedimentation can take place, and depending on the sedimentation regime, maintenance dredging could be required.
Conclusion
Receiving LNG ships at terminals requires special measures. The marine and civil design of FSRUs and conventional LNG terminals need more attention as compared to other terminals. There are various issues that need to be addressed while developing LNG terminals like exclusion zones, jetty optimisation for type of terminal, jetty design for types of ship, harbour tranquillity and navigation simulation. w
Based on a presentation by Suren Vakil, Managing Director, BMT Consultants
