TERMINAL SITING—Conclusion: Project viability hinges on waterway, land assessments

Table 1 provides guidance regarding typical areas across which a cloud could extend.

The risk of ignition and the consequences thereafter depend on the type and density of population within the spread of the cloud, making it important to know the population densities and activities within 2 miles of the terminal.

As LNG ship sizes increase, ship-arrival frequency decreases, but correspondingly, required storage volume for loading and unloading increases. Vessel delays due to unfavorable weather or ship problems directly affect LNG stock levels at both terminals and liquefaction plants. The volume and design of ship tankage therefore, largely determines the area required for onshore facilities.

Operating full-size LNGCs will require at least one 265-295 ft diameter tank. Operational considerations, however, suggest the need for more than one tank. Tank-containment philosophy, however, likely most affects the area required for the terminal (and also its location relative to its neighbors).^{2 3}

Single-containment tanks require large bounded areas and are only practical where space is readily available and little or no population nearby. Full-containment tanks provide the smallest likelihood of an accidental release and can be used almost anywhere but are expensive. Double-containment tanks cost slightly less than full-containment but still need a large terminal area.

A low throughput terminal (up to about 3 million tonnes/year) handling large LNG carriers (125,000 cu m or larger) will need two tanks. Industrial or residential areas require full-containment tanks, but in greenfield or unpopulated areas double or single-containment designs may be acceptable.

A terminal in an industrial area, with two full-containment tanks, would need about 40-50 acres.

Table 2 provides rough terminal areas for initial site-selection studies based on different containment configurations.

Shoreline lengths

A terminal might also require water intakes and outfalls and firewater or water for use in the vaporizers. Intakes are often large structures accommodated by settling ponds, pumps, and screens and are usually on shore. Water for the intake comes from a pipe placed close to the bottom of an adjacent waterway but away from the shoreline to avoid bringing too much sediment into the system. Outfalls are simpler structures located away from the intake to avoid recycling water between the two structures.

If insufficient shoreline exists to provide enough distance between water intake and outfall to prevent mixing cooling water, the choice of vaporizer may be limited to either a submerged combustion vaporizer or shell-and-tube exchanger vaporizer.

Soil, ground

Poor soil and ground conditions at a site can raise material and construction costs, typically requiring pile foundations and soil enhancements.

LNG tanks should be founded on rock, but firm sand would also avoid use of piled foundations. Soft materials such as silts and soft clays, often found in estuarine locations, will almost certainly require piling under the larger structures, such as tanks and vaporizers.

Risks associated with flooding, environmental concerns, and the cost associated with ground preparation can make areas such as marshes and swamps unsuitable.

Seismic conditions

Earthquakes can damage storages tanks and pipelines.

Japan, one of the world’s largest users of LNG, has many LNG storage tanks and LNG pipelines. Even during its most severe earthquakes, however, no LNG tanks were damaged, even though LNG pipelines were.

Site elevation

Low site elevation may contribute to the risk of site flooding. Events that may result in flooding include:

1. Poor drainage.
2. High river elevations created by upstream conditions.
3. High sea levels created by low atmospheric pressure and high winds.
4. Tsunamis.
5. Hurricanes.

All of these events can damage facilities, lead to partial or complete shutdown, or disrupt terminal operations. The US Federal Energy Regulatory Commission requires a storm-surge study on any prospective site. This study determines site elevation relative to 100-year flood levels and its susceptibility to flooding.

An area that normally floods, however, does not preclude its selection. A number of measures can mitigate risks associated with siting a terminal in such an area. These include using pile foundations, raising site elevation, or building a berm around the facility. Each of these options adds to the facility’s cost.

Environmental limitations

Large LNG carriers can normally berth in winds up to about 20 knots. Some current terminals quote a limit of 30 knots, but this may apply principally to membrane carriers since they have a lower sail area than spherical carriers. It is also possible that a terminal will rely on additional tugs should this condition occur. LNGCs will usually remain at berth in winds up to about 40 knots, although the berth itself will usually be designed for 60 knot winds or higher.

Ship breakout from the berth can have serious consequences. Orienting the berth so that the LNG carrier will tend to be pushed toward it can help reduce this risk but may not always be feasible, as other conditions might take precedence. This possibility has prompted LNG discharge arms to be equipped with powered emergency release couplings to reduce any spillage stemming from exceeding the loading or unloading arm’s operating limits.

Waves and currents can also place large loads on a ship’s hull. Current loads on a ship’s hull are high if imposed beam on, with implications for berth design and operation. The speed of a current is less important than its directional uniformity, although current speeds greater than 2 knots will usually result in operational difficulties. Currents should be considered during vessel maneuvering as well as while the LNGC is at berth.

Jetty location

Marine terminals should afford protection from waves and currents and be away from vessel traffic. Berthing a vessel in a strong current (>2 knots) can be problematic and may make a location undesirable from a shipping viewpoint. Water depths should allow passage during all states of tide.^{4 5}

The seabed’s nature and underlying strata determine an acceptable draft at the jetty for vessel operations.^{5 6} Underkeel clearance typically measures 10-20% of the vessel draft, depending on wind, wave, and current conditions as well as the nature of the seabed and operator requirements. Vessels subject to large wave action will require a greater underkeel clearance than those in calm waters.

Berthing and unberthing operations should occur in a maneuvering area with a minimum diameter of twice the ship’s length unless it can be demonstrated the berthing maneuver requires less. The berth should be away from the navigation channel to afford protection to the docked LNG vessel. Aside from the heightened threat of collision with other ships a site near a channel would introduce, wakes can disrupt unloading operations if they cause excessive vessel motion. FERC requires analysis of passing ship wakes’ effect on moored gas carriers.

Local port infrastructure such as tugs, pilots, support craft, and operations such as refineries or chemical plants can increase the attractiveness of one location over another.

Marine terminals, however, should remain sufficiently separated from other operations with safety distances determined through risk analysis. Items addressed by risk analysis should include LNG spillage and collision risks from adjacent channel shipping. Analysis should also include vapor cloud dispersion models which take into account LNG spillage and calculating vapor dispersion resulting from site-specific wind speeds and directions as well as other environmental conditions.

Results of these models form part of the required permitting exercise in the US and much of the world and can affect placement of the berth relative to other berthing operations in the area and also relative to the channel. Once a site has been selected, design addresses any risks to operation by carefully choosing equipment and procedures.

Access to the terminal from the sea should be as direct and as short as possible, reducing both transit time through confined waters and the risk of interface with other shipping. Such access also permits quick departure to open water should an emergency arise. Vessels should not have to pass under any bridges or other structures spanning the channel if this situation can be avoided.

LNG vessels require unobstructed approaches from the open sea. Typically the LNG carrier will pick up the pilot(s) at the outer limit of the channel or the sea buoy where an anchorage should be and then be escorted into port.

Wave heights from 2-3 m may impose operating restraints on pilot pick-up or require the use of helicopters. Entrance channels should run straight with as few bends or sharp turns as possible. Buoys should mark the limits of the channel. Leading marks or lights should orient a ship for safe transit into the more sheltered inner port area.

Exposed outer channels may require an underkeel clearance of 20% of the ship’s draft to accommodate wave action in its approach to the port, squat (the increase in draft as a result of relatively high speed in shallow water), survey error, rolling, etc.⁶ Inner channels may only require an underkeel clearance of 10%.⁶ Both the outer and inner channel should be of sufficient depth to accommodate the largest LNG carrier envisioned at the terminal during all states of tide.

A Qmax vessel with a loaded draft of 12 m may require an outer channel depth of 14 m (46 ft) below chart datum and an inner channel depth of about 13.2 m below chart datum. Chart datum is the lowest predicted water level.

Lesser channel depths may be acceptable if the ship can enter and leave the port after low tide, or if the bottom is soft, but this is unlikely given the potential consequences of an incident, particularly in a heavily congested port area.

It may be possible to dredge a deeper channel, but this can be expensive, particularly if the seabed is rocky. Environmental issues associated with dredging and dredged material disposal also require attention. Ongoing dredging costs may become an issue if the selected waterway is subject to accumulation of sediment deposits. If none of these is an option, it may be necessary to limit the size of ship visiting the terminal.

Inner and outer channels also need sufficient width. In most LNG ports the port authority would ensure LNG carriers have a safety zone around them to reduce the risk of collision with other port users. The channel can then be treated as a one-way channel, with no other ships passing. Failure to provide a safety zone greatly increases the risk of ship collision, requiring a much wider channel and potentially straining the economics of the project.

Permanent International Association of Navigation Congresses provides guidance on acceptable channel width, accounting for various conditions which the vessel may encounter in its transit to and from the terminal. These conditions include whether the channel is an outer channel exposed to open water or a protected inner channel. Vessel speed, wind speed and direction, the presence of waves, current intensity and direction, aids to navigation, bottom surface and depth of the waterway, cargo hazards, and bank clearance also require consideration.

All channels require a basic maneuvering lane 1.3-1.8 times the beam width, with the previously listed factors adding to this basic width. Safety also requires bank clearance on either side of the basic maneuvering lane to help prevent damage to the ship’s hull from grounding. A damaged hull may result in cargo delay, channel blockage, or worse, breach of containment.

The nominal width of a maneuvering lane for a single LNG carrier transiting through an outer channel measures 6-8 times the beam. The nominal width required for an inner channel is around 5 to 6 times the beam. A 266,000 cu m Qmax requires bottom width of about 900-1,080 ft. Comparing calculated width for an inner channel with an actual channel often shows many are inadequate for larger LNG vessels.

Air draft

A vessel’s air draft measures the height of the highest point of a ship (usually the mast) from its waterline. It must therefore measure less than the distance between water level and the underside of any structure spanning the channel. Navigation charts provide river crossing information and can be used for initial site-selection exercises, but elevations on waterway crossings should be verified before completing final site selection.

LNG carriers should ideally avoid passing under any bridges or structures spanning the channel. The superstructure of LNG carriers usually measures 130-160 ft and modifications may need to be made in order for them to pass under lower structures. A waterway’s air draft (distance between the highest water level and the underside of the structural span) should therefore measure at least 135 ft for initial site selection.

References

1. GAO-08-141, “Maritime Security: Federal Efforts Needed to Address Challenges in Preventing and Responding to Terrorist Attacks on Energy Commodity Tankers,” Washington, December 2007.
2. Code of Federal Regulations, “49 CFR 193—Liquefied Natural Gas Facilities,” Rev. 2003.
3. National Fire Protection Association, “Production, Storage, and Handling of Liquefied Natural Gas,” NFPA 59A, 2001 edition.
4. Joint Permanent International Association of Navigation Congresses (PIANC)-IAPH Report, “Approach Channels-Preliminary Guidelines,” MarCom Working Group 30, 1995.
5. Joint PIANC-IAPH Report, “Approach Channels—A Guide for Design,” Vol. 2, MarCom Working Group 30, 1997.
6. PIANC, Report of a working group of Permanent Technical Committee II, “Underkeel Clearance for Large Ships in Maritime Fairways with Hard Bottom,” 1985.