In the drive to monetize stranded offshore gas fields, LNG and CNG are often thought of as competitors. This perspective misses a significant opportunity for synergy. CNG shipping is most effective in supplying regional gas markets while LNG shipping is most efficient is supplying global markets. Together, CNG and LNG can efficiently serve both the local and global markets adding significant value to offshore gas field development.

Subsea pipelines are the accepted method of delivering offshore gas to local markets. However, in cases where the distance is too long, the seabed too difficult, or the water depth too deep, a subsea pipeline can quickly become uneconomic. Pipeline economics can be further exasperated by a relatively short field life – since they cannot be moved. Marine CNG overcomes most of these difficulties and effectively reaches many local and regional markets that cannot be joined by pipeline.To overcome the shortcomings of subsea pipelines, industry first turned to the development of floating liquefaction and offloading plants (FLNG). However, FLNG vessels are proving to be complex and expensive, especially when the gas requires significant processing before liquefaction.

The American Bureau of Shipping (ABS) recently provided full class design approval for a CNG ship that uses very large CNG pressure vessels made with inexpensive, small diameter coiled steel pipe (a ‘coiled-pipe’ CNG ship) . This design dramatically reduces the cost of CNG shipping while offering exceptional safety and simplicity. The ‘coiled-pipe’ CNG ship is designed to be constructed using normal shipyard construction techniques, with Korea’s largest shipyards qualified as builders. Therefore the risk for significant cost and schedule overruns on a marine CNG project is limited. Each of the pipe coils are themselves contained with a framework and are thus integrated into the ship like any other ‘block’, and form part of the ship’s structure. With the advent of a CNG ship receiving full approval from ABS, CNG shipping is gaining industry acceptance as a viable and flexible option for the transportation of natural gas over water.

CNG receiving facilities have a very small footprint, since no regasification is needed. In circumstances where an absolute minimum shore impact is required, the scavenging compression can be installed on each of the ships so the only onshore facility would be a pipeline connection.

CNG ships used in conjunction with a conventional offshore Gas Processing Unit (GPU - floating vessel or fixed platform), make it possible to exploit smaller gas fields providing there are regional gas markets. For large gas fields, capable of supporting an FLNG project, marine CNG ships can still serve the local and regional markets, while LNG ships serve the global markets.

Floating LNG and CNG working togetherby David Stenning, P.Eng and Lyndon Ward

Sea NG Corporation

Floating options enable producers to overcome limitations of traditional subsea pipelines

Figure: CNG ship with large pressure vessels

Next Gen monetization: Sevan Cylindrical Gas FPSO with

Marine CNG Shuttle ShipsOne possible way to exploit a large gas field is to first deploy a gas processing unit (GPU) and CNG ships to serve local/regional markets and adding an FLNG vessel later to serve global markets. The GPU which will provide processed gas to the FLNG vessel thus saving considerable cost and complexity. Using this method it is possible to see a ‘standardized’ FLNG vessel that can be moved from one gas field to another since it will always receive gas processed to an LNG specification. Serving local/regional markets has considerable appeal since these markets are often using expensive liquid fuel or polluting coal. Solving the local gas supply issues can often enhance the acceptability of an FLNG solution to the local authorities.

The GPU, CNG ships and FLNG vessel can be re-deployed from field to field and thus allow the sequential development of multiple smaller gas reserves that would be impossible to exploit using pipelines or FLNG alone.

CNG technology should not be seen as a competitor to LNG, but as a complementary technology. A CNG ship effectively serves the function of a portable floating pipeline; it can be deployed in any water depth, with any seabed conditions, and its economic reach is much longer than a subsea pipeline. Gas fields once considered stranded can now be developed. By restructuring the natural gas supply chain to include CNG ships, the boundaries of what is politically, economically and technically feasible expand dramatically.

LNG and CNG can partner to extend the lifespan of onshore LNG plants by providing new sources of feedstock and expanding the scope of FLNG applications by adding regional CNG markets. Rethinking the transportation of natural gas opens hundreds of new opportunities to deliver offshore gas reserves to a range of consumers.

“ ...CNG ships effectively serve the

function of a portable floating pipeline. ”

by Fredrik Major, CBDOSevan Marine ASA

Norwegian outfit, Sevan Marine has focused on the development of a cylindrically shaped floater hull since they started in 2001. Today, elevan projects have materialized five Floating Production Storage and Offloading units (FPSOs), four Mobile Offshore Drilling Units (MODUs) and two Accommodation Units. The understanding of the benefits for this concept when compared to ship shaped floaters is increasing within the oil companies. A cylindrically shaped unit will respond to environmental forces from waves, wind and current in the same way regardless of the direction these are acting in. For this reason, there is no need for the unit to rotate or weather-vane in order to align with the main environmental forces. In this way the turret and swivel technology is not required, greatly simplifying the floater and resulting in significant savings both in capital and operational cost as well as reducing the risk for downtime caused by failure of this complex system.

In addition to FPSO, MODU and accommodation, applications of the Sevan hull for FDPSO, FLNG, floating powerplant (Gas to Wire), FSRU and well intervention has been developed.

For a gas FPSO processing and exporting compressed gas either to marine CNG tankers or to a pipeline, swiveling of the gas-rich wellstream and the export gas both at high pressure is creating challenges for the swivel that will be avoided for a floater moored in fixed orientation.

Sevan’s cylindrical FPSO design in combination with a submerged buoy system for gas transfer to marine CNG tankers represent a safe, efficient and proven concept for harvesting offshore natural gas fields.

Figure: Conceptual FLNG/FCNG configuration

1. Sea NG Corporation has received full class approval for its Coselle® CNG ship design from the American Bureau of Shipping. The Coselle is a patented CNG pressure vessel that is a large coil of six inch pipe contained with a carousel-like structure.

Sevan FPSO with traditional oil tanker

Loading Coselle CNG Ships: A Gas Process Perspective

by Dr. Mark Trebble

INTRODUCTIONThe ability to efficiently load a series of CNG containers, each comprising a large coil of small diameter pipe, within the hold of a ship is key to the economics of the overall value chain of marine CNG. Loading large quantities of natural gas into a ‘coiled pipe’ CNG ship is unique and required detailed evaluation through computer simulation and physical tests. This is a brief description of loading coiled pipe CNG ships from a gas thermodynamics perspective. GAS CONDITIONINGGas is generally treated prior to injection into the CNG containers to avoid the possibility of liquid formation inside. This generally means that the gas cricondentherm is kept below the minimum gas temperature within the CNG containers which ranges from 15 C to +10 C and the water content is kept below 7 lbs/MMscf.

GAS LOADINGLoading and unloading of the coiled pipe CNG containers offers some interesting processing challenges due to the unsteady state nature of the process. Ships arrive at a loading facility with a heel pressure as low as 150 psig. The

conditioned gas is compressed into the ship either through a single or multiple header system up to a pressure of 4000 psig. Compressor aftercooling is accomplished with air or water cooling although in some instances it may be economic to refrigerate the feed gas in order to reduce the maximum stored gas temperature and thus increase the density of the carried gas. The gas temperature within the coiled pipe is kept above -15 C in all instances to avoid a lowering the steel temperature (including safety factor) into a region that could affect its ductility.

THERMAL EFFECTSThe filling of the coiled pipe causes a temperature rise inside the pipe in the order of 40 C when aftercooling by air coolers is employed. This temperature rise is calculated by simple energy balance and is mitigated by the thermal capacitance

of the encasing steel. This energy rise was confirmed by pilot plant testing described previously in an article by Trebble and Jeffery (2008) . As well as the change in the overall average temperature, the temperature gradient along the length of the coiled pipe is also of interest. By using dynamic simulation we were able to match the simulated thermal profile along the length of the coiled pipe to the pilot plant data. The conclusion of the study was that the magnitude of the transient temperature profile was relatively small and was mitigated over a reasonably short time frame. As a result we have utilized steady state simulation to calculate the overall temperature rise during filling as well as the temperature drop upon depressurization at the receiving facility. For a typical application the drop in temperature at the receiving facility is quite close to the temperature rise upon filling.

Management of the temperature within the coiled pipe over an entire cycle has a few control elements. If the temperature trend is increasing over cycles it is possible to reduce the average temperature by either aftercooling more or by raising the heel pressure. Alternately if the temperature trend is decreasing it is a simple matter to aftercool less and leave the feed gas to the coiled pipe hotter.

CONCLUSIONEvaluating the thermal behaviour of the gas during ship loading presented some interesting challenges during the design process. Engineering and testing have shown that CNG ships using coiled pipe as the CNG container can be loaded in an efficient and cost effective manner using standard approaches to design and implementation.

i. The cricondentherm temperature is the highest dew point temperature seen

on a liquid-vapor curve for a specific gas composition over a range of pressure

ii. Trebble, M.A. and Jeffery, J., “Measurement and Modelling of Thermal

Transients in Coselle Based CNG Transport”, Presented at Chemeca, October,

2008, New Castle, AUS.

Figure: Floating CNG Value Chain high-lighting gas processing and loading