Over 600 tcf of offshore gas reserves are estimated to reside in approximately 600 stranded gas fields worldwide. Producers have been seeking a flexible means to extend their transport reach and unlock these reserves. The commercial availability of Compressed Natural Gas (CNG) ships combined with Floating LNG allows the rethinking of offshore development. Fields previously considered stranded, can now be monetised by using CNG ships for regional distribution and LNG for global markets.

Subsea pipelines are the accepted method of delivering offshore gas to local markets. 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 exacerbated by a relatively short field life – since they cannot be relocated. Floating CNG overcomes these difficulties and effectively reaches many local and regional markets that cannot be supplied by pipeline. Marine CNG floats above water depth and seabed concerns and is economic for shipping distances up to 2000 km. CNG ships, unlike pipelines, can be relocated to new fields as the original field declines. This allows a series of smaller offshore fields to be developed in sequence, effectively increasing the total project life and minimize stranded assets.

With the advent of the American Bureau of Shipping (ABS) approved Coselle® CNG ship, CNG shipping has become a viable and flexible option for the transportation of natural gas over water. The patented Coselle technology uses very large pressure vessels made with inexpensive, small diameter steel pipe. One of the advantages of conventional construction is that CNG ships can be produced in shipyards with known costs and schedules. Given that the majority of project CAPEX is in the fleet, the risk for significant cost and schedule overruns on a marine CNG project is thus very limited. This innovative technology significantly reduces the cost of CNG shipping while offering exceptional safety and simplicity.

Offshore monetization with CNG and FLNGby David Stenning and Lyndon Ward

Sea NG Corporation

Figure 1: C112 Coselle CNG ship structure reveal

Loading facilities for a marine CNG project are relatively simple and essentially the same as those needed for pipeline export; processing, dehydration and compression. CNG discharge facilities are even simpler, and typically comprise metering, pressure and temperature control, and scavenging compression. These facilities can be located onshore or offshore on a fixed platform or floating vessel. In smaller applications, some or all of the facilities can be installed on each CNG ship.

One application to exploit a large gas field is to first deploy a production platform or vessel (eg. FPO) and a fleet of CNG ships to serve local markets. This provides early-stage revenue by providing these customers with an alternative to expensive liquid fuel or polluting coal. An FLNG vessel can be added later with the production unit providing processed gas to the FLNG vessel, effectively separating processing from liquefaction. This separation makes it possible to see the potential for a ‘standardized’ FLNG vessel that can be moved from one gas field to another provided that it receives gas processed to an LNG specification. As a flexible monetization solution, the processing platform or vessel, Coselle CNG ships and FLNG vessel can be moved from field to field and thus allow the sequential development of multiple smaller gas reserves.

Coselle CNG ships effectively serve the function of a portable floating pipeline; they can be deployed in any water depth, with any seabed conditions, and their economic reach is much longer than a subsea pipeline. Both solutions 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. An enhanced natural gas supply chain that includes CNG ships expands the boundaries of what is politically, economically and technically feasible.

Figure: FCNG for local and regional markets | FLNG for global markets