CNG carrier: pipe dream or reality?

A number of oil majors are seriously considering a new technology for the carriage of compressed natural gas (CNG) in coiled pipes onboard ship. Enron, BG International, Chevron, Conoco, Exxon Mobil, Shell and Marathon have all contributed to developments to date. Now the company behind a preliminary study into the so-called Coselle technology ? Cran & Stenning Technology ? is seeking funding for a pilot scheme to demonstrate the project?s technical feasibility via live cycle testing. It is examining the possibility of shipping gas from Venezuela and Trinidad to the islands of Cura?ao and Aruba. The quantity of gas involved is modest (60 million m3) making the route particularly suitable for one or more Coselle tug/barge unit. This project is currently in the financing stage and Cran & Stenning Technology is confident it will have the $20-30 million required in place before long. Canadian inventors? David Stenning?s and James Cran?s Coselle technology could, if they can raise the funds, change gas field economics forever. Instead of using pressure bottles, they propose to use pipes coiled into a carousel to contain the gas. The Coselle (derived from ?coil in carousel?) technology involves coiling 10.6 miles (17km) of standard 6.6-inch (16.75cm) diameter pipe into a carousel measuring some 15.4m across by 3.4m high. Each carousel will have a gas carrying capacity of 71t. The inventors claim their proposed CNG ship design is capable of transporting 108 carousels giving a total capacity of 7,700t. While a 135,000m3 LNG carrier has far greater capacity ? about 60,000t of gas ? it also requires special cryogenic reception facilities ashore costing over $800m. Add this to the $170 million required to build each 135,000m3 LNG carrier and it means large gas reserves are required to make LNG economically attractive. The Coselle ship, on the other hand, could cost as little as $100 million if built in series and it could plug directly into a pipeline distribution network or to a nearby power generating plant that can use the gas, say its inventors. Stranded fields The main raison d??tre for CNG is that it helps make marginal fields more economically attractive and can provide an alternative to undersea pipelines, say Stenning and Cran. In numerous instances worldwide, offshore gas reserves are stranded because neither LNG nor pipelines can economically exploit them. A rough guide within the oil industry suggests a pipeline costs about $1 million per mile ? a cost that is difficult to bear if there are long marine distances, small reserves or difficult marine environments such as deepwater, ice scour, or environmental concerns. It is suggested the Coselle ship could exploit reserves at distances of 500-2,000 miles offshore. Using associated gas Associated gas, i.e. natural gas found while developing oil fields, is in many instances, such as in Nigeria, simply burned off, known as flaring, or re-injected back into the field, as in Canada. With oil extraction and production expanding to deeper waters, associated gas will become an increasing problem and is cited as the most limiting feature of the FPSO (Floating Production Storage and Offloading unit) field development concept for deep waters currently being exploited. These not inconsiderable quantities of gas would be ideal for Coselle CNG technology, say its developers, where a Coselle carrier could take the gas directly from the deep water FPSO and deliver it to a nearby market or into existing facilities in more shallow water. Strong candidates for this solution are the Hibernia and Terra Nova offshore oil fields off the Canadian East Coast where the associated gas is currently re-injected despite the presence of a lucrative market for this gas only 1,100 miles away in the Boston area. Taking this concept a stage further, a new type of FPSO, primarily designed to exploit gas fields, becomes an attractive proposition when linked into a Coselle CNG transport system. This would enable the commercially viable exploitation of deep water gas fields with limited reserves since the gas FPSO along with the mooring system and CNG carriers could be moved from field to field. Safety aspects Concern has been raised that the coiled Coselle pipe might rupture as a result of chafing when the ship is in motion but, because the thin walled pipe (pipe-wall thickness is 0.25 inches) is very ductile and less likely to suffer from propagating cracks, the probability of this happening is remote. Even if it were to happen, the gas flow is effectively choked by the pipe?s small diameter and so the decompression energy is released more slowly than would be the case in a large diameter conventional pressure vessel. To enhance safety, the Coselle stacks would be inerted with nitrogen in order to eliminate the danger of fire below deck. All valves and fittings are installed above deck to facilitate servicing. The Coselle CNG carrier design has received preliminary approval of the concept from class society ABS with the drawing up of guidelines for the new class of vessel. DNV found that “in general, the concept was accepted as providing equivalent safety as compared to a standard LNG tanker”. Lloyds Register has carried a brief review of the technical aspects and found that it agreed in principle with the work done by ABS and DNV but the study was not large enough for LR to verify the work in detail. Interestingly, under the current mandatory IMO standard (the International Gas Carrier Code) which governs LNG and other existing gas ships, CNG is not classified as a cargo but, in order to ensure a conservative and acceptable design, these rules were followed where applicable. The next step is to assess the concept in practice and to this end a full-sized Coselle will need to be built to examine the abrasive interaction between the coils while the CNG carrier is at sea. It has been found that only the topmost layers where there is insufficient pipe weight to keep the pipe from vibrating could cause a problem. Therefore, the design calls for the cribbing to be placed on top of the pipe so that the topmost layers are secured. The vast majority of pipe within the Coselle cannot vibrate since it is simply restrained by the surrounding pipe. There is no doubt however that the favourable response from the class societies is due largely to the excellent safety record of LNG carriers coupled with the fact that the Coselle CNG carrier has 200 times greater cargo division than an LNG carrier. In the event of a major tank failure therefore, the potential spillage would be correspondingly less. Another factor is that CNG is a gas and not a cryogenic liquid like LNG. In the event of an accidental release CNG will vent into the atmosphere while the low temperature of the liquefied gas could put the ship in jeopardy by making the ship?s steel brittle and dangerously weakening the hull. Industry reaction In a joint industry project a wide range of natural gas compositions were studied by Fluor Daniel, a leader in power plant design, construction, and complete life cycle services. It concluded that the Coselle?s can safely load, store and discharge rich associated gases provided they are dehydrated to prevent hydrate formation and that the temperature in the Coselle?s is controlled to prevent steel temperatures below ? 40?C. Fluor Daniel also concluded in a paper to the Gas Processing Association?s annual convention that Coselle CNG technology offers the best potential for commercially utilising the associated gas produced on Canada?s East Coast. In another paper by BG Technology (British Gas) entitled “Commercialising stranded gas reserves”, the authors noted that Coselle CNG is a new technology that has considerable promise. On the same subject, Worley International Inc, an Australian based global design and project services business, presented a paper in June 2000 “Natural gas development based on non-pipeline options – offshore Newfoundland” concluded that Coselle CNG offered significant advantages for stranded gas exploitation and that there were no significant technical hurdles. Finite reserves There appears to be no shortage of potential markets for the Coselle. With finite reserves of fossil fuel it will be increasingly imperative for every source to be exploited to the limit, particularly since more and more countries are switching to gas and worldwide consumption is expected to rise by 20% in the next decade. There seems little doubt that the invention by Cran & Stenning of the Coselle CNG storage system and the development of the Coselle CNG carrier promises to greatly improve the economics of shipping natural gas over short sea routes, unlocking many presently ?stranded? gas reserves. n