Grasping the potential
5 min read
07 Nov 2020
5 min read
07 Nov 2020
Kjell Ove Ulstein and Per Helge Madsen, Wärtsilä Gas Solutions, AS, Norway, review the latest trends occurring in the LNG regasification sector and some of the new regasification technologies and supporting systems currently available and in development for FSRU conversations.
The global demand for LNG is increasing at a fairly rapid rate. According to Shell’s latest LNG Outlook, demand rose by 12.5% to 359 million t in 2019, while consulting giant McKinsey expects average annual growth of 3.6% per year through 2035. This rise in demand places a strain on conventional shore-based regasification, which involves transferring the LNG to terminals for storage in tanks before it is regasified and pressurised with vaporising equipment, prior to it being delivered to the distribution networks.
During the past 15 years, alternative solutions have been developed that are faster and which, by extension, are also less expensive. Among these are Wärtsilä’s regasification modules designed for use onboard FSRUs, as well as shuttle and regasification vessels (SRVs).
Having the regasification (regas) equipment onboard the vessel allows high-pressure gas to be delivered to land-based networks, either via a floating buoy and submerged pipeline system from an offshore location or via loading arms on a jetty. Both FSRUs and SRVs provide greater flexibility than conventional land-based regas facilities, and the time from investment decision to start-up is relatively short.
By offering a fast-track means for opening energy markets, supply diversity is increased, costs are reduced, and environmental benefits are enhanced. A further advantage of utilising an FSRU, rather than a fixed land-based regas facility, is that it can be moved to a new offshore location should the business environment change.
SRVs, which transport LNG in large quantities while also using the onboard regas equipment to vaporise the LNG before sending it to a land-based network, typically work in pairs using separate mooring buoys. The brief overlap between one shuttle arriving and the other departing allows a continuous flow of high-pressure natural gas to be supplied.
A seawater heated regas system
Since 2006 up until mid-2020, Wärtsilä has developed and delivered 21 regas systems for FSRUs and SRVs, as well as one for an offshore jetty. The first two systems were steam heated using a water-glycol mix as the intermediate medium, but subsequently, 11 deliveries were made using seawater as the heating medium with propane utilised as the intermediate medium. These systems were delivered as complete modules for easy integration into LNG vessels, both for new-build vessels and for conversion projects to existing ships. The system depicted in Figure 1 has an intermediate loop of propane, where propane evaporates against the seawater in the plate heaters (PHEs). The propane condensates towards the LNG in printed circuit heat exchangers (PCHEs). By using the latent heat in the propane, only small amounts of it need to be circulated, while its low freezing point allows the use of printed circuit heat exchangers that are very compact and efficient. As a result, the entire system is rather compact and suitable for installation onboard ships with limited availability of space.
Figure 1. A seawater heated system with propane used as the intermediate medium
Figure 2. A seawater heated system with water glycol used as the intermediate medium.
Figure 3. Typical interconnection points for a 3 × 250 million ft³/d Wärtsilä regas module.
Switching to water glycol
Subsequent to these deliveries, Wärtsilä developed seawater heated systems using water glycol instead of propane as the intermediate medium. Propane is a very suitable fluid for this application, since it will not freeze and has good
thermodynamic properties. However, the use of hydrocarbons in the deck area is restricted by some of the industry’s major operators.
With water glycol, the heat exchangers need to be larger but the use of hydrocarbons as the intermediate fluid is eliminated. Since 2017, Wärtsilä has produced and delivered nine systems of this type. As shown in Figure 2, the system has an intermediate closed circulating loop of water glycol, which heats the LNG in two stages. The initial heating stage is in a shell and tube heat exchanger taking the LNG to a minimum heat of -15˚C, and the second stage is in a printed circuit heater, taking the temperature up to approximately 10˚C below that of the seawater.
The water glycol is circulated at a constant speed regardless of the LNG capacity, making control of the system very simple. The system can be arranged in such a way that the water glycol is channelled down to sea level, with the seawater/water glycol heater located there. This eliminates much of the power needed to lift the seawater to deck level.
Furthermore, since the water glycol loop is operated at a higher delta temperature (dT) than the seawater temperature in and out, the flow typically contains one-third of seawater. This means that the piping required to bring the water glycol up and down is much less than it would be for piping the seawater up and down.
The major benefits
Wärtsilä regas systems are delivered as complete modules, with all the engineering, component procurement, and construction of the module carried out entirely by the company’s execution team, making integration of the modules very easy. All the interconnection work and intercommunication activities with the parties executing the required utility systems is similarly very simple and straightforward.
In addition to those already mentioned, other benefits of Wärtsilä’s systems include: a fast delivery time of approximately 16 months; the compactness and flexibility of the plant configuration; the ease of operation and maintenance; and the fact that all equipment can be repaired or replaced on board a vessel.
Wärtsilä has been commissioned to build and deliver a large power plant to El Salvador, which will eventually supply approximately 30% of the country’s energy needs. The plant will operate on LNG.
In order to deliver LNG to the facility, a 290 m long Moss type LNG carrier, built-in 2002, is being converted by Wärtsilä into an FSRU. This is a major project that is being carried out on a tight schedule of just 14 months; having started in October 2019 with an expected completion date of the end of 2020. In order to be successful, the project requires close co-operation between all the parties involved.
The vessel is being fitted with a regas module, a compressor module, and a power module, all integrated as a single common package. The heat source will be seawater with water glycol as the intermediate medium. Wärtsilä’s experience and integration capabilities are considered an an essential element in meeting the various requirements of the project.
The regas module comprises four trains each of 70 million ft³ and has a total weight of approximately 450 t. It includes 300 lb of piping systems and has a length of 14.87 m, a width of 13.25 m, and a height of 18.18 m. The compressor module has two trains with noncryogenic
screw compressors and an approximate weight (including the skid) of approximately 200 t. It measures 22.46 m long, 8.11 m wide, and 7.49 m high.
The power module features three Wärtsilä 34DF dual-fuel engines capable of running on either LNG or conventional marine diesel fuels. The total weight of the module, including the crane, is approximately 750 - 850 t. It is 19.7 m long, 16.24 m wide, and has a height of 14.9 m. If the exhaust casing is included, the height is increased to 31.95 m. The three modules comprise the equipment required for an FSRU conversion. Supplied in this form, as complete modules, the conversion work and the interfacing with the yard are simplified. At the same time, the engineering, component procurement, and the production of the modules are carried out more efficiently, since Wärtsilä handles everything using the same project group and production yard.
It is hard to overstate the complexity of such a project, and the skills needed to bring it to a successful conclusion. When the conversion from LNG carrier to FSRU is completed, it will feature energy efficiency, robustness, and operational flexibility comparable to Wärtsilä’s other FSRU projects.
Figure 4. The layout of the former LNG carrier after conversion to an FSRU.
Figure 5. A Wärtsilä water glycol regas module ready for shipment. All connection co-ordinates are clearly marked.
Figure 6. The Wärtsilä Compact Reliq can be installed on existing vessels without extensive modification requirements.
Fulfilling gas chain supply requirements
Wärtsilä has products and complete systems serving the entire gas value chain. Among its offering is the capability to develop and deliver other products that may be required for the conversion of LNG carriers to FSRUs. One example is reliquefaction systems to handle the problem of boil-off gas (BOG).
The regas system itself has the capability to recondense BOG, but only while the system is being operated. Furthermore, when the regas system is operating on limited send-out capacity, the BOG recondensing capability is limited. In these situations, an additional system for handling the BOG is usually required. Gas combustion units (GCUs) are a common solution.
A GCU is a burner which combusts the BOG in a controlled manner without the risk of releasing unburned natural gas into the atmosphere. Although a possible solution for BOG handling, no useful energy can be recovered from a GCU, which is why it should primarily be recovered by other means.
Wärtsilä’s BOG reliquefaction process is based on reversed nitrogen Brayton cycle refrigeration technology. This means that the process is a closed nitrogen cycle for extracting heat from the BOG. Typically, the reliquefaction system is used to control the cargo tank pressure by liquefying BOG. The system has the capability to handle all BOG (100% capacity) or only excessive BOG not burned in the engines (partial liquefaction).
Adding onboard reliquefaction flexibility.
Another recent introduction to this field is Wärtsilä’s Compact Reliq reliquefaction plant. The system is compact in design, enabling it to be installed on existing gas carriers and LNG bunker vessels without extensive modification work.
The system is designed to reliquefy BOG, while also keeping the cargo cool under all operational conditions. The Brayton technology allows a portion of the BOG to be utilised as fuel for the vessel, while the excess can be liquefied and sold with the LNG cargo. Liquefying the BOG instead of burning it in a GCU also has a positive environmental effect, as no gases are released into the atmosphere.
Increasing demand around the world for LNG is disrupting the conventional means of its regasification in land-based facilities. Greater flexibility enabling faster and less costly means for regasification is clearly called for, which has, in turn, led to a fairly rapid increase in the use of floating alternatives. The technology is well established and the solutions are proven. One can assume that growth in this market will continue to gain pace.