Integrated Floating Power with LNG regasification facility
Augusto Bulte
Director Gas Monetisation / LNG
Amec Foster Wheeler

Global LNG businesses are becoming more competitive, driven by an oversupplied market and a requirement of flexibility by the LNG offtakers. Most LNG market analysts appear to concur that this situation will remain over the near-term while the LNG trade shifts toward a spot market environment.

Taking advantage of low LNG prices, many countries have developed LNG regasification capacity based on Floating Storage and Regasification Units (FSRU) technology to provide new channels of baseload feedstock for power plants.

A possible new development to further satisfy the market demand is the integration of a Combined Cycle Power Plant (CCPP) on a ship with a LNG regasification system.

There are some developments in the industry integrating both facilities on a ship to produce power in a range of 400 – 800 MW. These developments, which are still to demonstrate their feasibility and cost effectiveness in the years to come, would avoid the need of fuel structures in remote areas and would be easy to deploy and eventually to decommission. It is expected that a floating power plant could moor at a selected site for three to five years on a leasing agreement, or larger periods under a PPA.

A possible gas to power solution that would optimize installation cost would be to consider the refurbishment of existing surplus LNG carriers into power ships. Limitationsmight exist however, in terms of available plot for the large power plants due to the ship size. Current concepts of this solution limit the power output to 400 MW. Other concepts consider building a fit-for-purpose ship, which would likely come at a high upfront cost.

In this sense, alternative designs might be explored to maximize power output and reduce investment cost. Thus plant efficiency in regasification of LNG as well as in power production needs to be of the highest possible efficiency. As it is well known LNG regasification is an intensive thermal energy exchange process, such thermal energy can be recovered not only to optimise power plants production in different ways , but also for other usessuch as water desalinisation.

The proposed scheme is as follows:

Figure 1: Process Scheme

Maximum utilisation of the cold energy is achieved when it is used for both chilling the inlet air to the Gas Turbine and chilling the cooling water used for the Steam Cycle (SC).

Gas turbines are air-based engines, with operating efficiencies directly sensitive to ambient conditions.The effects of ambient air through an actual gas turbine, which are ignored in ideal models, must be considered.

In this case, the LNG is vaporised in a vaporiser by a heat transfer (HT) fluid (warm water from the cooling circuit of the steam cycle condenser). Downstream in the LNG Vaporiser, the HT fluid is used to cool the inlet-air stream to a gas turbine to approximately 7C, thus increasing the power output of the gas turbine due to the improvement of the specific mass per volume unit of the air. Typical gas turbine output increases by ~7% per 10C decrease of inlet air temperature. Air inlet of the turbine is set at such temperature becausecondensation of water could occur if temperature is lower than 7C,whichwould damage the turbine.

The HT fluid will flow downstream tothe condenser of the SC and will close the loop returning to the LNG Vaporiser. SC efficiency will also improve in comparison with a standard SC cycle of a CCPP, due to a lower vacuum pressure of the steam condenser achieved by the injection of the cold water of the cooling circuit. This enlarges the expansion phase and therefore more power is generated in the steam turbine. The impact of cooling water on the SC power performance is significant. The optimal amount of cooling water depends primarily on cooling water temperature and power demand.

Exhaust gases from the Gas Turbine of the CCPP are used to in the Heat Recovery Steam Generator (HRSG) to produce steam for the steam cycle. Once such gases passed through the HRSG they are delivered to be used in an Organic Rankine Cycle (ORC) that will be used to heat up sea water. The ORC is considered to use pentane as thermal fluid. In this system, the heat from the exhaust gases of the CCPP generates superheated pentane vapor in the vapor generator; the vapor is led to a turbine to move a reciprocating pump, and then condensed by the feed seawater which gets preheated. The reciprocating pump driven by the turbine pressurizes the preheated seawater going into the Reverse Osmosis (RO) unit.

As conceptualized, and besides the vaporisation of the LNG, such scheme would produce:
~ 150 MW of power output
~ 72 m3/h of fresh water

Power output of the GT, and therefore of the schematised CCPP, is limited to approximately 100MW. The reason for such a limitation is due to the consideration of the limits in size of the modules to be installed on a ship. In this sense, and in order to produce 750 MW, five integrated CCPP modules would be required.

The ORC system, as well as the RO unit would be installed below the main deck of the ship, and the regasification system, as depicted in the picture above, would be installed on a double sided mooring jetty which will also act as a mooring facility for the Floating Storage Unit (FSU) and the LNG carrier (LNGC).

Why LNG regasification on Jetty?

To achieve the highest possible efficiency within the regasification process, and provide an environmental friendly solution, the use of a submerged combustion vaporizer (SCV) with the combustor off should be considered. SCVs vaporize LNG inside stainless steel tubes that are submerged in a warm water bath, their thermal efficiency is about 98% and installation costs are lower than other options. As seen in the process scheme above, the heat energy for the water bath of the SCV is provided by the steam condenser of the CCPP. The SCV's combustor would only ignite as back up in case of peak send-out, or shut off of the CCPP's steam cycle. SCVs arehighly sensitive to movement and would therefore be integrated in a module to be installed on the jettyto avoid any malfunction derived from a floating platform.

A jetty would be required to properly moor the ships according to international standards applicable to LNG installations. Therefore the installation of the regasification module on the jetty seems to be the most appropriate option.

LNG transfer from the LNGC to the FSU is considered to be transferred Ship-to-Ship (STS). STS for LNG has been approved in international standards (see EN 1474-Part 2) on coastal weather exposed facilities for aerial configurations.