WO2018208143A1

WO2018208143A1 - System for transferring energy

Info

Publication number: WO2018208143A1
Application number: PCT/MY2018/050028
Authority: WO
Inventors: Arul Jothy A/L SUPPIAH; Yi Lih Ellie TEO
Original assignee: Ngltech Sdn. Bhd.
Priority date: 2017-05-08
Filing date: 2018-05-08
Publication date: 2018-11-15

Abstract

A system for transferring energy by converting gas from an offshore facility to electrical power using a fuel gas power generator located on a stationary modified tanker, storing the electrical power as charged electrolyte in storage tanks therein, transferring the electrolyte to storage tanks within a shuttle modified tanker, which can then transfer the electrolyte to an onshore facility for use thereof.

Description

SYSTEM FOR TRANSFERRING ENERGY

Field of Invention

The invention relates to a system for transferring energy, particularly but not limited to the transfer of energy from offshore platforms to onshore facilities.

Background

Gas flaring is a significant source of greenhouse gas emissions. It also generates noise, heat and makes large areas uninhabitable. The World Bank reports that 150 to 170 billion m³ of gases are flared or vented annually, an amount valued at about USD30.6 billion, equivalent to one-quarter of the United States' gas consumption or 30% of the European Union's gas consumption annually. Thus, a reduction or recovery of gas flaring is a crucial issue. Oil fields always contain a mixture of products including associated gases. Flaring these gases is now prohibited and in locations offshore, their management can be challenging. These gases can be used in onsite energy production. But in most cases, the amount of produced gas largely exceeds platform fuel gas requirement. Exporting this gas by a subsea pipeline can be quite costly, particularly as water depth increases. Various means are available for monetization of associated gas and non-associated gas from stranded fields. These include:

1. Offshore Liquid Natural Gas (LNG) production and storage

2. Gas To Liquid (GTL) production

3. Conversion of gas to electricity (GTW) and transmission to shore

4. Compressed Natural Gas (CNG) and transport to gas receival facility

All of the above are very expensive and are only suitable for large developments. In addition, in many cases the system results in further environmental emissions to monetize the flare gas. In most cases, the main ultimate consumers of gas production, be it in the form of pipeline natural gas, LNG, CNG, etc. are power stations to generate electrical power onshore. For example, the typical value chain for natural gas power generation systems is as follows for a typical offshore facility:

1. Production fluid is produced from wells

2. Associated or non-associated gas produced is treated (dehydrated) and compressed

3. Gas (or conditioned production fluid) is routed via subsea pipeline to an onshore gas terminal.

4. At the onshore plant, gas is dew-pointed, with NGLs extracted, re-compressed and delivered to gas transmission lines to various consumers including electrical power generation plants.

5. The power generation plant uses the gas as fuel to generate power which is then supplied to the power transmission grid.

As can be seen from the above, there are significant logistic, infrastructure, land requirements, safety issues and environmental emissions associated with the current mode to electrical power generation and supply.

It is possible to producing power offshore directly near the source of gas supply to eliminate a significant amount of the issues mentioned above, but then it might not be cost-effective to deliver the power to the grid or end user. Offshore to onshore transmission cable is one option but this is generally not viable for smaller facilities where field life is relatively short and/or production capacities are not big enough for economic viability. An aim of the invention therefore is to provide a system which overcomes the above issues.

Summary of Invention

In an aspect of the invention, there is provided a system for transferring energy comprising the steps of:

converting gas from an offshore facility to electrical power using a fuel gas power generator; and

storing the electrical power as charged electrolyte in one or more storage tanks; characterised in that the storage tanks are formed inside the hulls of modified tankers.

Advantageously the storage tanks can be used for bulk storage and transfer of power from flare gas. This removes the need for an expensive pipeline, and utilises the gas by converting into electrochemical energy which can then be provided to the onshore grid via the shuttle modified tanker. In addition, as the power generation takes place offshore, the onshore emissions are reduced. In one embodiment the fuel gas power generator is located on a stationary modified tanker. In an alternative embodiment the fuel gas power generator is located on the offshore facility.

Typically the stationary modified tanker remains moored to or adjacent the offshore facility.

In one embodiment the storage tanks are formed as flow batteries located in the stationary modified tanker and/or at least one shuttle modified tanker. In one embodiment a flow battery comprises two forms of charged electrolyte, an oxidised form and a reduced form. These are usually referred to as positive and negative electrolytes respectively.

Typically the electrolytes can be pumped past a membrane separating the electrolytes, and held between two electrodes, to generate an electrical charge.

In one embodiment the two forms of electrolyte are Vanadium solutions comprising different valencies. Advantageously cross-mixing of electrolytes across the membrane does not lead to the contamination of electrolytes.

In an alternative embodiment depleted or uncharged electrolyte can be emptied into the sea or obtained therefrom. Typically the electrolyte is salt water or seawater. In one embodiment the storage tanks and/or associated pipelines are provided in pairs to separately but adjacently contain and/or transfer two forms of charged electrolyte. Typically the two forms of electrolyte are capable of neutralising each other. Thus for transferring electrolyte to a tanker, two pipelines may be used side by side, one for transferring electrolyte in an oxidised or alkaline form, the other for transferring electrolyte in a reduced or acidic form, such that if the pipelines are simultaneously ruptured due to some accident, the electrolytes neutralise each other.

In one embodiment charged electrolyte is pumped from the stationary modified tanker to the shuttle modified tanker. Typically depleted electrolyte is pumped from the shuttle modified tanker to the stationary modified tanker.

In an alternative embodiment the fuel gas power generator supplies power to the shuttle modified tanker to charge the flow batteries therein directly.

In one embodiment the shuttle modified tanker is moveable to another location to discharge power or transfer the electrolyte. Typically power is discharged as high voltage DC electricity or converted into AC electricity. In one embodiment charged electrolyte may be pumped from the shuttle modified tanker to an onshore flow battery or storage tank, and/or depleted electrolyte may be pumped from the onshore flow battery or storage tank to the shuttle modified tanker.

In one embodiment the electrical power generated may be used to power one or more of the modified tankers.

In one embodiment the modified tankers include storage tanks lined with electrolyte- resistant material, such as polyvinyl chloride. Brief Description of Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

Figure 1 illustrates a system for transferring energy according to an embodiment of the invention (a) from an offshore platform to an onshore power grid; and (b) from an offshore platform to another offshore platform.

Figure 2 illustrates a vanadium redox flow battery. Detailed Description

With regard to Figure la, an embodiment of the invention is illustrated wherein an offshore host facility provides gas to a fuel gas power generator located on a stationary modified tanker, which includes storage tanks containing electrolyte. The generator receives and conditions the gas, generates electrical power therefrom, and stores the electrical power by charging electrolyte in the storage tanks via a flow battery stack.

While the stationary modified tanker is moored to the host facility, a shuttle modified tanker may be used to discharge power or transfer the electrolyte to another location, in this case an onshore power grid. The shuttle modified tanker is also provided with a flow battery stack and storage tanks, such that power can be transferred from the stationary modified tanker to the shuttle modified tanker via corresponding electrical systems and flow batteries i.e. the flow battery of the stationary modified tanker is used to charge the flow battery of the shuttle modified tanker. Alternatively the energy transfer from the stationary modified tanker to the shuttle modified tanker may be by transferring (by means of pumping) charged electrolyte from the stationary modified tanker to the shuttle tanker after displacing the uncharged electrolyte that may already be in the shuttle tanker to the stationary modified tanker. An empty buffer tank will be required on either the stationary modified tanker or the shuttle modified tanker to facilitate the initial transfer of electrolyte from stationary to shuttle tanker or of uncharged electrolyte in the reverse direction. The transfer of uncharged and charged electrolyte is sequenced between storage tanks of the tankers in a manner that would avoid cross mixing of electrolyte. Further depending on the type of electrolyte used (for example if seawater or salt water is used), it may be disposable in its uncharged state, in which case, after the shuttle tanker offloads the charged electrolyte at its designated location, it will return empty. In this case the storage tanks of the shuttle modified tanker are empty, the electrolyte can be pumped from the stationary modified tanker to the shuttle modified tanker. After discharge of the charged electrolyte from the stationary modified tanker, the tanks are filled with a fresh uncharged electrolyte which may be seawater. A similar system is illustrated in Figure lb, but where the shuttle modified tanker provides power to another offshore platform from its flow battery, via a power cable.

Yet another configuration of the system is where the shuttle tanker only transports electrolyte. In this case the shuttle will not be fitted with a battery stack for charging and/or discharging the electrical energy but instead will only transport charged electrolyte from the stationary modified tanker and offload to electrolyte storage tanks which may be onshore. Battery stacks to discharge power from the electrolyte receiving tanks may be provided for electrical power supply to the grid or for local power consumption. When the electrolytes in the electrolyte storage tanks have been discharged of power via the battery stacks, the electrolytes are offloaded to shuttle tankers which are in turn transport and offload to the stationary modified tanker for recharging of the electrolyte.

As the stationary modified tanker, shuttle modified tanker, shuttle tanker and the onshore storage tanks can have huge electrolyte storage capacity, this invention enables continuous supply of large amounts of power to the grid that is generated offshore.

According to the invention, in a typical crude producing facilities where associated gas may be flared, associated gas rates can be of the order of 20 MMscfd (Million standard cubic feet per day). The following example (see Table 1) demonstrates how the flare gas is recovered to produce power, store the power and transmit the power to the grid or wherever there is a need. Table 1

With multiple shuttle tankers to transport, deliver the power (either by offloading the charged electrolyte or by discharging the power stored in the electrolyte to the designated destination) and return for recharging with uncharged electrolyte, continuous power supply can be delivered to the designated location. For the above example, approximately 120 MW of electrical power can be delivered on a continuous basis to the designated location which may be to the grid or any other consumer. The existing prior art gas production to power generation value chain involves

Offshore gas processing to pipeline quality which includes dehydration and removal of acid gas components. The offshore production infrastructure involves gas flowrates upward of 200 MMscfd and 20 year project life. Compression to pipeline pressure typically taking 2% of the gas energy. Pipelines again are high cost investments of similar cost and life to the production infrastructure

Further gas treatment onshore to achieve sales quality

Transmission and distribution pipeline to power generation and other users Power production typically of 55% efficiency in modern combined cycle plants of 500 MW and upwards.

Although capital and operating costs vary widely it can be appreciated the traditional model involves investments at a large scale and requiring large gas reserves to support the long term nature of the business. Power generation costs are expressed in terms of levelized values which include capital and operating costs over the system life. For modern gas fired fired combined cycle plants in OECD countries these range from USD65 to USD140/MWh. Malaysian levelized cost is estimated at around USD65/MWh, equal to the low end OECD. Comparison of the offshore battery concept is difficult in that it operates at a much smaller scale targeting the otherwise wasted flare gas streams and small stranded gas field with field life's of less than 5 years and low production rates.

Nevertheless, as a rough estimate we can consider the above 20 MMscfd example of Table 1 producing around 134 MW and ultimately delivering 120 MW to the consumer, which may entail a total investment of $480m. We assume ship annual non-fuel operating costs of USD5 per deadweight tonne which we will assume as 300,000 which comes to USD4.5m per annum for three VLCC units. Bunker fuel consumption is not required as power supply to drive the vessel can be from the stored power in the batteries and this is already factored in to the net energy/power delivered. If we assume gas turbine non-fuel operating costs of a further USD 10m per annum and zero fuel costs we can assume an operating cost of USD14/MWh. Assuming a linear depreciation of capital cost over 20 years, capital cost component is equivalent to approximately USD24/MWh. Thus the total cost operating plus capital cost of monetizing offshore flare gas to deliver electrical power onshore is approximately USD38/MWh which is significantly lower than the rate delivered by OECD countries which ranges from USD65 to USD140/MWh for conventional onshore gas fired combined cycle plants.

Of course there are additional benefits associated with the invention is that it can be reused at other sites with minimal difficulty, which in comparison to a pipeline is much more cost-effective. A pipeline may cost USDlm per km or more, so if the offshore facility is 150km from land, it would cost at least USD 150m, and if the offshore compression, processing plant and tie in to an onshore gas plant for subsequent supply of fuel gas to a power generation plant is factored-in, the total capital cost is expected to exceed USD 500 million which exceeds the cost associated with the invention (in such circumstances). The invention is therefore very useful in smaller fields, and for dealing with the gas in an efficient and ecologically friendly manner.

Whilst current energy storage systems are capable are capable of storage in the MWh range, this invention enables storage to be increased to the GWh range thus enabling storage of large amounts of power for continuous supply of electrical power to the grid or other consumers.

With respect to Figure 2, a vanadium redox flow battery is illustrated. A redox flow battery is a type of rechargeable battery where rechargeability is provided by two chemical components dissolved in liquids contained within the system, usually separated by a membrane. This technology is akin to both a fuel cell and a battery - where liquid energy sources are tapped to create electricity and are able to be recharged within the same system. Different classes of flow cells (batteries) have been developed, including redox, hybrid and membraneless. The fundamental difference between conventional batteries and flow cells is that energy is stored as the electrode material in conventional batteries but as the electrolyte in flow cells. The battery capacity is thus only limited by the external electrolyte storage capacity.

One of the biggest advantages of flow batteries is that they can be almost instantly recharged by replacing the electrolyte liquid, while simultaneously recovering the spent material for re -energization.

Over the past 30 years, several redox couples have been investigated, including zinc bromine, polysulfide bromide, cerium zinc, all vanadium, etc. However, unlike other redox flow batteries, vanadium redox flow batteries use only one element (vanadium) in both tanks, exploiting vanadium's ability to exist in several states (see Table 2).

Table 2

Ion Salt Charge Discharge Electrolyte γ2+ VS0₄ † I Anolyte V³⁺ 0.5 V₂(SO₄)3 I † Anolyte

V0⁴⁺ or V0²⁺ VOSO4 I † Catholyte V⁵⁺ or V0₂ ⁺ 0.5 (VQ₂)2S04 † J, Catholyte

By using one element in both tanks, vanadium redox flow batteries can overcome cross- contamination degradation, a significant issue with other redox flow battery chemistries that use more than one element. The energy density of vanadium redox flow batteries depends on the concentration of vanadium: the higher the concentration, the higher the energy density.

Redox flow batteries store chemical energy and generate electricity by a reduction- oxidation (redox) reaction: i.e. a transformation of matter by electrons transfer, but differ from conventional batteries in two ways: 1. The reaction occurs between two electrolytes, rather than between an electrolyte and an electrode, therefore no electro-deposition or loss in electroactive substances takes place when the battery is repeatedly cycled.

2. The electrolytes are stored in external tanks and circulated through the stack. The electrochemical reactions occur at the redox flow battery core, i.e. the cells. Two simultaneous reactions occur on both sides of the membrane. During the discharge, electrons are removed from the Anolyte and transferred through the external circuit to the Catholyte. The flow of electrons is reversed during the charge; the reduction is now taking place in the Anolyte and the oxidation in the Catholyte.

The vanadium redox flow battery exploits the ability of vanadium to exist in 4 different oxidation states; the vanadium ions V⁴⁺ and V⁵⁺ are in fact vanadium oxide ions (respectively V0²⁺ and V0₂ ⁺). Thus, the VRB chemical equations become:

V0₂ ⁺ + 2H⁺ + e V0²⁺ + H₂0

V²⁺ V³⁺ + e^~

V²⁺ + V0₂ ⁺ + 2H⁺ V0²⁺ + V³⁺ + H₂0

where the water (H₂0) and protons (H⁺) are required in the cathodic reaction to maintain the charge balance and the stoichiometry Activated by pumps, flow batteries perform best at relatively large sizes of above 20kWh. They can deliver more than 10000 full cycles and are good for about 20 years. Similar to the fuel cell, the power density and ramp-up speed is moderate. This makes the battery best suited for bulk energy storage; less for electric powertrains and load levelling that requires quick action. Advantageously the same equipment is used to charge or discharge the battery. They are easily scalable by increasing the size of the electrolyte tanks, hold energy indefinitely, and have a discharge depth of 100%. There is no overheating or possibility of fire or explosions and the operating temperature range is suitable for most climates. Furthermore they have a high energy conversion efficiency (90% round trip) and tolerance to over-charge/discharge. Flow batteries are a proven and reliable technology with over 30 years of development, research and commercial deployment.

It should be noted that while Vanadium Flow Batteries are considered in the example above, any other type of electrolyte meeting similar or better merits may be considered for this invention. This includes other electrolyte solutions sodium polysulfide-bromide, zinc-bromine, lead acid, zinc-cerium, hydrogen-bromine, hydrogen-lithium bromate, hydrogen-lithium chlorate, iron-tin, iron-titanium, iron-chrome, titanium-manganese organic electrolytes, etc.

It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the system which does not affect the overall functioning of the system.

Claims

1. A system for transferring energy comprising the steps of:

converting gas from an offshore facility to electrical power using a fuel gas power generator; and;

storing the electrical power as charged electrolyte in one or more storage tanks;

characterised in that the storage tanks are formed inside the hulls of modified tankers.

2. The system according to claim 1 wherein the fuel gas power generator is located on a stationary modified tanker moored to or adjacent the offshore facility.

3. The system according to claim 1 wherein the fuel gas power generator is located on the offshore facility.

4. The system according to any preceding claim wherein the storage tanks are formed as flow batteries located in the stationary modified tanker and/or at least one shuttle modified tanker.

5. The system according to claim 4 wherein the flow batteries comprise vanadium solutions as electrolytes.

6. The system according to any preceding claim wherein depleted or uncharged electrolyte can be emptied into the sea or obtained therefrom.

The system according to claim 6 wherein the electrolyte is salt water or seawater.

The system according to any preceding claim wherein the storage tanks and/or associated pipelines are provided in pairs to separately but adjacently contain and/or transfer two forms of charged electrolyte capable of neutralising each other.

The system according to any preceding claim wherein charged electrolyte is pumped from the stationary modified tanker to the shuttle modified tanker and/or wherein depleted electrolyte is pumped from the shuttle modified tanker to the stationary modified tanker.

The system according to any preceding claim wherein the shuttle modified tanker is moveable to another location to discharge power or transfer the electrolyte.

The system according to any preceding claim wherein charged electrolyte may be pumped from the shuttle modified tanker to an onshore flow battery or storage tank, and/or depleted electrolyte may be pumped from the onshore flow battery or storage tank to the shuttle modified tanker.

The system according to any preceding claim wherein the fuel gas power generator supplies power to the shuttle modified tanker to charge the flow batteries therein directly.

13. The system according to any preceding claim wherein the electrical power generated may be used to power one or more of the modified tankers.

14. The system according to any preceding claim wherein the modified tankers include storage tanks lined with electrolyte-resistant material such as polyvinyl chloride.