Receipt plant for North Sea gas completes second train

John R. Oliver
Enron Power
Seal Sands, England

Train 2 of Enron Power's gas processing plant at Teesside, England, was completed in October 1995 after 22 months of planning and construction and is currently being commissioned.

Installation follows 2 years' successful operation of Train 1. Gas that flows through the plant feeds one of the world's largest gas-fired electrical power plants.

Fig. 1 shows the processing plant site.

North Sea developments

In 1958, the Schlocteren gas find was made in the Dutch Groningen province. Subsequent demonstrations that similar geological formations extended under the North Sea were followed in 1964 by the first licensing round in the U.K. sector of the North Sea.

Gas finds in the Southern Basin of the North Sea followed rapidly: West Sole in 1965; Leman, Indefatigable, and Hewett in 1966. The development of these and other fields led to the establishment of gas-processing facilities at Theddlethorpe, Bacton, and Dim ling ton/Easington.

In the 1970s, development of Frigg field and of associated gas from Brent and adjacent fields led to establishment of gas-processing plants at St. Fergus, Scotland. More recently, gas from Beryl and Brae fields has been added. These fields provided gas which was much richer in hydrocarbon liquids than those of the Southern Basin.

The Central Area of the North Sea has proven reserves in the trillions of cubic feet of gas which had been relatively unexploited with the exception of associated gas from 4/5 platforms in the Fulmar oil field transported to St. Fergus.

In 1990, representatives of Enron Corp. conducted negotiations with a range of parties to build a gas-fired combined heat and power (CHP) plant at Teesside which would take gas from the Central Area and produce electrical power and steam.

The steam and a proportion of the electricity would be used by ICI Petrochemical & Plastics division's Wilton Works, and the remainder would be exported.

This 1,875-mw CHP plant would be the largest in the world and, by developing this project which included the supporting gas processing with gas and liquid pipeline facilities, Enron and its partners provided the stimulus for this further stage of the development of the Central Area.

Supplies for the Enron-constructed and operated Teesside power station are taken from the Everest and Lomond fields operated by Amoco (U.K.) Ltd.

A 400-MMscfd processing train (Train 1; Fig. 2) with full liquids fractionation capability was commissioned in 1993. The plant was built by Costain Oil, Gas & Process Ltd. under a turnkey design and construction contract. A second plant, with a design capacity of 400 MMscfd, has now also been completed.

These two process trains provide a processing capability which represents more than 10% of the average U.K. demand for gas. Preliminary design for further processing facilities has been carried out in anticipation of the expected development of other fields in the Central Area.

Coinciding with the start of exploration in the British sector was the 1965 Gas Act which established a "monopsony" (single buyer) position for the Gas Council (predecessor of British Gas) as the sole purchaser of gas for supply within the U.K.

The 1982 Oil & Gas (Enterprise) Act, the 1986 Gas Act which privatized British Gas, and the subsequent 1988 Monopolies and Mergers Commission report effectively removed the monopsony position from British Gas. This allowed independent companies to compete for gas supplies.

In 1989, the European Economic Commission (EEC) issued a directive calling for reduced atmospheric emissions from power stations, and the U.K. government incorporated the requirements into the 1989 Electricity Act.

The Act encouraged competition in power generation and at the same time gave permission to burn natural gas in power stations. This liberalization of the gas market, combined with changes in electricity supply legislation, allowed Enron and partners to set up the Teesside project.

Market demands

Gas-processing customers require a processor to deliver gas at a nominated capacity, pressure, and quality. These requirements apply to the liquids extracted, as well.

If the gas is used directly as a power station fuel or petrochemical plant feedstock, security of supply and reliability become critical to the customer.

Gas from the Southern Basin has been generally lean with some fields having a high proportion of inerts.

In contrast, the gas from the Northern North Sea (generally associated gas) has been so rich in NGL that St. Fergus' gas plants can support large fractionation facilities some distance from St. Fergus at Mossmorran.

Gas from the Central Area falls between these extremes with a reasonably high NGL content. For this reason, Enron constructed combined processing and fractionation facilities with a supporting liquids-handling infrastructure.

Information on other fields in the Central Area suggests that future gas supplies will be high in H2S and CO2. Provision has therefore been made for these elements in design to cover both Trains 1 and 2 plants.

To meet contractual obligations and the requirements of downstream customers, it is important that processing facilities operate at a high reliability.

The design of both Trains 1 and 2 provides for spared equipment, where appropriate, to maximize reliability. Examples include duplicate product pumps, relief valves, and Joule-Thomson valves.

In the first 2 years of operation, the availability of the plant was approximately 99%. Availability is the proportion of time for which the plant could supply gas. Because the plant is supplying a large, base-load power station, this factor is an important measure of performance.

Seasonal and other variations in demand are expressed as load factors, defined as the average demand divided by the maximum supply capacity.

In the U.K., British Gas has traditionally designed the supply system to cater for a demand likely to be exceeded only once in 20 years.

In considering supply, it is preferable to think in terms of swing, that is, peak rate of deliverability divided by the average demand over the year.

Gas supplies from the Northern North Sea have limited swing because the gas is generally associated and the prime concern is the export of oil.

In the past, British Gas has achieved the swing by a combination of storage (the Rough storage field, for example), peak-shaving contracts, and interruptible supply contracts.

The Teesside processing facilities have been designed to provide the flexibility to allow conversion of a marginal therm of delivered gas into a gas, NGL, or power product.

As an example the plant is designed to allow maximum rejection of NGL into the sales gas (subject to Wobbe-number constraints) or deep recovery of NGL. This decision will be made based on the relative prices of an increment of natural gas and a comparable increment of liquid products.

A similar conclusion can be drawn between the price of a unit of gas and of electrical power.

J-T, fractionation plants

Train 1 of the Teesside gas-processing plant re ceives gas at 95-140 bara in the dense phase, thus preventing liquid drop out. The gas is then processed and supplied to the Teesside power station. LPGs and heavier components are removed and separated for sale.

An overview of the Train 1 plant is shown in Fig. 3.

The Train 1 plant consists essentially of three sections: The Joule-Thomson plant, the fractionation plant, and associated systems.

In the J-T plant, dewpoint is achieved by a "Drizo" glycol dehydration system. This is an enhanced glycol dehydration system which utilizes a stripping solvent to produce a much lower dewpoint than conventional glycol systems.2

Feed gas is received into the plant at 95-140 bara and 4 C. with a water dewpoint of -15 C. The dewpoint is reduced to 2 80 C. by contacting the feed gas with circulating triethylene glycol (TEG).

After contact with the feed gas, the rich glycol passes to a glycol-regeneration system where the water is stripped out before the glycol is recirculated to the contactor.

The Drizo unit employs solvent stripping to achieve high TEG concentration for deep water removal. A coalescer filter is fitted on the contactor gas outlet to remove any entrained glycol. The water removal is necessary to prevent freezing during subsequent processing.

The dry gas is then let down in pressure across Joule-Thomson expansion valves in two stages to the Teesside power station's delivery pressure.

This expansion results in low temperatures causing condensation of heavier hydrocarbons. The resulting NGL stream is then stabilized in a de-ethanizer column from which the overhead gas, essentially ethane, is compressed into the residue gas stream (Fig. 4).

In addition to the processed gas, gas can be supplied via a 24-in. pipeline from a British Gas metering station. This provides an alternative delivery route for additional supplies to the Teesside power station.

NGL separated in the J-T plant is then passed, via a buffer storage tank, to the fractionation plant which consists of two trayed columns.

Propane is withdrawn overhead from the depropanizer while butane and heavier hydrocarbons are taken from the base and passed to a debutanizer where they are separated. The butane product is taken overhead and the condense product (C5s+) is taken from the base.

All three products are transferred by underground pipelines to storage in the Seal Sands area (Fig. 5).

Process heating is supplied by a closed loop heating medium system which utilizes a gas-fired heater.

All process vents and drains are captured in a closed system and incinerated in a ground burner. (A more detailed description of the process is contained in Tomlinson.1)

The Train 1 plant was commissioned in April 1993 and has run continuously since.

From start up, several improvements have been carried out with the plant on-line. One of the most significant was the installation of a turboexpander generator to utilize pressure drop to produce electricity rather than simply let down pressure across a J-T valve. The turboexpander generator was procured as a package from Mafi-Trench and is capable of producing 2 mw of electrical power.

Construction of Train 2 was carried out with Train 1 operating fully. One of the key objectives of Train 2 construction was to minimize the impact on existing operations.

Before work began at the site, discussions took place with the statutory authorities to gain approval for the construction/operations segregation controls which would allow the Train 2 construction area to be treated as a "greenfield site." This segregation allowed construction to proceed without the need to subject construction work to the full constraints necessary if the work were carried out in an operational area.

A total of 50 process tie-ins and the 11 kv electrical tie-ins had to be carried out to link the Train 1 and Train 2 plants. These were either carried out on-line or during a 7-day plant maintenance shutdown.

A series of detailed control procedures was agreed upon with the major contractor and the subcontractors to ensure the safety of Train 1. Several of the existing plant operator technicians were released to manage the interfaces and participate in construction activities.

Preassembled units and skid assemblies were used to the fullest extent to minimize work on site.

NGL extraction, gas handling

The Train 2 plant is designed to process a further 400 MMscfd of natural gas to produce high-purity pro pane, butane, and condensate liquid products, as well as three gas products. These latter are exported to the national transmission system operated by British Gas, Transco, the Teesside power station, or other users.

Several refinements were made to the Train 2 unit to cater for operability issues identified in the Train 1 system.

The dried feed gas passes to the NGL extraction section where it is progressively expanded via two turboexpanders in series. This can result in cooling to as low as 2 60 C. and extraction of unstable NGLs.

The turboexpanders, which are mounted on a common skid, were supplied by Mafi-Trench Corp. The system is designed to allow deep or minimal liquids recovery.

The NGLs pass to a de-ethanizer where ethane and lighter components are removed, leaving stabilized NGL which passes to buffer storage.

Gas is produced from two sources:

• Residual gas from liquids extraction which remains after turboexpansion. This may be either exported directly to users, to the existing Teesside power station, or recompressed to a pressure sufficient for export to the National Transmission System via the turboexpander compressors.

• Overhead gas (ethane and lighter) from the de-ethanizer. This is recompressed in two stages by centrifugal, electrically driven, compressors. Gas can be exported at inter-stage pressure or after the second compression stage.

The gas streams at various pressures can be exported to the existing Teesside power station, to other users, or to the National Transmission System.

If exported to the National Transmission System, the gas passes through an adsorption system to reduce H2S levels to within pipeline specification. The system chosen was the ICI Katalco Puraspec process (OGJ, June 5, 1995, p. 52; Feb. 13, 1995, p. 74).

NGL storage; safety

The Train 2 NGL storage system provides surge capacity between the Train 2 NGL extraction section and the Train 2 fractionation section.

The NGL storage, fractionation, and heating medium systems are very similar to the Train 1 plant. The two heating medium systems can be linked to provide backup in the event of a failure of one unit.

An overview of the process is shown in Fig. 6.

The plant site, purchased from ICI which had originally purchased it from Tees & Hartlepool Port Authority, lies within an area of land reclaimed in the 1970s from the intertidal sand and mudflats of the Tees estuary.

The site lies within an area designated by the Cleveland County Councils Structure Plan for special industrial development and which already contains several petrochemical plants. Nearby, however, lie several sites of special scientific interest (SSSI); impact on these areas had to be carefully evaluated.

The 1982 EEC Seveso directive, enacted in the U.K. by the Control of Industrial Major Accident Hazards Regulations 1984 (Cimah), aims to prevent major industrial accidents and to limit the consequences to people and the environment of any that occur.

Cimah regulations re quire the controller of an industrial activity to demonstrate that the major accident hazards have been identified and that the activity is being operated safely.

To meet this objective, Enron developed a rigorous risk-management policy which ensures that the facilities are designed, constructed, and operated to minimize risk of a major accident.

The methodology used to assess the plant against the objectives set by Cimah regulations uses a linking-pathway approach covering the following sequence.

1. The incident giving rise to a major accident:

a. Identify events which could lead to an uncontrolled release of a substance, including failure and faulty operation.

b. Estimate the amount and concentration of substance released.

c. Identify routes of entry into the environment.

2. The potential means by which the event can happen.

3. The measures taken to prevent the potential causes from occurring or developing.

4. Measures taken after the event to reduce the consequences.

5. Consequences of the incident.

The Environmental Protection Act 1990 introduced the system of integrated pollution control (IPC) for "prescribed processes" which includes gas processing.

Enron's Teesside gas-processing plant was one of the first to be assessed under the new system. The IPC application detailed the reasons behind the selection of technologies (Best Practical Environmental Option) and equipment (Best Available Technique Not Entailing Excessive Cost).

The condensate product is treated as a potentially dutiable product by HM Customs & Excise. To ensure that no duty becomes payable, Enron obtained a "refinery approval" from HM Customs & Excise which permits handling of condensate.

The main component of the approval is a traders scheme of control which is essentially a quality-assurance program, developed by plant personnel, which ensures that product movements are carefully controlled and accounted for.

A wide spectrum of statutory and nonstatutory bodies were consulted during the project, and a range of permits was required.

Other examples are pipeline consents from the Health & Safety Executive, a lease of the foreshore from the Crown Estate Commissioners, and permission to construct marine works from the Ministry of Agriculture, Fisheries, and Food.

References

1. Tomlinson, T.R., "The Enron Teesside Gas Processing Plant," GPA European Chapter, London, September 1992.

2. Smith, R.S., "Gas Dehydration Process Upgraded," Hydrocarbon Processing, Vol. 67, No. 2, February 1990, pp. 75-77.