Tassie Shoal Methanol Project Overview
The Tassie Shoal Methanol Project involves the following major components:

• Natural gas supply pipelines from the NT/P68 gasfields to Tassie Shoal
• Concrete gravity structures (CGS) on Tassie Shoal to support methanol production facilities and contain product storage
• Methanol production process plant and associated utilities plant constructed on the CGS
• Accommodation platform adjacent to CGS and facilities connected by bridge-link
• Single point mooring (SPM) for loading methanol into export tankers
• Seawater cooling system for methanol production process

The project aims to construct two separate methanol plants, each on a CGS.

Why Methanol?

Methanol’s primary value is as a chemical intermediate or feedstock for the chemicals industry.  The dominant markets are in manufacturing adhesives used in the building industry and as solvents, but there are many products derived from methanol.  Methanol is being used as a fuel in China, and it is a far more practical fuel than hydrogen for future mobile applications such as hybrid vehicles using fuel cells.  Emerging markets include biodiesel, where 10% methanol is required in the feed mix, and DME, a diesel and LPG extender or substitute.  There are also plans to convert methanol to PE and PP plastics, as an alternative to using petroleum-derived naphtha.

The global methanol market is about 40 million tonnes per year (Mt/y), and is growing at around 4% each year.  The first of MEO’s two approved methanol plants will produce 1.75 Mt/y and target markets in north Asia.  Supplies from New Zealand and Chile to that region have been limited due to gas supply issues, and the nearest alternative sources are in the Middle East.  The freight cost advantage for a plant on Tassie Shoal exporting to north Asia is significant.

There are also process reasons why methanol is a good commercialisation option for some gas, as discussed below.

‘Island’ Substructure

The CGS forms the substructure for the methanol production and utilities plant.  This plant has an estimated topsides weight of 30,000 tonnes.  In effect, when installed on Tassie Shoal, the CGS becomes an artificial island.  It is proposed to construct the CGS's and install the methanol processing and associated equipment in Southeast Asia.  The completed CGS will then be towed from the construction site to Tassie Shoal.  

The CGS will be positioned over a prepared site and then grounded on the shoal by flooding the outer cells of the structure with seawater.  The CGS's require ballasting to achieve sufficient on-bottom weight to resist storm wave loads.  Only seawater is used for ballasting.  Following location and ballasting,, anti-scour ‘blankets’ and armour rock will be installed around the outer perimeter of the CGS to prevent scouring by the wave action and tidal currents.

Each CGS, or artificial concrete island, is approximately 170 metres by 90 metres at its base and 180 metres by 100 metres at the deck.  The internal structure is made up of two walls (8m apart) around the perimeter which provide structural strength, protection for the storage inside and space for ballast water.  There is also a pair of central longitudinal walls providing extra strength, ballast space and support for the process deck slab above.  The central walls also divide the internal storage space into two compartments, each approximately 150m by 30m and 24m high.  Below the storage deck slab there is a cellar with rib walls spreading the loads to the base slab 7.5m below.

The storage tanks inside the CGS may be regular steel API650 tanks as shown in the above illustration, or alternatively they may be rectilinear steel tanks similar to those that form ships tanks.  The main criterion is that the external walls of the tanks may be inspected and repaired if necessary.

The outer and internal walls of reinforced and prestressed concrete form an integral part of the structural strength of the CGS.  The wall and slab thicknesses range from 475 mm to 600mm.  Each CGS will require approximately 63,000 m³ of reinforced concrete and have a dry weight of around 170,000 tonnes.

The CGS's will provide a highly stable and secure base for all of the necessary methanol production and storage facilities.  Each CGS can store approximately 140,000 m³ (110,000 tonnes) of methanol product plus another 35,000 m³ of crude methanol, water and condensate or diesel storage.  The ballast cells on the perimeter of the CGS (which are filled to process deck level with seawater) serve to protect the internal storage area containing the methanol product tanks from any accidental collision.

Methanol Process

Each methanol plant will have a production capacity of approximately 5,000 tonne per day.  The steam methane reforming (SMR) methanol process design by Davy Process Technology (DPT) for the TSMP will be the second at 5,000 t/d scale after MHTL’s M5 plant in Trinidad, and follows DPT’s record of engineering the majority of the large-scale plants currently operating.  The ‘Improved Low Pressure Methanol’ (ILPM) variant of the SMR process offered by DPT involves a pre-reformer which converts ethane and heavier hydrocarbons to methane and hydrogen, and also acts as a guard bed to prolong the life of the main reformer catalyst to up to 5 years. It also includes extensive energy conservation measures, as heat integration is the key to an efficient process.

MEO only briefly considered oxygen-based reforming processes, for example auto-thermal reforming or gas-heated reforming, rejecting these to avoid the perceived risks of oxygen offshore and the lower availability of oxygen-based plants.  The SMR process was really the only choice because of its ability to benefit from high concentrations of carbon dioxide in the feed gas.   As a range of gas qualities exist in the Timor Sea resources and in NT/P68, the ability to accept high levels of carbon dioxide is a considerable asset. 

The high CO₂ content feed gas also enables a significant reduction in the size and cost of the reformer to produce the same quantity of synthesis gas.  As the majority of the CO₂ in the feed gas is converted methanol with this process, the plant effectively sequesters carbon, and can be a very useful complement to an LNG by minimising emissions.  

In addition to the process plant, the topsides utilities facilities include production of power, steam, desalinated water, nitrogen, fire protection and other systems.

The first plant could be commissioned and operational by 2012.  It is planned that the second CGS and methanol plant would be installed adjacent to the first about four years later, raising the total methanol production to 10,000 tonnes each day or 3.5 million tonnes per annum.  The methanol will be exported via tankers to customers predominantly in Asia.

For a computer-generated impression of the future methanol plant and other facilities on Tassie Shoal, click here.