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Offshore terminals for the transportation of Liquefied Petroleum Gas

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Author(s): René Raaijmakers, Business Development Manager, Bluewater Offshore Production Systems (USA) Inc., Texas, USA

Demand for Liquefied Petroleum Gas (LPG) is rising, particularly in the residential and commercial sectors of developing and more developed countries. The use of cleaner liquid and gaseous fuels is expected to continue to increase as populations grow and total demand for energy in these regions rises proportionally.

At the same time, oil and gas prices have risen to historic levels improving the economics of liquefied gas resulting in stronger transportation demands for LPG. With a number of LPG transportation projects already in the planning stage, many more are expected in the coming years. As access to LPG increases and markets open up, offshore terminals will increasingly play an important role in the overall safety and efficiency of these projects.

Overview

LPG consists of butane and propane and may contain minor quantities of other light hydrocarbons associated with the production of oil and gas. At ambient temperature and pressure these components exist as a gas but can be cooled and/or pressurised to facilitate efficient liquid storage and transportation. Loading the LPG directly into or out of a tanker offshore is essential for many projects in order to capitalise on the market demand.

Over the past four decades Single Point Mooring (SPM) technology applications have become a preferred method of cargo transfer from tankers to shore and vice versa. SPM technology has proven its reliability and cost effectiveness. In particular, near isolated coastlines, in areas where natural harbours are scarce or where the costs of constructing harbours and jetty structures are prohibitive.

Since transportation of large volumes of LPG requires refrigeration, Bluewater is anticipating the emergent requirement for low temperature transfer technology up to cryogenic conditions, and has adapted standard SPM technology to accommodate this growing market for the safe and efficient mooring and (off)loading of product tankers via an offshore terminal facility.
 

LPG storage and marine transportation

In the marine transportation market, LPG is typically transported by dedicated vessels suitable for carrying pressurised, semipressurised or refrigerated LPG.

The pressurised (18-bar, ambient temperatures) or semipressurised (5-8 bar, -10 to -20 degrees Celsius) LPG ships carry 3 – 10,000 m3 or 10 – 30,000 m3 respectively.

Larger volumes require refrigerated transport solutions (ambient pressure, but at temperatures as low as -43 degrees Celsius for 100% propane). Fully refrigerated vessels typically have cargo volumes ranging from 35,000 m3 up to 100,000 m3. Evaluating the markets for LPG transportation world-wide, a trend is developing towards the refrigerated transport of LPG in large volumes. The offshore terminal solution is suitable for all three types of transfer. The loading/un-loading cycle, including the mooring and departure procedures, can usually be achieved in about a day.

The offshore terminal consists of a mooring and fluid transfer system (SPM), connected by means of a subsea pipeline to the LPG storage facility onshore. The SPM mooring system enables the vessel to freely weathervane in response to the wind, wave and current conditions. The pipeline (single or dual) may be several kilometres long, depending on local bathymetry, vessel characteristics and the operating philosophy of the onshore terminal. Transfer of propane and/or butane at refrigerated or semi-pressurised conditions through the fluid transfer system may require the use of insulated pipelines to prevent the LPG from warming due to the environment The insulation ensures that the products are kept at the temperature agreed upon between the terminal operator and the shipping company.

Advantages of SPM based offshore terminals

SPM terminals offer a practical solution with a number of advantages. Congested harbours can be relieved and the difficulties and expense of upgrading quaysides and jetties can be overcome, particularly where any combination of larger vessels, limited water depth and rapidly changing weather conditions are prevalent. The SPM based terminal also allows very high availability and workability through its capacity to adapt to the environmental conditions.

 

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