Current Situation

In the North Sea, as with the majority of the big offshore basins around the world, two trends are being seen:

The major fields currently in production are entering their mature age where they are beyond their production plateau. These fields will reach a time when the production rate will fall to the point where it will no longer be economical to continue production using the in-place structures and current technology. This has already occurred on a number of fields, with the result that they have been shut down and abandoned.
The discovery of very large offshore oilfields, which justify huge upfront investment costs, is on the decline. New discoveries are increasingly small dispersed fields which are frequently isolated from existing infrastructures. The risks associated with the development of these so-called marginal fields are too high, under the prevailing technical and economic conditions, to justify the use of existing high cost solutions.

As a consequence of these factors, the development of a low cost production method would allow further field production to be economic at the lower production rates and short life cycles associated with depleting fields, and facilitate the exploitation of the growing number of marginal field discoveries.

The Oil Production Buoy

For such marginal and end-of-life fields – with an expected production rate of up to 10,000 bbl/d and maximum recoverable reserves of the order of 25MM bbl – the high cost conventional solutions (FPSO, small platforms with subsea production systems etc) offer no economic solution for their development or continued production.

The concept of the production buoy is that it offers a method of extracting oil from marginal fields where the construction of a pipeline to export the oil would not be cost effective. The buoy must therefore have the ability to process the oil to produce stabilised crude suitable for export via a tanker.

The method chosen for processing the oil is a thermally stabilised two stage separation process. In order to utilise the produced gas and to reduce the need for maintenance to a minimum the process will use 2-3 gas fired thermal fluid heaters, together producing around 3-4MW of heat.

Wellstream fluids will be received by the 1st stage separator, which will separate the bulk of the water reducing the content to 10% v/v.

The oil under level control will then pass to the 2nd stage separator via a plate heat exchanger where the temperature of the oil will be raised by 600°C. The oil will then be degassed to meet the tanker export criteria of 10psi RVP and a BS&W of 2%.

The processed oil from the 2nd stage separator is then pumped to the gravity base storage facility.

The buoy offers considerable cost savings (both CAPEX and OPEX) when compared to conventional offshore technologies due to:

The employment of a standardised base design to reduce design costs.
The use of smaller and lighter components than traditional structures.
A reduction in commissioning costs due to the lightweight and flexible nature of the buoy.
A reduced maintenance regime due to process equipment being protected from the elements by the BEC.
The utilisation of produced gas as an energy source as opposed to being flared.
Reduced personnel transport costs due to the longer maintenance intervals.
The facility to relocate the buoy rapidly at the end of a field’s life to another marginal or brown field site (the buoy can be fully operational as little as 72 hours after arrival).