In recent times, the lowering of oil prices has forced a review of offshore production techniques with a view to lower capital and operating costs using improved floating production techniques allied to sophisticated subsea wellheads and templates. This trend is further encouraged by the need to exploit smaller fields, made ever more marginal by lower oil prices.

A brief look at a map of the North Sea (Fig. 1) shows the position of existing pipelines. A new field will have to be big indeed to merit the provision of its own export line to shore. While many factors determine whether new fields can successfully tie-in to existing pipelines, such as the nature of the fluid (gas or oil), the make-up of the fluid, the amount and type of contaminants, gas, water or other chemicals, it may be safely assumed that for smaller fields there is only a potentially narrow corridor centred on each existing pipeline when tie-in could be applicable if, and only if, all other factors were compatible.(Fig. 1 is available in full paper)

It is clear then that many of the new fields which will be exploited in the future will require some form of shuttle tanker operation. Unless this off loading operation is efficient and cost-effective it will again limit the number of fields that can be developed. The use of systems which permit production only when there is a tanker available to load will render many fields marginal, whereas a different approach would make them attractive commercial propositions

Clearly then, many new fields and some existing ones would benefit from the provision of buffer storage. Such storage will enable production to continue independently of weather, mechanical breakdown, accident or other circumstances which prevent the off-loading directly to a shuttle tanker. Storage may also be needed to reduce the number of tankers required to service an offshore field, hence reducing associated operating costs Independent producing wells may deliver as much as 420 barrels per hour or 10000 barrels per day of oil, but this is a small proportion of the rate at which a tanker can load (say around 40000 barrels per hour). Systems that rely on direct production into a shuttle tanker are therefore inherently inefficient in terms of tying up a major piece of capital equipment and crew. The use of storage obviates this, and the tanker becomes a true shuttle remaining on station only long enough to load.

Having made a decision that storage is a desirable goal for platforms that do not justify a pipeline the question arises as to how this can be achieved. There are several options.

  • storage built-in to fixed platform

  • storage built-in to floating production unit

  • floating storage

  • subsea storage

  • hybnd system

There are many successful examples of the first, for example Mobil's Beryl "A". But, in general, a platform of this type implies high capital cost and hence a large field and may not therefore be applicable to the type of development described here

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