This chapter reviews the merits of semisubmersible and tanker-based production systems and traces some developments aimed at improving the economic performance. Special reference is made to the use of two-axis control Dynamic Positioning (DP) thruster configuration.
Today, when most of the major geological prospects in most offshore regions have already been explored, the discovery of large fields becomes less likely.
Offshore technology is being driven relentlessly towards finding better ways to produce the smaller discoveries which are often geologically more complex, and for which development information is generally much less certain At than that at which the larger field developments were initiated Attention is increasingly directed towards reducing the cost per throughput barrel of the production facilities.
The application of the floating production unit is now well established, and there are more than 25 systems in use in different parts of the world.
The expression "marginal field" is often used when discussing the economics of production, but this is a relative term Its meaning shifts not only with the production cost rate which is applied but also with the variations in the price of crude oil and applicable taxation regimes One effect of the recent price falls was to raise the required production rate for fields to remain within the "marginal" classification As a result of this, a general increase in the design capacity of floating production systems has been indicated.
The advances in offshore drilling techniques, together with the design of subsea completions, manifolds and risers, allow this type of equipment to be combined with floating surface facilities thereby yielding a wide range of approaches to the design of production systems which can act as viable alternatives to offshore platforms.
For the floating part of the system, the choice lies between using a semisubmersible and a monohull or ship-shaped vessel. The semisubmersible is basically designed to be a method of supporting an operational platform using a buoyancy configuration which is more transparent to waves than a barge, whereas the tanker is designed to operate in a particular orientation and features flared bows to reduce resistance to waves and water movement Both types of vessel require a method of mooring to retain them on the assigned location.
With the low utilization in today's market, tankers and drilling semisubmersibles are economically attractive possibilities for conversion into floating production units. To optimize their use, it is interesting to compare their various features, as follows.
A semisubmersible can be used to provide operational control, riser suspension, separation process and flaring facilities. This may be entirely sufficient to provide an extension to an existing facility, for instance a nearby production platform, but for an autonomous system in the sense of one which is not sufficiently near to an existing facility to enable the economic export of the crude oil via a reasonably short pipeline, production storage capacity will be required Although this can be provided by a shuttle tanker, such a method exposes the system to production downtime when the shuttle is not connected due to transit voyaging or laying-off due to weather conditions (Fig.1)