SUBSEA PRODUCTION TECHNOLOGY Part II: Subsea processing -- Can it happen?

Downsizing reduces operator willingness to take risks, deepwater programs hold future possibilities

John Cotton
Kvaerner Energy

The following is Part II of a three-part series surveying operator and supplier/developer opinions on the status of subsea processing. Featured in this segment will be John Cotton, product manager with Kvaerner Energy. Part III, which run in the February issue, will feature Paulo Pagot, coordinator of subsea separation systems for Petrobras, and Peter Lovie, vice president of engineering for Bardex Subsea Corporation. Featured in Part I, appearing in the December 1994 issue were Leofric Studd, senior development engineer with BP and Johan Harboe, manager of subsea technology with Norsk Hydro. The views expressed by the interviewees are their own and do not necessarily represent company positions. This survey was conducted and written by Peter Lovie of Bardex.

Success and Catch 22: A long and expensive program carried out by a major offshore industry leader in Norway has resulted in the convincing demonstration that the Kvaerner Booster Station works very satisfactorily under water, with its principle of separation followed by electrically powered pumping and compression.

The basic principles and technology by now should be well enough known and can be adapted to cover a wide range of duties such as local gas lift, re-injection of unsalable gas, multiphase separation with local handling of produced water and sand.

An impressive history, but why is there no long line of users? We found ourselves in a Catch 22 situation as a supplier of new subsea technology:

New technology will not be acceptable to the end user until it is proven by experience.
Only the end user has the facilities and environment where new technology can be proven.

So what are the factors that will enable oil companies to use new technology, even though it is not proven?

Costs and Benefits: Obviously, these affect the use of new technology. Cost is a definable factor. The cost of all the equipment and services needed to get, for example the KBS, down and running on a proposed application should be predictable to fairly good accuracy.

Benefits are not so accurately definable - although boosted performance is.

Studies using powerful and available tools show that any form of boosting will increase revenue and provide a higher NPV. How much depends on the assessment and prediction of the effects on reservoir performance.

This is not an entirely new area but an extension of performance evaluation and prediction within limits with which the reservoir engineer is familiar. There must be applications where the potential benefit to risk ratio is high enough.

Risk Reduction: In addressing this question on the KBS, we broke it down into three components:

Risk analysis: Quantifiable, using familiar and accepted methods with good experience numbers as input, giving good result-numbers. They are less quantifiable but nevertheless very useful, using predicted experience input. However, they are very subjective in the final choice as to what is acceptable.
All potential users of new subsea technology, KBS included, use the word risk in discussions on applicability and benefit and it is this aspect which causes most discomfort. Analysis will quantify risk, but will not remove it.
Small technological strides: This factor was identified very early in the KBS project and the KBS technology was deliberately chosen to use small risk-steps. This was achieved by using well known equipment from highly-reputed suppliers, modifying only to adapt it to the new environment.
Comprehensive testing: Testing is vital, but unfortunately very time consuming and expensive. Therefore it is in everyone's interest to define test programs very carefully, concentrating on asking the equipment the correct questions and not least to eliminate unnecessary work, which comes from testing the information which is already known.

The questions to be asked are those which will ascertain and give convincing answers that the equipment is fit for the defined usage, performance and reliability. The developer of new technology will never be able to eliminate all unknowns by testing.

The complex combination of events, environment, operation, personnel, and coincidence in equipment usage will inevitably produce the unplanable and unexpected. It is not unsuccessful because of that. The final answer will only come in real use.

Key Components: Subsea processing implies using a number of components which have hitherto not been used subsea, although they have been well understood and used on the surface. To those who may not have considered what the implications of going subsea are with subsea processing, the following may help the thought process we went through on KBS:

Separator: Its functional success will depend on the quantity and quality of the fluids to be separated and the configuration chosen to satisfy the process. Just as topsides, the inside of a separator has no idea what is going on outside. It will work under water.
Pump: Pumping hydrocarbons is an everyday experience. Pumping a mixture of hydrocarbons, water and particles is done on tankers. Those with experience say it is a more arduous duty than most subsea boosting. Electrically driven submerged pumps are in reliable use under many platforms, as well as in other areas of activity. The KBS pump is highly wear resistant and has proven to run happily on a higher gas fraction than any separator will produce. Lifetime should therefore be adequate in most applications.
Compressor:The same basic argument was used as for the separator. The inside of any compressor is always carefully separated from the outside. Likewise for the gear and explosion proof motors. The interconnecting shafts are normally outside, but with these inside as well, a common internal atmosphere is achieved by joining the casings together. The result is a waterproof compressor using all the internals familiar to the expert.
Water on the outside becomes an advantage because it improves the compressor performance by intercooling. The KBS compressor module has very successfully proved that it works under water. The separation process upstream feeds it with gas at an acceptable quality. The integrated design eliminates shaft seals and systems and the need for alignment and permits a very simple lube oil system. Gone are most of the reasons for normal maintenance. The KBS compressor is therefore potentially more reliable than a normal topsides unit.
Instrumentation: Subsea instrumentation also is an everyday experience. Subsea instrumentation and their cable penetrations are known technologies at a definable level of reliability. With all due respect, however, topsides equipment is frequently over-instrumented and it is instrumentation which requires most maintenance resources. Subsea requirements are simple by comparison, particularly on the KBS.
Controls: Well control subsea using electronics, electrohydraulics, and multiplex signal transmission is well known. The KBS control system uses most of the same technology and has again been proved to work. The next generation of electronics, fiber optics etc. should be able to satisfy the most complex duty.
Pressure Integrity - Hollow Parts: Wellhead shut-in pressure is usually presented as a danger. For example, designing separators to withstand initial high pressures is not always practical. Remember though, if there is high overpressure available relative to the water depth pressure, then boosting is not required.

Boosting comes into its own when the available driving pressure has fallen. However there are many who would welcome a specification break, such as allowing a lower pressure rating downstream from the wellhead, which would save much on flowline costs. This subject is being discussed, and with integrated control systems, should be feasible.

Collapse due to excessive water depth pressure is another perceived danger. In order to quantify this, we must ask the question "under what circumstances could this actually happen in practice, either following well considered procedures or upset conditions?" It should never occur.

Apprehension Reduction: Subsea processing is a new field for the majority of those involved in the decision-making processes of definition of use, criteria of acceptability, evaluation of suitability, technical and economic risk, and the final decision. These people are therefore naturally apprehensive of something on the fringe of or outside their experience.

It is a challenge to project oneself from the topsides world into the less-known subsea scenario, make evaluations, recommendations, and decisions to use new technology and live with the apprehension that one may be proved wrong.

The supplier must do the best he can by open discussion and information on the areas of apprehension about his equipment. He can never reduce it to zero. It is probable that the above will not satisfy the pessimist, but it will hopefully help the optimist.

John Cotton of Kvaerner Energy has been responsible for the Kvaerner Booster Station (KBS), which has been under development since 1985, in conjunction with Esso and Saga. KBS has a NOK120 million development budget. A wet testing program was conducted recently.