This paper initially discusses the tendency to wider the conceptual envelop of subsea systems in oil production installations. From the conventional boosting systems to more complex gas/oil/water separation equipment and even polishing devices for coarsely separated fluid stream, there is nowadays a tendency to increase the complexity of subsea production systems far beyond manifolds and other maneuver stations. In sequence the pros and cons of subsea processing as an alternative to conventional topside primary processing are also discussed.

Restrictions of the subsea environment and the consequent requirement for unconventional solutions and equipments comparing to topside traditional separation equipment are also mentioned. The limitations of the expression " subsea processing" are emphasized and both advantages and technological gaps of new " building blocks" for processing plant for topside and subsea application are discussed. Besides, operational aspects are also addressed so as to emphasize the new challenges subsea systems pose to operation crew: some important paradigm changes should be captured by operators when changing from a topside plant to a subsea system. The problems arising from having a new subsea system connected to an old production unit in a brown field are also discussed.

The drive for the qualification of new conceptions and new equipment is approached not only for subsea use but also for the new generation of topside production facilities.

The paper tries to bring some conclusions on the means to allow further development - filling up the gaps - and qualification of the new " compact" or " in-line" building blocks for subsea processing plants. However, it must be emphasized that the focus of this work is on processing technology not on equipment or marinization technologies. Thus, subsea engineering (hardware) qualification is beyond the scope of this work.

1. Introduction

In offshore oil and gas production systems, subsea equipment started with wellhead components and flow distribution manifolds three or four decades ago. Some time later, boosting systems (including dynamic equipment - pumps either monophase or multiphase, and, eventually compressors) were conceived as means of turning feasible production of remote marginal fields with long tie backs to production platforms. But even these initiatives were initially assumed as part of the strategy of flow assurance in the offshore production fields. The concept of primary processing of produced fluids was kept in accordance with the usual expression " surface oilfield operations" and that " surface", in the offshore case, was considered as being provided by the deck on the offshore production unit. Seabed was not regarded as supporting site for primary processing facilities.

The idea of having at least part of the primary processing (mainly phase separation) at the subsea environment was driven by the necessity of boosting liquid and gas streams to flow to a greater distance - again flow assurance demand. Subsea separation was initially regarded as potentially complicated from the operational point of view, and as a consequence, in parallel to the new conceptions of subsea gas-liquid separation, an impulse on the development of multiphase pumping system was also boosted.

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