ABSTRACT

Underwater-mateable electrical connectors find their major application in subsea systems associated with the production of oil or gas. Example are in wellhead control systems and seabed-installed pressure-boosting stations. A wide choice of connector types is available. The principles of operation on which they are based are conductive, inductive or capacitive coupling. The last two types exist both in a face-to-face and a coaxial form A hybrid type combining the conductive and capacitive principles is under development for high power applications.

After an explanation of the various types and their pros and cons, the selection criteria are discussed, together with an indication of the requirements. Tabular and graphical means are presented to assist the user of subsea-mateable connectors in their selection. The results are converted into a selection flow chart. Capacitive connectors are recommended for general use because of their reliability and ease of use, while the other are useful in a number of special applications.

INTRODUCTION

The production of oil gas from subsea wells often dictates the use of an electrohydraulic control system. The numerous components of such a control system include subsea-mateable electric connectors (also referred to as couplers), which are generally found at various locations throughout the distribution system. They may transmit signals or power, between 50 VA and 5 kVA. Experience has shown that underwater-mateable electrical connectors often constitute the weakest link of a control system. The failure of an electrical connector may entail considerable expenses related to the interrupted production and the cost of the repair proper. Even if the job can be fixed in a few days, the total cost may be of the order of $ 0 5 million. Hence, high reliability of underwater connectors in such applications is of paramount importance. In addition, the lifetime requirement, often up to 25 years, makes underwater electrical connectors more than a trivial component.

Other usually less demanding applications of underwater electrical connectors are in ROVs, defence systems, wet civil works and naval installations.

The oldest principle of electrical connector is the conductive (also called pin-to-pin or pin-and-socket) type Poor experience with these couplers stimulated the development of inductive couplers. These have proven to be very reliable, but have some other drawbacks, such as their large size and limited efficiency. This, in turn, has led to further developments, in particular, more efficient coaxial inductive couplers and capacitive connectors.

More recently, marginal oil and gas prospects have created an interest in the pumping of unprocessed well fluids through seabed-installed pumps over large distances (15 to 50 km) to existing facilities. The power requirement of such a system may vary from 0 5 to 5 MVA. A typical subsea pressure-boosting system may comprise a platform-installed power supply, variable speed drive and step-up transformer, a 10-30 kV subsea cable with a length of up to 50 km, a subsea step-down transformer, a subsea-mateable high-power electric connector and a motor/pump assembly. The connector and motor will operate at some 1 to 5 kV. Here again, reliability is the prime concern.

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