How Subsea Constraints Influence Design Choices for Protection Systems

摘要

One of the objectives announced by some of the major players in the oil and gas market is placing subseaprocessing on the sea bed thus eliminating all topsides. Such full-subsea processing plants will be poweredand controlled from the shore station and will export extracted products there directly. Due to the longtie-back distances from the shore to the subsea processing plant, it will be necessary to have a subseapower distribution system to supply the loads. It is not economically feasible to supply subsea loadsindividually from the power source as is done today from platforms or floating structures. Projectedsubsea processing plants could consume as much as 60 MW. The technology that is commerciallyavailable today uses 50 and 60 Hz alternating current (AC) power distribution.There are many challenges to achieve this goal. One of these is the design of the protection system forthe complete power distribution system. Under the term onprotectionon are the typical overload, over currentand earth fault protection functions, but also overvoltage protection. Even though subsea processing plantswill be unmanned since they are inaccessible, protection of personnel must still be considered since thesubsea equipment is connected to surface equipment.Due to the difficulty and cost of accessing the subsea equipment for replacement in case of failure, itis very important to know where the fault occurred. Since human intervention at depths where subseaprocessing will take place is not possible, maintenance personnel must be able to rely on the informationfrom the protection system to know which equipment to retrieve. The protection and fault location of thepower umbilical itself is very important due to both the cost and the importance of this asset. Fault locationis critical since that is where the cable will be cut for repair, assuming that repair is possible. Positive faultlocation is a critical design criterion for subsea protection systems.Another challenge is the distinguishing between load currents and fault currents. The long cable lengthsintroduce large impedances in the short-circuit current path, especially for the low voltage (LV) modulesdue to the impedance of their step- down transformers. The protection devices used must be able topositively determine when a fault has occurred and not cause an unwanted trip. LV loads are often subjectto modification and it is also a design requirement that the protection settings of LV devices be able tobe remotely set. Modifications of protection function requirements should be able to be handled by thereconfiguration of existing devices. The use of the new IEC 61850 standards for onCommunicationNetworks and Systems in Substationson for subsea applications facilitates this since protection signals can be reliably transmitted by the communication system using the fiber optic cables installed for monitoringand remote control.