The requirements to reliability and accuracy have resulted in requirements for DP systems to class notations compliant to the International Maritime Organization (IMO) equipment class 2 or 3. For both class notations it is required 3 position reference systems, whereof at least 2 systems should be based on different principles for the class notation to be maintained. Installation of satellite based systems and hydro-acoustic systems normally accomplish this. However, both of these systems are subject to degradation or drop-outs. In areas where frequent degradation or fall-out is encountered, loss of class will result in decreased drilling time, as operation will have to come to a halt and preparations for riser-disconnection may have to be initiated. This has motivated the development of a third position reference system based on measurement of riser angle offsets. In this paper the design principles for such a riser angle position reference system are presented.
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