Marine Duty Harmonic Mitigation on DC Propulsion Saves Oil Service Vessel Program

An offshore oil and gas service vessel equipped with dc propulsion drives retrofitted with revenue generating equipment, such as remote operated vehicles (ROV) and cranes, could not pass sea trials due to multiple failures (including loss of navigation) caused by harmonic distortion. Marine propulsion systems represent a dominant load on most vessels and dynamically positioned (DP) offshore installations. These propulsion systems are typically nonlinear in nature with very high harmonic injection, which can have a significant impact on vessel wide systems and subsystems. AC adjustable speed drive (ASD) propulsion systems characteristically inject current harmonics into the generator system impedance creating voltage distortion. DC ASDs produce even higher voltage distortion levels due to voltage notching caused by the thyristor firing which makes their harmonic mitigation requirement much more challenging. In order to eliminate the numerous harmonic problems, the dc propulsion system of the oil service vessel was retrofitted with harmonic mitigation equipment. The case study discusses: 1) operational issues including component failures in crane controls and ROV equipment while operating in DP mode; 2) field testing to document the harmonic distortion conditions created by the propulsion and thruster dc ASDs prior to treatment; 3) computer simulation of the dc drive propulsion system to determine a harmonic mitigation strategy; and 4) extensive sea trial testing of the power system with harmonic mitigation to demonstrate compliance with ABS harmonic limits, compatibility of the harmonic mitigation with the ASD equipment, and resolution of all connected equipment failures and operational problems.