Optimizing electric energy production on-board offshore vessels: Vessel power consumption profile and production strategies using genetic algorithms

Abstract

This paper will focus on typical diesel electric propulsion systems found on board offshore vessels. The offshore vessels category include vessels such as PSV (Platform Supply Vessels), AHTS (Anchor Handling Tug Supply vessels) and CSV (Construction Support Vessels). In short at least all thrusters and often also the main propulsion is electric. Hence, the electric energy is produced on-board by medium speed diesel engines driving generators. Diesel engine and generator pairs is often referred to as generator sets or just gensets. Although the engines themselves has been optimised and may produce electricity at close to 181g fuel/kWh (see figure 1), this is only at ideal conditions. A major challenge to reach optimum efficiency is % load. Typically, the engine has a peak efficiency at 80–90% load. Due to operational and regulatory requirements it is usually impossible to operate at such high utilisation. This is due to three major factors: varying load, soft grid and redundancy requirements. The load varies depending both on weather but and mode of operation. The vessel may be in relocation/transport mode or work mode. Work mode could be standby or anchor handling or dredging. The work modes usually require DP (Dynamic Positioning), in addition anchor handling requires a lot of energy to the deck machinery and dredging will require a lot of energy for propulsion (bollard pull). Hence, the on-board generation capacity must be sized for large loads even if the vessels average power consumption may be low. The on-board power grid is a very soft grid since it is not connected to any other grids. Hence, any sudden variation in load must be handled by the spinning generators, and it has to happen immediately. A blackout on-board a vessel will be a very critical situation in any circumstance and potentially catastrophic. Especially if the vessel is close to an offshore installation the vessel could damage the integrity of the installation as well. As a consequence there must always be good margins on spinning production capacity. Redundancy requirements has been introduced by the class societies and is also required by the oil platform operators. Usually a vessel will have two power grids (A and B grid). The two grids may be joined into one grid by closing the bus tie bar, but in DP mode and close to an offshore installation it will not be allowed to have the bus tie bare closed (except if the vessel have specially designed and approved quick opening bus tie bars). Hence, splitting the grid into two grids makes the grids even softer and the % load per genset even lower. This paper will illustrate the effect of these constrains on an existing vessel and data retrieved from the offshore vessel during a 90 day period is analysed and visualised. An approach to select better and differently sized engines is discussed and a method using genetic algorithms for finding the optimal sizes is proposed.