Floating power platforms for offshore cold-ironing

The colloquial term ‘cold-ironing’ refers to connecting a ship to shore power when it is at berth, as its main and auxiliary engines, made of steel/iron, are shut down, literally becoming cold. In the current environment where strict emission regulations govern virtually every ship operation, shore power has become an essential service sought by ships at berth in emission controlled ports. At present, these ships in port can receive shore power only when they are at berth, usually during the transfer of cargo or passengers. However, there are many more ships anchored in and around the ports awaiting access to berths. Efficient supply of shore power to these anchored ships is a challenge and thus they rely on their on-board power generation systems to supply essential loads. As the waiting time can vary from a few hours to weeks, emissions from these ships are significant, especially as they are clustered together in close proximity to land. Therefore, if an efficient and effective way of powering anchored ships with clean or low emission power is available, it can significantly contribute towards reducing emission around busy ports as well as the operating cost of anchored ships.

In an attempt to address this problem, authors propose a floating power platform for ‘offshore coldironing’. The proposed system consists of a fuel cell, a battery bank, and a small LNG engine driven generator set installed on a floating platform such as a barge and moored in close proximity to the anchored ships to supply them with their required electrical power. Nowadays, with the advancements of the fuel cell technology, installation of a suitable fuel cell stack on a floating platform such as a barge has become feasible. Nevertheless, fuel cells response slowly and thus fast dynamic loads such as dynamic positioning load in adverse sea conditions can easily push the fuel cell beyond its safe operating range, possibly creating power system instabilities that can result in blackouts. In addition, the floating platform itself needs dynamic positioning or some form of control to maintain its position relative to supplied ships, further influencing the dynamic load.

The battery bank can support the fuel cell to cope with such loads. However, in extreme situations, even the battery bank will discharge rapidly and the fuel cell may struggle to charge it. Therefore, when the state of charge of the battery bank drops below a defined lower threshold, the LNG engine-driven generator set starts and provides boost power to charge the batteries. As the generator is used only when the battery charge is low, and the emission from the LNG is relatively low in comparison to other fossil fuels used on-board, the proposed system can be considered as a low emission technology solution. The feasibility of the proposed concept of floating power platform, from the power system control perspective, is investigated in this paper through modelling and simulation, with the results clearly showing the efficacy of the proposed hybrid power system to supply the dynamic loads encountered on anchored ships.