In recent years, there has been a great awareness and inclination towards reducing emissions and improving fuel efficiency across the marine industry. At the same time, there has been a great demand for new sources of energy, ultimately leading to zero greenhouse gas emissions. The aim of the 2030 and 2050 emissions targets established by IMO is to reduce total GHG emissions from international shipping in order to achieve the levels of ambition established by the Paris Climate Agreement. Such changes have had significant implications on the design and operation of DP vessels, as due to Class redundancy requirements, the power plant structure for class 2 and class 3 DP vessels cannot match the performance of equivalents in other marine applications.
One of the designs for below 20 MW DP vessels comprises the DC grid solution. In this design, generators run at variable optimal frequency and voltage which offers higher engine efficiency and lower emissions. It is worth mentioning that when a diesel engine runs at a constant voltage and frequency, the engine efficiency can be as low as 20% to 30% at low loads. If engines are allowed to run at optimal low load speed, the SFOC, namely specific fuel oil consumption is immensely improved saving up to 30% on fuel consumption.
As mentioned earlier, some engine manufacturers are trying to replace other sources of energy instead of diesel to have zero greenhouse gas emission engines. One of the alternatives being tested with some degree of success is engines running on Ammonia. The variable speed engine makes it possible for vessel owners to shift to this type of fuel if the technology is completely proven and commercialised in the near future.
Introduction of Energy Storage (ES) media into a DP power system can greatly enhance its performance from different points of view. Peak shaving, allows the engines to take up a constant load, giving them higher efficiency where the ES system accounts for the load variations. Zero emissions in port can easily be achieved using energy storage systems. Pertinently, regenerative power can be injected directly into the DP power system whilst thrusters’ loads are to be diminished or while auxiliary equipment such as cranes are lowered.
The above arguments would make the DC power grid an excellent choice for a DP power system, however the existing technology uses a maximum DC voltage of 1000 V for this application. This voltage level imposes restrictions on the power handling of such a system which is currently limited to 20 MW to 25 MW. Higher power handling requires higher voltages and currents which introduces complexities with the inverters, converters and drives.
For medium and high-power DP vessels, the power plant solution is an improved AC network. Nonetheless the concepts of variable frequency and voltage engines, also the introduction of energy storage media and closed ring operation remains intact in this type of design. Albeit, there are challenges to overcome for this design to be safe for a DP class 2 or class 3 vessel operation. In order to meet Class requirements for a DP class 2 or class 3 vessel, the power system must be fault tolerant and be able to inhibit any hidden failures. Additionally, fault propagation must be avoided as quick as possible, before DP capability is impaired beyond the worst-case failure design intent (WCFDI).
To achieve this, design is based on block protection scheme, which in the event of a fault, only a block will be isolated and DP capability will remain within the WCFDI. The latest communication protocol IEC 61850, and advanced generator and engine monitoring system have paved the way for fast detection and segregation of any faults in the power system.
Reza Yaghoobi is Techincal Adviser - Marine at IMCA
