DNV GL introduces hybrid power concept for offshore units

DNV GL is proposing a new hybrid power concept for offshore units using batteries in combination with traditional power generation equipment. The concept has already delivered multiple advantages in the maritime and automotive industries, but has not yet been applied in the O&G industry.

“We are inviting the oil & gas sector to jointly develop a solution for incorporating hybrid power systems in the offshore industry. This will reduce costs, fuel consumption and emissions” says Christian Markussen, subsea business development director at DNV GL Oil & Gas.

Oil and gas operators are under constant pressure to reduce both cost and emissions – two requirements that are conflicting. An important means of achieving these reductions is by improving energy efficiency, which DNV GL suggests can be done by using a hybrid system.

“We are therefore launching a Joint Industry Project which aims to capture the lessons learnt from the maritime industry, evaluate the business case and provide practical guidelines for implementation,” says Markussen.

The concept that DNV GL is proposing is similar to that used in the automotive industry, for example in the Toyota Prius. The maritime industry started to adopt this technology five years ago not just because of the environmental benefits, but also because it adds financial value and performance benefits. In the automotive industry there is typically about a 20% fuel saving with the use of a hybrid system, but in some maritime applications there is up to a 40% saving and DNV GL expects to see the same for the O&G industry. Currently there are 17 hybrid ships either in operation or being built, plus one electric ferry. DNV GL studies show that the payback period is typically only about two years for the maritime application.

“While this hybrid technology has matured in the maritime industry it has not yet been used in the offshore industry. We think it is now ready to be applied to new offshore developments. Such a solution can combine conventional gas turbine generators, power from shore through a subsea power cable, another nearby offshore facility for power sharing or even renewable energy sources,” says Markussen.

Markussen further explains that the reduced size of the power generator is a major advantage with a hybrid approach. Today, a conventional offshore facility is typically powered by three gas turbine generator sets, plus a spare one for redundancy. Here the total installed power is dimensioned according to the peak power requirement, but is normally operated at a much lower power level. In addition, the gas turbines are operated at low partial-load in order to have reserve capacity to be ready to react to sudden power demands. This result in low power generation efficiency and thus the fuel consumption and CO2 emissions are higher per unit of power.

“However, with a hybrid system the power generation becomes optimised for an average load rather than a peak load, and hence the power generation capacity can be reduced. A battery, like those used in maritime vessels, will act as a buffer and supply power when needed or charge up when demand is low,” he clarifies.

The initial studies show that hybrid technology can add value to a project in several different ways:

Fuel consumption and emissions: It can reduce fuel consumption and emissions as much as 40% as described above.

Green field: For a new build the number of gas turbine generator sets or the size of them can potentially be reduced, since the power generation system can be designed based more on average power loads rather than peak loads. This will reduce CAPEX for the generators. If the number of generators is reduced then the maintenance costs will drop proportionally. Maintenance cost for an offshore gas turbine is about 18% of the turbine CAPEX per year.

Brown-field: For a brown-field development where more power is required then it is easier to install batteries below deck rather than free up deck space for additional generators. Top side modification is very expensive and results in extended production shut-down. The battery can be modularised for easier handling and installation below deck.

Deck-space: If fewer generators are installed then this will free up valuable deck-space that can be used for other purposes or potentially reduce the size of the facility. On a typical pre-salt FPSO with 75 MW of installed power, the power generation equipment takes up 10-15% of the available deck space.

Performance: Batteries can deliver instant power compared to a gas generator which needs time to spin up when increasing the load. This is very important in dynamic applications such as dynamic positioning systems or heave compensated lifting systems.

Energy harvesting: The batteries allow energy to be stored which can be used to recover energy from, for example, electric active heave drawworks used for drilling or cranes. Today, this energy is burned off as heat.

Reliability: Since the power generators will run at much steadier and ideal loads, this has a positive effect on the reliability of this equipment, thus reducing unplanned maintenance.