There is a potential to reduce substantial amounts of CO2 and NOx emissions through improved energy efficiency on drilling rigs. Transocean has implemented energy efficiency measures on its rigs in Norway’s taxable waters with support from the NOx Fund.
Transocean is one of the world's largest offshore rig companies with a total fleet of thirty-seven units. Of these, seven semi-submersible drilling rigs operate in NOx-taxable waters in Norway. These rigs, used for drilling, completion, and well maintenance are leased to Equinor, among others. The NOx Fund visited the Transocean Endurance to review the measures implemented and supported. Transocean Endurance is a semi-submersible CAT D rig built in 2015 and is currently located outside Bergen drilling on the Troll field for Equinor. NOx emissions on the rig reached over 600 tonnes per year but reduced to 400 tonnes in recent years due to NOx-reducing measures that have been implemented, with support from the NOx Fund and through its own measures. All the measures supported by the NOx Fund which finished in 2020-2021, account for an annual NOx reduction of 125 tonnes.
Technical Manager at Transocean Ove Stavø shows senior adviser Anne-Lise Fleddum from the NOx Fund the various emission measures conducted at Transocean Endurance.
Energy efficiency measures conducted on the rigs
There is great potential for reducing significant amounts of both CO2 and NOx with energy efficiency measures that reduce fuel consumption on rigs. A typical semi-sub rig without measures that runs on diesel / electric load has a fuel consumption of 30-40 tonnes per day. With energy optimization of both power production and consumption, significant reductions (10-20% or more) can be achieved in fuel consumption and emissions, depending on operating conditions. Since 2011, the NOx Fund has paid out more than NOK 150 million to the Transocean rigs, where the measures have resulted in a NOx reduction of approx. 1 000 tonnes, and 30 000-40 000 tonnes of CO2 reduction. Transocean also has other emission reduction projects planned.
Heat recovery
Transocean has received support from the NOx Fund to install heat recovery systems on several of the rigs, including Transocean Endurance to reduce fuel consumption by utilizing surplus heat from the exhaust and cooling systems.
Heat recovery systems on the exhaust system to recover waste heat, here at Transocean Endurance.
- On many vessels, only a limited proportion (44%) of the energy in diesel is utilized. The rest of the energy disappears as heat loss to cooling water and exhaust. Thirty percent of the energy disappears in heat out of the exhaust. With heat recovery systems, 10-16% percent of the energy can be utilized for other purposes such as heat and hot water, explains Technical Manager at Transocean Ove Stavø.
The system also has a central and local heat recovery system that provides an overview of the heat distribution. The energy utilized from the exhaust heat is measured by energy meters and the saving can then be documented. Energy not utilized is dumped into the sea through heat exchangers.
Principle sketch of the energy distribution which shows where the energy in diesel goes and how you manage to capture a proportion of the energy from the exhaust gas.
Engine engineering conversion
Component replacement and engine adaptations on older engines can significantly reduce fuel consumption and NOx formation. The Engine designers and manufacturers are among the suppliers who have a lot of expertise and long experience in rebuilding their engines. Engine engineering conversion can give NOx reductions from 5-20% depending on what changes have been done and what the starting point was originally. When the engine is upgraded, the engine life will be extended and a new EIAPP certificate will be issued after the conversion. Such upgrade has been implemented on the Transocean Spitsbergen and is also ongoing on the Transocean Barents.
Frequency converters on seawater pumps
VFD (Variable Frequency Drive) has been installed on the cooling water pumps on several of the rigs. This is done to avoid running pumps at higher power than necessary. The motor runs only as needed. This measure provides a power-saving for the pump of up to 80%.
Installed VFD on seawater pump at Transocean Endurance with support from the NOx Fund.
Closed bus
The Transocean Endurance was early to install a closed bus without support from the NOx Fund. Rigs often use Dynamic Positioning (DP) when operating and separate the switchboards into separate distribution systems to provide energy to rig equipment. These systems are controlled from a power control center, or power management system, but the systems remain separate to minimize the effect of a partial loss of power.
- By upgrading the existing rig power control system to operate safely in a closed bus mode, the switchboards are effectively connected together to provide a single source of energy for all rig equipment. The power system can operate with fewer diesel engine-generators and with more optimal loads, depending on weather conditions and drilling operations. This means that the system can deliver more efficient power and at the same time reduces fuel consumption by 10-20%, Stavø explains.
Central control for closed bus at Transocean Endurance.
- In addition to the closed bus, Transocean Spitsbergen has also installed a battery in connection with the propulsion system so that in the event of a fault in the primary power supply, one will never lose power for positioning the rig while the fault is isolated, and the system recovers and restores power. This provides a benefit in that you can have fewer diesel generators connected with higher and more even loads so that you save both fuel and emissions, says Stavø.
Important with environmental technical solutions to secure contracts
Transocean has a high focus on being energy efficient and is constantly looking for opportunities for optimization. To secure contracts on the shelf, environmental technical solutions are becoming increasingly relevant.
- The NOx Fund gives us a unique opportunity to finance projects that otherwise would not have been financially justifiable. Transocean has conducted several measures in the last 5-6 years and has many more planned. By systematically reducing the power demand on the rigs, it will be possible to operate with fewer machines. In 2021, we started with DP operations on five rigs with only two machines in operation. This is only possible when the weather allows it, but experience indicates that there are days one would think that this is possible. We experience that there are great opportunities to be able to get further down on only one machine during periods of the year when the weather and operational demand allows it. This is a future state that requires further development as the power supply must be stable and have a large degree of redundancy. When you get as far as performing drilling operations with only 1 or 2 of 6 (8) machines connected, then you have come a long way and you can start looking at other emerging technologies, including for example, alternative energy sources, and a road to zero emissions. The collaboration with the NOx Fund is essential for developing modern technology to achieve our goals, Stavø concludes.