Emissions.

Our Company, proactively and at its own expense, carried out solid R&D work aimed at finding ways to reduce CO2 emissions and to find possibility of using alternative energy sources when drilling from mobile offshore units. We came to these results. 

 

The following basic requirement apply to offshore drilling :
–> modularity and compactness of all equipment for placement in limited space,
–> operation in a fully autonomous mode for drilling one-two wells without bunkering fuel,
–> stability in providing energy to the well construction. Rig power failure, even when it lasts a few minutes, entail costs incomparable with the initial savings. 

 

Using a direct diesel drive for drawworks and mud pump is not rational for energy saving. The selection of energy source consumed by equipment during drilling, both on land and on water, should be alternating current (AC) electricity. 

 

The power of the generator, required for drilling a well of medium complexity and at shallow water depth (hook load 350 metric tonnes) differs little from that of a land drilling rig and averages 3 Megawatts (MW). The minimum power is 2.6 MW, the maximum is 4 MW. The load on the main generator during operation fluctuates significantly, from 30 to 100%. 

 

We have not yet come to an answer of what is better in terms of fuel economy : to have 3 CAT generators with a capacity of 1.3 MW/unit, with connection units as the load grows, or one compact MTU genset of 3 MW that would operate in different modes. It seems to us that a three times more compact solution from MTU is a priority.

 

To generate electricity for an offshore drilling unit, we tried using the following options :
–> power supply from the shore,
–> electric generator on board, diesel fuel,
–> electric generator on board, heavy fuel oil (HFO),
–> electric generator on board, liquified natural gas (LNG).

 

It is advisable to organize electricity supply from the shore only when :
a) not a significant distance from the site to the shore,
b) the designed wells are not at a distance from each other,
c) electricity on the shore is cheap and available, representing a power grid from hydroelectric power stations (example, Norway),
d) the Operator plans to organize electricity supply from shore for further production.
If any of these conditions are not met, powering a single mobile offshore drilling unit from the shore does not make sense.

 

The organization of wind generation on the shelf is practically relevant only when drilling has already been implemented in whole or in part. It is applicable to exclude the electrification of a field from the shore. It is purely a production solution which in some cases is more preferable than a hydroelectric power station on the shore.

 

HFO as a fuel for powering a mobile drilling rig does not lead to significant savings due to the relatively low consumption of fuel by individual rig per day, comparable to the consumption of cargo vessels. At the same time, the risk of generator failure increases due to a worse degree of fuel purification.

 

The use of LNG is very promising, but for onshore drilling only. Compared to diesel generators, the use of natural gas in generators significantly reduces emissions. And when organizing early production system it is possible to use own field's gas for powering all operations. However, there are significant limitations to using natural gas for drilling from a mobile offshore unit :
a) a high fire hazard when LNG tanks are located close to the axis of the well due to limited space offshore,
b) the placement of natural gas requires a larger volume of tanks than for the placement of equivalent diesel fuel,
c) bunkering, as a rule, is completely excluded at sea.
Thus, the savings in the cost of fuel on average 100,000-150,000 Euros per one well with LNG does not justify all difficulties that drilling contractors who decide to repeat these experiments will face.

 

In this regard, diesel fuel is the only energy for drilling autonomous wells from mobile offshore units. Due to its widespread availability and maximum ease of working with this type of fuel. 

 

We see the following ways to reduce emissions :

1. In the optimal planning of all operations in such a way that for the construction of one well the electrical power is always optimally collected from the available diesel fuel, and not ten times more than needed.
2. In fundamental refusal to use dynamic positioning, that is, self-limitation in work at sea depths above 2,000 m (anchor chains for moored rigs are usually made for water depths of up to 500 m, but technically their length can be designed to work up to a water depth of 2,000 m). This seems to be a huge contribution to reducing atmospheric emissions, since the main generator is switched off at night when the shift personnel take a break from work, which is impossible to implement on drilling rigs with dynamic positioning.

 

Therefore, selecting the exact brand and model of the main diesel generator, which ensures stable operation of the rig in the mode we require, is critical to the goal of reducing emissions from the well construction process and achieving record energy efficiency.