I Report – Miniproject TET4190 Power Electronics for Renewable Energy – Fall 2012 Project 16 Siemens: Energy Storage for Reducing Energy Losses in Drilling Platforms Contact persons: Espen Haugan, Stig Olav Settemsdal, Siemens Group C Brian Kilberg Fernando Papi Gomez Axel Redse Bratfos Ferdinand Meltzer Dahl
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I
Report – Miniproject TET4190 Power Electronics for Renewable Energy – Fall 2012
Project 16 Siemens:
Energy Storage for Reducing Energy Losses
in Drilling Platforms
Contact persons: Espen Haugan, Stig Olav Settemsdal, Siemens
Group C
Brian Kilberg
Fernando Papi Gomez
Axel Redse Bratfos
Ferdinand Meltzer Dahl
II
Summary
In this project we analyzed drill rigs and how energy losses can be reduced by harvesting the energy
of the platforms upward and downward movement. To do this we first needed to understand how
drill rigs work and exactly where and how energy losses occurred. We found that on floating drill rigs
the drill string needs to stay stationary relative to the seabed. To obtain this a motor raises and
lowers the string to compensate for the ocean waves. When the rig goes up, braking occurs, and this
braking energy we found to be about 4MW. To store this energy temporarily we suggest using a
battery. This mainly because batteries have high energy density as compared to a capacitor bank
even though batteries can’t handle huge peaks in current.
Using data from Siemens and results from the energy and grid analysis, we suggested using a bi-
directional step-down converter. But after consulting with Siemens a Buck/boost DC-DC converter
was chosen. This because this type of converter can have any level of output and input voltage, and
having fewer limitations on the converter is desired.
Since this converter type can have any output voltage on either side dimensioning the converter was
done for a voltage level between 0 and 3000V. A maximum voltage and current ripple of 1% was
chosen to ensure small current ripple to the battery and small voltage ripple to the grid. The
inductance and capacitance was then calculated and plotted as a function of desired output voltage
using the 1% constraint.
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Content
Chapter 1: Introduction 1
Chapter 2: Offshore drilling and Heave Compensation 2
Chapter 3: Energy Storage 5
Chapter 4: Energy Calculations 6
Chapter 5: Electrical system 7
Chapter 6: Converter 8
-6.1 Control of the converter 8
-6.2 Dimensioning the converter 10
Chapter 7: Conclusion 13
Chapter 8: References 14
Appendix 1: Calculations for current ripple 15
Appendix 2: Calculations for voltage ripple 16
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Chapter 1: Introduction
In this project we will investigate drilling operations and electric installations on drilling rigs.
Siemens manufacture and deliver electric systems to drilling rigs, and have an ongoing
project to reduce energy used in drilling operations. We will look on the drilling part of the
platform grid, where power electronics is essential to control motors and handle varying
power flows. The main objective of this project is to look at how energy generated by heave
compensation can be stored and reused on the platform. To do this we first look into drill
rigs operations to get a general understanding of what is going on. After that we will discuss
different options for storing the energy, and look at how much energy that can be saved in
this manner.
Furthermore the topology and parameters of the electrical system of the drill rig is analyzed
so that a proper selection and topology of converter can be made. Lastly, we will do an in-
depth analysis of the converter based on the given constraints. We will try to find a
converter type needed to connect a battery/capacitor bank to the electrical system, and
then calculate the proper dimensions of it based on the data we get from Siemens.
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Chapter 2: Offshore drilling and Heave Compensation
Underwater oil reserves are accessed through the use of off-shore drilling platforms or ships.
There are two types of drilling platforms; semi-submersible rigs and fixed platforms. The size
and shape of the rigs can be anything from a mobile drill-equipped boat to a massive off-
shore metropolis. Many of these rigs rely on electrical machinery and power to drill for and
extract the petroleum deposits beneath the sea. Power electronics play a very important
role in regulating the electricity used by this vast assortment of electronics on an off-shore
oil rig.
The main components of drilling apparatus are an electric drive motor, a drill bit, and a long
drill string that connects the drive motor and drill bit. The drill string allows for the transfer
of torque from the top drive motor to the drill head. This string can be very long, since it
must span from the surface, where the rig is located, to drill head under the seabed. One
issue that arises is that waves cause the floating rig to rise and fall relative to the drill head,
thus changing the length of the drill string. To compensate for this, an electric winch controls
the length of the string and adjusts for the rise and fall of the rig. For example, when the
platform rises, the winch lets out slack to lengthen the string, preventing the drill bit from
pulling out of the sea floor. When the winch does this, it also brakes the upward movement
and acts as a generator. The power produced by this braking energy is dumped into braking
resistors that just transfer the power as heat to surrounding water. Harnessing and storing
this power could make drilling more efficient and conserve energy.
In this project, Siemens is trying to store the energy temporarily in storage devices such as
lithium-polymer battery bank or capacitor banks, so that it can be reused on the platform
grid. This is connected either directly to the DC-bus, or via any type of bi-directional chopper
or inverters used as bi-directional choppers. The energy can be stored in the battery during
operation that causes regeneration and re-used by the motors during periods without power
generation. The energy storage will then act as a dampener of the system.
To gain a better idea of how to solve this issue, the anatomy of a drill rig must be further
investigated. On the rig there is a tower, called a “derrick”. Inside this tower the drill string
are put together. The drill string is composed of segments of 15 meters.
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The drill string needs to be kept stationary relative to the seabed. This is called “Heave
compensation”. One way of to achieve heave compensation is through passive heave
compensation, for example using a spring in the derrick. This has several disadvantages, so it
is more common to use an Active Heave Compensation (AHC) system with a complex control
system compensating the heave. The draw works is the winch that raises and lowers the drill
string, and is placed on the platform floor (or seabed).The drill string is 8 inches in diameter,
10-12 km long, and made of steel. As a result of this, a lot of energy is involved in raising and
lowering the drill string.
Fig. 2.1 - 5. Draw work motor, 7. Draw-works winch, 14. Derrick, 25. Drill string, 26. Drill bit
When braking, the machine acts as a generator, delivering energy back to the platform’s
grid. Braking of the drill string motion occurs in the following three scenarios:
1: When a new segment of the drill string is added.
2: When the drill string is raised when the drill head needs to be replaced once a
month. This is because one raises the drill string quickly to reduce the rig’s down
time. As a result of this speed, considerable braking is required when the drill head
reaches the top.
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3: Heaving of the rig during normal operation leads to its vertical motion. This motion
relative to the sea floor must be compensated by raising and lowering the drill string.
When the platform moves up, the drill string needs to be lowered relative to the
platform at the same pace as the waves. Therefore braking is needed and energy is
generated. This results in the generator producing power, oscillating with the same
frequency as the waves with a wavelength of 7-15 seconds.
In this project we focus on the energy that can be harvested from the Heave Compensation.
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Chapter 3: Energy storage
There are several ways to harvest the energy from heave compensation, and several
alternatives have been discussed.
One of the alternatives has been to let waste energy burn up in resistors and produce
heated water. But then there must also be a demand for heated water on the platform. The
problem with this solution is that the temperature of the heated water varies greatly, and is
therefore not so usable.
Use of a flywheel may also be considered for storing energy, but the technology is
insufficient, especially on a moving drilling rig or ship.
Electrical energy is the most viable method of storing this energy. There are two ways to
achieve this; battery or capacitor banks. Traditionally, batteries have a large internal
resistance, which cause them to overheat. Another problem with batteries is that they
cannot handle to big peak currents. The strength of batteries is that they have a large energy
density which is important on an oil platform with limited space.
We have looked at two types of capacitors for storing energy. A “Supercap” is built for
storing energy and can take large peak currents, but then needs several minutes to
discharge. It has been designed for high energy density but also has the same overheating
problem as batteries do. Electrolyte capacitors are another option; they handle peak
currents well, but they also have low energy densities.
Modern lithium polymer batteries can handle some peak currents. They also have high
energy density and can deliver more power over time. This makes batteries a good choice.
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Chapter 4: Energy calculations
In order to calculate how much energy can be saved with energy storage for heave compensation,
we have made assumptions based on information from Siemens and Cameron.
Mass of the drill string: ⁄
Length of drill string:
Wave height (waves may be much higher, but the platform/ship does not move that
much)
Time between wave tops:
Energy from heave compensation in one wave:
Power generated:
This means that over one year (8760 hours) we can save 35 GWh on one drilling rig.
It is assumed to be about 300 drilling rigs operating and about 150 of this are floating. This means
that we can save approximately 5.25 TWh a year. In comparison, Norway uses about 100TWh
electrical energy per year.
CO2-emissions Diesel turbines have an efficiency of 30%, and diesel has an energy equivalent of 9.7kWh/l. 1 liter of
diesel gives 2.66 kg of CO2. We can then calculate the CO2 saved:
Tonn CO2 pr. Year
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Chapter 5: Electrical system
Figure 5.1 shows a one-line-schema of the drilling part of a drilling rig’s electrical system.
Fig. 5.1: Drilling part of the drilling rig’s electrical system
The platform grid is normally supplied by diesel or gas generators. The drilling system
contains a large amount of power electronics and is separated from the rest of the grid by a
transformer and a rectifier. Machines for the draw works are connected to the 930V DC bus
through a frequency transformer and a dc-ac buck (step down) converter. The draw work
machines are of the same size and are on the same shaft. There are six 1200kw resistors,
where energy from the machines is dissipated.
The batteries cannot be connected directly to the bus because voltage a current to the
battery must be controlled. In the next chapter we will discuss what kind of converter can be
used for this purpose. We then look at a case with 8 batteries connected to the bus.