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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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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.
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Chapter 6: Converter
Fig. 6.1 - Cascade connected Buck-Boost bidirectional DC-DC converter:
A bi-directional step-down DC-Dc converter was first suggested, but after consulting with Siemens a
cascade connected, Pulse Width Modulated (PWM), Buck-Boost bidirectional DC-DC converter is
chosen (shown in Fig. 6.1) so that two quadrant operation is possible and so that the customer can
choose a flexible battery voltage. Also, discontinuous conduction mode is not an issue, since the
bidirectional property allows the current to go both ways, i.e. be both positive and negative. In PWM
the transistor, i.e. the switch, is turned on and off at a constant switching frequency, fs. The average
output voltage is then controlled by adjusting the duty cycle, D. The duty cycle is the time interval the
switch is on pr. switching period divided by the switching period, Ts. Ts being the inverse of the
switching frequency.
This converter can either be controlled as a Buck + Boost, meaning it acts either as a buck or boost
converter separately, or as a Buck/Boost, acting as a Buck-Boost converter. This gives the following
modes of operation depending on powerflow direction and desired battery voltage:
6.1 Control of the converter
Buck + Boost:
Power flow to the right:
, Step-down: Switch S1 is PWM. All other switches are off. D4 conducts during ton interval,
D2 conducting during toff.
, Step up: Switch S1 is on all the time, switch S3 is PWM, all other switches off. D4 conducts
during toff.
Power flow to the left:
, Step up: Switch S4 PWM, all other switches off. D1 conducts during ton, D3 conducts
during toff.
, Step down: Switch S4 on all the time, switch S2 is PWM. D1 conducts during toff.
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Buck/Boost
Power flow to the right:
Switches S1 and S3 are PWM, all other off. D4 and D2 conduct during toff.
Power flow to the left:
Switches S2 and S4 are PWM, all other off. D1 and D3 conduct during toff.
Direction of power flow
The power flow through the converter is controlled by the voltage on the DC bus. When the draw
works motor acts as a generator and delivers power to the grid, the voltage increases. When the
voltage reaches a certain value the control for the converter detects this and starts to charge the
battery. Then, when the draw work acts as a load and consumes power, the bus voltage decreases
and below a certain voltage level the control for the converter starts to supply power from the
battery. I this case the bus voltage at which the converter starts charging the battery, given by
Siemens, is: = 1050V. The bus voltage where the converter start delivering power back to
the bus is =880V. At these two starting voltage levels, when the converter starts delivering or
consuming power, the voltage on the bus is assumed kept constant. This because the Power
delivered or consumed is set equal to the generator’s production or consumption. If the voltage level
were to increase in the case where the battery delivers power, the threshold voltage of =
1050V would be met, and the converter controller would start to consume power again. This would
defeat the purpose of the battery in the deliverance time interval.
One could also control the direction of power flow by looking at the power in the whole system, but
it is an advantage to have so called “autonomous units”, meaning that each units operation only
depend on the state at that point. The rest of the system then is treated like a black box.
Switching frequency
According to Siemens, a switching frequency in the range of 1-5 kHz is normal. When the switching
frequency exceeds 2-3 kHz on the IGBTs associated with these kinds of operations, the switching
losses become considerable. As can be seen from this equation:
This shows that the switching losses are proportional to the frequency. A constant switching
frequency of 2 kHz is chosen to minimize these switching losses.
Ripple:
The battery doesn’t handle high ripple currents, which can cause overheating and degeneration of
the battery in a process called thermal cycling. The maximum allowed current and voltage output
was selected to be 1%. As for the voltage ripple, the battery is, according to Siemens, viewed upon as
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an ideal voltage source, thus eliminating any voltage ripple on this side. As for the bus side a
capacitor was used to lower voltage ripple.
6.2 Dimensioning the converter
Capacitance, C
It was possible to calculate the relationship between and the capacitance at the boundary
condition of 1% output voltage ripple when operating under buck/boost control with power flowing
to the bus (eq. 2.6, appendix 2):
Fig. 6.2 – Plot of Capacitance versus Minimum Vd
Using this figure, it is possible to determine what capacitance is needed in the filter capacitor in order
to keep a 1% ripple voltage.
Inductance, L
To find the required inductance L with the constraint of 1% current ripple to the battery both the
step up, step down and buck/boost mode of operation was considered. A battery voltage from 0 V to
3000 V was chosen.
Step up:
Rearranging equation 1.4 from appendix 1:
L(Vo) =
0 200 400 600 800 1000 1200 1400 1600 1800 20000
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
Minimum Vd
Capacitance R
equired(F
)
Vd and Capacitance at 1% Voltage Ripple Boudary
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Fig 6.3 - Matlab plot of Inductance as a function of step up output voltage
Lmax,su = 0.1H
Step down:
Rearranging equation 1.11 from appendix 1:
L(Vo) =
Fig 6.4: Inductance as a functions of output voltage, step down.
Lmax,sd = 8.3mH
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Buck boost:
Rearranging equation 1.8 from appendix 1:
L(Vo) =
Matlab plot:
Fig 6.5: Inductance as a function of output voltage, Buck/boost
Lmax,bb = 0.07H
From these equations and plots the lowest possible inductance is 0.1H using the step up mode of
operation. If using step down in the 0-1050 V region, and Buck/Boost in the 1050-2000V region the
lowest inductance would be 0.07H.
As for the power flow to the left, i.e. battery discharge, the inductance will be sufficient, since the
bus voltage never exceeds 1050V, and certainly not 3000V.
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Chapter 7: Conclusions In this project, we have seen that a lot of energy can be saved in drilling operations. We have focused
on the energy from active heave compensation. A rough calculation showed that about 5.25 TWh
can be saved yearly on a global basis, and reduce CO2 emissions with 5800 ton. These numbers are
significant and are interesting in the ongoing debate in Norway about emissions from the offshore
industry.
Several options were considered for storing energy from AHC. Batteries were chosen, mainly because
the limited amount of space on the platform, and because the necessary technology is available.
The drilling part of the rig’s electrical systems contains a lot of power electronics. A converter is
needed to control power flow and voltages on the battery. We chose to use a bidirectional buck
boost converter. This allows us to choose a battery voltage than can be higher or lower than the DC-
bus. We then looked on how the size of the inductance and the filter in the converter depends on the
battery voltage.
This project has shown us how power electronics can play an important part in reducing energy
losses.
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Chapter 8: References
1. Mohan, Undeland, Robbins, Power Electronics, 3rd edition
2. Espen Haugan, Stig Settemsdal, Siemens, Trondheim
3. Na Su, Dehong Xu, Min Chen, Junbing Tao, Study of Bi-Directional Buck-Boost Converter with
Different Control Methods, IEEE Vehicle Power and Propulsion Conference (VPPC),
September 3-5, 2008
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Appendix 1: Calculations for current ripple
Step up:
(1.1)
(1.2)
⇒
,
(1.3)
⇒
(1.4)
Buck-boost:
⇒
(1.5)
During (1.6)
(1.7)
⇒
(1.8)
Stepdown:
(1.9)
(1.10)
⇒
(1.11)
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Appendix 2: Calculations for voltage ripple
Buck/Boost:
(2.1)
(2.2)
(2.3)
(2.4)
(2.5)
(2.6)
