Thesis - SEABED ELECTRIFICATION_Olawale Bamidele SAMUEL_Offshore and Ocean Technology with Subsea Engineering

CRANFIELD UNIVERSITY
OLAWALE BAMIDELE SAMUEL
SEABED ELECTRIFICATION
SCHOOL OF ENERGY, ENVIRONMENT AND AGRIFOOD
Offshore and Ocean Technology with Subsea Engineering
MSC THESIS
Academic Year: 2014 - 2015
Supervisor: Dr Weizhong Fei
September 2015

CRANFIELD UNIVERSITY
SCHOOL OF ENERGY, ENVIRONMENT AND AGRIFOOD
Offshore and Ocean Technology with Subsea Engineering
MSC THESIS
Academic Year 2014 - 2015
OLAWALE BAMIDELE SAMUEL
Supervisor: Dr Weizhong Fei
September 2015
This thesis is submitted in partial fulfilment of the requirements for
the degree of Master of Sciences Offshore and Ocean
Technology with Subsea Engineering
© Cranfield University, 2015. All rights reserved. No part of this
publication may be reproduced without the written permission of the
copyright holder.

i
ABSTRACT
Oil and gas exploration has progressed from onshore to near offshore and more recently deeper
offshore. There is a need to improve and increase exploitation at lower cost, therefore, a
cheaper way is the seabed electrification of subsea systems. Power requirement for field
exploration has majorly been from non-renewable sources by the use of turbine generators
driven by fossil fuels but as the demand for power subsea increased from kilowatts to
megawatts, recent researches, innovations and inventions has allowed power from onshore and
even offshore.
This paper extensively discusses equipment, technologies and topologies required for Seabed
Electrification. Various onshore, offshore renewable and non-renewable sources of power
generation for offshore fields were technically reviewed and discussed. It has also addressed
how the generated power from these sources are transmitted to platforms and then seabed or
directly to seabed before they are distributed to the devices they power. The HVAC and HVDC
are the major transmission options for the generated power to offshore locations.
It further discussed the challenges of seabed electrification power generation by grouping the
world’s oil and gas fields into five regions to review their distribution of energy sources. A
qualitative and quantitative analysis of HAVC and HVDC and its topologies and five case studies
of seabed electrification projects were considered.
The main achievements of this research includes analysis of onshore and offshore sources of
power, an availability, cost and environmental matrix of these sources of power, comparison of
HVAC and HVDC technologies and topologies, case study review of existing fields and deployed
electrification system. It also recommended newer technologies and topologies to enhance and
make power available to more offshore locations.

iii
ACKNOWLEDGEMENTS
I thank the Almighty God for the grace and favour to successfully complete my postgraduate
studies.
I want to express my heartfelt gratitude to all members of my family for their support and
prayers especially Olabisi Kofoworola.
I am very grateful to Dr Kara Fuat, Dr Weizhong Fei and all the friends I made in Cranfield
University during my studies.

v
TABLE OF CONTENTS
ACKNOWLEDGEMENTS.........................................................................................................................iii
ABBREVIATIONS.................................................................................................................................. viii
ABSTRACT..............................................................................................................................................1
1. INTRODUCTION.............................................................................................................................2
1.1. MOTIVATION FOR SEABED ELECTRIFICATION .........................................................................2
1.2. AIM .......................................................................................................................................2
1.3. OBJECTIVES............................................................................................................................3
1.4. SCOPE....................................................................................................................................3
1.5. DRIVERS FOR SEABED ELECTRIFICATION.................................................................................3
1.5.1. SUBSEA SEPARATION.....................................................................................................3
1.5.2. SUBSEA PUMPING.........................................................................................................3
1.5.3. SUBSEA COMPRESSION .................................................................................................4
1.5.4. FLOWLINE AND PIPELINE HEATING ................................................................................4
1.5.5. COST .............................................................................................................................5
2. ELECTRICAL POWER SYSTEM AND TECHNOLOGIES ........................................................................5
2.1. POWER GENERATION FOR SUBSEA ELECTRIFICATION.............................................................5
2.1.1. RENEWABLE SOURCES OF POWER GENERATION............................................................5
2.1.2. NON RENEWABLE SOURCES OF POWER GENERATION..................................................10
2.2. POWER TRANSMISSION AND DISTRIBUTION FOR SUBSEA ELECTRIFICATION.........................12
2.2.1. HIGH VOLTAGE ALTERNATING CURRENT......................................................................13
2.2.2. HIGH VOLTAGE DIRECT CURRENT ................................................................................13
2.3. CRITICAL COMPONENTS FOR SUBSEA ELECTRIFICATION.......................................................16
2.3.1. TRANSFORMERS..........................................................................................................17
2.3.2. SWITCHGEARS.............................................................................................................17
2.3.3. VARIABLE SPEED DRIVES..............................................................................................18
2.3.4. SUBSEA TRANSMISSION AND DISTRIBUTION CABLE .....................................................19
2.3.5. SUBSEA CONTROL SYSTEM ..........................................................................................20
2.3.6. SUBSEA ELECTRICAL CONNECTORS ..............................................................................20
3. METHODOLOGY...........................................................................................................................21
4. RESULTS AND DISCUSSIONS.........................................................................................................21
4.1. ANALYSIS OF SOURCES OF POWER GENERATION FOR SEABED ELECTRIFICATION..................21
4.2. COMPARISON OF HVAC AND HVDC TECHNOLOGIES.............................................................24
4.3. CASE STUDY REVIEW............................................................................................................27
5. CONCLUSION AND RECOMMENDATIONS ....................................................................................29
6. REFERENCES ................................................................................................................................30

vi
LIST OF FIGURES
FIGURE 1: A PUMP MOTOR UNIT DURING INSPECTION AFTER A TEST POOL BY LEEDS, UK (SULZER, 2013). ......................4
FIGURE 2: GORDON POWER STATION SHOWING A WIND TURBINE AND HOW IT TRANSMITS POWER (HYDRO TASMANIA,
2015). ............................................................................................................................................6
FIGURE 3: A GRAVITY-ARCH DAM - ALDEADÁVILA, DUERO RIVER, PORTUGAL (WIJAYA, 2010). .....................................7
FIGURE 4: TIDAL POWER GENERATION SHOWING FLOW THROUGH THE TURBINE (GALLOP, 2012). ..................................8
FIGURE 5: BUOYANCY UNIT - POINT ABSORBER (MOURANT, 2014)........................................................................8
FIGURE 6: AIR MOTION IN AN OSCILLATING WATER COLUMN POWER GENERATOR (ATHAVALE, 2012).............................9
FIGURE 7: WAVE ENERGY CONVERTER A LINE ABSORBER (OCEAN POWER DELIVERY LTD., 2014)....................................9
FIGURE 8: PHOTOVOLTAIC POWER GENERATING SYSTEM (HITACHI, 2013). ...........................................................10
FIGURE 9: NUCLEAR POWER GENERATING PLANT (MOFANIM, 2012)..................................................................12
FIGURE 10: LAYOUT SHOWING VARIOUS STAGES IN THE GENERATION, TRANSMISSION AND DISTRIBUTION SYSTEM. ............12
FIGURE 11: TRANSMISSION SYSTEM FOR HVAC (MARTÍNEZ , ET AL., 2009)............................................................13
FIGURE 12: WAVESHAPES OF CURRENT AND VOLTAGE FOR A DC CONVERTER BRIDGE (WOODFORD, 1998). ...................14
FIGURE 13: HVDC TRANSMISSION MODES (PERSSON, 2011).............................................................................14
FIGURE 14: HVDC OPERATION CONFIGURATIONS AND MODES (PERSSON, 2011). ...................................................15
FIGURE 15: TRANSMISSION SYSTEM OF HVDC LCC (MARTÍNEZ , ET AL., 2009).......................................................16
FIGURE 16: TRANSMISSION SYSTEM OF HVDC VSC. IMAGE SOURCE (MARTÍNEZ , ET AL., 2009)..................................16
FIGURE 17: SUBSEA TRANSFORMER INSTALLABLE FOR 145 KV AC, 900 A AND 3000 METERS (ABB, 2015)...................17
FIGURE 18: - SUBSEA SWITCHGEAR SYSTEM PREPARED FOR FACTORY ACCEPTANCE TESTING (HAZEL, 2011). ..................18
FIGURE 19: A CHART SHOWING THE DISTRIBUTIONS OF VARIOUS SOURCES OF POWER BY LOCATION. ..............................22
FIGURE 20: A DISTRIBUTION CHART SHOWING TOTAL CONTRIBUTION FROM ALL SOURCES OF POWER BY LOCATION. ...........23
FIGURE 21: A CHART SHOWING THE LEADING TOP 10 COUNTRIES IN THE PRODUCTION OF COAL, GAS, HYDROPOWER,
NUCLEAR, OIL AND WIND POWER (WORLD ENERGY COUNCIL, 2013).............................................................24

vii
LIST OF TABLES
TABLE 1: POWER REQUIREMENTS OF A SUBSEA OIL BOOSTING PUMP CONSIDERING DIFFERENT RANGES OF WATER DEPTHS AND
PIPELINE DISTANCE ..............................................................................................................................4
TABLE 2: INSTALLED HYDROPOWER CAPACITY BY REGION. DATA SOURCE: WORLD ENERGY COUNCIL ACCESSED 28, JUNE
2015 ..............................................................................................................................................6
TABLE 3: PERCENTAGE OF ELECTRICITY FROM COAL FUEL (IEA STATISTICS, 2013). ....................................................11
TABLE 4: CLASSIFICATION OF SWITCHGEARS, THEIR RATINGS AND USE. ...................................................................18
TABLE 5: FIVE MAJOR CLASSIFICATIONS OF SUBSEA POWER CABLES. .......................................................................19
TABLE 6: DISTRIBUTION OF SOME SOURCES OF POWER AVAILABLE FOR SEABED ELECTRIFICATION. ..................................21
TABLE 7: CUMULATIVE SOURCES OF POWER BY REGION.......................................................................................22
TABLE 8: MATRIX ANALYSIS OF THE AVAILABILITY AND ENVIRONMENTAL IMPACT OF RENEWABLE AND NON-RENEWABLE
SOURCES OF POWER. .........................................................................................................................23
TABLE 9: COMPARISON BETWEEN LCC AND VSC HVDC TRANSMISSION TOPOLOGY..................................................26
TABLE 10: A SUMMARIZED COMPARISON SHOWING SEABED ELECTRIFICATION PROJECTS OF FIVE VARIOUS FIELDS.............27
TABLE 11: FURTHER ANALYSIS FIVE FIELDS CONSIDERED. .....................................................................................28

viii
ABBREVIATIONS
AC Alternating Current
CAPEX Capital Expenditure
DC Direct Current
DMC Dry Mate Connector
EPR Ethylene Propylene Rubber
GW Gigawatt
HDPE High Density Polyethylene
hp Horse Power
HV High Voltage
HVAC High Voltage Alternating Current
HVDC High Voltage Direct Current
LCC Line Commutated Converter
LDPE Low Density Polyethylene
km Kilometers
kW Kilowatt
MW Megawatt
OPEX Operating Expenditure
PV Photovoltaic
PE Polyethylene
RMS Root Mean Square
TWh Thousand Watt Hour
VSD Variable Speed Drive
VSC Voltage Sourced Converter
WMC Wet Mate Connector
XLPE Cross-Linked Polyethylene

1
Olawale B. Samuela, **
and Dr Weizhong Feib, *
a
Cranfield University, School of Energy, Environment and Agrifood, Bedford, MK430AL, UK
b
Cranfield University, School of Energy, Environment and Agrifood, Bedford, MK430AL, UK
This paper follows the Journal of Petroleum Science and Engineering template
(www.elsevier.com/locate/jpetscieng)
ABSTRACT
Oil and gas exploration has progressed from onshore to near offshore and more recently deeper
offshore. There is a need to improve and increase exploitation at lower cost, therefore, a
cheaper way is the seabed electrification of subsea systems. Power requirement for field
exploration has majorly been from non-renewable sources by the use of turbine generators
driven by fossil fuels but as the demand for power subsea increased from kilowatts to
megawatts, recent researches, innovations and inventions has allowed power from onshore and
even offshore.
This paper extensively discusses equipment, technologies and topologies required for Seabed
Electrification. Various onshore, offshore renewable and non-renewable sources of power
generation for offshore fields were technically reviewed and discussed. It has also addressed
how the generated power from these sources are transmitted to platforms and then seabed or
directly to seabed before they are distributed to the devices they power. The HVAC and HVDC
are the major transmission options for the generated power to offshore locations.
It further discussed the challenges of seabed electrification power generation by grouping the
world’s oil and gas fields into five regions to review their distribution of energy sources. A
qualitative and quantitative analysis of HAVC and HVDC and its topologies and five case studies
of seabed electrification projects were considered.
The main achievements of this research includes analysis of onshore and offshore sources of
power, an availability, cost and environmental matrix of these sources of power, comparison of
HVAC and HVDC technologies and topologies, case study review of existing fields and deployed
electrification system. It also recommended newer technologies and topologies to enhance and
make power available to more offshore locations.
© Cranfield University 2015. All right reserved.
Keywords: Subsea compression systems; HVAC; HVDC; Energy sources.
*Corresponding author. Tel.: +44 (0)1234 750111; E-mail address: w.fei@cranfield.ac.uk (Dr Weizhong Fei)
**Author; E-mail address: samuelolawaleb@yahoo.com

Cranfield University
O. B. Samuel, 2015
2
1. INTRODUCTION
The global population growth, coupled with the economic and industrial demands of Nations of
the World has greatly impacted the exploitation of oil and gas. An increasing demand for energy
has caused a huge shortfall in the exploration of easy oil. As such, the surge has birthed a frontier
in deep water exploration of oil and gas, even to deeper offshores. Technology has advanced
and major investments in various projects are ongoing to reduce the cost of operation while
maximizing the returns on investments. Deeper water exploration requires newer challenges,
one of such is getting oil and gas to the surface at a cheaper and faster rate while preserving the
capacity and life expectancy of the oil and gas field.
Power requirements for offshore exploitations has increased from topside facilities and
equipment to a new field of seabed electrification. Seabed electrification covers the various
aspects of power generation, transmission and distribution while controlling how components,
equipment, processes and operations at seabed are configured and laid out. Renewable and
non-renewable sources are considered to meet the power demands for offshore explorations.
There are challenges on how the generated power is transmitted and distributed, firstly offshore
and secondly to the seabed. The transmission channels and equipment alongside its matching
distribution layout has witnessed constant growth, as more researches and innovations are
ongoing in this sector.
Various power distribution systems have been designed to reduce topside processing and
hydraulic seabed control of oil and gas exploration to electrical seabed controls so that such
fields become more profitable over its life. Heavy equipment requiring power in Kilowatts to
Megawatts are increasingly deployed to seabed to accommodate the challenges in separation,
compression, pumping and even the heating of flowlines.
1.1. MOTIVATION FOR SEABED ELECTRIFICATION
As the demand for energy increased in the early 1960s, investors began to think of other ways
of exploiting oil and gas. This led to the development of many projects on the possibility of
carrying out some exploration activities at the seabed. The first subsea well was built in the Gulf
of Mexico in 1961 after which several developments followed (Hansen & Rickey, August 1995).
The need to accelerate production and at the same time reduce CAPEX on exploration, topside
processing and equipment, led to the introduction of subsea processing which includes
separation, pumping and boosting amongst others.
These processes require high power to make them perform efficiently. As such, there is a
demand to supply electricity to the seabed where these equipment are placed. Seabed
electrification has been largely significant in alleviating constrain on topside host capacity,
increasing recovery and extending field life. It has aided in overcoming flow assurance and flow
management challenges through electrical means and has allowed long distance tie-backs.
1.2. AIM
This project takes a look at the development of electrically powered systems, their benefits and
how these systems have played a key role in driving forward subsea solutions for the oil and gas
industry. Besides, the project looks at opportunities for further benefits of power advances and
explores what we might see in the future.

O. B. Samuel, 2015
3
1.3. OBJECTIVES
This thesis will consider renewable and non-renewable sources of power generation, how the
generated power can be transmitted and distributed from its source to seabed. The project
would review all existing seabed power system technologies and their critical components. It
would further consider the current challenges of seabed electrification and discuss the seabed
systems future of technologies and topologies.
1.4. SCOPE
The project will cover a review of various literatures on the sources of power generation from
shore and offshore, transmission and distribution from shore, offshore power hub or topside of
a platform or vessel. It would use various statistical tools to analyse data and compare various
technologies.
1.5. DRIVERS FOR SEABED ELECTRIFICATION
The major driver of seabed electrification is subsea processing. This is the ability to handle and
treat produced fluids (Hydrocarbons) in order to avoid flow assurance challenges before the
products are being transported topside or onshore for processing (Bai & Bai, 2010). Constant
innovations and researches have continued on seabed processing to yield a better efficiency of
the systems, more power in kilowatts and now megawatts is required to effectively and
efficiently drive these processes. Subsea processing systems requiring electrification are
described below.
1.5.1. SUBSEA SEPARATION
The choice to separate reservoir fluids at the seabed is to reduce the water cut and improve
recovery from such fields. Separation is used to put apart heterogeneous phases of solid, liquid
and gas (Viska & Karl, 2011). Bulk of the subsea separators do not require electricity as they use
gravity for their phase separations. Examples include: Caisson Separation System, Compact and
Dynamic Separators, Gravity Separator and the Semi Compact Gravity Separation System.
1.5.2. SUBSEA PUMPING
There is a need to transport the products to their respective destinations for further processing
or as re-injection after separation. Pumps could either be single phase or could be rotor-dynamic
pumps (RDP). They could also operate as hydraulic pumps that transfer kinetic energy to low
pressure fluid from high velocity (Bai & Bai, 2010). Multiphase pumps however, have become
the most accepted and widely applied for subsea processing. They are useful for onshore and
offshore, with the Twin Screw Pump (TSP) and Helico-Axial Pump (HAP) used for subsea
applications while the Electrical Submersible Pumps (ESP) and Progressive Cavity Pump (PCP) for
downhole applications. These pumps require huge amount of power in the range of 100 – 3000
kW to be transmitted to the seabed to enable them function efficiently. The table below shows
power requirements of multiphase pumps at various water depths and pipe lengths.

O. B. Samuel, 2015
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Figure 1: A pump motor unit during inspection after a test pool by Leeds, UK (SULZER, 2013).
Table 1: Power requirements of a subsea oil boosting pump considering different ranges of water depths and pipeline distance
1.5.3. SUBSEA COMPRESSION
It is critical to transport the produced hydrocarbon gas from the subsea separation to onshore
or topside facilities. An onshore compression facility or an offshore platform can be replaced by
a subsea compression system, such as Ormen Lange (Shell) and Snøhvit (Statoil), with no
offshore processing facilities but a subsea-to-shore solution.
1.5.4. FLOWLINE AND PIPELINE HEATING
In order to mitigate against flow assurance challenges, technology has improved to allow
flowline and pipeline heating. Electrical cables are being designed to gradually heat up the lines
due to the very low temperature at seabed. As the current flows through the lines, they are
heated and as such help control the challenges.

O. B. Samuel, 2015
5
1.5.5. COST
The drive to drastically reduce the OPEX and CAPEX of oil and gas exploitation has led to many
innovations in subsea technology development. Having an all electrical equipment at subsea can
help reduce the need for regular maintenances and help avoid constant failure from hydraulic
systems. The initial cost may require huge investment but it will eventually pay off.
2. ELECTRICAL POWER SYSTEM AND TECHNOLOGIES
The electrical power system and technologies required for seabed electrification have been
classified into sub-categories to cover the areas of power generation, transmission and
distribution for seabed electrification and their required critical components.
2.1. POWER GENERATION FOR SUBSEA ELECTRIFICATION
The works of Alessandro Volta, Andre Ampere, Benjamin Franklin, and Michael Faraday in the
mid years of the 19th
century has been the root of electricity generation for the modern age
(Breeze, 2014). The early days of power generation witnessed the use of hydro and steam power
but they had a setback as they could not produce the needed high speed rotations to effectively
drive the generators. In 1884, Sir Charles Parson invented the steam turbine, an invention that
addressed the prevailing challenge. In 1878, the first recorded power station was constructed in
the Bavarian town of Ettal. However, Godalming (Surrey, United Kingdom) in 1881, built the first
public power station, which used two water wheels in driving an alternator to produce power
for two circuits (Breeze, 2014). The sources of power generation for subsea electrification can
be classified into the renewable and non- renewable power sources. These can be further
categorized into on-site or platform (offshore) and onshore power generation (Bai & Bai, 2010).
2.1.1. RENEWABLE SOURCES OF POWER GENERATION
2.1.1.1. WIND POWER
The overall installed capacity of wind power has seen a rapid growth in this 21st
century. After
the hydropower, it is the second most significant renewable source of electric power. Its
installed capacity grew above 51GW, increasing the global total to about 370GW (GWEC, 2014).
The offshore wind sector is still emerging as most of the recently added capacity has been from
onshore wind. The cost of building such offshore is very expensive when compared to an
onshore installation but this is evened out by better wind system and the capacity to construct
bigger windfarms using larger turbines andan improvedplanning consent (Breeze, 2014). Power
rating of the wind power has improved from 30kW record in the 1980s to 2-3MW range for
onshore installations while offshore machines of about 5MW are now on the increase. Bigger
machines that can deliver up to 15MW are currently being planned. The offshore wind power
installations have better advantages than the onshore as they are more rugged because of the
harsh environment they operate in. Their sizes can be made larger which saves the cost of having
a foundation and they have less environmental restrictions. They allow easy design and
construction of a wind regime.

O. B. Samuel, 2015
6
Figure 2: Gordon Power Station showing a wind turbine and how it transmits power (Hydro Tasmania, 2015).
2.1.1.2. HYDROELECTRIC POWER GENERATION
The Hydroelectric power generation is believed to be the first mechanical power source and
oldest energy in the world dating back to 85 BC. Greek poem and Roman texts make historic
reference to the use of wheels to drive mills and grind harvested grains. Iron paddles replaced
the conventional wood due to the Industrial Revolution in England in the early 18th
century.
(Breeze, 2014). The Global Status Report for 2014 total hydropower capacity by REN21
(Renewable Energy Policy Network for the 21st
Century) indicates how hydropower has grown
from 715GW in 2004, 990GW in 2012, 1018GW in 2013 to 1055GW in 2014. In 2012, it put the
global electricity generation to 3700 TWh which represents about 16% of the global electricity
generation (Al-Zubaidy, 2015). Many countries of the world have shown increased annual
investment, net capacity addition and production in year 2014 with China leading the world,
Brazil, Canada, Turkey and India amongst others have followed closely.
Table 2: Installed Hydropower capacity by region. Data source: World Energy Council accessed 28, June 2015
Hydropower plants are classified into smaller categories depending on their sizes. Those with
capacity less than 100kW are termed Micro, 100kW – 1MW as Mini, 1MW – 10-30MW as Small
and above 10 – 30MW as Large plants (Breeze, 2014). They are sometimes classified based on

O. B. Samuel, 2015
7
their structures, four broad categories include the Arch, Buttress, Gravity and Embankment or
Earth. The Arch dam is named such because of its shape which gives it the needed strength. It
uses less material, as such it is cheaper with a narrow site construction space. However, this
design structure needs a strong abutment to make it withstand the water impact. The Gravity
dam makes use of a lot of concrete which gives the needed weight to hold the dam in place.
Buttress dams have either a flat or curved face and it is held up by series of supports. In the
case of the Embankment dam, earth and rock are used as piles to fill and make a huge weight
to resist the flow of water.
Figure 3: A gravity-arch dam - Aldeadávila, Duero River, Portugal (Wijaya, 2010).
2.1.1.3. TIDAL POWER
The resultant energy of the moon and sun’s gravitational influence on the ocean is defined as
tidal power. In coastal areas, tidal currents are created from height difference between high and
low tides and the currents are powerful to drive turbines (Maehlum, 2015). Tidal barrages are
used to capture the required kinetic motion of ebb and surge of tide for power generation. A
barrage is like a dam, which holds water back during a high tide. However, unlike a dam, it has
an opening, the sluice gate, almost at the base to allow the water through and a final part
containing the turbine and generator (Tidal Energy, 2015).
Its greatest advantage is that it is a green energy source and it is renewable. They are predictable
and as such a proper planning of production and maintenance can be easily implemented. This
source of power is rather new with few companies investing in the technology. Notwithstanding,
there are tidal power plants in operations with many projects still in the implementation stage.
The first large scale tide energy project which was opened in 1966 is the La Rance tidal power
station. Located North-West of France, it is in the river Rance. The total installed capacity is
240MW, which is generated from 24 turbines. It has an annual production of electricity of about
600GWh (Maehlum, 2015).

O. B. Samuel, 2015
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Figure 4: Tidal power generation showing flow through the turbine (Gallop, 2012).
2.1.1.4. WAVE POWER
Melham (2013), wrote that wave energy has a tremendous global potential of generating
electricity. He further explained that if this energy source is totally exploited, it can carter for
almost 40% of the world’s demand, an equivalent of up to 800 nuclear power plants. Wave
energy converts a kinetic or motional energy in the wind into waves as it hits the ocean surface.
It has about five times higher a density of energy transported under the ocean surface than that
of wind energy 65 feet above (Maehlum, 2015). Many energy companies have a cumulative of
over 1000 various methods of utilizing the wave energy with only a few in operation. Of all
methods, the three which look most promising are:
2.1.1.4.1. Buoyancy Unit / Point Absorber
In this arrangement, electricity is generated when waves drive a pump. A floating unit below the
water surface or on the wave is fixed to the bottom as a result of the upward and downward
motion of the wave. About 1MW of ocean wave energy unit is generated as an increase in
production is expected with increasing innovations.
Figure 5: Buoyancy Unit - Point Absorber (Mourant, 2014).

O. B. Samuel, 2015
9
2.1.1.4.2. Oscillating Water Column (OWC)
Power generation is done by converting mechanical energy into useful electricity. The oscillating
water column is moderately immersed in water, it has an opening below the surface line that
allows the upper part to be filled with air. The increasing and decreasing water level in the
column causes a compression and decompression of air which effects the rotation of the
turbines in a way that its rotation is nondependent of the direction of airflow.
Figure 6: Air Motion in an Oscillating water column power generator (Athavale, 2012).
2.1.1.4.3. Surface-following Attenuator (Line Absorber)
The movement of a point absorber which is made up of long surface floating units, connected in
series by the action of the wave is used to generate electricity.
Figure 7: Wave energy converter a line absorber (Ocean Power Delivery Ltd., 2014).

O. B. Samuel, 2015
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2.1.1.5. PHOTOVOLTAIC POWER
The most available energy source on Earth and to its inhabitants is the solar energy. It help
creates wind, as such it plays a vital role in wind energy. Its role in evaporation of water and
rainfall shows the vital role it plays in hydropower, not forgetting the ocean thermal and wave
power are products of its isolation (Breeze, 2014). The received power of the sun on the earth’s
surface is about 1.4 x 105
TW with 3.6 x 104
TW of it being usable. The world power (2012) was
rated at 17 TW which is less than the usable 3.6 x 104
TW (Hosenuzzaman, et al., 2015). The first
recorded solar thermal power generating station was built in the 1960s in the city of Italy.
However, major innovations were as a result of the energy crises in the 1970s.
Becquerel Antoine-Cesar, a French scientist discovered the effect of photovoltaic as light fell on
an electrode which generated voltage. In the end of the 19th
century Charles Fritts, coated
selenium with gold to capture light energy (Breeze, 2014). This however was not efficient until
the discovery of silicon solar cells in 1914 by Russell Ohl. The photovoltaic or solar cell has
become one of the most significant sources of renewable-generated power. Nuclear reactions
within the sun generates solar energy. The generated energy is transmitted to the earth’s
surface via electromagnetic radiation. A radiation with composition of about 56% infrared, 7%
ultraviolet, 36% visible radiation and the remaining 1% representing spectrum not in the energy
ranges of the aforementioned. PV panels are made mainly of semiconductor materials like
silicon and are placed between electrical contacts. The longer these panels spend in direct
sunlight, the more electricity they generate. An electric current is created when loose electrons
combine after being knocked from some atoms by the sunlight strike. The strike and
accumulation of these electrons cause a flow in one direction as the semiconductor is positively
and negatively charged. A direct current (DC) is generated by the PV and this needs to be
converted to an alternating current so as to be used in homes and for businesses. An inverter is
used to convert DC to AC after which it is being transmitted and distributed (EDF Energy, 2015).
Figure 8: Photovoltaic power generating system (HITACHI, 2013).
2.1.2. NON RENEWABLE SOURCES OF POWER GENERATION
2.1.2.1. COAL POWER GENERATION
Power generation by coal is accountable for more than 40% of the world electricity production,
as it has become the most significant source of generating power today (Breeze, 2014). It has in
the last decade of the 21th century accounted for an annual production of about 8100 Terawatt
hour of the world’s total of 20,000 Terawatt hour (EIA, 2014). Coal deposition can be found
across many countries of the world. Many parts of Africa, Asia, Australia, Europe and United
States. Countries like Taiwan and Japan with little deposition depend on export of the
commodity. Power generation using coal uses a simple principle of operation. The coal is

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pulverised to increase its surface area and it is then heated and mixed with air. The product is
blown into the firebox of a boiler which turns water to steam. The steam is heated to reach
about 537.778 degrees Celsius and pressures up to 24.1316505 Kilopascals, and is piped to the
turbine. The turbine blades are caused to rotate by the steam which eventually turns the shaft
of the generator causing the magnetic spin within the wire coil to generate electricity. The steam
is cooled through a condensing pipe by water from a source such as lake or river and the cycle
continues.
Table 3: Percentage of electricity from coal fuel (IEA Statistics, 2013).
Figure: Coal power plant (World Coal Association, 2015).
2.1.2.2. POWER GENERATOR FROM DIESEL AND NATURAL GAS
The electric power generator converts mechanical energy into electrical energy. Michael
Faraday in year 1831 discovered the principle of electromagnetic induction and explained that
a moving conductor in a magnetic field can induce electric charge. It produces a voltage
difference between the ends of its conductor thereby initiating a charge to flow. Some of the
vital components of a generator are: Engine, a source of the mechanical energy part of the
generator. Its size is directly related to the maximum possible power output it can provide. The
fuel system of most generators would support mainly hydrocarbons in the form of diesel,
gasoline, liquefied or gaseous propane or natural gas that are stored in a tank. Gasoline is used
to drive the smaller engines while larger engines are driven by diesel (Diesel Service and Supply,
2013). An alternator uses the mechanical input supplied by the engine to produces its electrical
output.
The voltage regulator’s main function is to control the generator’s output voltage. It converts
AC voltage to DC current, which then feeds the exciter windings. The exciter windings does the
conversion of the DC current back to AC current. They are connected to the rotating rectifier
that converts the DC current to AC current. This is delivered into the rotor / armature, where it
creates an electromagnetic field. The rotor / armature does a final conversion of the DC current
to AC voltage that gives the required output AC voltage. Other important parts of the generating

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system are the cooling and exhaust systems, the lubricating system, the battery and its charger,
a control panel and finally its main assembly or frame
2.1.2.3. NUCLEAR PLANT POWER GENERATION
Power generation by a nuclear plant uses the basic concept of similar types of power generation
such as the coal, oil and natural gas by boiling water into steam to drive turbine so as to produce
electricity. The nuclear plant burns uranium fuel in solid ceramic pellets unlike other sources to
generate electricity by the technology called fission. The U-238 and U-235 are the major types
of uranium used as nuclear fuel with the former being dominant. The nuclear plants could either
be a boiling or pressurized water reactor (Nuclear Energy Institute (NEI), 2015).
Figure 9: Nuclear power generating plant (MOFANIM, 2012).
2.2. POWER TRANSMISSION AND DISTRIBUTION FOR SUBSEA ELECTRIFICATION
After overcoming the challenge of selecting the various combinations of the power generation
required for seabed electrification, another major hurdle is how the generated power can be
transmitted and distributed to their required destinations. Transmission technology is divided
into alternating current (AC) and direct current (DC) technology. It is done mainly by using high-
voltage (HV) so as to mitigate against a decreasing transmission loss and voltage increases
(Andersen, 2014). The HVAC and HVDC technologies are applied in power transmission with
both offering different cost implications and various technical solutions.
Figure 10: Layout showing various stages in the generation, transmission and distribution system.

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2.2.1. HIGH VOLTAGE ALTERNATING CURRENT
The alternating current power transmission is used to transmit bulk power as it has the ability
to renovate voltage to various levels by the use of a transformer. The HVAC allows the
bidirectional flow of power which has given it a better acceptance than HVDC. The HVAC system
is not suitable for long distances with lengths greater than 80 km (Boyle, 2012) because of the
losses in the cable.
Figure 11: Transmission system for HVAC (Martínez , et al., 2009).
The HVAC’s major disadvantage is handling its peak voltage sine wave as the maximum power it
can transmit over its line is proportional to the RMS value of the voltage of a sine wave 0.7 times
the peak value. A DC line has a higher power carrying capacity of 1.4 times of an AC line when
considering the same insulation and wire size on standoffs and its supporting equipment
(Warne, 2005).
2.2.2. HIGH VOLTAGE DIRECT CURRENT
The high voltage direct current transmission is applicable for long distance by the use of
overhead or submarine lines. The HVDC is used to join separate power generating systems
especially in a setup where AC connections are not useful. The HVDC takes electric power from
a source in a three-phase alternating current network and with a converter station, and converts
it to DC. It is then transmitted by an overhead cable to the receiving end and converted back to
alternating current by a converter station (ABB, 2015). It allows power transmission rate greater
than 100MW even to the range of 1,000 – 5,000MW.
When considering subterranean and subsea cabling, the HVDC is preferred. The AC system is not
suitable for long distances, with lengths greater than 50 km (Boyle, 2012). The Pacific Intertie
link which feeds the Greater Los Angeles area with power from various Columbian River
Hydropower stations was the first overhead HVDC bulk transmission link in Northwest of
America. Another ground breaking innovation in China is the transmission link between
Xiangjiaba-Shanghai (2,071 km). The project held the record in 2010, recording a high voltage
(±800 kV DC) with a power capacity of 6,400 MW (Saksvik, 2012).

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Figure 12: Waveshapes of current and voltage for a DC converter bridge (Woodford, 1998).
Connection types in the HVDC can be grouped into the following:
 Monopolar connection – In this connection, a single high voltage cable through which
power is transmitted is grounded in the conversion station. It gives a huge cut on cost
when considering long distances (Martínez , et al., 2009).
 Bipolar connection – This connection has two transmission lines. One line, the positive
voltage and the other, the negative voltage. It is a better reliable system that the
monopolar because a failure in one of the lines can still allow transmission of over 50%
in the other.
 Homopolar connection – This connection makes use of a third metallic conductor in the
middle of two conductors with the same polarity. The third cable transmits twice the
nominal current in each of the other two lines.
Figure 13: HVDC Transmission Modes (Persson, 2011).

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Figure 14: HVDC Operation configurations and modes (Persson, 2011).
The HVDC is classified into two types: the line commutate converter (LCC) which is a thyristor
based technology and the voltage sourced converter (VSC), a transistor based technology. It uses
an efficiently designed technology to deliver a huge amount of electricity with a very low loss
over long step out. Another good use of the HVDC is to interconnect various types of AC
networks, thereby stabilizing the grid.
2.2.2.1. HVDC LCC
The high voltage direct current line commutated converter makes use of two converters. One,
a rectifier terminal, takes power from the grid and converts AC to DC. It is then transmitted by a
DC link to the inverter terminal which then converts the electric power back to AC and feed it
into the grid. The converter transformer, a major component is used to increase transmission
voltage and most times reduce the harmonics (Ulsund, 2009). HVDC LCC requires an auxiliary
power set to supply valves when they are fired at the beginning of transmission. Two of these
converters usually in delta and star connections are necessary at two ends of the transmission
line e.g. onshore and offshore. Components that aid the thyristor based power converter are:
 AC and DC filter – In order to minimize the impact on a connected grid, filters aid to
absorb high content of lower harmonic currents generated by the converter. While the
AC filter supplies reactive power to the converter station, the DC filter deters the
generations of AC in the transmission cables.
 DC cables are used as a transmission medium between the source and its destination.
 Smoothing reactors – In order to avoid current interruption with minimum load, limit
DC fault currents, reduce harmonics (Martínez , et al., 2009) and prevent resonance,
smoothing reactors are used.
 Synchronous compensator (STATCOM) – Also called capacitor banks are used as valves.
The converter requires reactive power to operate efficiently.

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 The cheapest and simplest HVDC transmission system for a moderate power
transmission is the monopolar configuration which makes use of two converters and a
single transmission line.
Figure 15: Transmission system of HVDC LCC (Martínez , et al., 2009).
2.2.2.2. HVDC VSC
Unlike the HVDC LCC, the VSC can independently control an active and reactive power at its
terminals, thereby making transmission controllable and at the same time flexible. The
components that aid the transistor based (IGBTs - Insulated Gate Bipolar Transistors) VSC are
briefly discussed below:
 AC and DC Filters – The HVDC VSC does not require reactive compensation. As such, the
filters are smaller.
 Cable pairs required for HVDC VSC are the polymeric extruded cables.
 Transformers are either used to step-up or stepdown the transmitted voltages.
 Smoothing reactors for the VSC will also be smaller than the LCC as the switching
frequency is higher.
Figure 16: Transmission system of HVDC VSC. Image source (Martínez , et al., 2009).
2.3. CRITICAL COMPONENTS FOR SUBSEA ELECTRIFICATION
The critical components discussed below are vital parts of the seabed electrification. They are
required in aiding or completing the generation, transmission and distribution of power form
source to destination. A few of these critical components are discussed below:

O. B. Samuel, 2015
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2.3.1. TRANSFORMERS
Power generation from synchronous machines are at low voltages ranging about 20kV.
Transformers are used to step-up voltages from low to high, extra-high and even ultra-high in
order to reduce losses and increase transmission capacity of the lines (EL-Hawary, 2008) and at
the destination, stepped down to the desired voltages for distribution. Transmission is made
possible in a voltage level of 115 – 750 kV or even higher to various destinations and even
offshore where it is at different points stepped down for various distribution purposes. The
transformer operates majorly by Ampere and Faraday’s voltage laws using the number of the
windings on its sides; it may contain two or more windings interconnected by a mutual field. The
alternating voltage source is joined to the primary winding. This causes a flow of an alternating
flux with a magnitude that is directly dependent on the voltage and number of turns on the
primary winding (EL-Hawary, 2008). An induced voltage with a value also proportional to the
number of windings on the secondary winding is linked by alternating flux to the output.
Companies such as ABB, Aker Solution, General Electric, and Siemens amongst others have made
major advancement in the design and manufacture of seabed transformer with various voltage,
current and water depth requirements. Transformers are required to undergo some standard
tests before they are deployed for offshore installations. Some of these tests include the
compatibility test, compensator endurance test, component pressure tests, electrical test (IEC),
and thermal test, vacuum test of housing and welding qualifications.
Figure 17: Subsea transformer installable for 145 kV AC, 900 A and 3000 meters (ABB, 2015).
2.3.2. SWITCHGEARS
Another vital component in generating, transmitting and distributing power for seabed
electrification is the switchgear. Its functions can be summarized based on its use for isolating
damaged or faulty equipment, breakdown a large network into sections to allow easy repair,
control other equipment and to reconfigure the sections into whole so as to restore power
(Stewart, 2008).

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Figure 18: - Subsea Switchgear System prepared for Factory Acceptance Testing (Hazel, 2011).
Switchgears function as circuit breakers, disconnectors or isolators, earthing switches, fuse-
switch combinations and switches (Warne, 2005). As a circuit breaker, it is used to allow or
disallow the passage of current in a system under normal condition and at abnormal condition
such as short circuits. Disconnectors withstand normal working system voltage and over-
voltages by maintaining a safe working gap. The gap is left open or closed if there is a surge in
current and if there is no change in the potential difference of the conductor. The earth switch
is useful for earthing and assists in the short-circuiting of circuits. A fuse and a switch can work
in a combination such that the fuse works when current exceeds the breaking capacity of the
switch. The HVAC switchgear are less expensive because when switching off, the transmission
line will produce an arc in the voltage across the switch contacts. This arc extinguishes itself once
the contacts gets far apart because the voltage will drop twice to zero during the sine wave cycle
of the AC. The HVDC is more expensive because the voltage is constant and there is no cycling
to zero. This causes a HVDC switch to draw a longer arc which will require very expensive
switching equipment to assist in supressing the arc.
Table 4: Classification of Switchgears, their ratings and use.
2.3.3. VARIABLE SPEED DRIVES
VSD are used to provide a variable torque or speed for electric motors (Phipps, 1999). Also
referred to as variable frequency drive as it varies the frequency power and supplied voltage by

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controlling the speed of its AC induction motor (Turke, 1999). Their engines are either electric
motors or mechanical engines. The regulation of fuel fed into the engine controlled by throttles
help it achieve variable speed.
2.3.4. SUBSEA TRANSMISSION AND DISTRIBUTION CABLE
Cables are best described as conduit through which electric current flows from source to
destination. Submarine cables are used to transmit HVAC or HVDC to electrical components at
the seabed. They have a diameter between 70mm to 210mm for early designs and can reach up
to 300mm depending on the current-carrying capacity and the required amount of armour
protection. Subsea telecommunication cables are selected based on criteria such as good
consideration of the grid synchronization type, route length, transmission capacity and its
voltage amongst others provide the requirement for the required cable (Subsea Cables UK,
2015). Subsea power cables are manufactured either from copper or aluminium with the former
being more expensive and dominantly used. The choice of copper is as a result of its smaller
cross section which reduces materials content of the outer layer (Worzyk, 2009). A combination
of both cables can be used, such as in the Estlink project where a part of the cable was aluminium
and other parts copper (Ronström, et al., 2007) and they can be jointed together.
Conductors are further categorized below based on their shapes into solid conductor,
conductors stranded from round wires, profiled wire conductors, hollow conductors for oil-filled
cables and milliken conductors. The insulation of the cable could be made of polyethylene (PE)
with varieties as LDPE (low density), MDPE (medium-density), and HDPE (high-density). These
varieties have a density between 0.9 and 0.97 g/cm3
. The cross-linked polyethylene (XLPE) have
replaced the PE and the ethylene propylene rubber (EPR), an extruded dielectric is used for
making submarine cables (Worzyk, 2009). Additional protection sheath such as the water-
blocking, lead, aluminium, copper, polymeric sheaths are used to improve the water resistance
of the cable. Extruded synthetic dielectrics have replaced the traditional lapped paper dielectric
impregnated with oil under pressure (Hammons, 2010). The use of thermoplastic polyethylene
and cross-linked polyethylene (XLPE) cables has been on the increase due to properties such as
elimination of impregnants, low dielectric losses and simple maintenance amongst others.
Innovations and more research work has yielded advancement for this technology, helping
achieve higher voltages for subsea systems.
Table 5: Five major classifications of Subsea power cables.

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Armouring, which provides tension stability and mechanical protection to the subsea cable also
require good consideration while manufacturing them. They could be manufactured from non-
magnetic materials like aluminium, brass, bronze or copper. Armour made from stainless steel
are more expensive but are good for low-loss non-magnetic armouring which provides
resistance against seawater and have high tensile strength. HVAC submarine cables are best
used for distances not exceeding 80km making their manufacture cost way cheaper than HVDC
cables when considering transmission over the same distance. They come as three phase cables
which could be laid either as a whole bundle in a three core formation or separately as three
different cables. A fourth cable is sometimes added to serve as a spare to replace a bad cable.
HVDC cables unlike the HVAC depend on the selected system. They exist either as Monopolar or
as Bipolar as they contain two cables laid together (co-axial) or separately. The XLPE are
preferred dielectric over EPR, LDPE and HDPE (Hammons, 2010).
2.3.5. SUBSEA CONTROL SYSTEM
The subsea control system controls and monitors activities of various units that make up the
system. It serves as a link between the topside and seabed equipment that are responsible for
various activities of oil and gas production and transportation. They control the opening and
closing of various valves on units at the seabed (Bai & Bai, 2010) and also regulate various
activities at the seabed while receiving and transmitting signals from different transducers and
sensors (Bavidge & NES Gloal Talent, 2013). The subsea control system is made up of some of
the units: Subsea Power and Communication Unit (SPCU), the Human Machine Interface (HMI),
Master Control Station (MCS), Electrical power unit (EPU), Hydraulic Power Unit (HPU), Topside
Umbilical Termination Assembly (TUTA) and the Subsea Umbilical Termination Assembly (SUTA)
amongst others.
2.3.6. SUBSEA ELECTRICAL CONNECTORS
Subsea electrical connectors are used in terminating electrical cables carrying communication
signals and low voltages between components in subsea control system (Bai & Bai, 2010).
Connectors for subsea applications are categorized into Wet Mateable/mate Connectors
(WMCs) and Dry Mateable/mate connectors (DMCs). The DMCs require that they are coupled
above waterline before they are installed while the WMCs are coupled below waterline or
seabed (Jenkins, et al., 2013). (Legeay, 2014) Explained that there are requirements for design
of these subsea connectors. Some of the key design parameters to consider include the aft-end
technology and minimum wall thickness, contact density, current and voltage rating, frequency
range of operation, key, keyway heights, mating sequence, O-rings, pressure at depth of
operation, temperature rating of intended site of installation, water depth (Newell, et al., 2005).

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3. METHODOLOGY
The methodology adopted for the thesis is a comprehensive literature review of various
components and technologies that make up power generation, transmission and distribution for
seabed electrification. A qualitative and semi-quantitative comparison of these technologies at
different stages were considered.
First, a statistical analysis of primary data from (IEA Statistics, 2013) and (World Energy Council,
2013) were used to compare power generation from renewable and non-renewable sources.
Assumptions, such as using the recoverable reserve data of 2011 as the generated power from
coal, gas and oil while the installed capacities of hydropower, nuclear, solar and wind as at 2011
were used for analysis. A matrix scale further explains the availability, the environmental impact
and cost of these sources of power.
In addition, this project compared the HVAC and HVDC technologies and topologies.
Furthermore, a compressed summary review of five case studies: Goliat, Safaniya, Troll-A Gas,
Gjøa and Valhall Fields were used to discuss various subsea electrification technologies
deployed.
4. RESULTS AND DISCUSSIONS
4.1. ANALYSIS OF SOURCES OF POWER GENERATION FOR SEABED ELECTRIFICATION
Operating oil and gas fields were grouped into five regions using primary data from (IEA
Statistics, 2013) and (World Energy Council, 2013). Table 6 and Figure 19 below, shows East Asia,
Southern Asia and Pacific, South and Central Asia Region has the highest reserve of Coal (36.4%)
and installed Hydropower (40.1%) capacity. This translates that bulk of the power for national
consumption and available for seabed electrification in the region comes from Coal. Europe has
the highest installed capacity for Nuclear (43.4%), Solar (73.7%) and Wind (40.2%). This justifies
why the North Sea and Norwegian shelf are foremost in seabed electrification projects. North
Africa and Middle East region have the highest reserve of Gas (42%) and Oil (52.4%) as such
majorly depend on turbine engines installed on platforms offshore, for power generation.
Table 6: Distribution of some sources of power available for seabed electrification.

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Figure 19: A chart showing the distributions of various sources of power by location.
Further results in Table 7 and Figure 20 below shows Europe (35%) has the highest cumulative
source of power available for seabed electrification, most of which come from green sources.
Various legislatures on carbon emission and its associating cost has greatly aided the growth in
this region. East Asia, Southern Asia and Pacific, South and Central Asia region (26%), have a
good combination of all sources of power similar to North America, Latin America and The
Caribbean region (23%). North Africa and Middle East region (14%) and Africa (2%) are the
regions that require the most exploitation and investments in renewable sources of power. This
infers that there are currently more available sources of power being exploited for seabed
electrification in Europe, Asia and America than in Africa and Middle East.
Table 7: Cumulative sources of power by region.

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Figure 20: A distribution chart showing total contribution from all sources of power by location.
Table 8: Matrix analysis of the availability and environmental impact of renewable and non-renewable sources of power.
The Matrix analysis in Table 8 above from expert knowledge and literates, shows a scale of
availability, cost and environmental impact of these sources of power. Renewable sources of
power have no major environmental impact as they do not produce CO2 emissions but may
affect aquatic life (erosion and flooding) as in the case of hydro power or solarinstallations which
covers arable land. Their availability except for Hydro is average because they are solely
dependent of climate. The cost implication (CAPEX) of constructing a power generating station
from these sources are very expensive but have a low OPEX. The initial investments required has
affected its acceptance and implementation. Non-renewable sources have better availability as
they are none dependent on climate. Their CAPEX, however is average in term of the
infrastructure required but in the long run have more OPEX as the raw materials – Coal, Diesel,

O. B. Samuel, 2015
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Gas and Uranium are consumed to generate power and needs to be replenished regularly. They
also have high environmental impact because of their CO2 emissions and the risk of radiation
exposure from nuclear plants. A proper analysis of CAPEX and OPEX should be properly
considered in determining the best combinations of power sources to adopt for specific projects.
Figure 21: A chart showing the leading top 10 countries in the production of coal, gas, hydropower, nuclear, oil and wind power
(World Energy Council, 2013).
4.2. COMPARISON OF HVAC AND HVDC TECHNOLOGIES
The selection of a transmission technology is dependent on a number of indices that serve as
determinants. The cost of implementation is a determining factor in selecting a transmission
technology and can be determined by main system equipment needed for the project. The HVDC
can be capital expensive because of the need for a converter station and its footprints, this has
made it totally impossible to rule out transmission by HVAC. It is preferred over the HVDC due
to the huge financial investments required for its availability, control, conversion, switching and
overall maintenance. It is difficult to make circuit breakers for DC as mechanisms must be
contained in the design to bring current to null else, arcing and contact wear will be so large and
it will accommodate dependable switching.
The use of transformers in HVAC can easily assist in renovating the voltage to the desired level
during transmission but a major challenge is its thermal limit (Grigsby, 2001). On the other hand,
HVAC are not suitable for transmission distances over 80 km (Subsea) as the cost will equal and
surpass implementation by HVDC which has a smaller footprint requiring an almost invisible
(Farret & Simoes, 2006) or the use of overhead lines which consumes less installation land area
as HVAC. Less quantity of transmission cables are therefore required in HVDC transmission
compared to equivalent HVAC, hereby, saving significant expenditure cost (ABB, 2014). HVDC
unlike the HVAC requires a smaller construction space and can use the ground as a return path
(Meah & Ula, 2007). HVDC has lower transmission losses than HVAC over long distances
(Liebfried & Zöller, 2010) and the ability to transmit more power per conductor because it has a
constant voltage in its line which is lower that the peak voltage experienced in an AC
transmission line.

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This is possible because the peak voltage in an AC line is greater that the constant voltage in a
DC line for the same power rating. It also allows transmission of unsynchronised AC distribution
systems and grids, thereby increasing system stability while containing failures (Halder, 2013).
(Saksvik, 2012), explains that HVDC can help achieve complete control of power flow, thus,
allowing proficient power trading amongst regions and the stability of the grid has a controllable
power flow. In the event of failure, the HVDC can use its neighbouring grids as a “black start” to
recover while noting that the magnetic fields from its transmission lines are insignificant when
compared to that of AC lines. Another advantage of the HVDC is its ability to combine and
synchronize various transmission frequencies (Meah & Ula, 2007).
Other factors that has placed the HVDC over the HVAC technology for transmission are discussed
below:
 CORONA LOSSES: Air around various phases of a conductor acts as an insulator. As the
potential difference increases, there is an ionization of the atoms around the conductor,
causing the ions to attract and repel each other, thereby resulting in a collision until they
are attracted to the conductor. The diameter of the conductor is increased as a result of
ionized air becoming a virtual conductor. A weak bright glow of violet colour
accompanied with a hissing noise appears and an ozone gas production noticeable by
its odour as the potential difference increases in the lines (Sharma, et al., 2012). If this
continues, a Critical Breakdown Voltage will be attained and will produce a flash over,
constituting a Corona Discharge Effect.
 SKIN EFFECTS: The ability of an AC to make the current density close to the surface (skin)
of its conductor greater that the core by distributing itself with the transmission line is
known as skin effect (Halder, 2013). This causes an increase in the resistance of the line
by increasing the frequency of the current. In the DC transmission, the conductor has a
uniform current as such skin effect is absent (Khemchandani, et al., 2014).
 THERMAL LIMIT: The power flow in a conductor depends on the thermal limit of the
line. This is to peg the maximum temperature the line can attain thereby preventing loss
of tensile strength and sag of the conductor. The thermal limit is directly proportional
to the cost of insulation of the conductor and this cost transcends to an increase in the
cost of switch gear, terminal equipment and transformers (Halder, 2013).

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4.2.1. DIFFERENCES BETWEEN HVDC LCC AND HVDC VSC
A detailed comparison between the LCC and VSC was done by a review of (Eeckhout, 2008) and
(Kure, et al., 2010), with the following as major differences.
Table 9: Comparison between LCC and VSC HVDC Transmission Topology.

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27
4.3. CASE STUDY REVIEW
Tables 10 and 11 shows Seabed Electrification projects that has been implemented. The Goliat Field [ (Terdre, 2010), (Siemens Energy Sector, 2010)],
Safaniya Field [ (Al-Rashed, 2015), (Bari, 2015)], Troll-A Gas Field [ (Statoil, 2010), (ABB, 2015)], Gjøa Field [ (Lo, 2014) (ABB, 2010)] and Valhall Field
were compared using various parameters to interpret the selection of the chosen technologies.
Table 10: A summarized comparison showing Seabed Electrification Projects of five various fields.

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Table 11: Further analysis five fields considered.

29
5. CONCLUSION AND RECOMMENDATIONS
The demand for greater power requirements for seabed electrification and its environmental
impact requires the adoption of green (renewable) sources of power. Regions of the world can
achieve this by reducing and discouraging further use of non-renewable sources so as to reduce
CO2 emissions. Africa, North Africa and Middle East needs to invest and explore opportunities in
Nuclear, Solar and Wind, Tidal and Wave sources of power generation so as to meet its seabed
electrification demands.
Transmission and distribution of power from their sources of generation to seabed depends on
considering the electric system architectures of the field, the field size and seabed processing
technologies to be used. Transmission distance, power requirement of all equipment,
transmission voltage, intervention and maintainability amongst others. The available
transmission schemes are HVAC and HVDC. The HVAC is very economical for distances not
exceeding 80 km offshore because cost of equipment for HVDC technologies over the same
distance are higher. However, HVDC and its arrangement topologies are better for longer step-
out over 100 km, as the cost evens out and become cheaper than HVAC over greater distances.
Transmission and corona losses, skin effects, thermal limits and huge cost of cable are major
limiters of HVAC transmission at longer distances. The HVDC technology also provides an easy
synergy of all sources power. This means different variants of renewable source of power can
be easily summed up to one system. A holistic analysis of the OPEX and CAPEX seabed
electrification from generation to distribution has to be done to decide on the best technologies
and topologies to adopt.
Future developments can consider the constructions of smart grids that can combine all onshore
and offshore sources of power into one system. Offshore Substation platforms may be
constructed and located strategically to transmit and distribute bulk power to fields around its
location for their seabed electrification.

30
6. REFERENCES
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ABB - ASEA Brown Boveri, 2014. Introducing HVDC. Available at:
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ec7508118cf7c1257c670040069e/$FILE/Introducing+HVDC.pdf [Accessed July 7, 2015].
ABB - ASEA Brown Boveri, 2015. ABB wins installation order for electrification system to
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[Accessed July 28, 2015].
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Al-Rashed, R., 2015. Safaniya Electrification Using High Power Field Power Challenges: Design,
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