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, * aCranfield University, School of Energy, Environment and Agrifood, Bedford, MK430AL, UK bCranfield 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: [email protected] (Dr Weizhong Fei) **Author; E-mail address: [email protected]

http://www.elsevier.com/locate/jpetscieng

mailto:[email protected]

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.


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.


4

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.


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 wind farms using larger turbines and an improved planning 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.


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


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).


8

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).


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).


10

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


11

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


12

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.


13

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).


14

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).


15

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.


16

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:


17

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).


18

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


19

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.


20

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).


21

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.


22

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.


23

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 solar installations 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,


24

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.


25

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).


26

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.


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.


28

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

ABB - ASEA Brown Boveri, 2010. Gjøa receives power from shore. Available at:

http://www.abb.com/cawp/seitp202/b6dcda123bb5d28bc125778500352b28.aspx [Accessed

July 29, 2015].

ABB - ASEA Brown Boveri, 2014. Introducing HVDC. Available at:

http://www04.abb.com/global/seitp/seitp202.nsf/c71c66c1f02e6575c125711f004660e6/d8e7

ec7508118cf7c1257c670040069e/$FILE/Introducing+HVDC.pdf [Accessed July 7, 2015].

ABB - ASEA Brown Boveri, 2015. ABB wins installation order for electrification system to

Norwegian platform. Available at:

http://www.mena.abb.com/cawp/seitp202/cf86c94ac420344c85257a670045e7b7.aspx

[Accessed July 28, 2015].

ABB - ASEA Brown Boveri, 2015. HVDC Classic (LCC). Available at:

http://new.abb.com/systems/hvdc/hvdc-classic


ABB - ASEA Brown Boveri, 2015. Subsea transformers. Available at:

http://new.abb.com/products/transformers/special-application/offshore-and-subsea


Al-Rashed, R., 2015. Safaniya Electrification Using High Power Field Power Challenges: Design,

Procurement and Shallow Water Installations. Proceedings of Offshore Technology

Conference, Houston, (OTC 25777), p.1495. Available at:

https://www.onepetro.org/conference-paper/OTC-25777-MS

Al-Zubaidy, M. S. K., 2015. Green Energy: Examining Thier Effects on Heritage Sites and Climate

Change Mitigation. Open Journal of Civil Engineering, Issue 5, pp. 39-52.

Andersen, A. D., 2014. No transition without transmission: HVDC electricity infrastructure as an

enabler for renewable energy?. Environmental Innovation and Societal Transitions, Issue 13,

pp. 75-95.

Athavale, C., 2012. http://www.akshardhool.com/2012/12/the-wave-power.html. Available at:

http://www.akshardhool.com/2012/12/the-wave-power.html [Accessed June 30, 2015].

Bai, Y. & Bai, Q., 2010. Subsea Engineering Handbook. Burlington: Gulf Publishing Company.

Bari, S. J., 2015. Electrical Power Transmission to Offshore Facilities: A Case Study. Proceedings

of Offshore Technology Conference, Houston, (OTC 25704), p.711. Available at:


Bavidge, M. & NES Gloal Talent, 2013. Husky Liwan Deepwater subsea Control System.

Proceedings of Offshore Technology Conference, Houston, (OTC 23960), p. 540. Available at:


Boyle, G., 2012. Renewable Energy: Power for a Sustainable Future. 3rd Ed., USA: Oxford

Universiy Press.

31

Breeze, P., 2014. Power Generation Technologies. 2nd Ed., Oxford: Elsevier (Newnes

publications).

Diesel Service and Supply, 2013. How Generators Work. Available at:

http://www.dieselserviceandsupply.com/How_Generators_Work.aspx [Accessed July 4, 2015].

EDF Energy, 2015. How electricity is generated through solar power. Available at:

http://www.edfenergy.com/energyfuture/solar-generation [Accessed July 1, 2015].

Eeckhout, B. V., 2008. The Economic Value of VSC HVDC compared to HVAC for offshore wind

farms, Belgium: K. U. Leuven.

EIA, 2014. The International Energy Outlook 2014, Washington: US Energy Information

Adminstration.

EL-Hawary, M. E., 2008. Introduction to Electrical Power System. Hohoken, New Jersey: John

Wiley & Sons, Inc.

Farret, A. F. & Simoes, M. G., 2006. Integration of Alternative Sources of Energy. Hoboken, New

Jersey: John Wiley & Sons.

Gallop, M., 2012. Tidal Power Energy. Available at: https://prezi.com/q66dpqtci2an/tidal-

power/

[Accessed June 28, 2015].

Grigsby, L. L., 2001. The Electrical Engineering Handbook Series. Alabama: CRC Press.

GWEC, 2014. Global Wind 2014 Report, Belgium: Global Wind Energy Council.

Halder, T., 2013. Comparative Study of HVDC and HVAC for a Bulk Power Transmission. West

Bengal, India, International Conference on Power, Energy and Control (ICPEC), pp. 139-144.

Available at:

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6527639

Hammons, T. J., 2010. Power Cables in the Twenty-First Century. Electric Power Components

and Systems, June 21, 31(10), pp. 967-994.

Hansen , R. L. & Rickey, W. P., August 1995. Evolution of Subsea Production Systems: A

Worldwide Overview. Journal of Petroleum Technology, 47(08), pp. 675 - 680.

Hazel, T., 2011. Impact of Subsea Processing Power Distribution: Subsea Switchgear Module A

Key Enabling Component in Subsea Installations, Paris: Schneider Electric Industries SAS.

HITACHI, 2013. Photovoltaic Power Generatoin Sysytem Outline. Available at:

http://www.hitachi.com/products/power/solar-power/outline/index.html [Accessed August

10, 2015].

Hosenuzzaman, H. et al., 2015. Global prospects, progress, policies, and environmental impact

of solar photovoltaic power generation. Renewable and Sustainable Energy Reviews, Issue 41,

pp. 284-297.

Hydro Tasmania, 2015. About wind power. Available at:

http://www.hydro.com.au/energy/about-wind-power [Accessed June 25, 2015].

32

IEA Statistics, 2013. Energy Statistics of OECD Countries, France: International Energy Agency.

Jenkins, D., Christiansen, M. & Thumbeck, S., 2013. Essential Design And Risk Management For

A Next Generation Ocean Dry Mate Connector.

Available at: http://www.ametek-ecp.com/~/media/AMETEK-

ECP/Files/ProductDownloadableDocuments/DataSheetsSCP/Ocean-Dry-Mate-Connector.ashx


Khemchandani, R., Singh, A. N. & Khanna, H., 2014. New Comparison of HVDC and HVAC

Transmission System. International Journal of Research (IJR), November.1(10).

Kure, H. M., Larsson, C. O., Lefstad, T. S. & Müller, L. A., 2010. Power Electronics for Renewable

Energy, Trondheim: Norwegian University of Science and Technology.

Legeay, J., 2014. HV Wet-Mate Connectors and Penetrators Help Enable Subsea Processing.

Available at: http://designsmarterfaster.te.com/downloads/wet-mate-connectors.pdf


Liebfried, T. & Zöller, T., 2010. Transmission of electrical power through subsea-cables over

long distances. 3rd Evolving Multiphase Boosting Technology (EMBT) Conference, Hannover,

pp. 187-193. Available at: http://www.embt-conference.com/assets/EMBT-Conference-

Proceedings-2010.pdf

Lo, C., 2014. Onshore power for offshore platforms. Available at: http://www.offshore-

technology.com/features/featureonshore-power-for-offshore-platforms-4330517/ [Accessed

July 29, 2015].

Maehlum, M. A., 2015. How Does Ocean Wave Power Work?. Available at:

http://energyinformative.org/wave-energy/ [Accessed June 30, 2015].

Maehlum, M. A., 2015. How Does Tidal Power Work?. Available at:

http://energyinformative.org


Martínez , I. d. A. et al., 2009. Transmission alternatives for offshore electrical power.

Renewable and Sustainable Energy Reviews, 13(5), pp. 1027-1038.

Meah, K. & Ula, S., 2007. Comparative Evaluation of HVDC and HVAC Transmission Systems.

Wyoming, IEEE Xplore.

MOFANIM, 2012. The problem with Nuclear Power. Available at:

https://mofanim.wordpress.com/2012/06/12/the-problem-with-nuclear-power/ [Accessed

July 2, 2015].

Mourant, A., 2014. Ready to explode. Renewable Energy Focus magazine, 14 January, Issue

January/February 2014.

Newell, C., Brown, G. & Brantner & Assocs. Inc, 2005. Underwater Optical and Electrical

Connector Systems, Technical Innovations. Proceedings of Offshore Technology Conference,

Houston, (OTC 17309), p. 887. Available at:


33

Nuclear Energy Institute (NEI), 2015. How Nuclear Reactors Work. Available at:

http://www.nei.org/Knowledge-Center/How-Nuclear-Reactors-Work [Accessed July 13, 2015].

Ocean Power Delivery Ltd., 2014. The Pelamis Wave Energy Converter. Available at:

http://hydropower.inl.gov/hydrokinetic_wave/pdfs/day1/09_heavesurge_wave_devices.pdf


Persson, G., 2011. HVDC Converter Operations and Performance, Classic and VSC.

Available at: http://www.sari-

energy.org/PageFiles/What_We_Do/activities/HVDC_Workshop_Sep_2011/presentations/HV

DC%20Converter%20Operations%20and%20Performance,%20Classic%20and%20VSC_ABB.pdf


Phipps, C. A., 1999. Variable Speed Drive Fundamentals. 3rd Ed., New Jersey: The Fairmont

Press.

Ronström, L., Hoffstein, M. L., Pajo, R. & Lahtinen, M., 2007. The Estlink HVDC Light®

Transmission System, Estonia: CIGRE.

Saksvik, O., 2012. HVDC technology and smart grid. Hong Kong, The 9th IET International

Conference, Hong Kong November 2012. Advances in Power System Control, Operation and

Management.

Sharma, S., Goel, K., Gupta, A. & Kumar, H., 2012. CORONA EFFECTS ON EHV AC

TRANSMISSION LINES. International Journal of Scientific Research Engineering & Technology

(IJSRET), 1(5), pp. 160-164.

Siemens Energy Sector, 2010. Siemens to erect shoreside power supply system for offshore

platform in the Barents Sea – Fifty-percent reduction of CO2 emissions. Available at:

http://www.siemens.com/press/en/pressrelease/?press=/en/pressrelease/2010/power_trans

mission/ept201012026.htm&content[]=ET&content[]=EM [Accessed July 26, 2015].

Statoil, 2010. Electrification of offshore installations. Available at:

http://www.statoil.com/annualreport2010/en/sustainability/health,safety,climateandtheenvir

onment/climate/pages/electrificationofoffshoreinstallations.aspx [Accessed July 29, 2015].

Stewart, S., 2008. Distribution Switchgear. 2nd Ed., London: The Institution of Engineering and

Technology.

Subsea Cables UK, 2015. Submarine Power Cables.

Available at: www.subseacablesuk.org.uk [Accessed July 15, 2015].

SULZER, 2013. Sulzer Pumps and FMC Technologies Sign a Long-term and Exclusive

Collaboration Agreement on Subsea Pumps. Available at:

http://www.sulzer.com/en/Newsroom/Group-News/2013/130205-Sulzer-Pumps-and-FMC-

Technologies-Sign-Collaboration-Agreement-on-Subsea-Pumps [Accessed June 26, 2015].

Terdre, N., 2010. Goliat subsea power cable designed for long-distance service in harsh

conditions. Available at: http://www.offshore-mag.com/articles/print/volume-70/issue-

7/sweden/goliat-subsea-power-cable-designed-for-long-distance-service-in-harsh-

conditions.html [Accessed July 26, 2015].

34

Tidal Energy, 2015. What is Tidal Energy?. Available at:

http://www.tidalenergyltd.com/?page_id=1370


Turke, S. S., 1999. Understanding Variable Speed Drives. Available at:

http://ecmweb.com/content/understanding-variable-speed-drives-part-1 [Accessed July 14,

2015].

Ulsund, R., 2009. Offhsore Power Transmission (submarine high voltage transmission

alternatives), Norway: Norwegian University of Science and Technology.

Viska, M. & Karl, K., 2011. Separator Vessel Selection, and Sizing (Engineering Design

Guideline). Malaysia: KLM Technology Group.

Warne, D. F., 2005. Newnes Electrical Power Engineer's Handbook. 2nd Ed., Burlington: Elsevier

(Newnes publications).

Wijaya, W. Y., 2010. Energy Storage Technologies for Electricity Grid Infrastructure, Indonesia:

Wordpress.

Woodford, D. A., 1998. HVDC Transmission, Manitoba: Manitoba HVDC Research Centre.

World Coal Association, 2015. Coal & Electricity. Available at:

http://www.worldcoal.org/coal/uses-of-coal/coal-electricity [Accessed June 27, 2015].

World Energy Council, 2013. World Energy Resources, London: World Energy Council.

Worzyk, T., 2009. Submarine Power Cables. Dordrecht: Springer.