IEC TS 62600-20:2019

Marine energy - Wave, tidal, and other water current converters - Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC) plant - General guidance

IEC TS 62600-20:2019 establishes general principles for design assessment of OTEC plants. The goal is to describe the design and assessment requirements of OTEC plants used for stable power generation under various conditions. This electricity may be used for utility supply or production of other energy carriers. The intended audience is developers, engineers, bankers, venture capitalists, entrepreneurs, finance authorities and regulators.
This document is applicable to land-based (i.e. onshore), shelf-mounted (i.e. nearshore seabed mounted) and floating OTEC systems. For land-based systems the scope of this document ends at the main power export cable suitable for connection to the grid. For shelf-mounted and floating systems, the scope of this document normally ends at the main power export cable where it connects to the electrical grid.
This document is general and focuses on the OTEC specific or unique components of the power plant, particularly the marine aspects of the warm and cold water intake systems. Other established standards are referenced to address common components between the OTEC system and other types of power plants and floating, deep water oil and gas production vessels, such as FPSOs and FLNG systems. Relevant standards are listed within this document as appropriate.

General Information

• Status: Published
• Publication Date: 17-Jun-2019
• ICS: 27.140 - Hydraulic energy engineering
• Technical Committee: TC 114 - Marine energy - Wave, tidal and other water current converters
• Drafting Committee: ahG 14 - TC 114/ahG 14
• Current Stage: PPUB - Publication issued
• Start Date: 18-Jun-2019
• Completion Date: 13-Jun-2019

Overview

IEC TS 62600-20:2019 - “Marine energy - Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC) plant - General guidance” - provides high-level, practical principles for the design assessment of OTEC plants. The Technical Specification targets stable electricity production from ocean thermal gradients and covers land‑based (onshore), shelf‑mounted (nearshore seabed), and floating OTEC systems. It focuses on OTEC‑specific and marine aspects of plant design (notably warm and cold water intake systems) while referencing established standards for common power‑plant and offshore system components.

Key topics and technical requirements

• Site and metocean parameters: Guidance on seawater temperature profiles, wind, waves, currents, sea level variation, marine growth, seabed conditions and other environmental inputs required for design assessment.
• Cold Water Pipe (CWP) and intake systems: Technical considerations for CWP sizing, structural loads, dynamic response, suction collapse, deflection due to currents and platform motions, and qualification approaches for deep‑water versus land/shelf installations.
• Warm water intakes & biofouling: Requirements for intake screens, warm water ducting, pump arrangements and biofouling control strategies.
• Process systems: Selection considerations for working fluids, heat exchangers, materials compatibility, and process hazards unique to closed‑ and open‑cycle OTEC plants.
• Platform and station‑keeping: Design guidance for floating platforms, mooring and station‑keeping arrangements, and distinctions for grazing (no export cable) vs non‑grazing plants.
• Power export and interfaces: Scope limits for power export (ending at the main export cable connection) and considerations for platform equipment, transmission cable routing and land‑based tie‑in.
• Energy carriers & storage: Overview of energy transfer options (electricity export and alternatives such as hydrogen, ammonia, methanol, batteries) as part of system planning.
• Risk‑based design, commissioning & lifecycle: Risk assessment methods, prototype testing, transportation & installation, commissioning, operations, inspection, maintenance and decommissioning guidance.
• Environmental and plume analysis: Guidance on discharge arrangements and expected seawater plume behavior.

Applications and intended users

IEC TS 62600-20 is intended for:

• Developers, engineers and designers of land, shelf and floating OTEC plants
• Investors, bankers and venture capitalists assessing technical risk and feasibility
• Entrepreneurs and project managers planning OTEC deployments
• Regulators, finance authorities and insurers setting acceptance criteria and underwriting risks

Practical uses include feasibility studies, concept selection, system design reviews, risk assessments and tender/specification preparation for OTEC projects.

Related standards

• References to established standards for common power plant and offshore structures (for example ISO and offshore platform standards such as ISO 19900 family) are included to address shared components and certification pathways.

Keywords: IEC TS 62600-20, OTEC, Ocean Thermal Energy Conversion, marine energy, cold water pipe, warm water intake, floating OTEC, land‑based OTEC, metocean, plume analysis, heat exchanger, working fluid.

Standards Content (Sample)

IEC TS 62600-20 ®
Edition 1.0 2019-06
TECHNICAL
SPECIFICATION
colour
inside
Marine energy – Wave, tidal, and other water current converters –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC)
plant – General guidance
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IEC TS 62600-20 ®
Edition 1.0 2019-06
TECHNICAL
SPECIFICATION
colour
inside
Marine energy – Wave, tidal, and other water current converters –

Part 20: Design and analysis of an Ocean Thermal Energy Conversion (OTEC)

plant – General guidance
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.140 ISBN 978-2-8322-6915-2

– 2 – IEC TS 62600-20:2019 © IEC 2019
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 11
2 Normative references . 12
3 Terms and definitions . 13
4 Abbreviated terms and acronyms . 15
5 Site specific and metocean design parameters . 15
5.1 Environmental factors influencing design . 15
5.1.1 General . 15
5.1.2 Seawater temperature . 16
5.1.3 Wind . 16
5.1.4 Waves . 16
5.1.5 Water depth and sea level variations . 17
5.1.6 Currents . 17
5.1.7 Marine growth . 17
5.1.8 Other meteorological and oceanographic information . 17
5.1.9 Water chemistry . 17
5.1.10 Third party (collision, anchor impact, trawling, Unexploded Ordinance
(UXO) . 17
5.1.11 Soil/seabed conditions . 18
5.2 Biological impact . 18
6 Floating OTEC – General information and guidance (closed cycle, deep water) . 18
6.1 Seawater considerations . 18
6.2 Cold seawater system . 19
6.2.1 Systems engineering considerations . 19
6.2.2 Cold water pumping power considerations . 20
6.2.3 CWP dynamic response . 20
6.2.4 Static Loads and bending moments . 21
6.2.5 Suction collapse . 21
6.2.6 Deflection by current and platform motions . 21
6.2.7 Analysis of loads and displacements . 22
6.2.8 Recommendations for qualification of the Cold Water Pipe (CWP) . 22
6.2.9 Analysis approach . 22
6.3 Warm seawater system . 22
6.3.1 Warm water intake (screen) . 22
6.3.2 Warm water ducting and pumps . 23
6.3.3 Biofouling control . 23
6.4 Seawater discharge arrangement and plume analysis . 23
6.4.1 Seawater discharge ducts . 23
6.4.2 Seawater pumps . 23
7 Process system . 24
7.1 Working fluid selection . 24
7.2 Heat exchanger (HX) selection . 25
7.3 Materials compatibility . 25
7.4 Process system risks and hazards . 25
8 Platform type . 25

8.1 General . 25
8.2 Mooring/Station keeping . 26
8.2.1 Grazing OTEC plants (no power export cable required) . 26
8.2.2 Non-grazing OTEC plants . 26
9 Power export . 27
9.1 General . 27
9.2 Design considerations . 27
9.3 Platform based equipment . 27
9.4 Transmission cable . 27
9.5 Land based equipment . 28
10 Energy storage and transfer system . 28
10.1 General . 28
10.2 Hydrogen . 28
10.3 Ammonia . 28
10.4 Methanol . 28
10.5 Battery storage . 28
11 Land and shelf-based OTEC . 29
11.1 General information and guidance. 29
11.2 CWP design for land and shelf-based OTEC plants . 29
12 Risk based approach for the design and operations of OTEC plants . 30
12.1 Risk assessment . 30
12.2 Risk based design. 30
12.2.1 Risk assessment process . 30
12.2.2 Prototype testing . 31
12.3 Risk based operational guidelines . 31
12.3.1 Floating plant . 31
12.3.2 Operating plant . 31
12.3.3 Product export risks/hazards . 31
13 Transportation and installation (T&I) . 32
14 Commissioning and handover . 32
15 Operations, inspection and maintenance . 33
15.1 General . 33
15.2 Operations . 33
15.3 Inspection and maintenance . 34
15.4 Hazards and safety . 35
15.4.1 Hazards . 35
15.4.2 Safety . 35
16 Decommissioning . 36
Annex A (informative) OTEC potential and its history . 38
A.1 OTEC potential . 38
A.2 Installation sites . 39
A.3 Previous OTEC projects . 39
A.4 Open cycle OTEC . 40
Bibliography . 41

Figure 1 – Tropical ocean temperature-depth profile . 7
Figure 2 – Working principle of closed cycle ocean thermal energy conversion [2] . 8

– 4 – IEC TS 62600-20:2019 © IEC 2019
Figure 3 – Major power cycle components of a closed cycle OTEC plant . 9
Figure 4 – Open cycle OTEC system . 10
Figure 5 – Example of a typical process for developing and testing an OTEC system
(land-based and floating) . 12
Figure 6 – Seawater differential temperature with 95 % confidence intervals. 14
Figure 7 – Example of OTEC power definitions . 14
Figure 8 – Seawater flow considerations for floating OTEC . 19
Figure 9 – Major components of a closed cycle OTEC plant working fluid process
system . 24
Figure 10 – ISO 19900 offshore standards relevant to OTEC platform design . 26
Figure 11 – Simple risk evaluation matrix . 30

Table 1 – Indicative design consideration in selecting Cold Water Pipe parameters . 19
Table A.1 – Notable OTEC systems – Past and present . 39

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MARINE ENERGY – WAVE, TIDAL, AND OTHER
WATER CURRENT CONVERTERS –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion
(OTEC) plant – General guidance

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. In
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Technical Specification are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 62600-20, which is a Technical Specification, has been prepared by IEC technical
committee 114: Marine energy - Wave, tidal and other water current converters.

– 6 – IEC TS 62600-20:2019 © IEC 2019
The text of this Technical Specification is based on the following documents:
Draft TS Report on voting
114/286/DTS 114/299A/RVDTS
Full information on the voting for the approval of this Technical Specification can be found in
the report on voting indicated in the above table.
A list of all parts in the IEC 62600 series, published under the general title Marine energy -
Wave, tidal and other water current converters, can be found on the IEC website.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
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A bilingual version of this publication may be issued at a later date.

INTRODUCTION
Seventy percent of the Earth’s surface is ocean. Most solar energy striking the ocean is
absorbed within the upper 100 m and is retained as thermal energy. Expanding slightly as it
warms the surface seawater layer is reheated by additional sunlight resulting in temperatures
often exceeding 25 °C in tropical latitudes. Deep seawater is much cooler, typically,
about 4-5 °C at depths varying from 800 m to 1 000 m, as shown in Figure 1. This deep cold
water is replenished from the polar regions by the thermohaline ocean circulation. From the
temperature difference that exists between these upper and deep layers of the ocean,
significant quantities of energy can be sustainably extracted by a process called Ocean
Thermal Energy Conversion, OTEC.

Figure 1 – Tropical ocean temperature-depth profile
The temperature difference between the ocean layers in the tropics changes very little during
daily or even yearly cycles and shows a moderate and predictable seasonal variation. This
steadiness creates an attractive characteristic in that OTEC can generate non-intermittent
(sometimes referred to as ‘base-load’) power. Due to the relative simplicity of the process,
OTEC is expected to have a very high capacity factor compared to most other forms or
renewable energy. Capacity Factor is the ratio of actual electrical energy output over a given
period of time, relative to the maximum possible electrical energy output over the same
amount of time. The maximum possible energy output of a given installation assumes its
continuous operation at full nameplate capacity over the relevant period of time. OTEC power
output reliability and predictability is appealing when compared to the intermittency and hence
low capacity factor of most renewable energy sources.
a) Working principle
OTEC converts a sustainable, low-grade heat source, ocean thermal energy, into electricity by
applying a thermodynamic cycle. The theoretical maximum thermal conversion efficiency is
determined by the Carnot cycle, where absolute ocean temperatures are applied in Kelvin. An
example of the Carnot efficiency with a hot source of 27 °C and a cold source of 4 °C is:
η_Carnot= 1-T_cold/T_hot  = 1-(4+273,15)/(27+273,15) =7,66 %
This efficiency assumes that the conversion is done by an ideal, reversible heat engine. In
practice, the actual heat transfer is irreversible due to temperature differences in the heat
exchangers and other factors. These heat transfer losses and the actual performance of the

– 8 – IEC TS 62600-20:2019 © IEC 2019
turbine and generator shall be accounted for when calculating the actual efficiency. The non-
ideal, actual efficiency would thus be in the range of 3 % to 4 %.
The OTEC process can be configured with different cycles: open, closed and hybrid. The
choice of which system will be optimum will normally be based on site characteristics, such as
local power and fresh water demand.
b) Closed cycle
Closed-cycle OTEC systems are based upon the Rankine thermodynamic cycle and use a
refrigerant-type process working fluid, contained within a closed piping system. Liquid
working fluid is pumped into an evaporator heat exchanger where heat from the warm
seawater causes the working fluid to vaporise. This vapour is piped to a turbine where its
enthalpic energy drives a turbine-generator. The turbine’s vapour exhausts to a condenser
heat exchanger, where it condenses to a liquid by the cooling effect of the cold seawater.
The liquid working fluid then drains to the working fluid pump, completing the cycle. Major
components and flows of a Closed Cycle OTEC plant are illustrated in Figure 1 and Figure 2.
Design considerations associated with these components will be discussed in Clause 5.
Within the evaporator, the warm seawater transfers its heat to the boiling working fluid,
becoming less warm. Similarly, heat from the condensing vapour causes the cold deep
seawater passing through the condenser to become less cold. The heat flow from warm water
is 3 % to 6 % larger than the heat flow into the cold water. This difference is the energy
usefully extracted by the turbine or lost due to friction.
The working fluid will have fluid properties that vary with the specific type used, such as R717
(anhydrous ammonia), R32, R134a or others. The evaporation and condensation properties
and heat exchanger design performance should normally be selected to attain optimum
efficiency for a particular working fluid. Within the process system, the highest pressure
occurs at the working fluid pump outlet, the lowest pressure occurs in the condenser and the
most significant pressure drop will take place within the turbine.

Figure 2 – Working principle of closed cycle ocean thermal energy conversion [2]
____________
Numbers in square brackets refer to the Bibliography.

Figure 3 – Major power cycle components of a closed cycle OTEC plant
c) Open cycle
Open-cycle OTEC uses a vacuum process to exploit the different boiling pressures of warm
and cold seawater. The working fluid is used only once and is continually replenished, hence
the term “open” cycle. The process is as follows: Warm seawater enters a large evaporation
chamber at approximately 96 % vacuum, where a small fraction of the seawater vaporizes to
low pressure steam and the remaining seawater supplies the needed heat of vaporization.
The cooled warm seawater is pumped from the evaporator. The low-pressure steam passes
through a mist separator, drives a low pressure turbine and exhausts into the condensing
chamber, which is maintained at approximately 98 % vacuum. The steam condenses directly
onto cold seawater droplets within the condenser chamber and the slightly diluted cool
seawater mixture is pumped from the condenser. Continuously-running vacuum compressors
maintain the chamber vacuum by removing dissolved air and other trace gases that enter with
the seawater flows.
Alternately, a large condensing surface heat exchanger can segregate the steam from the
cold seawater, yielding quantities of fresh water suitable for drinking water or irrigation. Thus
open cycle OTEC can be configured to produce both electricity and fresh water.
Both closed cycle and open cycle OTEC use the Rankine thermodynamic cycle. The primary
difference is that open cycle systems use large vacuum chambers and a very high-volume low
pressure steam turbine, whereas closed cycle uses heat exchangers, a smaller turbine and a
working fluid pump. A schematic diagram of the open cycle OTEC system is given in Figure 4

– 10 – IEC TS 62600-20:2019 © IEC 2019

Figure 4 – Open cycle OTEC system
d) Hybrid cycle
A hybrid cycle combines features of both the closed-cycle and open-cycle systems to yield
both electricity and desalinated water. Heat exchangers, vacuum chambers and other
components may be arranged in numerous stages to extract additional thermal value from the
“used” warm and cold seawater flows.

MARINE ENERGY – WAVE, TIDAL, AND OTHER
WATER CURRENT CONVERTERS –
Part 20: Design and analysis of an Ocean Thermal Energy Conversion
(OTEC) plant – General guidance

1 Scope
This part of IEC 62600 establishes general principles for design assessment of OTEC plants.
The goal is to describe the design and assessment requirements of OTEC plants used for
stable power generation under various conditions. This electricity may be used for utility
supply or production of other energy carriers. The intended audience is developers,
engineers, bankers, venture capitalists, entrepreneurs, finance authorities and regulators.
This document is applicable to land-based (i.e. onshore), shelf-mounted (i.e. nearshore
seabed mounted) and floating OTEC systems. For land-based systems the scope of this
document ends at the main power export cable suitable for connection to the grid. For shelf-
mounted and floating systems, the scope of this document normally ends at the main power
export cable where it connects to the electrical grid.
This document is general and focuses on the OTEC specific or unique components of the
power plant, particularly the marine aspects of the warm and cold water intake systems. Other
established standards are referenced to address common components between the OTEC
system and other types of power plants and floating, deep water oil and gas production
vessels, such as FPSOs and FLNG systems. Relevant standards are listed within this
document as appropriate.
The flow diagram, shown in Figure 5, illustrates the main design process associated with
floating, shelf-mounted or land-based OTEC systems.

– 12 – IEC TS 62600-20:2019 © IEC 2019

Figure 5 – Example of a typical process for developing and testing an OTEC system
(land-based and floating)
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60079-0:2017, Explosive atmospheres – Part 0: Equipment – General requirements
IEC TS 62600-1, Marine energy – Wave, tidal and other water current converters – Part 1:
Terminology
ISO 13628-5: 2009, Petroleum and natural gas industries – Design and operation of subsea
production systems – Part 5: Subsea umbilicals
ISO 13628-11: 2007, Petroleum and natural gas industries – Design and operation of subsea
production systems – Part 11: Flexible pipe systems for subsea and marine applications
ISO 19900, Petroleum and natural gas industries – General requirements for offshore
structures
ISO 19901 (all parts): Petroleum and natural gas industries – Specific requirements for
offshore structures
ISO 19901-1, Petroleum and natural gas industries – Specific requirements for offshore
structures – Part 1: Metocean design and operating considerations
ISO 19901-7:2013, Petroleum and natural gas industries – Specific requirements for offshore
structures – Part 7: Station keeping systems for floating offshore structures and mobile
offshore units
ISO 19902, Petroleum and natural gas industries – Fixed steel offshore structures
ISO 19903, Petroleum and natural gas industries – Fixed concrete offshore structures
ISO 19905 (all parts), Petroleum and natural gas industries – Mobile offshore units – Jackups
ISO 19906, Petroleum and natural gas industries – Arctic offshore structures
ISO 21650, Actions from waves and currents on coastal structures
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 62600-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
seawater differential temperature
temperature difference between the warm surface and the cold deep ocean sea water
Note 1 to entry: Figure 6 illustrates how this temperature differential may vary during the course of a year.

– 14 – IEC TS 62600-20:2019 © IEC 2019

Figure 6 – Seawater differential temperature with 95 % confidence intervals
3.2
rated, nameplate or nominal capacity
maximum net power that can be generated by an OTEC plant
Note 1 to entry: Note the relationship between generated power and seawater differential temperature (compare
Figures 6 and 7).
Note 2 to entry: In the example shown in Figure 7, the rated, nameplate or nominal capacity is 120 MW. The
maximum gross power for the OTEC plant would be 160 MW (August, September and October), where gross power
is the electric power generated at a defined set of seawater flows/temperatures and measured as the electrical
output of the alternator or alternators. Net power should be measured at the interface to the electrical grid and is
the power remaining after all system power losses (self-power, transmission losses, transformer losses) have been
realized.
Figure 7 – Example of OTEC power definitions

3.3
working fluid
gas or liquid heat transfer medium, which drives the turbine
Note 1 to entry: Closed cycle OTEC systems use industrial refrigerants/working fluids such as anhydrous
ammonia, R32, or other suitable fluids. Open cycle OTEC systems use vapour from the flash evaporation of warm
sea water producing low pressure steam.
3.4
Front End Engineering Design
FEED
body of engineering that takes a conceptual design forward to allow a cost estimate for
project control purposes to be derived
Note 1 to entry: Typically, it may represent between 20 % and 25 % of the total engineering of a project.
4 Abbreviated terms and acronyms
CWP Cold Water Pipe
EJ Exojoule (1.0E+18 Joules)
FMEA Failure Mode Effect Analysis
FLNG Floating Liquefied Natural Gas
FPSO Floating, Production, Storage and
Offloading
HX Heat Exchanger
HIRA Hazard Identification and Risk
Assessment
IMO International Maritime Organisation
IRM Inspection, Repair and Maintenance
ROV Remotely Operated Vehicle
SCADA Supervisory Control and Data
Acquisition
VIV Vortex Induced Vibrations
5 Site specific and metocean design parameters
5.1 Environmental factors influencing design
5.1.1 General
The phenomena listed in 5.1.2 through to 5.1.10 shall, based on region specific data, be
addressed in the design.
These phenomena shall be described by physical characteristics and supporting statistics.
The joint occurrence of different values of parameters shall also be defined once suitable data
are available. From this information, appropriate environmental design conditions shall be
established that will consider the following:
a) The type of structure being designed.
b) The phase of development (e.g. construction, transportation, installation, etc.).
c) The limit-state considered.
Usually two sets of conditions should be established that take into consideration the following:

– 16 – IEC TS 62600-20:2019 © IEC 2019
• Normal meteorological and oceanographic conditions that are expected to occur frequently
during the life of the structure. These conditions are needed to design systems for fatigue,
to plan field operations, such as inspection/maintenance operations and to develop the
actions caused by the environment associated with particular operations or serviceability
checks;
• Extreme meteorological and oceanographic conditions that recur with a given return period
or probability of occurrence.
Extreme, normal and other meteorological and oceanographic parameters shall be determined
from actual measurements at the site or by suitable validated hindcast model data.
5.1.2 Seawater temperature
Seawater temperature profiles should typically be measured at the site at least monthly for at
least one year to optimise the selection of warm water intake depth, optimum cold water
intake depth, and the mixing of discharged seawater.
Seawater temperatures at the selected intake and discharge depths should be measured at
least hourly for at least one year to quantify daily and seasonal variability. Minimum,
maximum and average temperatures should be reported.
Adequate data shall also be collected at high resolution (~ 1 min) to quantify dynamic
temperature variations for warm and cold seawater caused by tides, storms or internal waves.
This data shall be evaluated with nearby long-term observations and/or validated hindcast
data to calculate Annual Energy Production (AEP) and to support financial modelling. The
data should also be used for environmental mixing studies as part of the permitting/licensing
process. Guidance on additional data collection requirements should be obtained from
relevant permitting agencies.
The warm water intake should be located deeper than typically 10 m to avoid flotsam and
effects of surface waves. Cold seawater intake depth will generally be based upon an
optimization of power output versus cold water pipe cost. The selected discharge depth is
likely to depend upon the desired use of the discharge water and permitting considerations.
Representative depths may be 15 m for warm water intake, 900 m to 1 100 m for cold water
intake and 75 m to 100 m for a mixed discharge.
5.1.3 Wind
Actions caused by wind acting on a structure shall be considered for both the global and local
design. Site-specific information on air density, wind speed, direction and duration shall be
determined.
Wind is usually characterized by the mean value of its velocity over a given time interval at a
given elevation above the mean water level. In specific cases (for example, design of flexible
and compliant structures such as oil and gas flare-towers), the frequency content is of
importance and shall be addressed.
Wind spectrum should be considered for moored floating OTEC systems, including second
order slow drift effects.
The variability with elevation and spatial coherence shall be considered. Reference to
ISO 19901-1 is highly recommended. Another beneficial reference is DNV GL RPC205 [5].
5.1.4 Waves
Actions caused by waves acting on a structure shall be considered for both the overall
structure and mooring, as well as individual components that are exposed to wave forces.
Waves are usually characterized by wave spectrum, significant wave height and peak period.

a) For land-based plants, data collected and analysis should conform to the procedures
listed in ISO 21650 as well as ISO 19905 or ISO 19902 as relevant.
b) For plants sited in deep water, data collected and analysis should conform to procedures
listed in ISO 19901-1. Also beneficial is DNV GL RP-C205 [5].
5.1.5 Water depth and sea level variations
The water depth shall be determined. The magnitude of the low and high tides and positive
and negative storm surges shall be addressed.
The possibility of ground subsidence shall be considered when determining the design water
depth.
5.1.6 Currents
Phenomena such as tidal, wind driven, cyclone induced, global circulation, solitons, loop and
eddy currents shall be assessed to determine whether they should be considered in the
design process.
Currents shall be described by their velocity (magnitude and direction), variability with water
depth and persistence.
The occurrence of fluid motion caused by internal waves shall be considered.
5.1.7 Marine growth
Potential marine growth shall be considered and then defined by estimated thickness,
roughness, density and variation with depth.
The design may rely on periodic marine growth cleaning or anti-fouling systems during the
platform life. Any such reliance shall be documented and the cleaning program defined over
the life of the platform including cost and feasibility. The consequences of not maintaining this
program should be determined and formally documented taking into account the implications
relative to the original design parameters.
5.1.8 Other meteorological and oceanographic information
Other environmental information such as precipitation, fog, and variability of the density and
oxygen content of the sea water shall be assessed from a possible operational point of view
and documented whether this is necessary to assess.
5.1.9 Water chemistry
For both warm and cold water the following information at intakes and proposed discharge
zone depth range should be determined:
• Nutrient concentration.
• Turbidity.
• Micro and macro plankton.
• Oxygen, carbon dioxide, etc., content.
5.1.10 Third party (collision, anchor impact, trawling, Unexploded Ordinance (UXO)
These factors all can potentially affect an OTEC plant design and should be reviewed and
addressed as required.
– 18 – IEC TS 62600-20:2019 © IEC 2019
5.1.11 Soil/seabed conditions
Seabed conditions shall be precisely defined in the OTEC installation area by conducting
geophysical surveys and geotechnical surveys. This includes surveys in the vicinity of
mooring locations and along the cable route(s) or seawater pipe routes (for land or shelf-
mounted OTEC systems). Geohazards shall also be assessed for the proposed installation
site.
5.2 Biological impact
The following marine organisms have potential to affect the reliable and thermodynamically
efficient operation of an OTEC system:
• Plankton.
• Fish.
• Plants.
• Coral and biofouling build up.
• Mammals in the location of the OTEC system.
• Other Benthic organisms.
Experience from the offshore oil and gas sector has shown that the potential effect from these
biological factors can be controlled with considered initial design and careful in field
operation. Because the thermodynamic efficiency of an OTEC system is low, small reductions
in efficiency can significantly reduce power output and profitability. This illustrates the
importance of controlling potential biological impacts and also ties in with the importance of
prototype testing, which is covered in 12.2.2.
6 Floating OTEC – General information and guidance (closed cycle, deep
water)
6.1 Seawater considerations
Figure 8 illustrates terms and possible seawater intake and discharge layouts for a generic
floating OTEC plant, nominally based on a Spar buoy type hull form as an example. Many
other types of hull form have also been proposed.
Primary design considerations include:
– Optimised architecture for construction, installation, operation, mooring/propulsion,
maintenance/refit and decommissioning.
– Appropriate stability and strength during storms, and long-term structural fatigue capacity.
– Ensured crew safety and health during visits or long-term occupancy.
– Efficient seawater flow paths to minimize parasitic power losses.
– Seawater intake design optimised to manage entrainment and impingement of sea life
addressing location, flow velocity and intake flow direction.
– Sea water discharge plumes located and directed to prevent their re-ingestion into th
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