Concrete for ocean thermal energy conversion structures

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Technical Note N-1448

CONCRETE FOR OCEAN THERMAL ENERGY CONVERSION STRUCTURES

By

H. H. Haynes and R. D. Rail

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August 1976 es Givin

DOCUMENT |

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Division of Solar Energy
U. S. Energy Research and Development Administration
Washington, D.C. 20545

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CIVIL ENGINEERING LABORATORY
Naval Construction Battalion Center
Port Hueneme, California 93043

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TITLE (and Subtitle)

CONCRETE FOR OCEAN THERMAL ENERGY
CONVERSION STRUCTURES

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TYPE OF REPORT & PERIOD COVERED

Final; Jun 1975—Jan 1976

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PERFORMING ORG. REPORT NUMBER

AUTHOR(s)

H. H. Haynes and R. D. Rail

8. CONTRACT OR GRANT NUMBER(s)

. PERFORMING ORGANIZATION NAME AND ADDRESS
Civil Engineering Laboratory
Naval Construction Battalion Center
Port Hueneme, California 93043

PROGRAM ELEMENT, PROJECT, TASK
AREA & WORK UNIT NUMBERS

44-017

CONTROLLING OFFICE NAME AND ADDRESS
Energy Research and Development Admin.

Division of Solar Energy
Washington, DC_ 20550

REPORT DATE
August 1976

NUMBER OF PAGES

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SUPPLEMENTARY NOTES

KEY WORDS (Continue on reverse side if necessary and identify by block number)
Concrete structures, ocean structures, pressure-resistant structures, underwater
ocean thermal energy, research, state-of-the-art, cylindrical shells, hydrostatic

pressure, offshore structures

ABSTRACT (Continue on reverse side if necessary and identify by block number)

The purpose of this study was to assess the state

of the art of concrete technology

and construction practices as they are related to the construction of massive floating
structures to house ocean thermal energy conversion (OTEC) systems. The relevant
capabilities and limitations of available concrete technology and construction practices
are described and deficient areas are identified. Recommendations for research and

FORM
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EDITION OF 1 NOV 65 1S OBSOLETE

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20. Continued

development are given by which reasonable improvements can be made in the near term to
provide greater assurances of long-term safe and reliable operation of the OTEC systems and
to provide lower cost structures.

Library Card

Civil Engineering Laboratory

CONCRETE FOR OCEAN THERMAL ENERGY CONVERSION
STRUCTURES (Final), by H. H. Haynes and R. D. Rail

TN-1448 47 p. illus August 1976 Unclassified

1. Concrete structures 2. Underwater ocean thermal energy I. 44-017

The purpose of this study was to assess the state of the art of concrete technology and
construction practices as they are related to the construction of massive floating structures to
house ocean thermal energy conversion (OTEC) systems. The relevant capabilities and limitations
of available concrete technology and construction practices are described and deficient areas are
identified. Recommendations for research and development are given by which reasonable
improvements can be made in the near term to provide greater assurances of long-term safe and
reliable operation of the OTEC systems and to provide lower cost structures.

i Unclassified

SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered)

CONTENTS

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OlnjecEhye o 6 0156 6660500 0 0
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REQUIREMENTS FOR OTEC STRUCTURES .. .

SCOV oo 6 0 60 0 Oo OO 0 0 OO
System Requirements ....
Structural Requirements... .

SUM O12 AYE G6 5 6 5 6 00 00 0

Floating and Submerged Concrete
Macerte 9 56 0 © 00000 0 0
Iyoealm 5 560.06 0-00 00.00 0
GCOMmStrctereslei 565656000000
(Ojoyeralicsl@in 6 G6 0660 0000 6 06

Structures

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RESEARCH AND DEVELOPMENT RECOMMENDATIONS .. .

Materials 6 i. in et cn ews we a
DASE 6 6 6 610 0,00 0 0.0 6
GCOMBIERSMICELEM 46 6600066000
Oparealelein 5 © 6 6.000 6 0 0
Selnacwttl 6 56666000 6

SUMMWINRSS 5 59 6000000085000 0

INFRIANANGIES 6 6 0 0 06 6008 0 0

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INTRODUCTION
Objective

The purpose of this study was to assess the state of the art of
conerete technology and construction practices as they are related to
the construction of massive floating structures to house ocean thermal
energy conversion (OTEC) systems. The relevant capabilities and limita-
tions of available concrete technology and construction practices are
described and deficient areas are identified. Recommendations for
research and development are given by which reasonable improvements can be
made in the near term to provide greater assurances of long-term safe and
reliable operation of the OTEC systems and to provide lower cost
structures.
Background

OTEC power plants are being developed to convert solar-derived
energy stored as heat in the world's oceans to electrical or other man-
usable forms of energy. Since the temperature differences between the
warm ocean surface waters and the cold deep waters are small, the size
of the thermal engines and the quantities of warm and cold water needed
are very large. Therefore, the OTEC power plants are not standard con-
struction items but represent a new type of ocean facility that has
never been built before: huge floating structures on the order of
several hundred feet in both vertical and horizontal directions. From
the main platform hangs a cold water inlet pipe at least a thousand feet
long and perhaps one hundred feet in diameter. Both moored and free
floating (dynamically positioned) concepts have been proposed. The

main platform may be a surface vessel, a semisubmersible, or a structure

wholly submerged several hundred feet with only an access tower to the

[-5] . ;
surface. The design and construction of such novel structures will
require imagination and utilization of past experiences drawn from many
engineering disciplines.

This report discusses only the topic of concrete. The main platform,
the cold water intake pipe, and the anchor for OTEC structures could be
built of concrete today using available technology and construction
practices; however, further study at this time of selected problems with

reachable near-term solutions can significantly lower costs, lessen

risks, and provide longer structure life.

REQUIREMENTS FOR OTEC STRUCTURES
Scope

This study addresses the requirements of OTEC structures as a class
and not the requirements of individual configurations or concepts. At
present, OTEC structural concepts range from long, vertically oriented
structures to moderately long, horizontally oriented structures, and
from single-cylindrical-hull surface structures to multiple-cylindrical-
hull submerged peanoras 2) Requirements common to all OTEC concepts
are the focus in this paper for describing current concrete technology and

identifying areas for profitable research and development.

System Requirements

The basic system requirements for OTEC structural systems include:

fo) Capability (to perform the mission)
co} Availability (to begin the mission)
fe) Reliability (to continue performance throughout mission life)

The fundamental requirement is, of course, to be able to construct and
deploy structural systems capable of performing their service functions
in the ocean environment for the design life of about 25 to 40 years
(not specifically defined at this stage).

Capability. The service function of the platform is to house men
and equipment in a dry one-atmosphere environment.

The cold water pipe is required to transport huge volumes of cold
water with a minimum of power, head loss, and temperature change from

depths of one thousand to several thousand feet.

For moored OTEC plants, an anchor or anchors is required to hold
the structure on station in water depths to 20,000 feet under all
environmental conditions.

Availability. The OTEC plant must be constructable and deployable.
The current target schedule for OTEC development is to complete concept
design by the end of 1979, a 25- to 50- Mw, prototype by 1982, and
an operating 100-Mw demonstration plant by 1987. The approach to
meet this schedule is (1) to utilize state-of-the-art engineering,
construction, and marine operations capabilities, (2) to adapt the state-
of-the-art to the OTEC application by engineering investigations,
engineering development including validation testing of critical compon-
ents, and extension of marine and construction methods; and (3) to
generate new information from research and technology

Reliability. Throughout the 25- to 40-year life the standard of
safety and degree of long-term reliability--including survival in extreme
conditions--need to be high. Total loss of the facility or loss of
personnel is unacceptable; however, a partial structural failure that

would interrupt power generation for a limited time would be acceptable.

Structural Requirements
To provide the system requirements of capability, availability and
reliability will require certain structural characteristics which are
described below.
The structural requirements to provide "System Capability" are:
1. Hydrodynamic Stability. The structure must be stable as a floating/
moored vessel during construction, deployment, service, and modular

assembly and disassembly (if used).

2. Positive Buoyancy. The platform supports its own self-weight, the
cold water pipe, thermal and electrical equipment, and vertical
component of the mooring force.

3. Controlled Variable Buoyancy. Platform buoyancy will need to be
constantly compensated for changes in the density of ambient seawater,
vertical forces due to waves, and - most important - the vertical component
of the mooring force, which is influenced by currents, wind, and waves.
Long-term increases in weight will also need to be considered because of
biofouling and seawater saturation of concrete.

4. Pressure Resistant Hull. The platform will need to be pressure
resistant to depths of several hundred feet in order to provide the
buoyancy and the one-atmosphere housing for men and equipment.

The structural requirements to provide "System Availability" are:

1. Design and Engineering. Design requires a knowledge of the loadings,
the structure's capability to resist the loadings, and analysis methods
to relate the loadings to the structure's resistance.

2. Loadings. The structure will be subjected to a variable hydro-
static head} repetitive loadings and vibrations from waves, machinery,
and mooring lines$ and dynamic, concentrated loads at the cold water pipe
connection. The cold water pipe will be subjected to tensile loads by
its own weight and to significant bending and shear forces (and possibly
large and small amplitude vibrations) by currents and, perhaps, internal
waves. Contingent loadings such as impact due to collision and grounding
must be considered.

3. Load-Resistance Capacity. To resist service and environmental

loadings, the structure must provide adequate strength to bending, tensile,

compressive, torsion, and shear loads and provide overall and local struc-

tural stability.

4.Structural Analysis. Established analysis methods of working stresses,
cracking strength, ultimate strength, elastic and plastic deformations,
and stability--based on behavior of materials and of structural elements-—-
will be required to analyze the OTEC structure. Other methods such as model
testing, surveys of existing structures, and probabilistic design may need
to be employed. Consideration will need to be given to time-dependent
design for fatigue, creep, and relaxation, and to change to material properties
with time.

5.Construction Methods. To construct large concrete structures methods
need to be available to construct in a floating mode in shallow or deep
protected waters. Also, open sea assembly of large floating components
may be required. Total time of construction should be minimized.
Variable positive buoyancy will be required during construction and
deployment of the platform and the cold water pipe. The concrete anchor,
if used, will require positive buoyancy during tow-out, slight negative
buoyancy during lowering, and then heavy selfweight or engagement
to the seafloor; this may require the capability to fill the anchor with
grout or concrete at great depths by remote methods.

6.Availability of Resources. Size dominates the
requirements for manpower, materials, and facilities. Concrete is not as
labor intensive nor as demanding as steel construction for sophisticated
skills; however, skilled and unskilled labor will be needed, probably
around the clock. Cement, aggregates, fresh water, reinforcing

and prestressing steels, and forming materials must be available at, or

readily transportable to, the construction site. A large on-land near-
shore construction site, preferably with a graving basin, will be needed
as will a protected shallow water site. Anear-shore, protected deep
water site with a deep passage to sea is highly desirable to
preclude or minimize the need for expensive flotation and to minimize
construction time in the open sea. The construction site will likely
need to be at or near an industrial area to most economically provide
trained manpower, construction equipment and supporting services.
The structural requirements to provide "System Reliability" are:

1. Design for Service and Survival Modes. The structure will probably
be designed to operate in a service mode during a defined environmental
condition or a defined accident condition. For sea and accident conditions
beyond the service mode, the structure will be designed for survival.
That is, the structure will maintain its integrity and remain afloat so
that men and the facility are safe; however, power is not being generated
and repairs may be needed before the plant is again in operation.

2.Safety. Safety depends on probability of failure and consequences of
failure. Thus safety requires structural characteristics that will mini-
mize probability of failure by good quality design and construction, durable

materials, and so forth and that will minimize the seriousness of failure

by designing for an acceptable mode of failure.

3. Noncatastriphic Failure Mode. Structural failure should be gradual
and partial rather than sudden and catastrophic. This requires the use
of structural redundance, the provision of structural resilience (to
absorb energy without failure) and structural ductility (to accommodate
deformation without failure), and the avoidance of instability failure.

‘Accident tolerance may require sacrificial protective structures such

as fenders, compartmentalization (perhaps double hulls), and guaranteed
reserve buoyancy in case of flooding. A progressive failure mode must
be avoided.

4, Fire Safety. The material and structure should be highly resistant
to damage by fire and capable of limiting the spread of fire.

5.Durability. The construction material (the concrete itself and
the embedded reinforcing and prestressing steel) must have the long-term
capability to resist degradation below a specified standard in the marine
environments of atmospheric zone$ splash zone} shallow water submerged zone;
deep water submerged zone$ and, for the anchor, near the seafloor zone and
under the seafloor zone.

6. Long-Term Engineering Properties of Materials. The engineering
properties of the construction material must be known over long-term
exposure to the marine environment. This includes the behavior of con-
crete materials in the stages of partial and complete saturation with
seawater.

7.Maintainability and Repairability. To provide long-term reliability
and economy, the need for structural maintenance and repair must be
minimized; at the same time, inspection, maintenance and repair
capabilities must be available in order to detect, prevent, control and
overcome material degradation that does occur. Some, but not all, OTEC
concepts use a modular design to permit periodic removal and replacement
of major modules to reduce downtime and permit shipyard overhaul and
refit, particularly the heat exchangers; this modularity requires methods
for at-sea assembly and disassembly of major structural eonmenenec lt)

Some concepts visualize that remote replacement of the critical anchor-to-

mooring line connection, on a scheduled or as needed basis, will be

8

necessary to meet the design rite IS] All OTEC power plants, including the
modular ones, are too large to drydock and therefore must have the capa-
bility for at-sea inspection, maintenance, and repair.

8.Summary. Because the final structure is an integration
of material behavior, structural design, construction approach, and main-
tenance and repair operations, the structural requirements are summarized

in Table 1 as they relate to each of these major areas.

Table 1. Structural Requirements

MATERIALS
Durability

Concrete
Embedded Steel

High Strength

Low Unit Weight

Dimensional Stability

Ductility

Low Water Absorption; Low Permeability

Saturated Concrete Engineering Properties

Special Materials to Improve Engineering Properties
Antifouling Concrete

DESIGN

Service Loads

Environmental Loads

Load Resistance Capacity
Structural Analysis Methods
Fatigue

Shear

Impact

Failure Mode

CONS TRUCTION

Graving Dock Construction

Construction Afloat

Slip-forming Methods

Conventional Intermittent Casting Methods
Precast and Segmental Construction Methods
Joining Techniques

Tolerances

Placing Concrete on Seafloor

Quality Control

Availability of Resources

OPERATION

Inspection Methods
Maintenance Methods

Repair Methods

Module Replacement
Long-Term Buoyancy Changes

10

STATE OF ART
Floating and Submerged Concrete Structures

Concrete production is a world-wide industry using primarily local
manpower and materials. Reinforcing and prestressing steels are readily
available in all developed countries.

There is a great deal of long-term experience that started around the
1890's with surface and submerged concrete structures for coastal and
harbor facilities, bridge piers, floating structures, and ship pili
In the past 4 years there has been a tremendous surge of development
of large concrete structures for use in the open ocean, particularly the
North Sea, for offshore oil drilling, production, and storage.

Concrete structures for coastal protection, dock and harbor works,
and large bridge foundations in fresh and salt water are frequently con-
structed by combining precasting of large components (floated to the
site and submerged) with in-situ concreting underwater by bucket and
tremie placement, grout intrusion into prepacked aggregate, and other
methods. Representative examples are the San Francisco-Oakland Bay
Bridge caisson-piers, the largest of which is 197 feet long, 92 feet
wide and over 500 feet high (1930's), the Richmond-San Raphael Bridge
"bell piers" (1950's), the Oakland Estuary highway tunnel (early 1960's)
and the 3-1/2 mile-long San Francisco Bay Area Rapid
Transit (BART) Tunnel (late 1960's). Concrete multiple-pontoon floating
bridges, each several thousand feet long, have been constructed, two
across freshwater Lake Washington and one across saltwater Hood Canal,
near Seattle (1940, 1955 and 1961), and one in Tasmania (1940). Precast

pontoons, for example 360 feet long, 50 feet wide and 14 feet deep, were

IL

towed to the site and connected by high strength bolts or by post-tensioning
of epoxy—bonded sommes ol

Many ship hulls of regular weight and lightweight reinforced concrete
were built in the United States in response to steel plate shortages during
World Wars I and II (about 15 ships in WWI and about 104 in WWII). A few
reinforced concrete ships were built in Europe in each of the world wars.
These concrete ships saw service as tankers and dry cargo careediains (29 64
Typical WWIL sizes in the U. S. were 366-foot length, 54-foot beam, 35-foot
depth, and 11,000-ton diepiscemencl Concrete hulled lighters and barges
were also used. An experimental U. S. Navy landing ship of prestressed
concrete successfully performed many test landings on beaches in 1946 but
was not put into serve det The concrete ships demonstrated good perfor-
mance particularly in resistance to vibration, fatigue, and abrasion, but
were uneconomic in the post-war periods, due, in part, to imitative designs
that did not utilize the advantages of concrete, and to high self-weight-
to=-cargo-weight ratios. Lo?! 4 Commercially successful prestressed concrete
ocean-going barges (2,000-ton size) have been in regular service in the
Philipines for the past 9 years and have performed well 2??4] Reinforced
concrete ocean-going barges have been used in the U. S. Gulf states,
Mexico, South America, and AfricaL In 1962 a floating oil refinery was
built in Belgium on atwo-way, post-tensioned, compartmentalized, concrete
barge about 180 feet long by 80 feet wide and was towed to Africa for
service there. A major reason for choosing concrete was concrete's
superior resistance to fire LA

A 3-year long program of detailed inspection (reported in 1972)

of many USSR concrete floating dry docks found them to be, in general, in

UZ

excellent condition with very little concrete deterioration or rein-
forcing steel corrosion after 10 to 40 years service in various seas
with different climates. The drydocks, up to 8,500 tons lifting capacity
(about 425 x 105 x 48 feet in size) had been constructed by precast
and in-situ methods of dense, high-strength concrete and had required a
minimum of Tenmeenencen tel

A precast prestressed concrete floating platform has just been con-
structed at Tacoma, Washington. The 68,000-ton displacement vessel,
461 x 136 x 57 feet was outfitted as a liquified petroleum gas
(LPG) processing and storage facility; it was recently towed to
Indonesia and moored in the Java Sea. The platform was segmentally con-
structed in a Pate aeeey basin and then floated. The hull, including the
precast curved bottom shell elements which weighed 35 tons each, was post-
tensioned longitudinally and nmaeaeaiy El

The successful emplacement of the Ekofisk concrete oil storage tank
in the North Sea in 1973 precipitated a wave of orders for concrete
gravity (bottom-sitting) structures for offshore oil agate Three
drilling and production platforms were installed in 1975. At least 9
more, currently under construction at shallow and deep water sites in
Norway, Scotland, Sweden, and the Netherlands are scheduled for deployment
in 1976 and 1977 in water depths to 510 feet ['] These very large struc-—
tures are of particular interest to the OTEC program since they are
floating during most of their construction and during deployment. They
are usually constructed as follows. The base or bottom section is started
dry in a dewatered basinnear the shoreline. When the walls of

the base are sufficiently high the basin is flooded and the bottom section

13

is moved to a protected deep water site and moored. Construction con-
tinues, usually by slip~forming the tank and tower walls on a round-the-
clock basis. Sand and water ballast maintains the working level at about
a 30- to 40-foot freeboard for the major part of the construction whrace ed

The Brent B CONDEEP platform, built in Norway by the above method, is
representative of the concrete offshore platforms. Brent B has an installed
displacement of about 400,000 tons. Its base, composed of 19 cells, is 330
feet across; three 525-foot-tall concrete towers support the steel super-
structure. The structure was towed 250 miles across the North Sea and
emplaced in 460 feet of water in August 1975.05]

Because of the present offshore construction activity, concrete
societies around the world have committed much effort to defining the state-
of-the-art and developing recommended standards of practice for concrete
ocean seruceure ecm) This work is continuing.

Also of particular interest for OTEC applications are the outstanding
advances made in the past decade in construction of large concrete structures
on land such as ultrahigh-rise buildings, nuclear reactor containment ves-

sels, and liquid natural gas (LNG) storage tanks.

Material

Well—designed, high quality concrete is an excellent marine construc-—
tion material for massive, floating structures. Experience has shown a
relatively long life for many marine structures with a minimum of mainte-
nance and an ease of repair.

The following topics are various material considerations that have

importance to OTEC-type concrete structures.

14

Durability. Durability is the long-term resistance of concrete to
disintegration of the concrete itself, to corrosion of embedded steel, or
both, which may fnrecraare 20521) In general, durable concrete for OTEC
applications can be produced with a high degree of assurance by strict
adherence to established methods for producing high quality, dense, sul-
phate-resistant concrete with low Memneabaltieyees 227 The major consid-
erations include proper mix design (particularly a low water/cement ratio,
< 0.45, and a high cement factor, > / sacks/yd?), use of proper materials
(sulphate-resistant, e.g. ASTM Type II, cement with a CA content between

5 and 9%, sound, nonreactive aggregates, careful limitation of

chlorides in water, aggregates, and admixtures); proper placement, com-
paction and curing procedures; and proper structural design and detailing
(adequate cover of embedded steel, control of eran) 24

Durability problems that do exist are frequently the result of not
following the known procedures and are often associated with extreme en-
vironments such as disintegration of highway bridge decks due to the
combined effects of freezing and thawing, abrasion and high salt concen-
erection 2224) For the OTEC structures, the splash zone is the most severe
environmental condition. The concrete is subjected to chemical attack by
sulphates, chlorides, carbon dioxide and oxygen, and physical stresses due
to alternate wetting and drying, temperature differentials, shock loads
from waves, and other conditions. The concrete industry is actively
studying these durability problems; for example, federal and state high-
way engineers are developing practical field methods for polymer impreg-
nation of existing bridge decks 25°24 Such developments will likely be

available for the OTEC structure.

15

Corrosion. Corrosion of reinforcement and other embedded steel is
considered to be potentially the most serious durability problem although,
again, corrosion can usually be controlled by using appropriate materials
and procedures 24 Steel embedded in high-quality dense concrete with a
high cement factor is protected in two ways: (1) the high pH environment
created by the cement passivates the steel and (2) the concrete's low
permeability rate prevents resupply of seawater with dissolved carbon
dioxide (which could reduce the pH) and oxygen (which is necessary for
corrosion of steel). Chloride ions penetrate even dense concrete in time
periods of months 24 When in contact with embedded steel the chloride
will decrease passivation protection for a given pH level. However, cor-
rosion still does not occur if either the pH is high enough or the rate
of permeability (rate of oxygen resupply) is low saan e729)

Other corrosion prevention methods have been used. Cathodic protec-
tion is not considered practical in many large structures since all the
reinforcing steel must be electrically bonded together. Metallic (zinc,
cadmium) and nonmetallic (epoxy, chlorinated rubber) coatings of rein-
forcing and prestressing steel have been tried in the laboratory and in
the field with mixed syeceesce ed Currently, uncoated steel is preferred
for both reinforced and prestressed concrete. Uncoated posttensioning
tendons are grouted in watertight ducesrad Galvanizing, if used, should
be treated with small amounts of chromate to prevent hydrogen gas for-
mation 24) Steel coated with nonmetallic materials must be prepared and
handled very carefully (which increases cost) since even a small pinhole
in the coating (due to lack of coverage or to a knick or scratch) may

cause localized accelerated corrosion.

16

Concrete in a submerged zone is more resistant to reinforcement
corrosion than concrete in the tidal, splash, or marine atmospheric
zone where alternate wetting and drying permit a more rapid supply of
oxygen to the reinforcing Bree 2)

The above discussion applies to the general mass of the concrete in
the structure. However, portions of the structure may be attacked by
corrosion for such reasons as cracked concrete due to impact or tensile
loading, or other electromotive forces whose driving forces are unknown.
OTEC structures will experience deep ocean pressures and extreme oxygen
gradients; the effects of these factors on corrosion are not known.24

High Strength Concrete. Ocean structures can utilize higher strength
conecretes than are utilized conventionally. Field use of concrete with
compressive strength of 6,000 psi is common. For floating structures, higher
strengths (on the order of 8,000 to 12,000 psi) can result in thinner struc-—
tural elements and thus lower weight and reduced draft. Minimum draft is
usually very important during construction and tow out. Significant cost
savings are realized during construction if auxiliary buoyancy structures
are not required.

The state-of-the-art exists to produce high strength concretes, but
problems are encountered in developing quality assurance procedures for
proper field handling, placement, and curing.

Saturated Concrete. The effect of partial and full saturation on
the compressive and tensile strength, modulus of elasticity, Poisson's
ratio, and creep rate of concrete is unknown. A limited study explored
the changes in compressive strength and quantity of seawater absorption

in concrete at various simulated ocean depths [3] It was found that

7)

concrete strength is dependent on the degree of saturation and that 6-
inch-diameter, 12-inch--long cylinders under a pressure head of 500 feet
were not completely saturated after two months. Also, it was observed
that compressive strength of partly saturated concrete under hydrostatic
pressure showed small increases in uniaxial strength but that completely
saturated concrete showed strength decreases of 10% or more com-

pared to fog-cured concrete.

The lack of knowledge about the behavior of saturated concrete is
an outstanding deficiency in the state-of-the-art. The pressure-
resistant hull is a highly stressed, critical component of OTEC and the
behavior of the construction material must be known.

Lightweight Concrete. Structural lightweight concrete can be readily

produced with compressive strengths of 5,000 psi, and from some lightweight
aggregates, strengths of 6,000 psi. Lightweight reinforced concrete sat-
urated with seawater has a unit weight in the range of 115 to 125 pcf in
air and thus less than 60 pcf submerged. This represents an in-air weight
saving of 20% and a submerged weight saving of 30 to 35% or
more compared to normal weight reinforced concrete which, when saturated,
has a unit weight of about 155 pcf in air and 90 pcf when submerged.
Lightweight, as compared to normal weight, concrete has a lower modulus
of elasticity (about 60 to 75%) and, on the average, somewhat
greater creep and cintinteane (05)

Aside from the obvious advantages of lower weight, which could greatly
affect the design of the cold water pipe and which might be critical in
being able to produce a structure of sufficient buoyancy, lightweight

concrete can be used in other ways. For equal weight structures, thicker

18

sections of lightweight concrete could be used than normal weight concrete
and permit additional space for prestress or reinforcing steel and allow
for improved impact and punching shear resistance. Highly stressed lo-
cations of the hull can be fabricated with thicker sections of lightweight
concrete and thus reduce the average stress. The inelastic behavior of
lightweight concrete can also aid in significant redistribution of stresses
in overstressed hull locations.

An example of expanded shale lightweight concrete usage in an ocean
structure is given by the USS SELMA, a 7,500-ton vessel built in 1918.
The vessel is presently grounded on a beach in Galveston, Texas, and
reportedly the durability of the concrete and lack of steel corrosion are
outstanding Fectumes [Oo

Lightweight aggregate concretes have not been tested for their suit-
ability in pressure-resistant structures subjected to several hundred
feet of hydrostatic head. In particular, information is lacking on the
permeability of lightweight concrete subjected to such pressures. Only
limited information is available on seawater absorption of expanded shale
lightweight concrete; experimental results were snomnelmaine Led The effect
of partial and full saturation with seawater on compressive and tensile
Benedeths of lightweight concrete is not known. In summary, there is a
lack of knowledge of the engineering properties and behavior of lightweight
concrete for ocean structures.

Antifouling Concrete. OTEC structures will experience marine bio-
fouling. For example, the North Sea structures in 100-foot water depths
show about 4 inches of vegetation and animal growth from the tide zone

to 30-foot depth after several years of operation. Below 30 feet, 4 inches

LY)

of animal growth of mostly snail tubes is found. The maximum depth for
snail tubes growth is not known at this time but existing concrete plat-
forms in 450-foot depth will yield this data for the North Sea in the
future. Growth of this magnitude will decrease buoyancy and significantly
increase mooring forces. The Civil Engineering Laboratory (CEL) has pioneered
in the development of an antifouling Ponererencdl Toxic chemicals are
incorporated into concrete by first impregnating porous expanded shale
aggregate with the chemicals and then mixing this aggregate with the
other concrete ingredients. The antifouling concrete has successfully
prevented marine growth for up to 4 years (limit of test) in surface
waters and at a depth of 120 feet.
Coatings have been an age-old technique for short-term prevention of
marine growth. New products, such as dense-polyurethane and dense-epoxies
which contain no solvent to evaporate, have appeared on the market and hold promise
for long~term prevention. The dense-polyurethanes and epoxies exhibit
highly smooth surfaces which may prove easy to clean. However, coatings
may lead to intensified galvanic cell corrosion by creating locations of
differential electrical potential, because some concrete sections aie wet :

and others are relatively dry.

Recent Developments. New materials and techniques have been researched

over the last 10 years that may have application to ocean structures.

Fiber-reinforced concrete is beginning to have field acceptance. Fibers
of steel, glass, or synthetics are incorporated in the concrete as it is
mixed.E 4] The notable improvements in engineering properties are an in-

creased tensile strength, increased ductility, and improved crack control.

20

Polymer-impregnated concrete (PIC) shows compression strengths of 20,000 =<

psi, tensile strengths of 1,500 psi, and elastic moduli of 6.0 x 10° psi,
and, as compared to conventional concrete, an 80 to 90% decrease

in permeability and in creep 25) A major disadvantage of PIC is its lack

of ductility, but research is now being conducted to improve this prop-
oper Bal Other disadvantages are higher cost of PIC and lack of estab-
lished fabrication/construction methods and experience for large structures,
although such methods are currently under development.

A newly developed dry casting technique that has not yet been field-
tested to any great extent yields concrete with a water-to-cement ratio
of 0.30 and thus has the potential for producing precast concrete members
of higher strength and lower permeability at competitive oriees 29) In
this method the ingredients are mixed dry, placed in the forms without
water, and compacted, after which water is introduced to fill the voids
by capillary action.

The history of concrete is rich in novel approaches to improve con-
crete. Examples of successful innovations that are still practiced
include: vacuum removal of excess water from in-place concrete, pre-
packed techniques in which the coarse aggregate is initially placed in
forms and then intruded with grout special compaction techniques; and

use of various concrete admixtures to reduce water, control setting
time, increase strength, durability, and workability, and otherwise im-
prove the engineering properties of the fresh and hardened aomnerana 2a
A new super-water-reducing admixture recently introduced to the industry
permits use of lower water-to-cement ratios while still providing a

workable concrete mix.

21

Design

Environmental Loads on Structures. The design for OTEC structures

to resist environment loads will be only as accurate as the input loading
data. Oceanographic and meteorological data need to be compiled from
probable operational sites so that valid historical data are available on
currents, waves, and wind. The dominant environment load is from waves,
and analytical techniques exist to predict forces due to waves. The pre-
dictions are based on wave scatter diagrams that cover significant wave
heights and zero crossing periods that enable both maximum and cumulative
forces to be assessed for static and dynamic conditions.

Structural Analysis Methods. Once the environmental loads are deter-

mined, the forces within the structure may be calculated. Powerful analytical
methods exist, namely finite element and finite difference techniques, to
predict the response of structures to external loads. The internal forces,
stresses, deflections, and strains are predicted well for linear and non-
linear materials. Limitations on these methods are the accuracy of input

data on material behavior and skill of the structural analyst in subdividing
the structure into elements.

Design for Hydrostatic Loading. Studies on pressure-resistant con-
crete structures have been directed at developing design approaches to
predict implosion strength. For cylinder structures, research on the
effectsof length to diameter, wall thickness to diameter, and different
typesof end closures has been conducted. Design guides have been pub-
lished in the form of a nendbool a2]

Several deficiencies exist in the available design procedures when

related to OTEC structures. No studies have been conducted on the effect

Dep

of large penetrations or out-of-roundness of cylinder hulls. These para-
meters will significantly influence the behavior of the structure. Other
parameters that need to be considered are the effect of vertical and
horizontal stiffeners in concrete shells and the effect of the pressure
gradient between the bottom and top of cylinder structures. Horizontally

oriented structures have the problem of being loaded into an out-of-round

shape. For vertically oriented structures, the end-condition effects and the
variable hydrostatic pressure load along the structure length complicate the

definition of the critical section.

Design for Long-Term Loading. The ability to design for long-term
loading of pressure-resistant structures is marginally available. Con-
servative estimates of the maximum stress level to which concrete struc-—
tures can be safely loaded can be made from studies on concrete column
members and from concrete spherical structures placed in the deep ocean.
A test on 18 spherical structures with 66-inch outside diameter and
4-inch wall thickness is still in progress after 4 years of exposure to
the hydrospace environment at depths ranging from 2,000 to 5,000 feet [4]
This study is producing results that have direct application to OTEC
structures.

The ability to predict strength changes and creep behavior of con-
crete over long periods of exposure to the ocean environment is not well-
established. This deficient technological area was discussed above in
the Materials section.

Design for Fatigue. The fatigue behavior of reinforced and prestressed

concrete in an ocean environment is not well-known. Throughout the life of

23

a structure, waves can impart 10° cycles of load, which is a significantly
large number of cycles. On-land concrete is known to possess good fatigue
resistance if the level of stress does not exceed 50% of the com-

pressive stasnech ed With periods of rest between cycle loadings, the
autogenous healing properties of concrete assist to improve the overall
fatigue resistance. In the ocean, the randomly varying loads due to waves
and lack of rest periods may require a re-evaluation of the existing design
guides for fatigue.

Design for Shear. Shear is one of the more troublesome loading con-
ditions for concrete because of the tensile component of force. It is
recognized that shear stresses can be reduced by introducing precompression
forces. However, design guides are not available to assist in the design
of large shell structures to resist shear loads.

Punching shear is another problem for concrete shell structures.
Curvature assists in resisting punching shear failures, but guides for
design are lacking.

Design for Impact. The impact behavior of concrete structures has
not been adequately researched. For OTEC structures, it is most desirable to
understand the mechanism of impact resistance for concrete and have a
means of quantifying impact behavior. Concrete has considerable capacity
for strain energy absorption by reinforcing with closely spaced bars or
fiber reinforcement. Guides for the design against impact loading are

lacking.

24

Construction

Construction Methods. Three approaches are available for the con-

struction of large concrete structures: (1) slip-forming, or similar
methods in which the concrete is cast in an essentially continuous manner
without construction joints; (2) conventional sequential concrete pours
with water-stop cast in the construction joints (or other waterproofing
methods used); and (3) joining precast elements together to build the
main structure or subassemblies which are then in turn joined to each
other or to the structure.

Construction Sites. As described above, construction is usually
started on land and then continued afloat. The critical item for con-
struction of OTEC structures is the availability of deep, protected
waters for the construction site and natural deep waterways leading to
the open ocean. For OTEC, the water depths may need to be as great as
400 feet. A preliminary search for construction sites in United States waters
found no such sites. If none are available then other construction
approaches will have to be developed, such as reusable auxiliary flota-
tion structures to supply buoyancy in shallow waters or the use of
modular assembly in the deep but less protected waters.

Political considerations, effects of which are not known at this time,
may also strongly influence construction site availability. States such
as California have legislation for the conservation of shoreline that
might prohibit the construction of new flooding basins.

Other Considerations. Manpower, construction materials, and construction

equipment are available and in abundant supply for building OTEC structures.

ZS)

In 1971, the Navy surveyed the impact of building a mobile, ocean basing
system (MOBS) a concrete structure of sufficient size to land C5A air-
craft, and found that one MOBS structure (the size equivalent to about

20 OTEC structures) required only a small percentage of the annual
quantity of construction materials, especially conan 25)

Joining Techniques. Precast construction using concrete elements is

an expanding industry for on-land structures. This technology is beginning
to be used, quite effectively, in constructing the North Sea structures
and large floating barges. The advantages are shorter construction time,
less congestion at the site, and better quality control. To use precast
construction requires that the elements be joined together with adequate
structural integrity and watertightness under hydrostatic head. Reliable
joining techniques exist; however, improvements in the technology would
expand application and utilization of precast concrete. For example,

in lieu of slip-forming the walls, a more rapid approach might be to use
precast wall elements and slip-form the vertical joints. The critical
item is the quality of the joint.

Also joining methods encompass means to couple together large struc—
tural components. This approach is frequently used for underwater con-
struction of bridge piers and subaqueous tubes. Experience in its
application to large floating structures, other than pontoon bridges in
protected waters, does not exist; however, the technology does exist.
Significant advancements in construction engineering techniques for
joining large floating elements together could reduce construction time

considerably.

26

Quality Control. Quality control procedures for concrete were ad-

vanced significantly by the requirements of concrete nuclear reactor
containment Pesecie 2S) Quality control is rigorous today; however, a
major flaw still exists. The basic approach in concrete quality control
is to inspect constituent materials and batching, mixing, and placing
equipment before use to prevent problems. Concrete is sampled at time of
casting and the samples tested several days later for strength. This
procedure is archaic and should be replaced by a method of testing con-
crete for its quality just prior to casting. It is quite important for
structures resisting hydrostatic load to have concrete of uniform strength
and elastic modulus. The quality of the concrete needs to be known before
it is placed in the structure and not after. Technological development
in testing fresh concrete is advancing. The United States Army's Construction
Engineering Research Laboratory recently held a conference on rapid test-
ing of fresh concrete; the state-of-the-art was summarized for techniques
to determine the water and cement content of fresh eonezetede dl Present
methods using chemical-mechanical or nuclear analyses are available for
field use. They require about 15 minutes to analyze fresh concrete for
water or cement content. However, limited field use was reported.
Operation

Although the North Sea structures have not yet undergone inspections,
such inspections are required by law and will be conducted in ne. future.
Therefore, by the time OTEC structures are placed in service, substan-

tial advancements in the state-of-the-are of inspection, maintenance, and

repair of concrete ocean structures will have occurred.

Zi

Inspection. A vigorous inspection program will be required in order
to detect problems early so that corrective action may be taken. Three
zones need inspection for different mechanisms of deterioration; these
are the submerged, splash, and atmospheric zones.

The submerged zone requires underwater inspection to check for cracking
of concrete due to fatigue stress, overloading conditions, or corrosion of
reinforcing steel. Corrosion of reinforcing steel is unlikely in the
submerged zone. Inspection for sulphate attack is warranted and will be
revealed by a surface-softening effect which will permit abrasion by
currents or water jets used to clean the concrete surface. 3] A rough,
exposed aggregate surface may indicate sulphate attack, and hence, a
more detailed inspection would be indicated.

The splash zone will be most susceptible to reinforcing steel cor-
rosion. At advanced stages of corrosion, rust marks bleeding from the
concrete surface will be observed. More serious corrosion will cause
spalling of concrete cover. Less obvious corrosion problems will be harder
to detect. Electrical potential difference tests can be conducted to
determine the potential between different sections of concrete. When
potentials are greater than 0.35 volts the probability is 95% that
steel is corroding. [-"] Successful field application of the half-cell
potentionmeters has occurred on highway bridge-decks and buried concrete
pipe.

The atmospheric zone is also susceptible to reinforcing steel cor-
rosion but to a lesser degree than the splash zone. Particular problem

locations are inside corners on overhanging or vertical sections. It is

28

recommended to avoid sharp corners by making rounded contours and the
use of fillets.

Maintenance. Scheduled (preventive) maintenance will include such
items as removal of fouling and replacement of coatings, if used. Un-
scheduled (corrective) maintenance will be due to unanticipated loadings
or material behavior.

Repair. Two types of repair are envisioned. One type is repairing
sections of concrete having corrosion of steel reinforcement and the
other type is repairing damaged sections of the structure caused by im-
pact or overload. Both types of repair are not well~developed; however,
approaches to the problem are available. Remedial measures for corrosion
include (1) cathodic protection, (2) repair of spalled and laminated con-
crete, and (3) isolation from the environment by coatings on the steel or
by coating the exterior surface of the eonerece ls) The recommended ap-
proach is to remove all concrete covering the first layer of reinforcement
and replace this concrete after appropriate cleaning and sRepacationded
A new approach with much promise is polymer impregnation of existing
concrete. Corrosion can be terminated at whatever stage of damage it has
caused by using polymer-strengthened concrete technology. Polymer im-
pregnation of deteriorated concrete has been applied to bridge decks as
an experimental method. The same techniques could be applied to concrete
in marine structures. The result would be a stronger concrete than the
original material and an impervious concrete that would prevent oxygen
and chloride ions from reaching the steel. This same technique could be

used as a preventive system; however, the cost of polymer-strengthened

29

concrete is several times that of regular concrete.

Damaged sections of the concrete hull can be repaired by removing
cracked or crushed concrete from the damaged area. Reinforcing or pre-
stress steel is exposed so that new reinforcement can be attached by
welding or mechanical means. Prestress steel can be stressed to proper
levels by jacks or screws. New concrete is cast in the damaged section.
Similar repair procedures have been conducted on concrete ships and
vessels built in WWI and II. Recently, the Italian government required
that repair procedures be proven effective in returning a concrete dry
dock to its original conditions of strength before approval was granted
to build the erates bal Repair methods were proven for reinforced and
prestressed concrete and the drydock is presently under construction.

Buoyancy Changes. Buoyancy changes in an OTEC concrete structure
can occur from animal and vegetation growth on the structure. One cause
for buoyancy change that is special to concrete is water absorption with
time. The designers of the structure must incorporate means for adjusting

the buoyancy as water is absorbed into the concrete.

30

RESEARCH AND DEVELOPMENT RECOMMENDATIONS
Materials
Saturated Concrete. A study on saturated concrete is required

to determine the effect of partial and full saturation on

the engineering properties of concrete. It is important that the material
behavior used in the analysis be that of concrete representative of the
concrete condition in the OTEC structure. The field condition for the
conerete in the OTEC structure will be partially saturated to fully saturated,
reinforced, and prestressed concrete subjected to sustained and intermit-
tent service loads in a hydrostatic environment. In this environment,
seawater enters concrete at ambient pressure filling a portion of the void
volume. Some portion of the void volume will remain at atmospheric pres-
sure, and the concrete experiences a triaxial loading effect. The engi-
neering properties of concrete during this partial saturation stage is
dependent on the seawater absorption rate. Therefore, the rate of sea-
water absorption must be monitored along with the engineering properties
of concrete when specimens are tested in a hydrostatic environment for
compressive strength, tensile strength, modulus of elasticity, Poisson's
ratio, and creep. These same engineering properties need to be obtained
for fully saturated concrete. Comparison of these results to continuously
fog—cured specimens is essential. This type of study will yield results
for the design engineer so that an accurate analysis of the structure can
be conducted using known concrete properties.

Corrosion of Reinforcement. Basic research needs to be conducted to

understand the fundamental mechanism of corrosion and sources of electro-

31

motive force in steel-reinforced concrete ocean structures. The effects
of differential oxygen concentrations and hydrostatic pressure on corrosion
need to be investigated. The electromotive forces between structural
elements of reinforced concrete and steel need to be analyzed. Certain
portions of the OTEC structure are likely to be massive plain steel com-
ponents (for example, the mooring line ), The question of whether or not
the reinforcing steel will be anodic to the plain steel needs to be
studied. Also, information is needed on the potential effects on rein-
forcing steel of possible "stray electrical currents" associated with an
in-water power plant, and of the presence of huge quantities of other
metals such as aluminum or titanium in the heat exchangers.

Development of methods to prevent corrosion and also to passivate
existing corrosion needs to be conducted. Techniques or products that
produce a less permeable concrete are desirable. The effectiveness of
existing methods to seal pores with crystalline compounds needs to be
tested. New methods such as impregnating concrete with corrosion inhibi-
tors should be studied. Coating techniques also should be evaluated;
this includes coatings such as polymer—impregnated concrete.

A document discussing the state of art needs to be published to dis-
cuss potential corrosion problems, methods of designing against corrosion,
nondestructive methods to detect incipient corrosion before the concrete
is damaged, and remedial methods to inhibit further corrosion and to repair
corrosion-damaged concrete. This document will provide a guide to research

and development studies.

32

Subsequently, a handbook type of document should be published. Included
in this document would be data obtained by a systematic survey of existing
ocean concrete structures, particularly those that are well-documented as
to specifications and actual construction. Within the United States, there
are numerous such structures that would yield valuable data spanning a 50-
year time period. For example, the effects of the two different environ-
ments - seawater and fresh water - on Washington State's floating
concrete bridges might provide valuable data on the role of chlorides in
the process of corrosion.

The North Sea concrete structures are being built to specifications
that reflect present-day knowledge of durability and corrosion resistance.

A program should be instituted to monitor these structures. The concrete
technology used in these structures will most likely be used in the OTEC
structures. One of the better assurances that the OTEC structure will
perform properly will be to record how the North Sea structures perform.

The North Sea structures will be inspected periodically by regulating
agencies, such as Det Norske Veritas or Lloyd's of London. The inspection
will be for the serviceability of the structure (i.e., is the structure

still safe for personnel and environment?). It is not likely that the in-
spection will cover detailed research topics that would exceed the minimum
level of effort to obtain certification. Perhaps a joint venture between

an Environmental Research and Development Agency contractor and a regulating
agency can be arranged (with the cooperation of the owner of the structure) to
conduct additional studies on these North Sea structures and thus obtain data not

otherwise obtained in routine inspections.

33)

Antifouling Concrete. Development work on antifouling concrete by

using toxic-impregnated aggregate needs to be conducted. High strength
concretes with sufficient antifouling properties for long-term effec-
tiveness need to be developed. The long-term effectiveness of the mixes

may be improved by use of more viscous chemical solutions. Higher strengths
may be obtained by using chemicals that do not interfere with the hydration
or bonding processes of the portland cement. Tests need to be conducted on
specimens having surface areas of several square feet and exposed to ocean
sea conditions.

The bond strength of antifouling concrete when cast against normal
concrete needs to be investigated. It is envisioned that antifouling
concrete could be used as an overlay coating to normal concrete. It is
unlikely that antifouling concrete would be used for the entire wall-
thickness.

Lightweight Concrete. Research to determine the behavior of light-

weight concrete for use in ocean structures -in particular, pressure-
resistant structures -should be conducted. The permeability and seawater
absorption characteristics need to be determined. Some tests for compres-
sive strength, tensile strength, and modulus of elasticity of partially
saturated lightweight concrete should be conducted.

Advanced development work on new lightweight aggregates would be
beneficial. Aggregates that show small seawater absorption values and pro-
duce concretes of compressive strength of greater than 5,000 psi would find
application in OTEC structures. Due to the additional cost of lightweight

concrete compared to normal weight concrete, it is unlikely that the entire

34

structure would use lightweight concrete. However, the cold water pipe,
upper portions of the structure, and congested reinforcing steel areas
could use lightweight concrete.

Rapid Analysis of Fresh Concrete. Advanced development needs to be

conducted on equipment and techniques to analyze fresh concrete. Present

methods have not been field-tested and accepted to any significant degree,

and their processes require 15 minutes to analyze the concrete. This

time period should be reduced to 5 minutes,and the analyzing equipment and
techniques should be designed for the end application of inspecting every batch of
concrete placed in the OTEC structures. A program should be developed to

gain field experience and to validate the techniques by comparing the field

prediction with results obtained from standard control cylinders.

Design

Environmental Load Criteria. Environmental load criteria need to be
developed for OTEC structures. The entire structural design is aimed at
producing a structure with a given factor of safety under a given loading
condition. For ocean structures, the most difficult part of the design is
defining the environmental loads. Considerable effort is required on this
topic area,and the importance of the item is high. For example, the early
North Sea concrete structures were designed for a 100-year wave of 70 feet.
After four years of additional data collection, the size of the design 100-
year wave increased to 100 feet. Work is required to compile environmental
data on potential OTEC sites and reduce this data to loading criteria.
The use of probabilistic loading definition and design methods should be

considered.

35)

Hydrostatic Loading. Additional studies are required to improve

procedures for designing against hydrostatic load. OTEC structures will
be pressure-resistant hulls for the entire life of the structure. Risk
analysis places hydrostatic loading as a critical item because if failure
occurs the probability of catastrophic loss of the entire structure is
high. The effect of large penetrations, out-of-roundness, and axial and
hoop stiffeners on the implosion strength of cylindrical hulls needs to
be studied. The effect of pressure gradient needs to be studied but this
topic depends on whether OTEC structures will be oriented horizontally or
vertically.

Computer Design Methods. Improvements to computer analysis, such
as finite element or finite difference methods, should be made by incor-
porating valid constitutive relations for the construction material. The
constitutive relations for concrete materials needs to account for applied
multiaxial loading effects while the concrete is partially and fully sat-
urated with seawater.

Design for Shear. Guides need to be developed to design for shear in

large shell structures. For example, the cold water pipe at the base of
the structure is under high bending and shear forces due to currents act-
ing along the length of the pipe. The pipe will be prestressed in the
axial and hoop direction. An allowable design shear strength is required
for the concrete in the shell. This shear strength should be based on
appropriate experimental data on saturated concrete.

Design for Impact. Guides need to be developed to design for impact

loading of concrete structures. Local and large area impact loads need to

36

be considered. These loading conditions cover the topic of punching

shear failure. Methods that quantify the strain energy absorption of

concrete need to be proposed and studies conducted to verify the theories.
Design for Fatigue. Guides to design for fatigue in an ocean environ-

ment need to be developed. Tests should be conducted on reinforced and

prestressed concrete members that are saturated with seawater and remain

submerged during test. Initial tests should explore differences between

on-land and in-seawater fatigue behavior for concrete.

Construction

Construction Methods. A comprehensive study should be conducted on

construction methods. No other item can result in as significant a cost
saving for the structure as improvements in the method of construction.
Innovations and developments in this field usually evolve by building
many of a certain kind of structure and from competition. Research and
development has the potential of by-passing some of this evolution.

Significant advances in construction methods are possible. For
example, the following items are highlighted:

1. Develop methods to utilize more precast elements in construction.

Precast elements can be built by many subcontractors, and with proper

timing, the elements only have to be assembled at the construction site.
The objective is to minimize steel work and cast-in-place concrete at the
job site. Innovative techniques might show that precast segments combined
with slip-forming methods for the joints will increase the speed of con-
struction over that of an entire slip-formed operation.

2. Develop improved joining methods for precast elements. New types of

concretes and synthetic materials open the possibility for joining precast

37

elements with less time and complexity than present methods. For example,
polymer concrete and sulphur concrete may permit high strength joints to
be made in the field in the time period of hours.

3, Develop methods to assemble large floating structural components.
The completed structure may be assembled from components that were built
at different locations. The components, which are major structures them-
selves, will be assembled while floating. An advantage to this approach
is that the components may be built in relatively shallow water which
makes available more Conserucetion sites. The assembly technique should
be reversible to provide for replacement of modules as discussed in the

"Requirements" section.

Prestressing Systems. Development of a noncorrodible prestressing

system would improve the overall reliability of the structure during long-term
exposure to the ocean environment. Highly stressed steel of rather small
diameter is vulnerable to corrosion and subsequent failure of the steel
tendon. Synthetics, such as the new Kevlar fibers, and canemnies should be
investigated for prestressing materials for concrete. Of particular in-
terest are the failure mode and whether or not such materials undergo creep

or progressive failure under long-term sustained loads. The economic and
technical feasibility of using noncorroding metals and alloys, such as
titanium, should also be studied.

Out-of-Roundness Measurements. Methods to measure out-of-roundness

of full-scale structures need to be developed. The structural capacity of
pressure-resistant concrete hulls is dependent on the out-of-roundness

deviations from true circular form. Designs are based on a specified

38

out-of-roundness and the structure needs to be checked for conformance
with specifications. Local deviations are not too difficult to determine
but overall deviations from cross-sectional geometry are quite difficult
to determine. For concrete structures, the failure or instability shape
is dependent more on overall out-of-roundness deviations than local devi-
ations. (For thin-walled metallic structures the reverse is true where

the instability shape is controlled by local deviations.)

Operation

Inspection. Instrumentation and nondestructive test methods need
to be developed to aid in inspecting and monitoring the long-term integrity
of the structure and quality of the materials. Long-term strain measure-
ments can be obtained with vibrating strain gages and Carlson stress
gages; however, these measurements monitor the behavior at discrete loca-
tions. Advanced systems that monitor the integrity of structural compo-
nents are desirable. Acoustic emission technology is an approach that
has potential for monitoring large segments of the OTEC structure (whether
fabricated of concrete or steel). When concrete is overloaded or fatigued,
material breakdown progresses from microcracking to macrocracking. Every
propagating crack releases strain energy and results in an emission of
acoustic energy. By monitoring the sound amplitude and number of occur-
rences of cracks an indication is obtained as to the severity of material
distress. Sensors can be placed at various locations on the structure and
by use of triangulation, the location of distress can be identified for

more detailed investigation.

3Y)

Although a high-risk Research and Development item, acoustic emission
technology should be developed to monitor the material behavior during the
operational life of the structure. This nondestructive method can be
employed to signal danger of a possible structural failure during adverse
weather conditions or incipient failure due to otherwise undetected
progressive deterioration.

Repair Methods. Repair methods will be required and need to be
developed. These items, however, do not need immediate attention insofar
as the North Sea Concrete structures will undergo inspection and probable
repair. Techniques and methods will evolve from the North Sea experiences
and will have direct application to OTEC structures.

Schedule

A schedule for the recommended research and development is given in
Table 2. The schedule is coordinated with ERDA's current time-phased
plan: (1) concept design completed by the end of 1979; (2) 25- to 50-Mw
prototype plant completed by end of 1982; and (3) demonstration power
plant in operation in the ocean by 1987.

The Research and Development topics are ranked in order of relative
priority and risk. The time scale reflects an indication of duration
for each topic. In some cases the length of time shown could be reduced
if funding were high; in other cases the full time is required to obtain
results and the work could not be accelerated effectively by an increase

in funds.

40

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SUMMARY

Research and development topics for concrete technology related to
the construction of ocean thermal energy conversion (OTEC) structures have
been identified. The topic areas are in the major categories of materials,
design, construction, and operation. A schedule of recommended research
and development is given in Table 2 to lead to near-term advancement in
deficient technological areas and to increase the data base from which

OTEC structures will be designed.

42

REFERENCES

1. Lockheed Missiles and Space Company, Inc., Ocean Thermal Energy Con-
version (OTEC), Power Plant Technical and Economic Feasibility, Vol I
TechnicalsReport, Vol II Supporting Data. Prepared for National Science
Foundation Research, Applied to National Needs Program, NSF/RANN/SE/
G1-C937/FR/75/1 and LMSC-DO-56566, Sunnyvale, CA, April 1975.

2. TRW Systems Group, Ocean Thermal Energy Conversion, Research on an
Engineering Evaluation and Test Program, Vol 1, 2, 3, 4 & 5, Final Report.
Prepared for Energy Research and Development Administration for Contract
No. NSF-C958, Redondo Beach, CA, June 1975.

3. University of Massachusetts. Progress report NSF/RANN/SE/GI-34979/PR/74/1:
Research Applied to Ocean Sited Power Plants, by W. E. Heronemus, et al.
Cambridge, MA, 30 April 1974.

“&. A. Lavi and C. Zener. Solar Sun Power Plants - Electrical Power from
the Ocean Thermal Difference, paper submitted to American Society of Naval
Engineers for the ASNE Day 1975 Program, 26 August 1974.

5. J. H. Anderson and J. H. Anderson, Jr. A Summary of the Anderson and
Anderson Analysis of the Sea Solar Power Process, 1964 to 1972. University

of Massachusetts, Technical Report NSF/RANN/SE/GI-34979/TR/73/5, March 1973.

6. Ben C. Gerwick, Jr., "Concrete Structures for 2000-meter Depth," in

Ocean 75, record of combined meeting of IEEE Conference on Engineering in

Ocean Environment and MTS llth Annual meeting, San Diego, CA, 22-25 September
1975, pp. 701-704.

7. Civil Engineering Laboratory, Technical Note N-1144, History of Concrete
Structures in a Marine Environment, by W. R. Lorman, Part 2 of Model Ocean
Basing System —- A Concrete Concept. Naval Construction Battalion Center,
Port Hueneme, CA, January 1971.

8. C. C. Nichols, "Construction and Performance of Hood Canal Floating
Bridge,'' Paper No. 9 of Special Publication SP-8, American Concrete Insti-
tute, Detroit, MI, 1964.

9. Arthur R. Anderson, ‘''Prestressed Concrete Structures (State-of-the-
Art),'' Pre-print of Paper No. 11C presented at Society of Naval Architects
and Marine Engineers, Vancouver, B.C., 14-17 May 1975, pp. 123-144.

10. Rowland G. Morgan, ''Concrete Ships," Proceedings FIP Symposium on
Concrete Sea Structures, Tbilisi, September 1972, Federation Internationale
de la Precontrainte,London, 1973.

11. Ben C. Gerwick, Jr., "Design and Construction of Prestressed Concrete

Vessels,'' Paper No. OTC 1886, Offshore Technology Conference, Houston, TX,
1973.

43

e
12. V. A. Mishutin, "Concrete Floating Dry-dock Bodies After 10 to 40
years of Use in Seas with Different Climates," Proceedings of FIP Symposium
Concrete Sea Structures, Tbilisi, September 1972, Federation Internationale
de la Precontrainte, London, 1973.

13. Ivar Foss, "Concrete Gravity Structures for the North Sea," Ocean
Industry, Vol 9, No 8, August 1974, pp. 54-58.

14. E. M. Q. Roven, K. Hove, and O. Furnes, Det Norske Veritas Rules for
Design and Construction of Fixed Offshore Concrete Structures, A Review
and Outlook. Paper presented at American Concrete Institute Symposium on
Offshore Concrete Structures, ACI Convention, Boston, April 1975.

15. Anonymous, "World's Biggest Oil Platform,'' Sea Technology, Vol 16,
No. 10, October 1975, pp. 24-25.

16. Federation Internationale de la Precontrainte (FIP), Recommendations
for the Design of Concrete Sea Structures, 2nd Edition, London, 1975.

17. Japan Society of Civil Engineers, Recommendations for the Design and
Construction of Concrete Sea Structures, Japan (in publication).

18. American Concrete Institute Committee 357, Offshore Concrete Structures.

19. Det Norske Veritas, Rules for the Design, Construction, and Inspection
of Fixed Offshore Structures, Grenseveien 92, Oslo, Norway, 1974.

20. ACI Committee 201, "Durability of Concrete in Service," ACI Journal,
Proceedings, Vol 59, No 12, December 1962.

21. Bryant Mather, Durability of Concrete Construction - 50 years of
Progress, Proceedings of the American Society of Civil Engineers, Journal
of the Construction Division, Vol 101, No C01, March 1975, pp. 5-14.

22. Ben C. Gerwick, Jr., 'Practical Methods of Ensuring Durability of
Prestressed Concrete Ocean Structures,' in Durabilility of Concrete,
Publication SP-47,American Concrete Institute, Detroit, MI, 1975, pp. 317-324.

23. Povinda K.Mehta and Harvey H. Haynes, Durability of Concrete in Sea-
water, Journal of the Structures Division, Proceedings of the American
Society of Civil Engineers, Vol 101, No. ST8, August 1975, pp. 1679-1686.

24. Roger E. Carrier, et al., "Factors Affecting the Durability of Con-
crete Bridge Decks," in Durability of Concrete, Publication SP-47, American
Concrete Institute, Detroit, MI, 1975, pp. 121-168.

25. G. W. DePuy and J, T. Dikeou, "Development of Polymer—Impregnated
Concrete as a Construction Material for Engineering Projects," Polymers

in Concrete, Publication SP-40, American Concrete Institute, Detroit, MI, 1973,
PDo SIeDOo

44

df

= 26. David W. Fowler, et al., 'Polymer-impregnated Concrete Surface
Treatments for Highway Bridge Decks," Polymers in Concrete, Publication
SP-40, American Concrete Institute, Detroit, MI, 1973, pp. 93-117.

27. Odd E. Gjorv, "Concrete in the Oceans," Marine Science Communications,
LG), 19755 joe. Bila,

28. R. D. Browne and P. L. J. Domone, The Long Term Performance of Con-
crete in the Marine Environment. Paper 5, Conference on Offshore Structures,
Institution of Civil Engineers, London, October 1974, pp. 31-41.

29. George J. Verbeck, "Mechanisms of Corrosion of Steel in Concrete,"
Corrosion of Metals in Concrete, Publication SP-49, American Concrete
Institute, Detroit, MI, 1975.

30. TT. E. Backstrum, "Use of Coatings on Steel Embedded in Concrete,"
Corrosion of Metals in Concrete, Publication SP-49, American Concrete
Institute, 1975, pp. 103-113.

> > 31. I.Cornet, et al., "Chromate Admixture to Improve Performance of Gal-
vanized Steel in Concrete Sea Structures,'' FIP Proceedings, Concrete Sea
Structures, Tbilisi, 1972, Federation Internationale Precontrainte, London,
LOS;

32. G. Somerville and H. P. J. Taylor, Conerete in the Oceans, Report
RR SMT-7402, Cement and Concrete Association, London, August 1974.

33. Civil Engineering Laboratory, Technical Note, Seawater Absorption and
Compressive Strength of Concrete at Ocean Depths, by H. H. Haynes, R. 5S.
Highberg, and B. A. Nordby, Naval Construction Battalion Center, Port
Hueneme, CA (in publication).

34. ACI Committee 213, "Guide for Structural Lightweight Concrete," Title
No. 64-39, American Concrete Institute, Detroit, MI, 1967.

35. Bryant Mather, Behavior of Concrete Exposed to the Sea, Proceeding of
the Conference on Civil Engineering in the Oceans II, American Society of
Civil Engineers, Florida, December 1969, pp. 987-998.

36. Civil Engineering Laboratory, Special Report 52-030, Seawater Absorp-
tion by Precast. Portland Cement Concrete Containing Lightweight Aggregate,
by W. R. Lorman, NCBC, Port Hueneme, CA, March 1973.

37. Civil Engineering Laboratory. Technical Note N-1392: Antifouling marine

concrete, by J. S. Muraoka and H. P. Vind. Port Hueneme, CA, May 1975.

—, 38. American Concrete Institute. Special Publication No. SP-44: Fiber
reinforced concrete. Detroit, MI, 1974.

45

39. Wai-Fah Chen and E. Dahl-Jorgensen, Polymer-Impregnated Concrete as a
Structural Material, Magazine of Concrete Research, Vol 26, No 86, March
UDA. poo AGR20.

40. L. H. Willis and W. E. Willis, IL, Dry-Cast 12,000 psi Concrete,
Journal of American Concrete Institute, Proceedings Vol 71, No 6, June
IQ7AS jo WIL.

41. Bureau of Reclamation, Concrete Manual, 7th edition, Denver, CO, 1966.

42. Civil Engineering Laboratory (SP-700), Handbook for the Design of
Undersea Pressure Resistant Concrete Structures, by H. H. Haynes, NCBC, Port
Hueneme, CA (in publication).

43. Civil Engineering Laboratory, Technical Report R-805, Long-Term Deep-
Ocean Test of Concrete Spherical Structures - Part 1: Fabrication, Emplace-
ment and Initial Inspections, by H. H. Haynes, NCBC, Port Hueneme, CA,
March, 1974.

44. ACI Committee 215, Considerations for Design of Concrete Structures
Subjected to Fatigue Loading, American Concrete Institute, Proceedings
Woll Vil, Mo 35 Wereen ID/aS p65 D7.

45. Civil Engineering Laboratory, Technical Note N-1144, Mobile Ocean Basing
System - A Concrete Concept, by J. J. Hromadik, et al., NCBC, Port Hueneme,
CA, January 1971.

46. ACL Committee 349, Criteria for Reinforced Concrete Nuclear Power
Containment Structures, Journal of American Concrete Institute, Proceedings
Vol 69, No 1, January 1972, pp. 2-28.

47. P. A. Howdyshell, Rapid Testing of Fresh Concrete, CERL-CP-M-128,
Construction Engineering Research Laboratory, Champaign, IL, May 1975.

48. J. R. VanDaveer, Techniques for Evaluating Reinforced Concrete Bridge
Decks, Journal of American Concrete Institute, Proceedings Vol 72, No 12,
December 1975, pp. 697-704.

49. Highway Research Board Bulletin 182, Corrosion of Reinforcing Steel
and Repair of Concrete in a Marine Environment, Washington, DC, 1958.

50. C. F. Stewart, Considerations for Repairing Salt-Damaged Bridge Decks,
Journal of American Concrete Institute, Proceedings Vol 72, No 12, December

1975, pp. 685-690.

51. B. C. Gerwick, Jr., Prestressed Concrete Ocean Structures and Ships,
Prestressed Concrete Institute, Chicago, IL, September 1975.

46

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