Deep Pipelines for Ocean Thermal Energy Conversion
 In 1998, Makai was contracted by the National Institute
of Ocean Technology (NIOT) in Madras, India, for the conceptual design of the
deep water intake pipeline, the effluent pipeline, and the mooring system for an
experimental floating OTEC Plant. The analysis included dynamic modeling of pipe bending,
shown at right.
The NIOT OTEC barge is scheduled to be 72 meters long and
will be supplied 1415 kg/s of deep cold seawater through a 1 meter diameter
pipeline from a minimum depth of 1000 meters. The mooring is 1220 meters deep.
In early work, Makai considered several concepts for the
pipe and mooring combinations. Basic concepts consisting of multiple point
moorings, dynamic station keeping, and single point moors were considered. The
design has lead toward a single point moor with the pipeline as a major
structural component of that mooring. The pipe is supported at the surface by a
large buoy. The OTEC barge can be moored to the buoy and a flexible transfer
hose is used to connect the wet sump in the barge to the top of the pipeline
below the buoy.
The pipeline material used for the main pipe and the
transfer hose is high density polyethylene (HDPE). This material is ideal for
this application because of its high flexibility, moderately high strength, high
fatigue strength, and corrosion resistance.
The pipeline was designed for long-term pumping loads, for
long-term operational dynamic loads, for short-term storm dynamic loads and
deployment.
The pipeline and mooring component characteristics were
optimized to be suitable under the most severe operating condition (4.25 m
significant, 7.3 sec. central period) specified by NIOT and to survive the storm
conditions specified by NIOT (8.25m, 12.7 sec). A finite-element dynamic model
was used to analyze the pipeline and barge under these and other conditions.
The net result of this work showed that a single point
mooring for the NIOT barge is a feasible, practical and cost-effective solution
to a very difficult and potentially costly problem. Each critical component was analyzed for feasibility and conceptual design specifications have been
generated. The mooring and pipeline can be constructed, deployed and recovered
with an adequate safety factor, including
determination of RMS fatigue stresses at various areas within the same pipe,
to meet NIOT's design goals.
The first down the slope pipeline design by Makai Ocean
Engineering was for the Natural Energy Laboratory of Hawaii at Keahole, Hawaii.
This State of Hawaii laboratory was engaged in OTEC research and required a
reliable source of deep cold ocean water in order to perform laboratory
experiments on heat exchanger bio-fouling and corrosion.
The design criteria for the pipeline called for a low
budget, 12" pipeline with a lifetime expectancy of 2 years. Some limited
bathymetry and side scan information was available but no on-site
observations had been made. Makai's role in the pipeline was to provide the
design, develop the deployment plan, monitor the financing, locate and purchase
the materials, be directly in charge of the pipeline assembly and provide
technical direction during the at-sea deployment.
 Because
of the very deep bottom with an unknown roughness, the Makai design for the
pipeline evolved toward the inverse catenary approach (see 40" CWP design, a
later but similar pipeline). The inverse catenary approach takes advantage of
the buoyancy, flexibility and high fatigue strength of the polyethylene material
in order to void the primary design problem: a very steep and rough sea bottom.
The 12' pipeline was assembled in a large harbor 20 miles
north of Keahole Point. The pipe fusing, attachment of all components, launching
into the harbor and testing was completed by Makai Ocean Engineering. Once ready
for deployment, Hawaiian Dredging & Construction served as marine contractor
to tow the pipeline to Keahole Point and place it on the bottom.
The basic controlled submergence process used in all the
pipelines at Keahole was developed with this first, 12" polyethylene pipe.
On the first day of deployment the pipeline was submerged to the bottom to a
depth of 500'. On the second day, the catenary portion of the pipeline was added
and lowered to the bottom.
Inspection of the 12" pipeline first occurred on its
one year anniversary. The pipeline was accurately placed and was functioning
well. After over 10 years, this pipeline (designed for two years) was still
providing water to NELH. No maintenance had been required in the deep water
portion of this pipeline. Only in the shallow water have the anchoring clamps
needed replacement and the pumps have been changed. After approximately 12
years, a shackle at the base of the catenary gave way due to corrosion and the
catenary was shortened and redeployed in shallower water.
At the time of installation, the catenary concept was a
radical deviation from conventional pipeline design. Today, because of the
12" pipeline success and longevity, the catenary approach is generally
considered as a viable contender for very large diameter OTEC pipelines.
Makai Ocean Engineering served as the primary design
engineer for the Phase 3 portion of the OTEC Cold Water Pipe At Sea Test
Program. Makai was a subcontractor to Hawaiian Dredging & Construction
Company, the prime contractor, on this federal Department of Energy research
program. The primary purpose of this study was to instrument, deploy, monitor
and analyze an 8-foot diameter test section of a fiberglass reinforce
plastic (FRP) pipe on underwater foundations along a 40 degree slope. This
deployment was completed in May 1984 and marked the beginning of a 12 month data
gathering effort to provide information critical to design of large diameter
CWPs for shelf mounted OTEC plants. Following this monitoring period, Makai
analyzed the data.
 The
figures to the right illustrate the in-place pipeline and the assembly process.
The demonstration and goal of this project was to develop a
down-the-slope pipeline concept utilizing an existing FRP pipeline
and to deploy it on a specific coral rubble 40o slope off the Big
Island of Hawaii. While the test section of pipe installed was placed in shallow
water, the techniques used were applicable to much deeper water.
The
design incorporated several features that are necessary for a large diameter
down-the-slope pipeline. The rigid pipe was laid in sections of
fixed length with a joint between each pipe section. This joint served several
purposes:
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The joints were easily coupled underwater utilizing remote sensing
techniques (video) without divers. This coupling technique was developed for
deeper water and was actually more difficult, yet successful, in the shallow
water demonstration because of barge surge and heave.
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A large FRP ball joints allowed the pipe to rotate or bend to
accommodate an irregular bottom.
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The joint acted as a seal for the pipeline. The seal was completed
by pumping tremie concrete into the pipe joint connector after it was placed on
the bottom. The lowered connector was the form.
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The joint was also the gravity anchor. The concrete greatly
increased the weight and stability of the joint foundation but did not restrict
the swiveling or rotating motion of the pipeline. By adding concrete later, the
lowering weight was reduced considerably.
The deployment demonstrated the
successful handling of very large masses and the precision coupling of these
masses utilizing only video camera feedback to the deployment superintendent on
the barge. The installed pipeline section is still in place having survived
several severe storms thus showing the use of large gravity anchors on a steep
coral rubble slope.
A second goal of the program was to utilize the installed
8' diameter pipe as a test sample to measure hydrodynamic loading on a large
diameter slope-mounted pipeline. Appropriate hydrodynamic data for a
pipeline of this size could not be obtained in a laboratory or at reduced scale.
Makai designed into the pipeline an instrumented mid-section test ring
which actually "floated" in the center of the pipe supported only by a
series of load cells. Other load cells were installed at the end of the pipe
section in order to measure the wave and current loads on the entire length of
pipe. In addition, multiple current meters and wave sensors were added to record
the environmental conditions under which the pipeline loads were measured.
Makai, in association with Edward K. Noda and Associates, reduced the data from
this program and determined pipeline drag and lift coefficients.
 Mini-OTEC was a joint partnership program of the
State of Hawaii, Lockheed Missiles and Space Company, Alfa Laval Thermal and
Dillingham Corporation to develop and operate the first closed-cycle
self-sustaining OTEC system operating at sea. Its purpose was to prove the
feasibility and usefulness of OTEC as an alternative non-polluting power
source. Makai Ocean Engineering designed the mooring, the cold water pipe, the
water supply systems and the deployment process. Makai was a subcontractor to
Dillingham Corporation during this work.
The Mini-OTEC facility is illustrated
to the right. A small,
complete ammonia-based OTEC facility was mounted on a small barge moored
1-1/2 miles offshore. A 2150' long, 2' in diameter polyethylene pipe
served as the cold water intake for this facility.
Makai was responsible for both the intake pipeline and the
mooring. Initial concepts involved single and multiple point moorings with a
separate pipe; keeping the pipe free from fouling on the mooring was a difficult
problem. Makai selected polyethylene as the ideal pipe material for this
application and, appreciating polyethylene's strength and flexibility, the final
solution was to use the polyethylene pipe as the actual mooring member. This
required the development of a very rugged and reliable end attachment to the
polyethylene pipe that would transfer the axial loads of the deployment and
mooring. Makai successfully designed and tested these attachments which have
subsequently been used in several down-the-slope polyethylene pipe
installations.
Mini OTEC was
deployment in July, 1979 and became the first successful at-sea OTEC plant
the following month by producing net OTEC power. In the following year, the
National Society of Professional Engineers awarded Mini-OTEC as being one
of the ten outstanding engineering achievements of that year in the United
States.
 Makai Ocean Engineering served as subcontractor to Hawaiian
Dredging & Construction Company in the suspended pipe test phase of the OTEC
Cold Water Pipe At-sea Test Program sponsored by the Department of Energy.
During this program, an 8-foot diameter, 400' long sandwich wall
fiberglass reinforced plastic (FRP) syntactic foam configuration CWP test
article was developed, constructed, deployed and used for data acquisition in
the open ocean near Honolulu, Hawaii. This instrumented CWP was suspended from a
moored platform in the spring of 1983. The CWP represented a scaled version of a
40-megawatt size CWP structure, nominally 30' in diameter and 3000' long.
Makai Ocean Engineering's role in the program was to assist in the FRP design
and planning, plan the pipeline deployment, design and plan the deployment of
the mooring in 1300' of water and provide general engineering assistance to
Hawaiian Dredging.
Through this program Makai gained valuable experience with
FRP pipelines, handling of large diameter rigid pipe sections and the
hydrodynamics of large pipelines. This same FRP pipeline was later used in a
down-the-slope research program that was also funded by the
Department of Energy.
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