Deep Pipelines for Ocean Thermal Energy Conversion
SWAC Aquaculture Services Papers

India OTEC Pipeline

 

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. 

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12" Down The Slope CWP

 
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.

MakaiLay Seismic - Planning

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.

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Down-The Slope 8′ CWP Demonstration and Research

 
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.
Traditional installation methods result in high tension.

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:

  • 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.

  • A large FRP ball joints allowed the pipe to rotate or bend to accommodate an irregular bottom.


  • 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.

  • 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.

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Mini-OTEC Intake Pipeline

 

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.

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8′ Suspended CWP, DOE Research

 

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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