Ocean Thermal Energy Conversion (OTEC) requires huge quantities of deep cold seawater and warm surface water to operate. The fabrication and installation of deep-water pipelines to provide this water represents the single most expensive portion of any OTEC plant and the highest risk during construction. Because of these associated costs and risks, it is the least demonstrated major component of a large OTEC plant.

     Hawaii has been the center for OTEC development in the United States over the past twenty years and OCEES and its strategic partners have been an integral part of that research and have gained very valuable experience in cold water pipe technology. The State of Hawaii is currently operating several pipelines at the Natural Energy Laboratory of Hawaii Authority (NELHA) , the largest of which is 55" OD. Pipelines as large as 8’ OD have been tested at the facility in both down-the-slope mode and suspended mode. In fact, all of the world’s large deep seawater pipelines have been deployed at NELHA. All phases of design, manufacture and deployment of each of these pipelines have been overseen by OCEES International, Inc.'s strategic partner Makai Ocean Engineering. The techniques learned and experience accumulated through the deployment of the deep water pipelines at NELHA and the research accomplished on large diameter pipelines performed in Hawaii have led to the confident conceptual design and deployment scenarios for OTEC pipelines up to three (3) meters in diameter. These segmented pipes of fiber-reinforced plastic can be deployed with the same controlled submergence techniques that have already been implemented in Hawaii and can be installed in either the gravity anchor mode, pendant mode or the long, inverted catenary mode.

     Pipeline configuration design and development requires analysis of the pipeline under two very differing environments: the near shore region and the deep-water pipeline.  

Near Shore Region

     We define the near shore region as the shore landing through to a depth of approximately 60 — 80 feet. In this region a designer is faced with a variety of challenges including severe current and wave loads, aesthetic and environmental considerations, pump station interface and offshore pipeline interface. Because the pipeline is in shallow water, wave action on the pipeline can be quite extreme, especially in areas susceptible to hurricane activity. This is where experience in ocean related design cannot be understated - OCEES International, Inc. and it’s strategic partner’s all possess the necessary ocean training and experience to confidently design, deploy and construct a working plan in nearly any ocean environment.

     Large hurricane waves and associated large design wave heights, as are common to tropical island communities, place very high lateral and lift loads on exposed pipelines and effectively eliminate consideration of a near shore traverse using gravity anchors alone. Several near shore crossing techniques are available for consideration, including:

• Trenched pipelines
  
  
• Bolt down pipelines
  
  
• Tunneled pipelines
  
  

     Buried or Trenched Pipelines are well protected from the environmental loads and, for multiple pipes, are the most cost effective approach for the near shore route. However, digging and blasting the trench can be environmentally damaging to reefs and the near shore region; therefore should only be pursued if other, more environmentally attractive means of traversing the near shore region prove unrealistic.

     Bolt Down Pipelines are sometimes utilized in applications where the near shore region is exposed to extreme wave conditions thereby exposing a hard seafloor or large boulders partially submerged in the sand of sufficient size to stabilize the attached pipeline under extreme wave conditions (hurricanes, etc.). Under these conditions, bolt down pipelines become extremely cost effective. Rock bolts are relatively inexpensive and divers can accomplish maintenance through periodic replacement of the sacrificial zinc anodes for corrosion protection. The disadvantage of this technique is that the seafloor is highly irregular and it is difficult to design in advance the exact location of each rock bolt and clamp on the pipeline.  

     Tunneling consists of two differing techniques for accomplishing the same effect — slant drilling and micro tunneling. Slant drilling uses oil drilling techniques with a drill oil rig onshore pressing a drill bit and drill pipe through the soil at a fairly shallow angle to the approximately 60 foot depth. In the micro tunneling approach, a large dry jacking pit is constructed onshore reaching well below sea level. A micro tunneling machine with a drill bit equal in size to the outer diameter of the desired tunnel is pushed through the vertical wall of this onshore pit and continues to drill by adding drill pipe sections until reaching the offshore regions of 60 — 80 foot depths. Under each of these tunneling configurations, the shoreline and seafloor regions are relatively undisturbed and prove the best protection for the near shore pipeline as well as the most environmentally favorable means of traversing the fragile shallow water region to a shore mounted OTEC facility.

Deep Water Pipeline

     The deep-water portion of any pipeline sees smaller environmental loads than the near shore region. Waves have less impact upon this portion of the pipe, there are less visible environmental concerns, and currents diminish with depth. In general, the design approach and installation procedures are completely different than the near shore region.

     Three different techniques for mounting deep-water pipelines have been developed in Hawaii, they are:

• Gravity anchored pipelines
  
  
• Pendant anchored buoyant pipelines
  
  
• Inverted catenary buoyant pipeline

     In many deep pipeline applications, it is necessary to incorporate a combination of the above approaches to accommodate different deep-water terrains.

     Gravity Anchored Pipelines are most appropriate in regions where the seafloor is relatively smooth over a gentle slope. Gravity anchoring consists of attaching a series of cement bottom anchors at regular intervals before deployment over the length of the pipeline with sufficient clearance to prevent damage to the pipe from the seafloor. A very detailed survey route and precise deployment of the pipeline is imperative in this technique to prevent damage to the pipeline during deployment — a pipelines most vulnerable moment. Because of its simplicity in design and minimal cost, it is the preferred method of anchoring the deep-water pipelines associated with an OTEC facility. However, since ideal conditions rarely exist in reality, as mentioned previously, this method is often combined with the other techniques over the course of the pipeline.

Gravity Anchored Pipelines  

     Pendant Pipelines are utilized under buoyant pipe conditions to traverse rugged bottom environments (sharp rocks, etc.) by allowing the pipeline to float above the bottom at considerable spacing to avoid damaging the pipe during deployment and periods of very large waves which may cause some lateral motion to the pipeline. A pendant system consists of a buoyant pipe attached to anchors via a cable system allowing the pipe motion flexibility without permitting contact with the seafloor. One drawback of this approach is the cost of buoyancy, which, if utilized over large portions of the pipe length, can become cost prohibitive.

     Catenary Pipelines take advantage of the deep water pipe’s natural buoyancy and flexibility to traverse very rugged and often very steep bottom terrain. When this technique is to be employed, detailed route surveys of the bottom are not necessary (only at the two ends of the catenary). Like the pendant pipeline, the catenary does not experience high inertial loads and, being mostly detached from the seafloor is not susceptible to earthquake damage.

     The catenary approach cannot be implemented everywhere, it is not suitable for shallow water applications or gentle slope considerations where the peak of the catenary could approach the ocean’s surface.

Pipeline Deployment

     The design of the installation process of the pipeline is the single most important aspect of the overall pipeline design. The most formidable obstacles to the installation of a deep-water pipeline are the extreme depths, currents and waves that the installer will encounter during installation. A pipeline will often experience its most extreme lifetime loads during deployment and certainly the highest risk of loss. The proper design process of a deep-water pipeline must constantly involve the deployment procedure.

     In most cases in engineering the role of designer and installer are clearly separated. However, this is not the case in deep water pipelines — it is unlikely the contractor hired to deploy the pipe has installed such an instrument, therefore, it is up to the designer to make sure the pipeline can be constructed and deployed both reliably and economically. It is inherently necessary for the pipe designer to provide detailed deployment plans and specifications together with the final design in order to make sure the contractor fully understands the intended installation procedure. Because of the extreme expense associated with marine construction operations, it is also necessary that the pipelines be designed for fast and efficient deployment as well.

     Most of the work on pipeline construction is performed on land or in a nearby harbor. All weights, pendants, and other attachments are attached to the pipeline with shore crews. Often the pipeline is assembled right at the shoreline in a harbor and as each section of pipe material is added to the pipeline, the other attachments are added and the pipeline is "pushed" into the harbor. The pipeline is designed to float when air filled; it supports the weights and other pipeline attachments. Therefore the whole pipeline is assembled and pushed into protected waters while using a minimal amount of marine equipment.

     At the time of deployment, the pipeline is towed to the site and aligned over the design pipeline path. The shore end of the floating pipeline is anchored in about 60 feet of water and attached to a set of flooding pumps at the shore end. While the pipeline is under tension (being pulled by barges at pre-designed speeds and directions), water is being pumped into the pipeline at the shore end. The non-flooded buoyant section of the pipeline supports the heavy flooded end at any given point and the pipeline takes on an S-shape. By carefully monitoring the pull of the pipeline, the internal pressure, the distribution of weight on the pipeline, and the temperature and time of deployment, the pipeline can be safely placed on the bottom to depths up to and exceeding 3000 feet. The alignment and flooding of a mile-long pipeline can be accomplished in one day.

     As one can imagine, careful deployment design and detailed planning is critical to the success and installation of a deep-water pipeline. The deploying contractor needs to pre-analyze all conceivable problems and be prepared for any contingency. Therefore, experience in the design and deployment of such a project requires the best, most experienced companies available — OCEES International, Inc.'s strategic partner Makai Ocean Engineering represents that experience and possesses the expertise necessary to confidently design and deploy the large diameter cold water pipelines necessary for the successful implementation of an integrated OTEC system.