55″ Seawater Supply System
Makai has designed for the State of Hawaii a larger warm and cold-water intake system for supplying both cold and warm water for aquaculture and ocean energy systems. Makai had full responsibility for the pipelines and pump station from the survey through to the final design plans and specifications. Estimated construction cost of the seawater supply system is 13 million dollars.
The 55″ seawater supply system is a dual ocean intake. One pipeline takes in seawater from a depth of 80′ and supplies over 50,000 gpm on shore. A second pipeline, approximately 10,000′ long and extending to a depth of 3000′ brings in 27,000 gpm of seawater at 4 deg C. Each pipeline is 55″ OD and primarily consists of polyethylene pipe.
The deep-water pipeline profile is illustrated above. The offshore region had been surveyed extensively with sidescan sonar that showed that the slopes were both steep and very rugged. The sidescan data on the steep and highly reflective slopes in Hawaii was inadequate for the detail required in a pipeline design. Makai surveyed the seabed with a high-resolution acoustic bottom roughness survey system to determine roughness resolution as fine as 6 inches. By knowing the capabilities, limitations, and costs of various polyethylene pipeline approaches (see the design approaches discussed below covering bottom, catenary, and pendant pipelines), an acceptable route and design approach could be joined. The resultant design was a pipeline that lies primarily on the seafloor, missing known obstacles. In the region of 500′ to 1600′ depth, the seabed is quite rough with old lava flows and ledges. The pipeline has been designed to be pulled tight over this region where it can span above the rough seabed due to the concave nature of the bathymetry. A series of pendants attached to the pipeline in this region help restrict movement due to high currents. A final survey was conducted with a series of submarine dives confirming the route and location of specific obstacles.
The deployment of the 55″ pipeline is to be completed by the controlled submergence process developed by Makai and used previously on deep-water pipelines ranging in size from 12″ to 40″ (see sections 6.2, 6.3 and 6.4). The primary difference for this pipeline is that it will be installed in one continuous section and will reach a depth of 3000 feet. By extrapolating from the success of earlier pipeline deployments, this larger and longer pipeline can be safely deployed to this greater depth.
The near-shore portion of this pipeline was particularly challenging because of the strict environmental constraints limiting trenching and the very extreme design waves (48 feet) and strong surface currents. Because of the large diameter of the pipelines, any surface mounted pipe would experience extreme inertial loads. Tunneling approaches were investigated: both slant drilling and micro-tunneling. Slant drilling was not contractually feasible, not being able to obtain a firm fixed bid from any contractor. A micro-tunneling approach was used and the design completed. Twin tunnels cross the shoreline from the pump station 500′ inshore to the breakout 500′ offshore at a depth of 80 feet. The tunnels are steel tubes with concrete carrier pipes grouted in place. The construction of this portion of the pipeline has been completed.
The warm water intake structure and the offshore cold water pipe attaches to the tunnels at 80′ depth. The deep-water portion of the pipeline is primarily gravity anchored with the exception of the immediate shallow region that has additional screw anchors for added stability. The intake structure likewise is stabilized with screw anchors.
The pump station is on shore, approximately 500′ back from the shoreline. Two pumps for each pipeline provide for the full flow, a third pump is installed for each pipe as a backup. The cost of installing large sumps in the very porous soil at Keahole, Hawaii prompted the design approach illustrated below. The large intake pipelines are located well below grade and open into an inlet screen well that separates any debris from the incoming seawater. From that point, the water flows to three individual wells, each with a vertical turbine pump, accessible from the surface. Sinking these pre-fabricated well structures in a flooded pit proved to be a more economical approach than the conventional large sump structure.
At present, the design is complete and the near-shore twin tunnels have been constructed. The remainder of the pipeline has been funded and is scheduled for construction in 2000 – 2001.
40″ Cold OceanWater Intake Pipeline: Design and Installation
The state of Hawaii is developing a Hawaii Ocean Science and Technology (HOST) Park on the Island of Hawaii to provide land and resources to developing companies in aquaculture and energy. The HOST park includes seawater supplies, both warm and deep cold water. Makai was responsible for the design of the offshore pipelines.
The intake cold-water pipeline was one of the most formidable engineering and construction challenges for the HOST Park development. The pipeline design called for a total flow of 50,350 lpm (13,300 gpm) of deep, cold water at 5 to 6 degrees Centigrade (39-43°F) from a depth greater than 640 m (2100 ft). The pipeline was designed to withstand a 100-year storm and have a lifetime of 10 years or greater. Because the final use for the cold water is to be for aquaculture and OTEC research, the pipeline materials contacting the flowing water must be inert.
The bathymetry at Keahole is ideal for a down-the-slope deep-water intake pipeline. The bathymetric profile and the pipeline installation is illustrated above. The desired intake depth of 640 m (2100 ft) is less than 1.6 m (1 mile) offshore. The shore elevation is approximately 3 m (10 ft) with a basalt cliff at the shoreline. In the shallow water are boulders with occasional bedrock outcroppings. At a depth of 15 m (50 ft), approximately 92 m (300 ft) offshore, to a depth of 61 m (200 ft), the bottom is coral rubble with slopes up to 40 degrees. Between the depths of 61 m (200 ft) and 152 m (500 ft) the bottom is smooth with sand and small pebbles. At 152 m (500 ft) the bottom slope abruptly increases along an old reef. The slope becomes 40 degrees or more at greater depths and the bottom is covered with old lava flows and occasional boulders 3 m (10 ft) or more in diameter. At the pipe intake area, the bottom is lightly sediment covered and smooth at about a 25 degree slope.
Keahole Point, being the western most point of the island of Hawaii, frequently has strong currents. The 100 year design currents are 2.9 m/s (9 ft/s) at the surface, 1.3 m/s (4.2 ft/s) at 152 m (500 ft) and .4 m/s (1.3 ft/s) at 305 m (1000 ft) and below. In addition, the pipeline has been designed for a 100 year storm wave at 14.5 m (47.5 ft) ft high at 18 seconds in the open ocean. Extreme wave and current events have been combined to yield a 100 year maximum load expected on the pipeline. In the shallow water, the pipeline loads are wave dominated and in the deep water they are current dominated.
The pipeline profile illustrates the general design approach used for this pipeline. The pumps are located in a sump 76 m (250 ft) inshore of the shoreline. From the pump station to 92 m (300 ft) offshore, the pipeline is buried. From a depth of illustration A1 21 m (70 ft) to a depth of 152 m (500 ft), a distance of 640 m (2100 ft), the pipeline is held just off the bottom with concrete anchors clamped to the pipeline. Beyond 152 m (500 ft) depth and reaching to the bottom anchor at 701 m (2300 ft) depth, a continuous band of polyethylene pipeline, 1030 m (3377 ft) long, freely floats above the bottom. This long, inverted catenary is held in place with two large gravity anchors at either end, 36.3 t (40 tons) at the top in 152 m (500 ft) of water and 18.1 t (20 tons) at 701 m (2300 ft).
Polyethylene had been selected as the pipeline material for this project. Polyethylene has significant advantages for this pipeline in that it is inert and will neither corrode nor contaminate the cold water. Polyethylene links can be heat fused together to form a long continuous pipeline with joints that are as strong as the pipe itself. With its excellent strength and flexibility, deploying a polyethylene pipe is far easier then other materials. Furthermore, the polyethylene is buoyant in water, a characteristic that provides for a great deal of design flexibility and deployment ease.
The lower portion of the pipeline is an inverted catenary, 1030 m (3377 ft) long, which floats several hundred feet above the steep bottom. This catenary design has been used to avoid contact with the rough and generally unsurveyed bottom beyond 152 m (500 ft) depth. The buoyancy in the inverted catenary is provided by the buoyancy of the polyethylene material plus, in the shallow region of high currents, fishing floats attached to the pipe. Even with this buoyancy, the pipeline sways in the strong currents at Keahole. Under 100 year current conditions, the center of the catenary will move nearly 152 m (500 ft) horizontally and 76 m (250 ft) vertically. Under moderate and strong current conditions, there will be vortex shedding on the pipeline and the pipeline may or may not resonate. Even if the pipeline does resonate, the cyclic fatigue limitations of polyethylene material will not be exceeded.
At the top of the inverted catenary the pipelines moves through the transition region to become a fixed anchored pipeline. The transition is a critical area of the pipeline becauset it is loaded by two dimensional bending, very high tension and torsion due to the buoyancy and movement of the catenary. At 152 m (500 ft), the catenary loads are transferred through a external pipe clamp to a mooring line attached to the upper 36.3 t (40 ton) anchor. Therefore the bending is taken by a relatively unloaded portion of the pipeline.
The bottom anchored pipe stretches between the depths of 152 m (500 ft) and 21 m (70 ft); it is 640 m (2100 ft) long. Pipeline anchors, with a wet weight of 3062 kg (6750 lbs), are clamped at varying intervals along the pipeline. The anchors support the pipeline 0.75 diameters off the bottom in order to minimize hydrodynamic loading and to clear small obstacles on the bottom. The spacing between weights varies since the loading and anchor requirements diminish in the deeper water. The installed pipeline in this region has an in place weight varying from 205 kg/m (138 lbs/ft) at 152 meters (500 ft) to 1235 kg/m (830 lbs/ft) at 21 m (70 ft). The very high weight per ft in the shallower portion of this region was achieved by adding saddle weights to the pipeline after it was placed on the bottom. The shallow portion of the pipeline is buried. The buried pipe stretches from the 21 m (70 ft) depth point, 92 m (300 ft) offshore, through the cliff shoreline and to the pump station, 76 m (250 ft) onshore.
The cold water pipe is a long suction intake delivering 50,346 lpm (13,300 gpm) to the submerged pumps onshore. The pipeline operates at 345 kPa (5 psi) suction total, 9.2 kPa (1.33 psi) is due to the static head from the high-density cold water. Because the pipeline suction requirement diminishes along the pipeline, the pipe wall thickness is staggered; the pipeline has a diameter to wall thickness (SDR) of 21 near the pump station and an SDR of 32.5 near the intake. The pump station is within the high surf zone and built entirely below grade. The 8 submersible pumps are installed in a covered sump 7.3 m (24 ft) deep. Three of the pumps feed the Natural Energy Laboratory of Hawaii and 5 feed the HOST Park.
The 1 m (40") HOST cold water pipe was assembled and deployed in July, 1987. Assembly of the pipeline took place at Kawaihae Harbor, 35.4 km (22 miles) north of the deployment site. The pipeline was fused together, anchor weights attached, and the assembly floated air-filled in the harbor at Kawaihae. Two separate tows were made to the site delivering first the near-shore portion of the pipeline, over 915 m (3000 ft) long and then finally the latter part of the catenary section, 610 m (2000 ft) long. At Keahole, the pipeline was controllably submerged and placed on the bottom.
The first section of pipe deployed was the shore section stretching from 21 m (70 ft) depth to the mid-catenary flange, a pipe length of 1131 m (3709 ft). This pipeline, air-filled and floating on the surface, was anchored at the shore end and connected to shore pumps. A large tug on the offshore end pulled the pipeline into alignment. By pumping water into the pipeline at the shore end, the pipeline was controllably submerged to the bottom. The internal pressure in the pipeline was controlled to prevent pipe collapse. By pulling strongly offshore, regulating the flow of water into the pipe and monitoring the air pressure inside the pipe, the pipeline submergence was always in static equilibrium and could be easily controlled. This submergence of the weighted portion of the pipe was a primary consideration in the design of the pipeline. There was a fine balance in the required offshore pull, minimum bend radii in the pipeline, internal pressure on the pipeline, external pressure on the pipeline, point loads on the pipeline due to anchor weights and temperature of the pipeline. The design of the deployment technique was an intricate part of the overall pipeline design.
Prior to submerging the near-shore portion of the pipe, the transition anchor and transition attachments were connected to the floating pipeline. The transition anchor was then lowered with the submerging pipeline. Once the transition anchor was placed on the bottom, the deployment of the first section was complete and the end of the first section, 366 m (1200 ft) down the inverted catenary from the transition, remained on the surface.
The second and final section of pipe was then towed from Kawaihae to Keahole and the two flanged pipe sections were bolted together on the surface. A pig was pumped through the pipeline to purge it of air, the intake end of the pipeline was attached to the lower anchor and this was lowered to the bottom in approximately 701 m (2300 ft) of water.
18"CWP, Pendant Pipeline Design
Makai Ocean Engineering designed for the Natural Energy Laboratory of Hawaii an 18" back-up pipeline to the 12" pipeline Makai had designed and installed 6 years earlier. This work was performed for the Research Corporation of the University of Hawaii.
This pipeline was installed approximately 1 mile south of Keahole Point, Hawaii, the location of the 40" and 12" inverse catenary design pipelines. American Divers, Inc. of Honolulu and Makai Ocean Engineering completed the deep-water deployment in October, 1988.
The main restricting design criteria for the 18" pipeline was cost. The State’s budget was limited for construction of this pipeline plus the State wanted to demonstrate new cost savings techniques for the installation of deep-water polyethylene pipes. Compounding this restriction, considerably less was known about the bottom conditions along this pipeline route as opposed to previously laid pipelines and the acceptable pipeline corridor was quite narrow.
The above figure illustrates the overall pipeline configuration and shows the primary difference between this pipeline and the preceding inverse catenary design concepts. The very shallow portion of the pipeline, from the shoreline to the depth of 70′, is rock bolted directly the hard bottom with a series of tubular steel supports that are highly versatile and adaptable to the very irregular bottom. Beyond the 70′ depth and out to 500′ depth, the pipeline traverses a fairly smooth but narrow corridor and is held in place with concrete weights similar to earlier designs. Beyond 500′ depth, the design changes are most apparent. The buoyant polyethylene pipe rises above the bottom but is moored with a series of pendants, approximately 40′ long, that are distributed along the pipeline down to the intake at 2100′.
There are several design, budgetary and deployment advantages to the pendant supported pipeline configuration. The pipe is supported well above the bottom and therefore an accurate survey for a smooth pipe route is not required. Secondly, some of the disadvantages of the catenary are eliminated: the handling of the large clump anchors at either end of the catenary during deployment is eliminated and the complex transition between the moving catenary and the stationary pipeline is not necessary. Among the disadvantages of the pendant system is the large number of suspended weights dangling during deployment and the pipeline is still somewhat affected by bottom features. Because of the considerable previous experience in handling and laying polyethylene pipelines, it was felt that the pendants could be safely handled without fouling.
Included in the development of this pipeline concept were low cost pipe attachments, not requiring heavy steel clamps as used in the past, such that the pipeline could be pulled safely at high tensions. This high tensile load is a requirement during deployment.
The full length of this 6000′ long pipeline was laid on the bottom using the controlled flooding and submerging techniques used in the past. This pipeline, however, used this technique to a conservatively greater depth. The pipeline was flooded from the shore end to the offshore end, and the bitter end was lowered with a cable to the sea floor. The submerging and lowering of the pipeline took less then 12 hours.
The polyethylene pipe portion of this pipeline was assembled at the Natural Energy Laboratory just prior to the deployment. The pipe was placed on rollers and launched into the ocean through a ramp at the shoreline. As the pipeline was pulled into the sea, pendants, weights and various pipeline components were attached to the pipe near the shoreline. The total launch took approximately 12 hours. The ability to launch a pipeline in this manner demonstrates that a harbor or protected shoreline is not necessary for installing a CWP. Prior pipelines had used the harbor at Kawaihae for their installation but parallel projects at Keahole made it advantageous to launch this pipeline on site.
The relationship between the State of Hawaii, American Divers, the contractor, and Makai Ocean Engineering, the designer, was unique. American Divers was selected to construct the pipeline early in the planning stage. During the design, Makai worked closely with American Divers in order to design a pipeline that could cost effectively be deployed by American Divers and still meet the operational requirements of the State of Hawaii. American Divers worked directly for the State on a time and materials basis and all parties acknowledged that there are significant risks that are inherent with any marine construction, the contractor would work to the best of his ability and the ultimate risk would be taken by the State.
One major difficulty with the 18" pipeline route was the location of rocks on either side of the pipe route at depths between 240′ and 600′. Currents at Keahole can be quite high and on the day of deployment exceeded 1.3 knots at times. The restricted deployment budget did not allow precise acoustic navigation techniques nor even surface electronic navigation. The pipeline was precisely deployed utilizing shore ranges and restraining lines moored to the bottom at key placement locations.
The pipeline was smoothly and efficiently installed in October, 1988. The pipeline was inspected the following December proving that the pipe alignment and pendant arrangement is as designed.