Figure 3
Several factors come into play for the best system configuration:
The offshore marine piping can be installed using several techniques including tunneling. trenching and surface routing. Tunneling techniques have been perfected during the past decade. Microtunneling machines can directionally drill tunnels up to 6 feet in diameter. Other tunneling techniques have been successfully used to bore significantly larger tunnels under large bodies of water. Tunneling generally is the most expensive offshore pipe installation technique, although the least obtrusive.
Marine piping can also be laid in a trench in the ocean or lake bottom. Conventional marine excavation or dredging is possible to depths approaching 50 feet. Dredging is possible in deeper water, although the costs become prohibitive. The trenched piping can be armored with rock or spoils from the dredging activity.
The least expensive configuration is simply laying the intake piping on the bottom surface of the body of water. The piping must be anchored to mitigate current induced pipe system shifting. Piping materials that are less dense than water require ballast systems to neutralize bouyancy forces. The piping that remains exposed to subsurface interferences and hazards, such as boat anchors, must be armoured.
Several materials are suitable for the marine piping system. The primary materials include:
The primary factors used to design the intake piping system are similar to those imposed on any water piping system. A trade off between piping system capital cost and pressure drop is used as a means to determine the proper line sizing. Pipe wall thickness of the intake system is related to the pipe material, pipe diameter, design pressure, temperature. system design life and the expected corrosion and biofouling rates.
From the heat exchangers, the water is returned offshore at an environmentally appropriate point from the shoreline. The offshore water discharge piping is typically shorter than the intake line. This is generally due to the requirement to return the discharge back to a zone of similar temperature and water chemistry. The outfall is fitted with a diffuser assembly to evenly distribute the discharged water in order to mitigate the formation of temperature gradients that may affect the environment. Hydrodynamic thermal modeling will assist the design of an optimal outfall configuration that will minimize the environmental impact.
- Coated and/or Lined Carbon Steel (CS)
- has been used for intake and outfall systems for decades with acceptable results. New technologies in external and internal coatings have decreased the potential for corrosion induced failures. Carbon steel is readily available, easily repaired, if accessible, and is relatively impact resistant.
- Cast Ductile Iron (DI)
- DI is a cost effective alternative to CS systems in diameters through 12 inches. Large bore DI is more expensive than CS.
- High Density Polyethylene (HDPE)
- Recent improvements in manufacturing and quality programs show that HDPE is a very viable material in intake piping systems ranging in size from 6 to 63 inch outside diameter. HDPE is an extremely tough product, is easily fabricated, has excellent flow characteristics, offers good insulation qualities and is benign to the surrounding water system. Heavy wall thicknesses are required with this material to design against pipe collapse due to the continuous or cyclic negative pressure induced during normal flow conditions. HDPE is lighter than water and therefore requires substantial anchoring ballast to sink and hold the pipe in place.
- Fiberglass Reinforced Plastic (FRP)
- FRP is commercially available in sizes in excess of eight feet in diameter. It is durable, offers good flow characteristics and is corrosion resistant. FRP is easy to handle and is patchable if accessible. FRP, however, is subject to impact damage.
- Polyvinyl Chloride (PVC)
- PVC is available in smaller diameters and could be an economic material for short intake systems with very low piping pressures.
- Reinforced Concrete (RC)
- RC is available for pipe diameters through 100 inches. Various joining techniques are available. RC is heavy and bulky; but, it could be an economic material for short intake and outfall systems.
A typical outfall diffuser assembly may take the form of simply a length of pipe fitted with multiple outlets equally spaced for uniform water distribution. Risers can be used to enhance thermal dispersion and can be of the break-away design to avoid recreational boat damage.
The offshore water pumping systems are shore based for simplicity of construction and operation. The system hydraulics and physical designs are influenced primarily by the pressure loss in the intake pipe and the elevation of the pumps and heat exchangers. These factors will determine the basic pumping system hydraulic grade line and the physical configuration. The pump systems can be designed as a deep well, wet pit type using vertical turbine type pumps or grade mounted pumps with priming systems.
The use of variable speed drives (VSDs) for the offshore water pump drives is evaluated based upon both the capital cost and energy savings. VSDs are generally economically justified in systems that are designed to follow varied cooling loads.
The addition of strainers or filters must also be considered both for equipment protection and to mitigate performance degradation due to fouling and marine growth. Generally, water found in the ocean or lake depths is clean and does not require significant filtration. Filtration is an effective method to deter or eliminate biological growth and fouling of piping and equipment.
Heat exchangers are selected based upon their capital cost, thermal performance, compatibility with the water type and operating costs. Several types of heat exchangers are available for this application. Shell and tube or plate and frame type exchangers are suitable for this service. Plate and frame type heat exchangers generally provide superior thermal performance, are smaller, more economical and are available in a multitude of materials and sizes. Plates fabricated from titanium are used in sea water applications while stainless steel provides good performance in fresh water applications. Aluminum plates are now being developed as a cost effective alternative to titanium. Approach temperatures of 1 - 3°F are attainable with plate and frame configurations, while shell and tube configurations are limited to minimum approach temperatures of 5 - 10°F. Plate and frame heat exchangers are commercially available in sizes through approximately 4,000 tons of cooling (based upon a 4°F approach temperature and a 150 psig design pressure).
The offshore water is usually pumped through a parallel bank of plate and frame heat exchangers. The use of heat exchangers configured in series has been considered to improve approach temperatures, but generally results in significantly higher costs.
The chilled water portion of the system consists of the pumps and chilled water side of the heat exchangers: the supply chilled water transmission line; the service load tie-in points; and the return chilled water transmission line.
As in the lake or ocean side pumping system, if the chilled water system is designed as a load following system the use of VSD pumps is generally economically justified. Conventional, centrifugal pumps with ductile iron and/or bronze internals can be used in this application due to the close control of water chemistry that can be imposed on the closed chilled water system.
The chilled water distribution system can be designed as an above or below ground system. Above ground systems are generally less expensive to construct; but, they are considerably more obtrusive to the surrounding area. Below ground installations include direct buried systems, installations in utility tunnels and installations in finished trench segments. The direct bury method is the most economical underground construction method.
Piping material selection depends in large part upon the service load distances from the DWSC facility and the elevations and resulting operating pressures. Welded, coated and wrapped carbon steel pipe with a corrosion allowance is a common choice as it accommodates nearly any pressure, is leak-tight and is usually economical. Other materials such as DI, RC, PVC or HDPE may also be considered given the particular application.
A system feature to consider in the design is the location of the pumps downstream of the heat exchangers to minimize the heat exchanger operating pressure. By locating the chilled water pumps downstream of the heat exchangers, the system pressure loss can be used to reduce the operating pressure below the system static pressure and minimize the design pressure of the heat exchangers. This allows for cost savings on the heat exchangers, pumps and the piping
Also, the addition of a cool-down cross-tie in large systems allows the chilled water to be circulated and cooled prior to accepting load. This feature also allows water circulation during winter lay-up periods to prevent pipe freezing.