Advanced methods (submerged seawater intake/discharge system)

— A problem related to the operation of existing units is reduction in overall plant efficiency that results from an increase in the temperature of cooling water;

— In light of this concern, the submerged intake and discharge was selected as the most appropriate scheme for the condenser cooling water system;

— The purpose of the submerged intake is to supply cool water from the stratified subsurface to the cooling system in summer, when thermal stratification appears in the near-shore area at the site;

— The regulatory requirements for limiting discharge temperature are met by designing the discharge structure to induce rapid mixing. Initial dilution of submerged discharge is maximized by increasing the number of diffusing ports on condition that spacing should be sufficient to avoid overlapping of plumes from adjacent ports. Figure 90 shows a comparison of the maximum excess temperature;

— Each segment of concrete channels for the submerged intake and discharge system is laid underwater by a heavy duty offshore crane as shown in Fig. 91.

7.1.2.2. Major structures of advanced cooling water system

The submerged intake/discharge cooling water system consists of the intake structure and the discharge structure. The main components of the intake structure include the submerged intake velocity cap, submerged intake tunnel, intake reservoir, intake structure and intake conduit. The main components of the discharge structure include the discharge conduit, discharge pond, submerged discharge tunnel and submerged discharge diffuser.

FIG. 88. Typical section of revetment (credit: KHNP).

FIG. 89. Typical section of wharf (credit: KHNP).

FIG. 90. Comparison of maximum excess temperature (credit: KHNP).

FIG. 91. Submerged intake/discharge conduits installation (credit: KHNP).

The cooling water from the sea flows through the submerged velocity cap, submerged intake tunnel, intake reservoir, intake pump house, intake conduit, main condensers or heat exchangers, discharge conduit, discharge pond and submerged discharge tunnel, and is then finally discharged to the sea at the submerged discharge diffuser.

Figure 92 shows a schematic view of a submerged cooling water system. The characteristics of the main components of a submerged cooling water intake/discharge structure are described in the paragraphs after the figure.

Submerged intake velocity cap

— Submerged intake velocity caps are precast reinforced concrete structures that are manufactured in the shop and installed by floating crane at a depth of approximately 20 m from the seawater surface to draw the lower temperature deepsea water;

— Submerged intake velocity caps are required to prevent fish entrainment from the vertical flow velocity change. The approach velocity to the velocity cap should therefore be within a range of 0.15–0.45 m/s to prevent fish entrainment;

— The inlet opening shall be located at a suitable level from the seabed to prevent the inflow of sediment;

— The spacing between the centres of velocity caps shall be determined to minimize the interference of each flow boundary.

Submerged intake tunnel

— A submerged intake tunnel is a structure with a square section to transport the cooling water drawn through the submerged velocity caps to the intake reservoir;

— The tunnelling method is selected considering the geological conditions and environmental constraints. Based on the applied tunnelling method, the structural types are largely divided into two categories: conduit type and bored type. The conduit type uses the open-cut, laying on and fill method, and the bored type uses the conventional (drill and blast) or mechanical (tunnel boring machine (TBM) and Shield TBM) tunnelling method;

— The submerged intake tunnel for an is a precast concrete conduit type, constructed by the immersed tube method in consideration of geological conditions;

— The optimum diameter of the submerged intake tunnel should be determined considering the head loss through the line and the gradual reduction of flow area due to marine growth. The design velocity should be within a range of 1.8–3.0 m/s.

Intake reservoir

— An intake reservoir is a structure of the retaining wall type to transport cooling water from the submerged intake tunnel to the intake structure;

— This structure is a transition area of the flow toward the intake structure. The bottom elevation of the reservoir shall be lower than or equal to that of the intake structure/pump house.

FIG. 92. Submerged cooling water system conceptual scheme (credit: KHNP).

Intake structure

— The intake structure is a reinforced concrete structure with shear walls;

— The intake structure is connected to the intake reservoir. It consists of the circulation water system (CWS) intake structure/pump house and the essential service water system (ESWS) intake structure/pump house, which contain pumps and travelling screens, and supply the cooling sea water to the main condensers in the turbine building and component cooling water system (CCWS) heat exchangers, respectively.

Intake conduit

— The intake conduits of the CWS and ESWS supply the cooling sea water from the intake structure/pump house to the main condensers inside the turbine building and the CCWS heat exchangers, respectively;

— The type of intake conduit is selected considering durability, constructability, economic effectiveness, etc. In general, the cast-in-place reinforced concrete box type is widely used.

Discharge conduit

— The discharge conduits of the CWS and ESWS transport the warm water from the main condensers inside the turbine building and the CCWS heat exchangers, respectively, to the discharge pond;

— The type of discharge conduit is selected considering durability, constructability, economic effectiveness, etc.

In general, the cast-in-place reinforced concrete box type is widely used.

Figure 93 shows an example of the layout of submerged intake/discharge conduits.

Discharge pond

— A discharge pond is a rectangular-shaped structure of cast-in-place reinforced concrete pond, functioning as a surge tank during transient flow conditions;

— It transports the cooling water from the discharge conduit to the submerged discharge tunnel.

Submerged discharge tunnel

— The submerged discharge tunnel is a structure used to transport the warm water from the discharge pond to the submerged discharge diffusers;

FIG. 93. Submerged intake/discharge conduits layout (credit: KHNP).

— The tunnelling method is selected considering the geological conditions and environmental constraints. Based on the applied tunnelling method, the structural types are largely divided into two categories: conduit type and bored type. The conduit type uses the open-cut, laying on and fill method, and the bored type uses the conventional (drill and blast) or mechanical (TBM and Shield TBM) tunnelling method;

— The submerged discharge tunnel is normally a precast concrete conduit type, constructed by the immersed tube method in consideration of geological conditions;

— The optimum diameter of the submerged discharge tunnel should be determined considering the head loss through the line and the gradual reduction of flow area due to marine growth. The design velocity should be within a range of 1.8–3.0 m/s.

Submerged discharge diffuser

— Submerged discharge diffusers are precast reinforced concrete structures that are manufactured in the shop and installed by floating crane at a depth of approximately 15 ms from the seawater surface to discharge the warm water to the sea. The discharged warm water is well mixed with the ambient water in the vertical turbulent entrainment;

— Discharge velocity should be more than 3 m/s in order to enhance the mixing effects by initial dilution;

— The spacing between each diffuser can be determined to minimize the plume overlap under the given current, jet velocity, water depth, etc.

7.1.2.3. Advantages

— The environmental zone affected by high temperature discharged water could be minimized.

— The thermal recirculation to the existing units could be reduced, thus not impairing the power output of the existing units.

— By adopting the advanced type (submerged seawater intake/discharge system), the original coastline can be preserved, minimizing the effect of encroaching and sedimentation. The advanced type is therefore an environmentally friendly system that protects the existing coastline.

7.2. CATHODIC PROTECTION