Offshore Intake System

Introduction




In the light of the increasing pollutants reaching the shores of the seas around the world the state of the ecosystem has changed a great deal over the last three decades. It has become unsafe to install onshore open intake systems without investing heavily in equipment to safeguard an interrupted flow. Going offshore and drawing the water from hundreds of meters away from the shoreline has become a sound alternative. Wave action effects, responsible for driving seaweed, sandand, silt, and edbris ashore, tend to subside in deep waters.

Locations relatively free of seaweed and less in fish concentrations and debris can be found in 10-15 meter (30-50 ft) depths. The wave turbulence is minimum in those depths and the water is usually very low in sand content and seaweed. Inferior designs of the upstream inlet of the pipelines can lead to creating high negative pressures at and around the inlet, which will result in the suction of sand, fish, seaweed and other debris.

The inlet has to be properly designed in a proactive and not in a reactive fashion. Wedge wire screens for example do not reflect a system that is specifically made for seawater intaking. They aboslutely represent a reactive solutions to a problem that was underway. They are operation and maintenance intensive. They have to be frequently backwahed and air purged to push away the impenging marine life and debris. The backwashed and purged material returns to the screens along with fresh supplies soon after the backwash pumps and air purgining compressors are turned off. The backwahed and purged material is not drained away but all of it or a large part of it would keep coming back and increase the frequency of backwash and air purge.

We have developed proactive technologies including:
The InvisiHead, a hydraulically invisible seawater intake velocity cap system fitted to the upstream pipeline inlet located offshore. The InvisiHead minimizes suction and drastically reduces the inflow of debris. It is hydraulically tuned in the lab. The entrance dimensions are not calculated as usually done in standard intake inlets and velocity caps but experimentally determined as the flow is tuned in.
The NatSep Separation Basin located onshore where natural separation of sand and debris takes place.


Open Water Systems


Many different configurations and designs of open seawater intake systems are used in the industry. Cooling water intakes can be divided into two types: Open channel and offshore pipe systems.



Open Channel Intakes




The open channel type of intake system used to be the most common, usually located on the shore of a water body. This kind of intake system is usually made of a conveying concrete channel. Simple and complex mechanical screening systems are always included in these types of intake systems. The size of the screening system depends on the quantity and type of the suspended matter present in the water. This type of system works with an acceptable degree of performance when small loads of debris are present. It becomes ineffective when the water gets saturated with large loads of debris and sediments such as weeds and sand. The open channel system is going under a phase out on a global scale in favor of the more advanced and effective offshore system.




Offshore Systems




The offshore type of intake system is common where shallow water and concentrations of weeds are present near the shoreline. The controlling factors of how far shourld the offshore distance be are:
the topography of the floor of the water body ,
the geotechnical critria of the intake site of both of its onshore and offshore segments,
the size of waves and depth of wave disturbances,
tide variations,
local environmental reguulations, and
weed concentration and movement patterns. Dead weeds - their concentrations and especially their patterns of movement - are the most difficult factor of all. The buoyancy of weeds varies according to ambient and atmospheric conditions. Dead weeds travel over the whole range of the water depth. Therefore, the prediction of their movement patterns is almost impossible, especially when the submarine current conditions vary with the water depth.

The problems involved with offshore systems are similar to those of the open channel, but with less severity and lower frequency. Traveling screens in settling basins onshore are still required and intake head cleaning has to be performed regularly. Large quantities of marine life including fish, fish larvae and small live organisms however are obliterated in the proces of the intake system operation and maintenance.

There has been a wide range of submerged intake designs and layouts, all aimed at reducing approach flow velocities to help control and reduce the inflow of debris but suffer from excessive level losses and reduced flow rates caused by seaweed, and Zebra Mussels which stick to wedgewire screen tees and sections in large numbers. Very few systems are designed to limit the impingement of seaweed or lake vegetation, sand, and other suspended matter inflow, which are the main concern among others of the Elmosa Intake System.



The Elmosa Offshore Intake System




Elmosa Offshore Seawater Intake systems consist of:
The InvisiHead (shown below) - the InvisiHead is positioned at the upstream end located between 50 to 10,000 meters offshore from the shoreline. The offshore distance depends on the site conditions and the requirements of the user. The depths of the of the upstream end where the InvisiHead is to be located range between 2 and 50 meters.
The intake pipeline - the pipeline connects the InvisiHead with the NatSep separation basin located onshore.




The NatSep separation basin - the NatSep is where whatever sand and debris that may flow into the InvisiHead get naturally separated and settle at the NatSep intake basin. The flow reaching the pumps is clean and free of sand and debris. Cleaning of the NatSep basin is done once or twice a year. Elmosa has discontinued the use of all screening systems including stationary and traveling screens. The NatSep patented design helped eliminate standard screening processes.







InvisiHead Intake Head System - the backbone of the process




After many years of research and development the InvisiHead system has been optimized and produced. The system is adjustable and virtually hydraulically invisible to suspended mater including fish, fish larvae, sea floor sand, seaweed, and debris. It is engineered to control and drastically reduce the inflow of debris, the ingress of fish, and the impingement of mussels and other marine life and entrained air. It is ideal to be installed in areas designated as fragile and critical such as spawning grounds or special habitats. By utilizing the continuity equation and the potential flow theory we developed this system. These two hydraulic principles have been used as the main tools for flow calculations and system design optimization. The system also makes use of the natural laws in its operation. Gravity is the only driving force of the flow into the InvisiHead Intake Head System.

The InvisiHead Intake Head SystemThe system's added costs are marginal, and it is usually less expensive and more economical than the conventional systems. It requires no backwash. Zebra Mussels have no effect on the operation of the Intake Head when they attach to it.

With minor alterations, the InvisiHead can also serve as an industrial effluent outlet by which the effluent flow can be dispersed in a way that it inhibits the fluid from rising to the surface and being washed back to the shore. It also

The clogging of seawater intakes generally occurs as suspended matter such as Zebra mussels, fish, shells, weeds and algae flow into the system. The clogging of these systems is becoming a major problem for installations along lakes and seacoasts. Many remedial solutions have been used like the construction on shore of intricate screening systems that require skilled personnel for operation and maintenance. Such skilled technicians are in short supply in many countries. To keep clean water flowing especially were seaweed occurs in large quantities, some highly automated systems have been used. These also require highly skilled staff to operate and maintain them.

To minimize or completely eliminate the need for automation and to reduce capital expenditures and operation costs, natural laws and forces have been positively utilized to design an adjustable water intake head system. The InvisiHead is virtually invisible to fish, seaweed and other suspended matter in terms of hydraulic effects.

The system is installed 1-30 meters (3-100 feet) below the water surface, away from waves and wave action. Before examining the details of the the InvisiHead, it is worthwhile to shed some light on traditional intake system techniques.

Many intake systems have either proven to be very costly to build, operate, and maintain; or have involved a great deal of guesswork that produced less than optimal results. Offshore intake systems can be upgraded and highly improved by a balanced and well-designed intake head system fully utilizes the potential flow principle in guiding water particles in smooth, uniform streamlines into the intake head. The InvisiHead Intake Head System is divided into four flow phases:

Approach
Stabilization
Acceleration
Steady Flow

It was a meticulous task to tune the InvisiHead and to divide the through flow regime into four sequentially mating phases. This process led to what the InvisiHead is. Super smooth flow transition from one phase to the next to lead the flow through into the pipeline upstream end made the head virtually invisible in hydraulic terms to suspended matter. There where the InvisiHead borrowed its name.







The approach phase:




Water particles start to move toward the Intake Head from all directions with a velocity of about 0.0027 m/s (0.009 fps) max. 5 meters (15 ft) away from the Head entrance. It rises to 0.03 m/s (0.1 fps) max. one meter (3 ft) away. The final entrance velocity is 0.09 m/s (0.3 fps) max. Natural currents, generated by thermal and density differentials, tend to have velocities much higher than these approach velocities. The pressure drop around the InvisiHead due to the suction caused by the head deferential at the intake settling basin located onshore is thus negligible, and does not disturb debris or cause it to travel toward the InvisiHead. This makes it virtually nonexistent as a sink point, and thus hydraulically invisible to suspended matter.




The stabilizing phase:




This phase expands from the point at the outer tip of the entrance to where the flow velocity becomes 2.6 times the entrance velocity. In this phase water particles move freely through the entrance ports of the InvisiHead. In this phase multi directionality of the flow overwhelms the radial flow and thus enhances flexibility and flow stability. Access for flow streams created by wind, thermal, and density differentials is permitted through the upstream part of the InvisiHead. This provides more security against the flow of debris into the InvisiHead, and smoothly divides and directs the water to the intake pipe inlet. Debris or fish that enter the InvisiHead flow out through the opposite end into the ambient again and will not flow down into the intake since the flow velocity at the InvisiHead outlet is much lower than that prevailing at this phase.




The acceleration phase:




This phase extends from the down-stream end of the guiding vanes to the inlet of the intake pipe. In this phase the flow proceeds smoothly toward the inlet and accelerates to reach steady state, that is the velocity of the flow prevailing inside the intake pipe. It is set a lab tested predetermined ratio of the InvisiHead entrance velocity providing smooth transition and eleminating any potential for eddies or vertices, thus reducing pressure losses and in turn allowing the use of smaller intake pipes. The depth needed for the settling basin onshore would also be reduced due to the reduction of the differential head required at the settling basin to drive the needed flow capacity.




The steady flow phase:




This phase begins at the downstream point of the InvisiHead and ends at the downstream end of the intake pipeline at the settling basin onshore. In this phase steady flow regime takes over.

The Intake Head dimensions are all based on the average steady flow velocity, thus an adjustable intake head designs and construction were made possible. In standard practice, a reasonable entrance velocity is usually chosen with a known flow rate. The area of the intake head opening is thus calculated using the continuity equation. Either the height or the diameter/width is assumed and then the other dimension is calculated. No
assumptions are made in the InvisiHead Intake Head system design and sizing. All values are calculated since they are all interdependent.

The InvisiHead is designed to take either individual or multiple forms. This is always decided on individual basis. The InvisiHead can be retrofitted to existing intake pipelines.



Outfalls for Power and Desalination Plants, Oil Refineries and Wastewater Disposal
The InvisiHead system works as an excellent offshore submarine outfall. It diffuses the effluent in such a manner that it is not allowed to rise up to the surface and be waved back - no recirculation. The dispersed effluent mixes with the ambient deep water and in a short time



assumes physical and chemical properties similar to those of the surrounding seawater, allowing the effluent to stay below the surface without negative impacts on plant operationon or on the local environment. The InvisiHead works as an efficient heat transfer system when used as a power plant cooling water outfall. It ensures adequate mixing and efficient heat dispersion. The heat plume does not reach the surface and it is confined to the area close to the point source - the crown of the outfall. Heat, TDS, or sewage sensors installed a few meters away from the outfall diffuser will register no change in their readings. Length of outfall is no longer a necessity.



Conclusion


A hydraulically balanced simple system, free of any operating or maintenance needs, makes the Elmosa water and seawater intake systems highly feasible and attractive. They function by positively utilizing the natural forces. In doing so, sediment flow is drastically reduced and pressure losses are minimized. The system features include:
Size reduction of pipeline pipe diameters and settling basins;
Initial costs reduction;
Operation and maintenance are also minimized since backwash of the intake pipes and head is eliminated;
Settling basins are also cleaned less frequently due to the reduction of sediment inflow;
No screens are attached to the InvisiHead thus Zebra Mussels have no negative impact on the intake system performance;
Low approach velocity 0.0027 m/s max.;
Low entrance velocity 0.091 m/s max.;
Component form for assembly at site;
Variable flow phases to:
promote head loss reduction that results in:
smaller pipelines,
shallower intake basins,
less debris inflow,
the elimination of backwash.
further reduce debris inflow,
perform self cleansing.
Metallic or nonmetallic construction;
No screens are used at any stage of the IH or the NatSep;
Adaptability for use in effluent outlets and plant outfalls;
Can be retrofitted to existing systems;
Immune to oil spills;
Low initial cost;
Lower O&M costs.

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