Elmosa Seawater Intake System Design

In the light of the increasing pollutants reaching the shores of 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, sand and, silt, and debris 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.


Elmosa Offshore Seawater Intake Systems consist of:

The InvisiHead

The InvisiHead is a hydraulically invisible seawater intake velocity cap system fitted to the upstream pipeline inlet located offshore. 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. Can be steel, steel lined with polyethylene, GRP, concrete, or high density polyethylene (HDPE) pipe.

The NatSep Separation Basin

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.

The omni directional InvisiHead intake head system adds to the performance of the offshore system. It is fitted to the upstream end of the intake system pipeline located some distance into the sea from the shoreline.

Entrance velocity is lower than 0.091 m/s (0.3 fps). The approach velocity is extremely slow –

Super slow entrance velocities also lead to lower head losses and lower level draw downs at the pump intake basin. The InvisiHead entrance section is hydraulically fine tuned in lab tests. The entrance dimensions are not arbitrarily selected but hydraulically calculated in multi dimensional approach and through model setup and testing. Each dimension is a function of the steady flow velocity. Eddies through this approach are totally eliminated thus head loss at the InvisiHead is reduced to a negligible level – < 0.2 millimeters.

Should the dimensions become out of phase as the case in standard intake head inlets and with the streamlines leaving the approach stage and reaching the entrance (which is usually the case when one of the dimensions like the height or the width is arbitrarily chosen and the other is calculated by dividing the area by the chosen dimension), eddies will form and multiply causing turbulence and flow disturbance at the head entrance. This causes a higher pressure drop that will result in higher suction and thus higher head loss and level reduction at the intake basin, and higher pumping energy. That is why we tune the flow with the height and the circumference of the InvisiHead. Once the three are in phase, the InvisiHead is kept to minimum in size, the head loss is kept to minimum and the level drop of water is kept to the absolute minimum at the NatSep intake basin.

When that is done, less sediment and debris flows through, smaller pipelines will satisfy the flow capacity required, smaller and shallower NatSep basins will be needed. All these optimization factors translate into lower initial investment and lower operation and maintenance costs for the intake system and for the overall plant costs.