Engineering hub for open-sea seawater intakes — the submerged, velocity-capped offshore headworks, risers, intake pipelines/tunnels and onshore pumping stations that feed desalination and power-station cooling plants. This page links our full intake knowledge base and sets out the science-based method we use to design biofouling-control chlorination: the Chlorine Demand & Decay (CDD) study.
Chlorination and antifouling strategy for the intake head, screens and pipeline.
Velocity caps, coarse bar screens and travelling band/drum screens.
Material selection for permanent immersion in chloride-rich seawater.
Low through-screen velocity and fish-protection design.
An open-sea intake draws raw seawater directly from the open water column through a submerged offshore structure, in contrast to a subsurface intake (beach well, infiltration gallery) that filters through the seabed. Open-sea intakes are the practical choice at large capacity and where the geology rules out subsurface options — most large seawater reverse-osmosis (SWRO) desalination plants and once-through power-station cooling systems use them. The trade-off is that the raw water carries the full marine load — suspended solids, plankton, larvae and the organisms that cause biofouling — so the intake must be screened, the wetted materials must survive permanent seawater immersion, and the system must be protected against fouling from the moment water enters the offshore head. The pages below cover each part of that scope, and the section that follows sets out how we size the biofouling-control dosing scientifically.
From the offshore headworks to the plant inlet — and the engineering discipline behind each stage.
Submerged intake head with a velocity cap to draw water horizontally and minimise organism entrainment, feeding a riser and an intake pipeline or tunnel to shore.
Coarse bar screens at the head and travelling band or drum screens onshore, sized for a low through-screen approach velocity.
Onshore pump station, sumps and rising mains designed for the intake hydraulics and seawater duty.
Super-duplex, GRP, HDPE and coated concrete selected for permanent immersion in chloride-rich seawater.
Fish-protection design to a low intake velocity, aligned with abstraction and environmental requirements.
Abstraction limits, discharge residual limits and marine-habitat considerations that shape the design.
A site-specific, science-based method for sizing intake chlorination — not a rule of thumb.
Open-sea intake water carries the larvae and adult stages of mussels, barnacles, hydroids and bryozoans. Left unchecked, they settle and grow on the intake head, screens, pipeline walls and downstream equipment, steadily restricting flow and raising head loss until the intake must be taken offline and cleaned. The established, lowest-risk control is oxidant dosing — chlorine, supplied as sodium hypochlorite or generated on site by electrochlorination. The correct dose is not assumed; it is derived from a Chlorine Demand & Decay (CDD) study that measures, for the actual site water, how much oxidant the seawater consumes and how quickly the residual decays — so the design holds an effective residual exactly where fouling organisms are exposed while staying within the discharge consent.
Dosed free chlorine (HOCl / OCl−) reacts almost instantly with the high natural bromide of seawater (~65 mg/L) to form hypobromous acid and hypobromite (HOBr / OBr−). It is this bromine residual — together with bromamines formed with any ammonia present — that does the biocidal work, and that field instruments report as total residual oxidant (TRO), expressed as Cl2. The demand is the oxidant immediately consumed by ammonia, organic matter (measured as TOC / UV254), sulphide and other reducing species; the decay is the subsequent, broadly first-order loss of residual with time. Both rise with temperature and organic load, so demand and decay are stronger in warm, biologically productive months and during harmful-algal-bloom events.
Collect seawater at the true intake depth across the seasonal range. Record temperature, salinity, TOC / UV254, ammonia, bromide, turbidity and algal indicators — these are the variables that govern demand and decay.
Dose a graded series of hypochlorite into fresh aliquots; after a fixed contact time, measure TRO (DPD or amperometric, as Cl2). Plotting dose against residual gives the chlorine demand and the dose needed to leave a chosen target residual.
Hold a representative dose and measure the residual at intervals (typically 0, 5, 15, 30, 60 and 120 minutes). Fit a first-order — or two-stage — decay coefficient k(T), capturing the fast initial demand followed by slower bulk decay.
Compute the hydraulic residence time from the offshore head to each critical point — riser, pipeline, screens, pump sump, plant inlet — from pipe length, diameter and design flow. Combined with k(T), this back-calculates the injection dose that keeps the residual above the fouling-control threshold along the whole intake.
Choose continuous low-level chlorination for steady suppression, or intermittent shock dosing to defeat the valve-closure ("Neptune") response of adult mussels. Inject at the offshore head so protection begins at the mouth of the intake.
Confirm the residual reaching the outfall meets the environmental consent, and assess halogenated by-products (bromoform / THMs, and bromate where electrochlorination is used). Size dechlorination — sodium bisulphite / metabisulphite — where a residual cap requires it.
Translate the result into the design dose, on-site hypochlorite or electrochlorination capacity, dosing-pump duty, injection-point detail, residual set-points and a flow-paced control loop with online TRO / ORP analysers — plus a seasonal re-validation schedule.
| CDD design output | Typical basis |
|---|---|
| Target biocidal residual (TRO, as Cl2) | ~0.2–0.5 mg/L maintained at the critical point |
| Chlorine demand | Site-specific; rises with TOC, ammonia and temperature |
| Decay coefficient k | First-order, fitted per season from the decay test |
| Injection point | Offshore intake head / diffuser (protection from the mouth) |
| Dosing regime | Continuous low-level, or intermittent shock for adult mussels |
| Discharge limit | Per environmental consent (low TRO); dechlorinate if required |
| Control | Flow-paced dose with online TRO / ORP feedback |
Intake Biofouling Control Chlorine CT Calculator Dosing Calculator
Understanding the settlement sequence is what makes a dosing strategy work.
Marine biofouling is not a single event but a sequence that begins the moment clean steel, concrete or polymer meets seawater. Within minutes a microscopic conditioning film of dissolved organic molecules adsorbs onto the surface; within hours pioneer bacteria attach and secrete a protective slime of extracellular polymers; over the following days diatoms, protozoa and fungi thicken that biofilm. Critically, this living film is itself the chemical cue that invites the larvae of the hard macrofoulers — barnacle cyprids, mussel and oyster veligers, tubeworms, hydroids and bryozoans — to settle, cement themselves and calcify. Once macrofouling establishes, it narrows the pipe bore, roughens the walls and raises friction head loss, shelters microbially-influenced corrosion (MIC) beneath the deposit, and periodically sheds debris that blocks screens and damages pumps. Because each stage has a different vulnerability, the most reliable intakes defend in layers rather than trusting a single barrier — and the oxidant dose from the CDD study above is most effective when it interrupts the sequence early, at the biofilm and larval stages, before adult shells form.
Dissolved organic molecules — proteins, polysaccharides, humic matter — adsorb onto the bare surface, changing its wettability and charge and preparing it for biological colonisation.
Pioneer bacteria attach and multiply, secreting an extracellular-polymer slime (EPS) that anchors the community and shields it from shear and, partly, from biocide.
Diatoms, protozoa and fungi colonise the slime to form a visible micro-mat. Its chemistry emits the settlement cues that the larvae of macrofoulers actively seek.
Barnacle cyprids and bivalve veligers detect the biofilm, settle preferentially in low-velocity sheltered spots, and cement themselves to the surface — the point of no easy return.
Settled larvae grow calcified shells and tubes. The colony restricts flow, raises head loss, drives MIC and breaks loose in fragments that foul screens and pumps downstream.
A low chlorine/bromine residual — sized by the CDD study — holds the biofilm and larval stages in check throughout the intake, the primary barrier for most open-sea schemes.
Short high-dose bursts defeat the valve-closure ("Neptune") response of adult mussels, achieving kill at a fraction of the total chemical and by-product of continuous dosing.
90/10 and 70/30 copper-nickel screens and pipework leach trace copper ions that inherently resist settlement, while super-duplex and titanium resist the chloride corrosion. See materials.
Foul-release and biocidal coatings, and the inherently smooth bore of GRP and HDPE, reduce both settlement and the friction penalty as any film develops.
Chemical-light or oxidant-free options for environmentally sensitive discharges and polishing duty, where a low total-residual-oxidant consent constrains chlorination.
Periodic heat treatment, pigging and screen washing physically clear established growth — the fallback that recovers capacity when chemical control is interrupted.
| Fouling organism / stage | Stage to target | Most effective control |
|---|---|---|
| Bacterial biofilm & EPS slime | Conditioning / slime | Continuous low oxidant residual |
| Diatom / algal micro-mat | Microfouling | Continuous oxidant + UV polishing |
| Barnacles (cyprid larvae) | Larval settlement | Continuous residual through settlement season |
| Mussels (Mytilus, Perna) | Larvae & adults | Continuous residual + intermittent shock dosing |
| Hydroids & bryozoans | Larval | Continuous residual; antifouling alloys |
| Tubeworms (Hydroides) | Larval / juvenile | Continuous residual; Cu-Ni surfaces |
Biofouling Control Methods Antifouling Materials Intake Dosing
Proving the intake hydraulics in software before anything is built.
An open-sea intake has to draw water at a low, uniform approach velocity — so it neither impinges fish on the screens nor entrains larvae — and then deliver swirl-free, vortex-free flow to the pumps onshore. Those are three-dimensional flow problems that drawings and hand calculations cannot fully resolve, and physical scale models are slow and expensive. Computational Fluid Dynamics (CFD) simulates the full flow field — through the velocity-capped head, along the intake pipeline or tunnel, across the band/drum screens and into the pump sump — so the geometry can be optimised, and acceptance criteria demonstrated, before construction.
Confirms a uniform, low through-screen approach velocity (a common fish-protection target is ≤0.15 m/s) with no hot-spots, and even flow distribution across the screen face.
Detects free-surface and submerged vortices, excessive swirl and uneven approach flow that cause cavitation, vibration and lost pump efficiency — assessed against ANSI/HI 9.8.
Models the interaction between the intake and the brine or thermal outfall, quantifying how much discharged water recirculates back to the intake — and sediment transport / scour around the head.
State precisely what is being assessed — sump vortices, screen velocity, recirculation — then build the 3D geometry of the head, pipeline and sump from the design drawings and set a domain large enough that the boundaries do not influence the result.
Generate a mesh refined at walls, the screen face (often modelled as a porous baffle) and free surfaces, then confirm the answer no longer changes with further refinement — the mesh-independence check that makes the result trustworthy.
Apply the design and extreme abstraction flows, tide and current, water levels and screen porosity — and temperature/salinity where a plume is modelled. Choose steady-state or transient depending on whether vortex behaviour over time matters.
Select the model to suit the question: RANS (k-ω SST) for general hydraulics, Volume-of-Fluid for free-surface vortices, species/energy transport for plumes, and scale-resolving (LES/DES) where transient vortex detail governs the design.
Run the solver while monitoring residuals and the engineering quantities that matter — swirl angle, velocity uniformity, recirculation fraction — until they settle to stable values.
Check the result against the ANSI/HI 9.8 acceptance criteria, against any physical-model or field data, and through sensitivity to mesh density and turbulence model, so the conclusion is defensible.
Iterate the geometry — flow splitters, anti-vortex devices, floor cones, bell-mouth and screen layout — until the criteria are met, then deliver the velocity fields, vortex assessment, head loss and recirculation results with a clear pass/fail against each requirement.
| CFD design output | Typical acceptance basis |
|---|---|
| Through-screen approach velocity & uniformity | Low and even (e.g. ≤0.15 m/s) for fish protection |
| Pump-sump swirl angle | Within ANSI/HI 9.8 limits (typically ≤5° time-averaged) |
| Free-surface / submerged vortices | No coherent air-core or strong sub-surface vortices |
| Intake–outfall recirculation | Quantified and minimised for the design conditions |
| Sediment transport / scour | Assessed around the offshore head |
| Flow-conditioning devices | Anti-vortex baffles, splitters, cones as required |
Where the chemistry of the CDD study meets the flow physics of CFD — the most reliable way to reduce biofouling.
A CDD study sets the dose that holds a biocidal residual for a given contact time — but it implicitly assumes the oxidant is perfectly mixed and present everywhere. A real intake is not a perfect reactor. Oxidant is injected at one or a few points into a large-diameter pipeline, then travels through bends, manifolds, screen chambers and a pump sump, forming a fast-moving core alongside slow recirculation pockets and dead legs. Those low-velocity pockets are exactly where the residual is weakest — and exactly where mussel and barnacle larvae prefer to settle. CFD species-transport modelling closes this gap: it carries the dosed oxidant as a reacting scalar through the true geometry, applies the measured CDD decay kinetics as a consumption term, and produces a map of residual concentration on every wetted surface. It converts "the average dose is 1 mg/L" into the answer that actually matters — "every surface sees at least the target residual, and not a milligram of chemical is wasted."
Resolves jet penetration, the number and placement of injection ports and the diffuser geometry — and the mixing length needed before the residual is uniform across the full pipe cross-section.
Renders the three-dimensional oxidant field and flags any wetted surface sitting below the biocidal threshold — the precise settlement-risk zones a coverage calculation alone would miss.
Applies the site-specific first-order decay coefficient k(T) from the CDD study as a volumetric sink, so the model predicts the genuine residual that organisms experience — not mere hydraulic dilution.
Tracks a pulse of oxidant through the system in time, confirming that the whole intake — including the slow pockets — experiences the lethal concentration-time window that defeats adult mussels.
Start from the converged intake-hydraulics CFD — the velocities, turbulence and free-surface solution — so the dosing study runs on flow physics that has already been validated.
Introduce the dosed chlorine/bromine as a transported scalar at the real injection location, modelling the actual hardware — a single quill or a multi-port diffuser — and its jet momentum.
Apply the measured first-order decay k(T) as a volumetric reaction term, so the oxidant is consumed as it travels exactly as it is in the real site water — the link that makes the prediction quantitative.
Resolve advection, turbulent diffusion and decay to a converged residual field — steady-state for continuous dosing, or fully transient for a shock/pulse regime.
Render the residual on every wetted surface and along the flow path, flagging recirculation pockets, dead legs and short-circuits that fall below the target total residual oxidant.
Reposition or add ports, redesign the diffuser, or retune dose and pulse timing — then re-run until the biocidal residual covers the whole intake at the lowest effective dose.
Deliver the confirmed injection layout, minimum effective dose, mixing length, shock-dose hold time and the discharge-residual check — feeding directly into the dosing-control set-points.
| CFD-dosing output | What it delivers |
|---|---|
| Mixing length / time to uniformity | Distance & time to ~90% uniform residual downstream of injection |
| Residual coverage | % of wetted surface at or above target TRO (goal: 100%) |
| Dead-zone & short-circuit map | Low-residual pockets identified, then designed out |
| Injection configuration | Quill vs multi-port diffuser; port count and placement |
| Shock-dose propagation | Concentration-time window achieved across the whole system |
| Minimum effective dose | Reduced average dose — lower chemical cost and by-products |
| Discharge residual | Overdose checked against the environmental consent |
The result is a dosing design that reduces biofouling more reliably and uses less chemical: by proving where the residual goes, CFD lets the dose be trimmed to the minimum that still covers every surface — cutting hypochlorite consumption, halogenated by-products and discharge load at the same time.
CFD Dosing & Mixing CFD Mixing Studies Dosing System Designer Dosing Calculator
The first decision in any seawater scheme.
Direct abstraction through a submerged offshore head. Scales to the largest plants and works on any coastline, but draws the full marine load, so it needs screening, biofouling control (the CDD study above) and corrosion-grade materials. The standard choice for large SWRO and once-through cooling.
Water is drawn through the seabed (beach wells, infiltration galleries), which pre-filters it and largely removes solids, algae and larvae — cutting pre-treatment and biofouling load. Limited by capacity and favourable geology. Explore subsurface intakes →
How each intake configuration generates its fouling community and demands a matched screen and dosing strategy.
Read MoreEngineering guide to biofouling vulnerability, cleaning and dosing for every intake screen type.
Read MoreThe full intake engineering picture.
Read MoreVelocity-cap headworks, risers and pipelines.
Read MoreChlorination and antifouling strategy.
Read MoreOxidant and pre-treatment chemical dosing at the intake.
Read MoreOnline instrumentation — TRO, turbidity, SDI and more.
Read MoreProtecting the intake during bloom events.
Read MoreMaterials for permanent seawater immersion.
Read MoreWhere open-sea intakes feed SWRO.
Read MoreFrom the offshore headworks to a CDD-based biofouling-control dosing design, Reynolds & Bauhm delivers the intake scope as part of an integrated, single-responsibility engagement.
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