Marine and Offshore Desalination Systems: An Engineering Guide for Shipowners, Platforms, and Yachts

Why Freshwater Supply Is Critical Offshore?

Fresh water is essential for survival at sea and crucial for the normal operation of maritime activities. For merchant ships, engineering vessels, and offshore engineering vessels, fresh water is directly related to crew health and the proper functioning of equipment. When freshwater becomes scarce, ships have to change course to find supplies. This results in lost time and increased operating costs.

Not to mention offshore platforms, whether they are drilling platforms, FPSOs, or offshore wind power platforms, all of which are far from land and stationed at sea for extended periods. A water outage means production stoppage, resulting in substantial losses. 

For yachts and small boats, onboard space is precious, and water storage is limited. Short voyage times due to water storage necessitate frequent shore stops for replenishment, directly reducing the user’s quality of life.

In a marine environment, the challenges of freshwater supply go far beyond simply “having water.” Salt spray corrosion, hull vibration, and rolling and tilting are common occurrences, resulting in a poor operating environment. Furthermore, there are maintenance difficulties and energy-sensitive issues. These combined problems make the marine freshwater supply a complex systems engineering challenge.

For this reason, more and more ship owners, offshore platform operators, and yacht users are choosing to equip their ships or platforms with seawater desalination systems to obtain fresh water directly from seawater. This fundamentally solves the problems of stability, independence, and economy in marine water use.

What Is a Marine Desalination System?

Marine desalination systems utilize reverse osmosis technology to convert seawater into freshwater. A high-pressure pump forces seawater into the reverse osmosis membrane module. Water molecules pass through the membrane, while salt, impurities, and microorganisms are trapped and discharged back into the sea with the concentrated seawater.

A complete system includes a pretreatment unit, a high-pressure pump, reverse osmosis membrane modules, and a control valve assembly. The entire unit features a modular design, occupying minimal space, making it suitable for installation in ship engine rooms, platform mezzanines, or unused spaces on yachts.

The core value of this system lies in its ability to continuously produce freshwater that meets drinking water standards, as long as there is seawater and electricity. It eliminates reliance on potable water replenishment and the need for bulky freshwater tanks. The produced freshwater can also be used directly for various purposes, such as equipment cooling and boiler feed.

Key Design Challenges in Marine Environments

The marine environment presents challenges to seawater desalination systems, including corrosion, vibration, and space constraints, making the design of offshore equipment far more complex than that for land-based applications. Knowing these challenges is a prerequisite for making the right selection.

Corrosion

Seawater, with its high salt content, combined with salt spray, humidity, and temperature fluctuations in the air, causes a much faster corrosion rate for metallic materials than on land. Ordinary stainless steel may develop pitting corrosion within months in the marine environment, while carbon steel is even less likely to survive. For SWRO systems, corrosion significantly impacts equipment lifespan.

Vibration

Ships and platforms constantly experience vibration during operation. Main engine operation, propeller rotation, and wave impact all generate this continuous mechanical vibration. Ordinary equipment is not designed for offshore conditions. Prolonged vibration can lead to problems such as loosening of pipe joints or instrument malfunctions, and most importantly, damage to the RO membrane.

Space

Extremely limited space is a common problem for offshore equipment. Every inch of space on board and on platforms is used, and pipework, cables, and other equipment already occupy most of the space. Traditional large-scale water treatment equipment simply cannot fit inside.

System Selection by Application Scenario

Shipboard RO systems

Shipboard RO systems are suitable for merchant ships such as cargo ships, container ships, and oil tankers, as well as engineering vessels and offshore vessels such as tugboats and cable-laying vessels. The stability, footprint, and maintenance complexity of the system need to be determined for marine applications.

In terms of stability, ships cannot obtain external technical support while at sea, so the system must possess high reliability. Key components such as high-pressure pumps, control valves, and membrane modules require redundant designs or models with long-term seaworthiness verification. The electronic control system should have self-diagnostic capabilities, automatically adjusting operating parameters in case of anomalies.

In terms of space, ship engine rooms are already very cramped. The system must adopt a compact design, keeping the overall footprint within a reasonable range. A modular structure allows the pretreatment unit, high-pressure pump set, and RO membrane modules to be installed separately in different locations within the engine room. While making full use of the available space in the engine room, ease of maintenance must also be considered.

Crew members generally do not have professional knowledge of water treatment. The system should be designed for easy disassembly and assembly, with simple and straightforward replacement of filter cartridges and membrane elements. The pretreatment unit needs an automatic flushing function to reduce the frequency of manual intervention.

A common pitfall is that when selecting equipment, one cannot only consider the price. The material and structural requirements of offshore conditions must also be taken into account.

Offshore platform desalination systems

Offshore platform water treatment systems are suitable for drilling platforms, FPSOs, offshore wind power platforms, and other fixed or floating marine engineering facilities. Offshore platforms prioritize continuous freshwater supply, remote maintenance, and resistance to corrosion and shock.

Offshore platforms cease operation immediately upon water outages. Therefore, the system requires a high-availability design, with critical components such as high-pressure pumps considered for a one-for-one backup configuration. Vulnerable parts should be standardized and universalized to reduce the variety of spare parts in stock. 

The water production capacity should be designed according to the platform’s maximum water demand, with a margin of safety. The control system needs to support unattended automatic operation and remote monitoring capabilities.

Regarding corrosion and shock resistance, platforms are constantly exposed to the marine environment. All components in contact with seawater or humid air must use marine-grade corrosion-resistant materials. The installation and securing of the equipment must also take into account the impact of waves and storms.

A common problem is insufficient pretreatment capacity. The water quality around the platform fluctuates greatly, and the content of suspended solids and microorganisms may be much higher than expected. Another easily overlooked point is the platform’s power limitations; systems with excessively high energy consumption should be avoided.

Yacht water makers

Freshwater generations are suitable for small to medium-sized yachts, sailboats, catamarans, and other recreational boats. The core requirements are small size, low noise, and simple operation.

Space on yachts is more limited than on merchant ships and platforms. Water purifiers typically require an integrated design, combining all water purification units into a small frame. For small yachts, a movable or foldable design should also be considered.

Yachts require a high level of living comfort, and the noise from the equipment must be kept at a low level. The high-pressure pump should be a low-noise model, and the whole machine needs to be equipped with vibration damping and sound insulation. 

The control system for yacht water production should ideally be straightforward, and the membrane elements should be easy to replace. System status, water quality, and fault indications should be clearly displayed and easy to understand.

A common misconception is that people only focus on water production and ignore water quality. Freshwater on yachts is generally used directly for drinking and bathing, requiring the water to meet drinking water standards.

System Configuration and Key Components       

Pretreatment

Impurities in seawater that directly enter the reverse osmosis membrane will rapidly clog the membrane element, leading to a decrease in water production and even membrane failure.

For offshore platforms that require continuous operation and cannot afford frequent shutdowns, improved pretreatment can significantly reduce membrane fouling and extend membrane lifespan. For ships and yachts, whose operating time is relatively limited, pretreatment requirements can be appropriately reduced.

High-Pressure Pump

Selecting the right high-pressure pump is crucial for marine environments. Besides considering the corrosion resistance of the materials, flow rate and inlet pressure are also essential. The import pressure should ideally be kept between 2 and 4 bar. 

The operating pressure required for seawater reverse osmosis is typically 54-80 bar, and some commercial marine high-pressure pumps can achieve outlet pressures up to 85 bar, providing a safety margin for system operation.

Reverse osmosis membranes

Modern marine reverse osmosis membranes mostly adopt a spiral wound structure, which provides a large membrane area and meets the constraints of limited space at sea.

The desalination rate of these membranes is typically above 99.5%, reducing seawater salinity from 36,000 mg/L to below 200 mg/L, producing water of superior quality to ordinary tap water. Membrane lifespan depends on operating conditions and maintenance quality. Generally, membrane elements are designed for a lifespan of 3-5 years.

Capacity and System Selection

When selecting an offshore seawater desalination system, the main considerations are freshwater demand and actual operating conditions. Capacity calculation should calculate the total daily freshwater consumption at sea, then calculate the average, and add a 20-30% safety margin. A modular system that can operate automatically, rinse automatically, and monitor automatically.

Yachts prioritize low-noise, compact systems. Vessels should be equipped with systems capable of continuous operation and easy maintenance. Platforms must be configured with systems featuring energy recovery devices and supporting remote operation and maintenance. 

The following are typical flow ranges for different application scenarios:

Energy Consumption and Efficiency

Energy consumption is a key economic indicator for selecting offshore desalination systems. Small vessels have limited power, while large platforms have requirements regarding operating costs.

For example, the typical energy consumption of a small marine SWRO system (without an energy recovery device) is 7-12 kWh/m³, while a system equipped with ERD can reduce it to 3-5.5 kWh/m³. If frequency converter control is added, the energy consumption will be further reduced to 2-2.5 kWh/m³. The following is a summary comparison table:

Maintenance and Reliability Considerations

Maintaining a marine system is far more difficult than maintaining one on land, so high reliability and low maintenance requirements must be the core principles for marine desalination systems. In terms of system maintenance, the pretreatment filter cartridges require the most frequent maintenance. Washable and regenerable filter cartridges can be considered. It is also recommended for anti-fouling marine desalination.

Best water makers for yachts usually have a simple structure, allowing crew members to perform daily operations, including filter replacement and membrane cleaning. These systems are typically designed for modular replacement, avoiding the need for offshore maintenance. 

Marine systems require stronger continuous operation capabilities and are generally equipped with dual-pump or multi-stage designs to ensure continued water production even in the event of a single point of failure. 

Systems used on offshore platforms must support remote monitoring and fault diagnosis to reduce the need for on-site intervention by more than 80%.

Choosing the Right System for Your Application

Whether a seawater desalination system can be used stably in a marine environment depends on whether the correct configuration has been selected according to the application scenario.

Yachts, with limited space and precious electricity, are well-suited for portable desalination devices. The portable series is small in size, plug and play, comes with an ERD, and can also be started with one button and automatically rinsed. A seawater desalination system for vessels needs continuous operation. It is recommended to install a compact skid-mounted system. 

Medium to large skid-mounted equipment can be selected for offshore platforms or large cargo ships. These systems support remote monitoring, come standard with an ERD, are explosion-proof and corrosion-resistant, and can withstand the most demanding marine environments.

NEWater can provide customized system design and selection advice tailored to your specific needs. Contact us for detailed technical solutions and quotations！

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Portable Desalination Unit

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