Ship reverse osmosis seawater desalination system

Case Study: Reverse Osmosis Seawater Desalination System Installation on "Haigong 101"

I. Project Overview

"Haigong 101" is an offshore engineering vessel primarily used for offshore platform resupply, near-shore engineering construction, and emergency rescue operations, operating long-term in both offshore and coastal areas. The vessel's original freshwater supply relied on shore-based resupply and ship-transported freshwater, resulting in long resupply cycles, high costs, and unstable supply. Especially during continuous operations in the open ocean, freshwater shortages severely constrained operational time and personnel living conditions. To solve the freshwater self-sufficiency problem, the project team decided to install a reverse osmosis seawater desalination system to achieve independent water production at sea, ensuring the needs for operational and domestic water use.

II. System Selection and Design

1. Technology Selection

Considering the characteristics of offshore engineering scenarios, such as high salinity, high humidity, frequent winds and waves, compact space, and limited power supply, marine-grade reverse osmosis (RO) seawater desalination technology was selected. This technology is a purely physical desalination process, boasting advantages such as small footprint, low energy consumption, rapid start-up, high automation, and stable water quality. It can adapt to the turbulent environment of the ocean, and the produced water directly meets drinking water standards.

2. Core Parameter Design

- Production Capacity: 200 m³/d, meeting the daily drinking, sanitation, and small-scale operational water needs of 80-100 people on board.

- Process Route: Seawater intake → Pretreatment (multi-media filtration + precision filtration + security filtration) → First-stage reverse osmosis → Post-treatment (pH adjustment, mineralization, disinfection) → Freshwater storage.

- Core Configuration: Employs imported high-desalination-rate spiral wound RO membranes, high-pressure pumps with energy recovery devices, and a PLC intelligent control system with automatic start/stop, fault alarm, and cleaning protection functions.

- Material Requirements: Piping and tanks are made of 316L stainless steel and corrosion-resistant composite materials, suitable for the high-salt-spray corrosive environment of the ocean.

III. System Installation and Construction Challenges

1. **Space Layout Optimization:** Due to the limited space on ship decks and in cabins, a modular integrated design was adopted. The five major modules—pretreatment, reverse osmosis, control, chemical dosing, and cleaning—are integrated into two skid-mounted units, compactly arranged in a dedicated cabin on the stern deck, reducing floor space and facilitating installation and maintenance.

2. **Vibration Resistance and Stable Installation:** To address the challenges of turbulent navigation and swaying operations, the equipment base is reinforced with high-strength shock-absorbing supports and the ship's bottom structure. Flexible joints and anti-sway supports are added to the piping to ensure stable operation under roll ±15° and pitch ±10° conditions.

3. **Seawater Intake and Pretreatment:** A low-level seawater intake is used, equipped with a trash rack and rotating filter to intercept debris and algae in the seawater. Pretreatment involves the addition of flocculants, scale inhibitors, and bactericides to remove suspended solids and colloids, preventing membrane fouling and scaling.

4. Electrical and Automation Adaptation
The system is connected to the marine low-voltage power grid, equipped with a dedicated regulated power supply and emergency circuit; the PLC control system enables one-button start/stop, automatic water production, online water quality monitoring, automatic cleaning, and unattended operation.

IV. System Operation and Results

1. Process Flow

1. Seawater enters the pretreatment system via the intake pump, passing through a multi-media filter and a 5μm security filter to remove impurities.

2. The pretreated seawater is pressurized to 5.5-7.0 MPa by a high-pressure pump and enters the RO membrane module. Water molecules permeate through the membrane to form freshwater, while salt and impurities are retained as concentrate and discharged.

3. The permeate water undergoes pH adjustment, mineralization, and ultraviolet disinfection before being sent to the freshwater tank for storage.

4. The concentrate is processed by an energy recovery device to recover residual energy, reducing system energy consumption.

2. Operational Indicators

- Desalination Rate: ≥98.5%

- Product Water Conductivity: ≤20μs/cm, meeting the "Standards for Drinking Water Quality"

- Electricity Consumption per Ton of Water: ≤3.5kW·h/m³ (with energy recovery)

- System Recovery Rate: ≥40%

- Continuous Operation: Stable water production for 24 hours, single maintenance cycle ≥30 days

3. Application Benefits

- Self-sufficiency: Completely eliminates dependence on freshwater replenishment, extending offshore operation time from 7 days to over 30 days.

- Cost Reduction: Cost per ton of water is approximately 9 yuan, far lower than that of freshwater transported by ship (25-35 yuan/m³), resulting in annual savings exceeding 1.5 million yuan.

- Flexible and Efficient: Produces qualified freshwater within 30 minutes of startup, producing freshwater on demand, adaptable to multiple sea areas and varying operating conditions.

- Environmentally Friendly and Reliable: Physical process with no chemical pollution, concentrated wastewater meets discharge standards, low system failure rate, and easy maintenance.

V. Project Summary and Value

The successful installation and application of the HaiGong 101 reverse osmosis seawater desalination system provides a mature freshwater self-sufficiency solution for marine engineering vessels. Through modular design, marine-grade adaptation, intelligent automatic control, and energy-saving optimization, the project addressed core pain points in marine engineering scenarios, including space constraints, operating conditions, energy consumption, and water quality.

This case study validates the applicability and economic viability of reverse osmosis technology in the marine engineering field, providing a replicable model for the freshwater supply modification of similar vessels, drilling platforms, and offshore facilities. It effectively enhances the long-range continuous operation capability and support level of my country's marine engineering equipment.