Offshore Platform Jacket Water Cooling System Retrofit

Scenario

Retrofit of thermal management for a 25-year-old FPSO (Floating Production Storage and Offloading) unit operating in the North Sea. The existing seawater-cooled heat exchanger showed declining performance due to biofouling and aging piping. Space, weight, and downtime were severely constrained: only 72 hours of vessel shutdown permitted, and no structural modifications to the seawater intake manifold were allowed. Regulatory compliance required maintaining jacket water outlet temperature ≤45°C under full engine load.

Given Data

• Mass flow rate of jacket water: 6.8 kg/s
• Specific heat capacity of jacket water: 4182 J/(kg·K)
• Temperature difference of jacket water: 9.3 K (inlet 82°C → outlet 72.7°C)
• Density of seawater: 1027 kg/m³
• Specific heat capacity of seawater: 3915 J/(kg·K)
• Temperature difference of seawater: 4.6 K (inlet 8.2°C → outlet 12.8°C)

Calculation

The tool computes seawater flow rate using energy balance:

1. Heat duty removed from jacket water: $$ Q = \dot{m}{jw} \cdot c{p,jw} \cdot \Delta T_{jw} $$ $$ Q = 6.8 , \text{kg/s} \times 4182 , \text{J/(kg·K)} \times 9.3 , \text{K} = 264,580 , \text{W} $$

2. Required seawater mass flow rate: $$ \dot{m}{sw} = \frac{Q}{c{p,sw} \cdot \Delta T_{sw}} = \frac{264,580}{3915 \times 4.6} = 14.67 , \text{kg/s} $$

3. Convert to volumetric flow rate using seawater density: $$ \dot{V}{sw} = \frac{\dot{m}{sw}}{\rho_{sw}} = \frac{14.67}{1027} = 0.01428 , \text{m}^3/\text{s} $$

Result and Decision

The calculator returned 0.01428 m³/s (≈51.4 m³/h). Based on this, engineers selected a compact titanium-tube plate-and-frame heat exchanger with integrated flow control valves and specified a 63 mm nominal bore seawater service pump (rated 0.016 m³/s at 32 m head) to accommodate future fouling margin and transient loads. Installation was completed within the 72-hr window using pre-fabricated skids.

Lesson

Always apply a 10–15% fouling margin to calculated seawater flow rates in retrofit projects — especially in biologically active waters like the North Sea — to avoid premature derating and unplanned shutdowns.