How do you assess the stability of FPSOs in harsh weather conditions?

1 Basic concepts of stability

Stability is the ability of a vessel to return to its original position after being tilted by an external force, such as wind or wave. Stability depends on the shape, size, weight, and distribution of the vessel and its contents, as well as the water density and depth. A stable vessel has a positive righting moment, which means that it tends to restore its equilibrium when tilted. An unstable vessel has a negative righting moment, which means that it tends to overturn when tilted. A neutrally stable vessel has a zero righting moment, which means that it remains in any position when tilted.

2 Stability criteria for FPSOs

FPSOs must meet specific stability criteria to ensure their safety and operability in different weather conditions. These criteria are based on international standards and regulations, such as the International Maritime Organization (IMO) Code on Intact Stability for All Types of Ships Engaged on International Voyages (IS Code). The angle of heel (tilt), metacentric height (GM), righting lever (GZ), and the area under the righting lever curve (GZ curve) must all be within a certain limit to ensure stability. For example, the angle of heel should not exceed 16 degrees, the GM should be positive and above 0.15 meters, the GZ should be positive and above 0.2 meters for angles up to 40 degrees, and the GZ curve should be above 0.055 meter-radians for angles up to 40 degrees. These criteria must all be met under any loading condition and wind or wave action.

3 Methods of stability analysis

Different methods of stability analysis for FPSOs exist, depending on the desired level of detail and complexity. Static stability analysis is a straightforward and fast option, but does not factor in dynamic effects of motion and fluid behavior. Dynamic stability analysis takes into account the vessel's six degrees of freedom (surge, sway, heave, roll, pitch, and yaw) and simulates its response to external forces. This method is more accurate but requires more data and computational resources. The most advanced and comprehensive method is coupled stability analysis, which combines dynamic stability analysis with fluid-structure interaction (FSI) analysis to consider the effects of fluid flow on the structure. This method requires the most data and computational resources.

4 Tools for stability analysis

Various tools and software are available for stability analysis for FPSOs, depending on the method and purpose of the analysis. For instance, hydrostatics software, such as GHS, NAPA, and HydroSTAR, can calculate hydrostatic properties and characteristics of the vessel, such as displacement, buoyancy, center of gravity, center of buoyancy, metacentric height, waterplane area, and transverse moment of inertia. Stability software can perform static stability analysis to check compliance with stability criteria for different loading conditions and environmental scenarios. Motion software can simulate motion response of the vessel to external forces and moments using numerical models and algorithms, while FSI software can simulate fluid-structure interaction (FSI) effects on the vessel and its contents. Examples of these software include AQWA, MOSES, SIMO, CFD, FEM, and SPH.

5 Best practices for stability analysis

Stability analysis for FPSOs is a complex and critical task that requires careful planning and execution. To ensure success, it is essential to define the scope and objectives of the analysis, collect and verify relevant data, choose the right method and tool, perform the analysis systematically, and interpret and communicate the results effectively. This includes considering design stage, operational requirements, regulatory standards, available resources, vessel geometry, weight distribution, loading conditions, environmental conditions, mooring system, riser system, fluid properties, accuracy and reliability of results, compatibility of software, established procedures and guidelines, assumptions and models, results and outputs, process and findings, key findings and implications, uncertainties and limitations. Finally recommendations and solutions should be provided.

6 Here’s what else to consider

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