Finite element analysis (FEA) has been used in the offshore industry for many years to design and analyze structures and mechanical components, and is generally viewed as standard practice by most design engineers. The rapid increase in affordable computing power and the development of user interfaces with general purpose FEA and computational fluid dynamic (CFD) software enables engineers to simulate and analyze increasingly complex problems cost effectively and efficiently.

Fire protection

AMEC has, for example, used these tools to optimize passive fire protection (PFP) on offshore structures. Traditionally, designing structures for fire resistance would involve applying a given heat flux to the entire structure, and insulating the surface of any steel whose temperature is above a threshold with PFP.

More recent studies have been performed to optimize the specification of PFP for a series of fire scenarios defined from safety studies and assessments, whilst demonstrating that the integrity of the critical structure is maintained for the required duration.

Corresponding heat loads are then applied to a finite element model to determine the structural response. As the design intent is to ensure that integrity of the critical structure is maintained for a specified duration, non-linear progressive collapse analyses are performed to determine the ultimate capacity of the structure. This approach has been used to justify significant reductions in PFP, and consequently produced substantial cost and weight savings.

The most recent CFD technologies now enable full transient and 3-D fire simulations to be carried out to model the actual release of hydrocarbons and provide corresponding heat loads, radiation and smoke distributions.

Such knowledge of the behavior of real fires, combined with the use of advanced analyses to determine the corresponding structural response, quickly provides the detailed understanding required to achieve significant design savings and safety improvements.

Helicopter safety

Another example of how CFD has been successfully applied to both improving safety and increasing production revenue is in the optimization of safe operating envelopes for helicopter approaches to an offshore installation.

Production capacity of the installation was governed by heavily restricted helicopter operations due to unknown environmental conditions near to the heli-deck. The thermal plume from the hot turbine exhausts and flare can interfere with safe helicopter operation. If a helicopter flies into a region of warm air, where the density is lower, the engines generate less power and the rotor blades generate less lift. In extreme circumstances this can result in an uncontrolled descent and heavy landing.

Civil Aviation Authority (CAA) guidance on offshore helicopter landing areas recommends that sufficient clearance between a +3.5°F (+2°C) isotherm and the helicopter flight path be maintained at the critical flight decision point. In circumstances where this is not possible, the helicopter operators must be informed.

In the event of a take-off or landing, the water injection turbines were shut down to ensure that this condition was met, resulting in significant disruption and delayed production. To enable a more informed decision to be made to improve production efficiency whilst maintaining safety standards, a CFD study was undertaken to more clearly understand the exhaust dispersion conditions.

An existing CAE model of the topsides was modified to incorporate all thermal exhausts from flares and turbines, and a computational mesh was embedded within it. A wide range of wind speeds and directions were analyzed and safe operating envelopes were determined from the results.

By assessment of the analysis results, it was found that, under the majority of wind speeds and directions, it was safe to keep the water injection turbines running whilst helicopters were in the vicinity.

FPSO topsides fatigue

Unlike the topsides on fixed installations, the assessment and mitigation of fatigue damage on the topside structures of floating production, storage and offloading (FPSO) vessels is an important design consideration.

Structural and facet models of the FPSO hull are developed together with structural models of the individual topside pre-assembled units (PAUs). Radiation diffraction analyses of the FPSO are performed for a series of regular waves with different periods and directions. The overall response of the FPSO is then benchmarked against model test response amplitude operators (RAOs).

The hydrodynamic loads are read and static and dynamic stiffness analyses performed to generate the stress transfer functions, which include components from inertia stresses due to FPSO motions, displacement stresses due to hull bending and fluctuating gravity stresses due to roll and pitch of the FPSO.

Fatigue lives are then evaluated at individual joints.

Fully understanding the basic principles and making use of the latest advancements in computing hardware and analysis software has enabled detailed spectral fatigue analyses of this nature to be fully integrated into fast-track development projects.

Asset integrity

Over the field life, significant changes in well fluid properties can occur, which may have a major impact on production efficiency or asset integrity. For example, increases in sand content, resulting in higher erosion rates, damage to vessel internals and blockages, are issues for many installations. CFD provides an extremely useful tool in predicting erosion rates, understanding the detailed loading regimes on vessel internals and optimizing the flow regimes in separators and other vessels.

With a good appreciation of the loading regime or degradation rate being provided from CFD analysis or field data, more reliable fitness-for-purpose assessments can be undertaken.

By combining a detailed understanding of the operating conditions being experienced by process equipment, with the ability to perform advanced fitness-for-purpose assessments, significant benefits can be realized from optimized inspection and maintenance activities and even shutdown deferment.

Conclusion

With the computing power and software currently available to the engineer imposing fewer restrictions on what is practical to analyze, the challenge lies in identifying opportunities to use advanced engineering analyses to remove conservatisms in conventional design approaches and enable more informed decisions to be made to improve efficiency, reliability and safety.