Integration of HSE Aspects In Engineering Design
– Raj Narkhede, Senior General Manager–Risk Assessment, Aker Solutions

Design engineers are in a unique position to reduce the risks in the life cycle of a plant and thus have a key role to play. The design process evolves from initial concepts to a detailed specification, and involves different teams and people at various stages. At each stage, engineers from all disciplines can make a significant contribution by identifying and eliminating hazards, and in some cases where elimination is not an option, reduce the likely risks. Design evolves as a part of an iterative process and design engineers should consider the life cycle of a project from concept/feasibility to detailed design, constructability, maintenance and refurbishment/demolition. This thought process should not suppress innovative design; instead design engineers must take advantage of this opportunity to stretch the boundaries of the industry to create practical, pragmatic design solutions. HSE in Engineering is a good business solution when it is a part of the design process, in the life cycle of a project. It can help to eliminate or reduce the potential hazards and risks through design.

It takes an average of 3 to 4 years to build a process plant, right from concept to commissioning. All critical features of the plant are decided during the early stages of the project. Subsequently, during operation and maintenance, very few design changes are possible.

Design choices for safety systems help to minimise risks but they vary in terms of factors such as cost, reliability and maintenance. Design engineers should manage these design choices in order to follow consistent risk tolerability levels, to understand how a specific facility or process fits into their overall business plan and to know what the cost limitations are for the safety component of a process.

When deciding among the hierarchy of mitigation options, design engineers should avoid the pitfall of 'project mentality' which is focusing only on minimising capital cost. As the following figure 1 suggests, inherently safer approaches may have a higher investment initially. However, the cost of maintaining an active system, to obtain an equivalent level of risk reduction can be significant. Therefore, following the correct approach is necessary while considering the life cycle cost of the design options.

Design Options
Inherently safer design eliminates or mitigates the identified hazard by using materials and process conditions that are less hazardous. For example, faced with the hazard posed by a flammable solvent, the engineers might seek to substitute the same with water. When large inventories of hazardous intermediates increase risk levels, there may be a way to reduce or eliminate these inventories.

Passive designs offer a high level of reliability by operating without any devices that sense and/or actively respond to a process variable. Examples of passive design solutions include incompatible hose couplings for incompatible substances and process components, equipment designed to withstand internal deflagration and other very high-pressure hazards and dikes that contain hazardous inventories, with a bottom sloping to a remote holding area.

Active design employs devices that monitor process variables and activate to mitigate a hazardous situation.

Active solutions, sometimes called engineering controls, are often less reliable than passive or inherently safer design solutions because they require more maintenance and more operating procedures.

The following are characteristics of active design solutions:
• A pressure safety valve or rupture disk that prevents vessel overpressure .
• A high-level sensing device interlocked with a vessel inlet valve and pump motor to prevent overfilling.
• Check valves and regulators.

Procedural design, also known as administrative controls, avoids hazards by being controlled manually. These actions might include reacting to an alarm, an instrument reading, a leak, a strange noise, or a sampling result. The involvement of a person in safety solutions mean incorporating human factors in the analysis. These human factors lead to inappropriate division of tasks between a machine and a person and results in an unsupportive safety culture. Thus, these factors contribute to making procedural solutions generally less reliable than other design solutions.

Selection of Design Options
Design engineers have a responsibility to select the most reliable approach. Inherently safer and passive solutions offer high reliability and low operating costs, but involve an initial cost. Active and procedural designs cost less initially but typically involve higher operating costs and are less reliable.

The following examples will help in clarification:
While designing one of our process plants, a flammable substance with a highly hazardous reaction was involved. There were previous incidents with the substance and two options were available for reducing the risk. The first option was an inherently safer approach which involved designing a vessel to withstand a maximum pressure level of 800 psi. However, the cost of design was very high. The second option was to activate an emergency pressure relief system. This required a reactor vessel with a lower pressure rating. While this approach was less expensive, it required the facility to deal with a hazardous release into the atmosphere and to address the reliability of the release system. This option was found to provide a tolerable risk level and a lower cost of implementation.

In another case, water-cooled heat exchangers were used in a process that included a material that reacts vigorously with water, producing corrosive and toxic by-products. The design engineers considered various combinations of passive solutions such as heat exchangers that use nonpressurized water, active solutions such as advanced leak-detecting sensors, and procedural solutions such as enhanced testing, inspection, and maintenance. All of the alternatives reduced risk levels. The choice of an alternate design that substituted a compatible heat transfer fluid for water. required a higher initial investment for equipment replacement but eliminated maintenance and administrative expenses during life cycle of the plant.

Cost Effective Design Option
Safety studies such as HAZID/HAZOP/QRA/RAM during design is sometimes viewed as an expensive way to achieve greater risk reduction. However, when risk assessment is left out of the design process, the plant may be overdesigned, with safety protection costing more than it should, or the plant may be unprotected from significant, unidentified risks. Systematic, risk-based design helps in the identification of significant risks, ranks them, and prioritises steps to address them. The result is that capital expenditures, operating expenses, staffing, and other resources are better allocated.

Following figure explains cost effective risk reduction principles.

Comparison of Prescriptive versus Performance Based Approaches
The prescriptive approach consists of applying local regulations, industry codes and standards and good engineering practices. The safety objectives are implicitly defined and the design process focuses primarily on the means to reach them.

The performance-based approach relies on the explicit definition of the safety objectives and functional requirements(eg, performance standards). The design process focuses primarily on the objectives, and the design is developed to fulfil these objectives in a more flexible manner as compared to the prescriptive approach. This allows the implementation of innovative solutions and optimisation of the design.

The performance-based approach applies in realistic hazard scenarios (eg, fire, explosion or cryogenic release events), which could even be deterministic (eg, worst case approach) or probabilistic(risk-based).

The strengths and weaknesses of both the approaches are summarised in the table below:

Conclusion
This article is focused on the integration of HSE aspects in engineering design. The multidisciplinary approach is based on the verification method for all design disciplines, in order to fulfil overall safety objectives of the plant. Focusing on interfaces and integrating HSE aspects can be achieved by:

1. Specialist HSE engineers who provide accurate, quantitative inputs allowing all disciplines to work in parallel, using experience gained from previous similar projects.

2. Design engineers who need to know the impact of HSE aspects and design rules of their discipline.

Finally, the experience and anticipation of HSE aspects during the engineering phase of the project can prevent over- and under-design, leading to an impact on cost and schedule and thereby ensuring the required level of safety in design.