Platform orientation is one design aspect that can play a significant role in the inherently safe design process. Traditionally, the platform orientation has been determined by engineering judgment, heavily weighted by past experiences. While this approach initially appears to be time- and cost-effective, it has the potential to lead to a non-ideal design solution that could cause safety and operational issues to go unaddressed and increase costs in later design stages. A recent study examined potential factors that could add to these costs.
The parameters considered for the optimization study were as follows:
• The natural ventilation (wind), which can reduce the potential accumulation of toxic and flammable gases as well as provide indications of potential vapor cloud explosion consequences;
• The helideck impairment, which can impact helicopter operations due to hot turbine exhaust gases, affecting both general operations and potential emergency operations;
• The wind chill, which can affect the ability for personnel to work on the platform. This is particularly important in cold climates and extreme weather areas where working conditions can influence the number of personnel required for operation;
• The lifeboat drift-off direction, which can impact the safety of the crew in an emergency situation; and
• The hydrodynamic drag, which can affect tendon fatigue life, hull integrity and structural design requirements.
Natural ventilation (wind)
In the event of an unintended hydrocarbon release, higher ventilation rates typically translate into the formation of smaller flammable gas clouds. This parameter is therefore intended to be maximized.
Civil aviation regulations dictate that restrictions be put in place to the helicopter operations if there is a temperature increase of 2 C (3.6 F) above ambient within the operational zone above the helideck. Temperature rise is used to define potential impairment to operations; in some cases this might limit operations altogether or require adjustments to payload weight, approach paths, etc. For many offshore facilities, particularly in extreme weather areas, helicopters are used as the primary means of transportation and evacuation during an emergency. Thus it is imperative that the helideck remains available through as many expected weather conditions as possible.
Wind chill is quantified by the perceived decrease in temperature felt by the body on exposed skin. Wind chill can impact the number of personnel required to operate a facility. In some cases, environmental effects such as wind chill have been known to increase the potential for operator error. To provide personnel with acceptable working conditions and maximize safety, wind chill effects are intended to be minimized. It is important to note that this can be counter to increasing ventilation for the reduction of flammable clouds during an unintended release of hydrocarbons. One intent of the optimization approach is to find a balance between these two potentially competing goals.
If a lifeboat is deployed during an emergency, it is imperative to maximize the potential survival of the craft by limiting exposure to potential hazards. A lifeboat deployment might also suffer from loss of power, thus left to environmental effects to reach safety. To maximize the potential for survival, the lifeboat should drift safely away from the platform, assisted by the current. Adverse driftoff, the length of time to reach a safe area and potential drift back into the facility should be minimized.
Tension-leg platforms are typically used in water depths reaching up to 2,134 m (7,000 ft). The stress in the tendons resulting from maintaining the platform in place despite wave impact and drag loading from the current needs to be minimized. Tendon requirements can lead to weight and structural design limitations as well as requiring unnecessary buoyancy complications during operations.
Why use CFD?
Good judgment is fundamental in solving any engineering problem. However, numerical simulations can help in making a good design even better. Today, with powerful multidesign exploration and multidesign optimization tools such as HEEDS, it has never been easier to make a design reach its best potential.
In the oil and gas industry, however, decisions relating to the platform orientation are still typically made solely based on previous experience and qualitative judgment, which can lead to unintentional biases. This study is aimed at improving the accuracy of experts’ predictions through the use of numerical tools to meet the following design objectives:
• Maximize ventilation;
• Minimize helideck impairment from exhaust;
• Minimize wind chill effects;
• Minimize tendon stress; and
• Minimize adverse lifeboat drift-off.
Of course, using formal models doesn’t come without limitations. There are a few challenges associated with using computational fluid dynamics (CFD) to resolve issues related to offshore platforms.
Firstly, from a technical point of view, offshore platforms are very large and have extremely complex geometries. This makes it difficult, if not impossible, to explicitly resolve all objects within the available time frame.
Secondly, from a project management point of view, projects are strongly schedule-driven. Stakeholders want their platform to start running as early as possible since each day of delay will cost upward of $10 million in deferred revenue.
In addition, the platform orientation is one of the first design aspects to be decided. However, in very early design stages, information is scarce. Many uncertainties need to be dealt with regarding the location of the equipment, etc.
Finally, the budget allocation for HSE is usually about 1% of the total project cost, which greatly limits the amount of influence technical safety bears on the final design.
The combined cost function shows that the optimum orientation of this particular platform, once all objectives were taken into account, is for its north to face true east-southeast. This result does not coincide with any of the ideal orientations found for the individual design objectives, but it is the best compromise between all these objectives.
The optimum orientation of the platform was obtained using simulation tools based on five design objectives: ventilation, exhaust, wind chill, lifeboat drift-off and tendon stress. The approach taken in this case study considers an early stage of design, with parameters covering both safety and operational issues. As the design progresses, the number of parameters considered is expected to change, as will their weighted contribution. The idea is that the orientation can be further optimized as the design process progresses or in some cases completely alters the selection based on safety and operational prioritizations. If a proper balance of previous experience, qualitative judgment and the use of formal models such as CFD are deployed, this function method can be used to achieve an inherently safe design. Further work could involve optimizing the facility layout based on turbine stack design and positioning, helideck positioning, module placement, flare tower design, etc.