Every single step of the design, construction and operation process is abound with risk, but also with opportunities to optimise the design. Control of major hazards is of key importance, and prevention of escalation from an initial incident is critical.
It is therefore imperative that a clear philosophy should be set before design starts, and that this should be based on a high level of understanding of major hazards, their potential consequences and the associated risks.
Concept and philosophy
The challenges of developing FLNG facilities are complex. Although land-based LNG and offshore Floating Production Storage and Offloading (FPSOs) facilities are ‘standard’ operations, combining the two technologies into a FLNG asset is demanding and costly. It is not surprising, given that a FLNG facility comprises a LNG production process squeezed into a small area, placed on top of the LNG tanks and with all personnel within close proximity, that the risks to personnel and the vessel itself can be considerable if not addressed through careful design.
Technical challenges such as safety, ship motion, topside processing, hull and containment, offloading, integration and operation, combined with the critical aspect of very limited space constraints, means that greater attention must be given to the design of the facility. The industry should also not lose sight of the construction and commissioning challenges that faces these facilities. Commissioning such facilities in a shipyard with high pressure gas, and cryogenic conditions is something that has not yet been undertaken. Early design decisions, if made correctly, can however, deliver an inherently safer and a fit for purpose installation.
Understanding the threat
There is currently no real operating experience for FLNG production facilities. Knowledge and understanding has to be extrapolated from other types of existing installations. One measure that is inherently safer is to separate, as far as possible, people from the hazards. Designers of Shell’s Prelude FLNG, which is the largest floating offshore facility ever built, for example, have optimised safety on the facility by locating storage facilities and process equipment as far from crew accommodation as possible.
In adopting this approach, one of the major threats to the personnel and facility is escalation from an initial release of hydrocarbons to a major loss of containment as a result of fire or explosion damage. Limiting the potential for escalation is a key step in managing the risks.
For example, one measure that can limit the explosion potential in onshore process areas is to have ‘safety gaps’ between congested process areas. They are effective, because blast pressures are generated by flame acceleration through the congestion. Breaking up the process areas allows the flame to decelerate in the gaps, reducing the magnitude of the pressure generated.
This conventional wisdom becomes more difficult to implement when faced with the constraints and costs of an FLNG vessel. Keeping process units relatively small and well spaced can limit the potential for any explosion to cause further loss of containment. However, making the gaps too big can effectively define the size of the vessel and ultimately, significantly change the economics. Greater distance equals greater mitigation but with restricted deck space - an optimum approach is required.
Testing the theory
Understanding the hazards and what controls and mitigates them, as well as setting the risk management philosophy out early on in the process will help guide the development of the design phase.
The unique experimental work carried out by DNV GL at the Spadeadam test facility in the UK is key to understanding the nature of these hazards.
Spadeadam Test Site is a unique facility carrying out research and technical service work for the oil and gas, process and energy industries, construction industries as well as government agencies. With qualified engineers who have decades of experience in fire and explosion testing, DNV GL has the facilities and expertise to carry out trials involving the use of high hazard materials for a wide variety of purposes are world leaders in the field of gas vapour cloud explosion tests.
This focuses on what can happen if a release of gas, LNG or refrigerant occurs, the consequences and the means of protecting fire and blast. This experience and intelligence is invaluable when working at the very outset of a project, from pre-FEED studies through to detailed design. It sets the standard across five layers of protection against a major accident:
- Elimination and minimisation of hazards by design (inherently safer design).
- Prevention (reduction of likelihood).
- Detection and control (limitation of scale, intensity and duration).
- Mitigation of the consequences (protection from effects).
- Evacuation, escape and rescue (EER) arrangements.
In addition to understanding the means of protection it is important to realise that the hazards cannot be completely removed, only effectively managed. As a result, many of the decisions are risk based, requiring advanced tools for assessing the risks. Here again, the experimental studies carried out by DNV GL provide unique validation for the risk tools and a stong foundation for decision making.
It is critical that an offshore safety regime properly accounts for technological, organisational and human factor defences – or barriers – in the prevention and mitigation of accidents throughout the lifetime of the offshore installation. Actively involving the offshore workforce in major hazards management, and explaining their role in effective barrier management, is a key benefit of this approach and can be easily applied to FLNG development.
A floating future?
Growing demand for energy and the end of ‘easy oil’ is pushing the frontiers of ‘floating’ innovation to release and harness a bounty of stranded gas reserves. One estimate puts the global volume (onshore and offshore) at more than 6000 trillion ft3, of which about 900 trillion ft3 (almost 40 years of US current consumption) are in offshore gas fields.
Technically and commercially, the advent of FLNG operations is highly challenging. With costly investment, each FLNG facility must be structurally secure if it is to be economically viable throughout every step of the project. Market sensitivity, LNG price, and demand and supply, will most definitely have an impact on its evolution.
Beyond the questions and the challenges, the next few years will see the birth of some of the largest, most ambitious and technically challenging FLNG facilities ever constructed and Shell’s Prelude and Petronas’ FLNG 1 operations will provide the benchmark for further innovation.
Many fear that the development of FLNG is a major accident waiting to happen. The technology is new and by the very nature of its cargo and the environment in which it operates, the risks are potentially high. Setting out the philosophy and expectations early on is important and getting this right at the early stages ensures a better design and a more stable start, which will flow throughout the project.
Written by Mike Johnson, Principal Consultant, DNV GL. Edited by Callum O'Reilly
Read the article online at: https://www.lngindustry.com/floating-lng/09072014/a_plea_for_early_safety_design_in_flng_945/