An innovative design for enhanced safety

On 1 January 2017, the Dunkerque LNG terminal entered into commercial operations after five years of construction. The 13 billion m³/year of gas terminal is a model of safety in the LNG industry: Borrowing proven technologies tested in other terminals and designed from the beginning in order to enhance the reduction of risk and of gravity of potential incidents.

EDF’s reputation as an energy company focusing on safety was at stake: It was the first large scale gas facility designed and executed entirely under the EDF Group’s planning and management. To accomplish this, a project team was put together at the start of the design stage, and very soon after, Dunkerque LNG, a company dedicated to the project, was founded. Dunkerque LNG, a 65% subsidiary of EDF, 25% of Fluxys and 10% of Total, is now the terminal’s owner and operator.

EDF’s strong engineering expertise and Fluxys’ expertise in LNG for more than 30 years have been combined to tackle operating constraints after the design phase.

In addition, the design of the terminal does not incorporate any one-of-a-kind technologies to avoid associated risks.

A design tailor-made for operation

Ensuring that the terminal design fits operating constraints was a priority from the beginning of the project.

Including the future operation manager in the project team from the beginning of the initial studies in 2007 ensured that operating constraints would be taken into account. At that stage of the project, his task was to incorporate the operating experience feedback into decision-making and into specialised safety reviews (LOPA, HAZOP, etc.).

Later, as soon as the Final Investment Decision was taken, the operating team has started to grow to reach its full size approximately one year before commercial start-up.

This early recruitment of operators made it possible to prepare operating procedures and to train operators as detailed studies progressed, and also meant that the future operator’s maintenance and operating teams could monitor the work’s execution. This made it easier to take over the facility as soon as it was put into service. Because the operator participated in engineering reviews, there was stronger checking of operability and maintainability aspects of the installations.

This decision to consider operating constraints from the beginning led to some specifics about the Dunkerque LNG terminal design:

• The upper platform’s jetty has no process equipment and/or pipework, as they laid on a lower level. This decision allowed very easy operation and maintenance of arms, which are sensitive devices on a terminal with a single jetty.
• Pipe-ways and equipment have been distributed to be accessible with an eye to roundsmen but also maintenance operations.
• Fixed and travelling cranes make lifts for maintenance of pumps and compressors with no need for mobile cranes, which cause many operating incidents.
• The tank platform is designed for manipulating devices (pumps, valves, actuators, frames) via a fixed crane installed on the dome as well as dedicated supports (safety valves).

An innovative design for safety

The Dunkerque LNG terminal was the first upper-tier SEVESO establishment authorised in France after regulatory changes following the AZF accident.

The demanding regulation and the EDF Group’s environmental/safety culture led to the introduction, starting with the terminal’s design and founded on accidentology (including accidentology shared within the LNG industry), of a few innovative concepts with the goal to minimise the risk level of the establishment.

Collection of safety valves: All safety valves (Pressure Safety Valves and Thermal Safety Valves) in the terminal are collected, either towards the evaporation grid (Boil-Off Gas) or towards the flare system, depending on the flow rate and the design of the valves. By limiting all risk of uncontrolled emissions of gas into the atmosphere, this configuration reduces the terminal’s environmental impact in terms of greenhouse gas discharge. Aside from the environmental aspect, this choice reduces the size of ATEX zones and therefore risks to workers.

LNG collection channels: All LNG and BOG pipes circulate in pipe-ways. A channel located near the LNG pipes allows for any potential LNG leakage to be collected. It can then direct this LNG to a retention area fitted with foam generators. This ensures that no LNG stagnates near equipment and limits the need to protect the equipment against thermal flux. Finally, such containment of accidental spreading of LNG limits the size of vapour clouds that follow vapourisation of LNG and makes it easier to predict intervention teams’ areas of focus in the event of a major incident, as it determines where the collected and retained LNG will be located.

Over-filling of tanks: this accident scenario is probably among the most serious that could arise in an LNG terminal. The rise in internal pressure could destroy the tanks’ walls and cause the roof panels to collapse, leading to a massive loss in containment. Therefore, two special provisions have been implemented in the terminal’s tanks in addition to the usual provisions (safety valves, vent valve, gauging, etc.)

• Every tank’s dome is frangible. According to the tank designer’s calculations, an increase in pressure of about 700 mbar inside the tank creates enough cracks in the dome’s concrete that internal overpressure is released into the atmosphere. At this same pressure, the walls remain undamaged, ensuring that the LNG is retained. However, the maximum service pressure of the tanks remains identical to the value usually maintained, and pressure tests of the tank have been carried out without difficulty.
• Furthermore, in addition to the usual automatic actions performed on the upper LNG level, a patented arrangement allows very high levels to be detected. This detector is made up of three temperature probes placed at the altimetry corresponding to the tanks’ very, very, very high (HHH) level. A dedicated small flow of natural gas maintains its temperature at a value much higher than that of the LNG, while an increase in the level of LNG would bring their temperature down to -160ºC. Besides, a dedicated device allows to test them from outside the tank.

Absence of High Pressure LNG header: Each of the ten high-pressure (HP) pumps is devoted to a single ORV. Regasification is structured into five groups of two Pump/ORV sets, all identical. This arrangement makes it possible to avoid shared HP pipelines of large diameter and their associated risks, as these groups of equipment are fed with low pressure (LP) LNG.

As a result, the number of HP valves in the terminal is reduced: Isolation during maintenance is carried out from LP LNG and from natural gas systems. Furthermore, the maximum HP flows to be considered in risk assessment are on the order of 200 tph instead of 3000 tph in the case of a single HP header.

ORVs are reliable enough so that this arrangement does not reduce the availability of the terminal’s regasification function. The seawater pumps, however, remain standardised.

Independence of protective systems: In addition to the operating system supervised by the operator, three independent systems ensure that installations are protected from deviations in physical parameters or when an abnormal situation is detected – they set off an automatic protection of installations.

Register for safety action: The devil is in the details. So as to check that safety was being taken into account throughout the project’s lifetime, all observations with safety implications were tracked from the beginning of the project up to receipt of the installations via a single register – the Safety Action Follow-up Register (SAFUR).

The contractor in charge of the process aspect was tasked with keeping this register up to date. This made it possible to measure how safety was being accounted for in detailed studies (number of new points opened) and whether all project owner requests were being properly accounted for (by effectively closing observations). The existence of this register made it possible to check contractors’ attention to safety as they developed their studies.

Dropped-off ideas

For time reasons, a few ideas for simplification were not implemented in this project.

The spacing between an ORV and the HP pump has been kept around 40 m, even though a larger reduction was targeted, or even to incorporate the HP pump and the heat exchanger panels into a single concrete support assembly, which would have required a more in-depth study of the degassing systems and of the continued cooling of the pumps and LNG systems. The possible reductions in line weights would have perhaps made this study profitable.

A ‘conventional’ reincorporation system has been adopted. Without going so far as to turn to liquefaction solutions integrated into tanks, the condensation time for gas bubbles in an LNG flow, under usual speed conditions as well as under the pressure and temperature of an LNG terminal’s pipework – less than one second – must make it possible to postulate ‘on-line’ reincorporation, potentially followed by a degassing tank that would be passive so as to guarantee the protection of the LP LNG pumps.

Developing this type of arrangement would allow to suppress ion on the drum on each associated control loop, which are complicated to apply and have caused industrial incidents recorded at several LNG reception terminals. The study of such a system could be carried out in collaboration with other LNG actors to mitigate previous drawbacks.

Conclusion

The safety and security of people and assets, an absolute priority for the three shareholders in Dunkerque LNG, has been taken into account ever since the design of the terminal was begun. The innovative solutions implemented allowed Dunkerque LNG on 1 January 2017 to put into service an industrial asset that is highly reliable in terms of safety, which is a key satisfactory factor for its two customers EDF and TOTAL, engaged for a 20 year period in regasification subscription. For the next 50 years of operations, Dunkerque LNG knows it can rely every day on the expertise of its partner Fluxys.

This is an article written for LNG Industry's November issue and abridged for the website. Subscribers can read the full November issue by signing in. Non-subscribers can access a preview of the November 2017 issue here.

Read the article online at: https://www.lngindustry.com/special-reports/19122017/an-innovative-design-for-enhanced-safety-part-one/