Storage in severe conditions

The Entrepose Vinci Joint Venture (EVJV) team, comprising of Entrepose Projets and VINCI Construction Grands Projets, has developed specific experience of such projects as an engineering, procurement and construction (EPC) contractor. For Entrepose Projets, it started with the Snøhvit project, which was built between 2002 and 2005 at Melkøya Island, Northern Norway, located beyond the polar circle. In 2013, Yamal LNG awarded EVJV the EPC contract for four large LNG storage tanks for the liquefaction plant to be located even further north in Northern Siberia, Russia, with winter temperatures below -40°C. Meanwhile, the companies are also serving as the EPC contractor for the construction of LNG and condensate tanks for the Wheatstone Project, which is located in the opposite, but still demanding, conditions of Western Australia, where hurricanes and temperatures above 40°C pose different challenges for an EPC contractor.

Adapt foundation designs to face extreme and remote conditions

Due to their height, their weight and the stringent settlement requirements related to their function, LNG storage tanks have always been a special structure when it comes to foundation design. LNG tank foundations must be optimised, while also taking into account the site’s specific soil and earthquake conditions. Bearing the load is not the only function of the foundations. Seismically liquefiable soil layers have to be compacted, and LNG tanks cannot accept more than 1:300 deflection along the slab radius, and an overall 1:500 tilt on any diameter. Such criteria often requires a combination of soil improvement and piled foundations.

In the permafrost of the Yamal peninsula, EVJV faced a completely new challenge, with ground that is both extremely rigid when frozen, but also heterogeneous and unstable at depth and when it melts. Instability of the soil has been mitigated by a thermo-stabilisation system, which refrigerates the ground in winter months to ensure long-term stability even in the summer seasons. The high stiffness of the frozen soil, associated with thermal deformation of the LNG tanks’ base slab, also makes it complicated to ensure the integrity of the pile-slab connections. An innovative pile system has been designed to allow, even at extremely low temperatures, the displacements of the pile-heads that are necessary to accommodate the slab deformations. Adaptation and innovation were a key success factor on this unique project.

The challenge in Western Australia was to keep a reasonable pile grid, while dealing with the liquefaction susceptibility of a soil layer under a seismic event. A construction-oriented soil improvement design was proposed, using the same equipment as for the piling activities in order to avoid the high mobilisation and demobilisation costs of extra equipment. The liquefiable soil layer was densified by closed-end steel inclusions in the liquefiable layer only, on a pattern compatible with the piling pattern; and by closing the end of some of the piles, in order to reach the desired compaction at each location in an optimised way. In a remote location, the company’s foundation engineers had to take into account the scarcity of equipment, materials and manpower to adapt the design.

Adopt a modular approach

In any LNG plant, the LNG tanks have to be built mostly on site. However, some external parts of the LNG tanks can be preassembled and shipped to the construction site, not only to save time and cost, but also to put less pressure on the manpower available on site. Externalising steel structures and piping assembly at specialised yards helps to improve the overall safety and quality performance.

This modular approach was implemented by EVJV for the remote Siberian and Australian tanks, while it has not been chosen for European projects. With modules, the detailed design of the tank roof pump platform and riser is very different and has to start earlier in order to anticipate the procurement and fabrication of the steel structure, piping, equipment and instrumentation. The main difference is the structural design of the roof platform and riser, which no longer rest on a concrete structure, but have to be integrated in a steel structure that is strong enough to allow lifting and freight of the modules.

Adapt procurement organisation

For remote sites, the procurement strategy has to be modified to account for longer, more expensive, and seasonally difficult shipments. All of the recent LNG projects ship items from around the world to the construction site. In the case of LNG tanks to be built in Northern Siberia, the only access to the site is by ship on the Arctic Ocean. Freight in winter requires the assistance of ice-breaker vessels, which is a costly and less predictable method in terms of delivery date. Procurement has to be anticipated and logistics adapted to such extreme conditions.

Safe delivery

Promote the highest health and safety standards

The more severe the conditions, the more crucial it is for the EPC team to remember the health and safety of the craft and staff involved in the construction activities. Specific training programmes are organised to raise awareness of cold burn or dehydration risks. A fully equipped nursery is set up on site to ensure life support in spite of the remote location. Personal protection equipment (PPE) is also carefully selected for adequate protection against the risks associated with the hostile environment, while being as comfortable to wear as possible.

Manage social and environmental performance

It is required that EVJV and it subcontractors help the project developers to manage the social and environmental performance of the EPC phase of the project. This phase has the most significant impact on both local communities, with thousands of workers coming in with potentially huge economic consequences, and the environment, with potential destruction or pollution if proper assessment and monitoring are not implemented. On the Yamal peninsula, EVJV has a permanent contact with the Nenets community to ensure that their feedback during the construction phase is positive. The stringent social and environmental protection policies are implemented throughout the subcontracting chain, to ensure their performance by all participants. In Western Australia, local communities provide some of the workforce, and the EPC contractor manages the environmental commitments that were made by the project owner to protect biodiversity, throughout construction.

Adjust construction methodologies

On a more technical ground, the construction methodologies have to be chosen and, if necessary, adjusted in accordance with the project’s context. With LNG tanks, a telling example is the tank wall, which is a crucial part of the structure. It is made of reinforced concrete and is heavily prestressed in order to resist in case of LNG spillage inside of the tank (an unlikely consequence of a major failure of the inner LNG containment). On the Wheatstone Project, in order to enhance the safety conditions and reduce the use of manpower and lifting equipment, the decision was taken to design and procure a self-climbing formwork, well adapted to the hurricane hazard and the scarcity of labour. On the Yamal Project, EVJV had to make the most out of the summer seasons and minimise wintertime concrete pouring. In order to fit in the summer windows and match the project schedule, EVJV and subcontracting organisations had to perform concrete works of two LNG tanks’ base slabs and walls within the May to October periods of both 2014 and 2015. The choice was made to use sliding formworks (or ‘slipform’). Each 40 m wall was poured in approximately 20 days, working without any interruptions.

Other adjustments are worth highlighting. Heavy concrete works, such as haunches (wall-roof connection) and roofs, were performed at temperatures of approximately -30°C on the Yamal Project thanks to a fully winterised concrete supply chain, and a protected and heated concreting area. Also, the hurricane hazard in Western Australia led the EPC team to design innovative site facilities with operational safety procedures in case of alerts.

In any situations, EPC contractors need an innovative construction-oriented mindset to be able to support projects in remote and potentially hostile locations.

Conclusion

The role of an EPC contractor is to accompany the LNG project developers anywhere and to keep proposing the most cost-effective and reliable EPC solutions, also ensuring strict compliance to policies and regulations, as well as adherence to sound practices on all levels of works’ performance. Remote locations and severe conditions require that the industry adapts its design and methodology through open innovation in order to deliver timely and cost-effective solutions for project stakeholders, while keeping up with the utmost safety, social and environmental performances.

Read the article online at: https://www.lngindustry.com/special-reports/06022017/storage-in-severe-conditions/