Common bridge-connected fixed platforms can be outfitted with standard LNG processing equipment. (iStock photo)
Onshore liquefied natural gas (LNG) facilities have been the predominant industry solution since the 1960s for the import or export of LNG, particularly for baseload capacities. With the constant change in market supply/demand dynamics, floating LNG (FLNG) facilities (offshore and near-shore) are becoming increasingly prevalent worldwide with a focus on lowering project costs, improving operating efficiencies, and reducing execution schedules. The first deepwater FLNG was inaugurated last year with more to follow. Onshore LNG and FLNG facilities can vary widely in capacity, complexity, cost, and schedule necessary to bring product to market.
A new, viable alternative, the platform-based LNG production and storage solution (PLNG), has been developed by EnTX GasTek Global Ltd. (EGT) and HongHua Offshore Oil & Gas Equipment Co., Ltd. (HongHua Offshore) and engineered by Wood to address mid-scale LNG export opportunities. This PLNG concept consists of multiple fixed, bridge-connected platforms capable of hosting different process areas, gas pretreatment, LNG processing, storage, loading, utilities, flare and accommodations facilities, each on its own platform or combination of dedicated platforms. The PLNG concept can also be adapted for LNG regasification developments as an alternative to onshore LNG import terminals or floating storage and regasification (FSRU) units.
The PLNG solution
Multi-platform, bridge-connected oil and gas processing facilities are common worldwide, but the application in the LNG sector is an innovative application of a decades-old proven solution: fixed jackets supporting a topsides production facility. These structures are coupled with proven LNG processing equipment successfully operating globally. The PLNG concept relies on conventional jacket solutions for shallow-water locations in water depths of up to 60 m. It is principally designed for addressing two industry segments: 1) the 1-6 million tonnes per annum (MTPA) mid-scale production, and 2) locations between near-shore and deepwater.
Key enablers for the development of the PLNG solution include fit-for-purpose structural design of the fixed platforms (both jackets and decks) developed by Wood based on its 30 years of worldwide offshore experience. The platforms are engineered to support substantial loads of LNG production packages and LNG storage tanks at full capacity. Advances in the flat-panel, semi-membrane, prismatic-shaped (FSP) Type B LNG containment systems have facilitated evolution of the PLNG solution. The FSP containment system was engineered by Braemar Technical Services in collaboration with TankTek Ltd, a unit of HongHua Offshore, and certified by Bureau Veritas (BV). Finally, strategic transportation and installation profiles adopted for specific project site conditions foster cost-effective and time-efficient project execution while reducing project risks.
Wood has completed concept development and pre-FEED feasibility studies for EGT and HongHua Offshore. The pre-FEED included optimization of the general design concept to harmonize with US safety and environmental regulations (including HAZID reviews), as well as siting and installation strategies based on the initial design of a PLNG terminal capable of processing over 2.8 MTPA with 240,000 m3of LNG storage. The conclusions developed from these studies received Approval in Principle (AIP) by the American Bureau of Shipping (ABS) for this initial PLNG terminal concept.
Comparing the alternatives
Alternative terminal solutions to the PLNG present the following characteristics:
• FLNG vessels are considered best suited for deepwater offshore applications, typically for accessing stranded gas reserves. However, they require significant capital investment.
• Onshore LNG terminals have a significant construction risk, with a large percent of contracts either delayed or over budget.
• Near-shore FLNG terminals are suitable for mid-scale segments. However, they can still have significant siting requirements, including access to infrastructure, and weather-related low utilization rates among other constraints.
One of the advantages the PLNG solution offers is its worldwide applicability. After due consideration of site-specific bathymetry, metocean, geotechnical/geophysical conditions, the PLNG solution can be deployed anywhere in the world. Proper site selection provides unencumbered infrastructure access, proximity to local infrastructure, enhanced online production with fewer weather-related interruptions, and eliminated port congestion and terminal maintenance.
Typical LNG terminal solutions onshore (above) and PLNG (below).
Onshore LNG terminals are predominantly developed for baseload (large-scale) capacities that exceed 6 MTPA, while near-shore FLNG and FLNG are typically ranging between 1-4 MTPA capacities, with a few special cases (i.e. lean gas feedstocks) reported beyond this range. A PLNG terminal can be designed in incremental intervals for total LNG production capacities to meet small, mid-, and large-scale demands, as required. The PLNG concept is considered favorable for the small and mid-scale LNG segment as a viable alternative to either onshore or floating LNG. The capacity of near-shore FLNG, or FSRU, could be widely constrained if rigorous feedstock gas processing requirements are needed as limited by the available deck space for a given hull size. Whereas the modularized PLNG has no such constraint due to the construction of stackable levels and/or additional platform(s) to meet such requirements.
Another parameter regarding terminal capacity is the size and type of LNG storage. For an onshore terminal, storage is essentially unrestricted as long as land accessibility, terrain, siting and other requirements do not prevent construction of additional LNG storage tanks. Depending on the type of hull used for near-shore FLNG, storage is directly linked to hull size. In some instances, storage can be increased via additional floating storage units (FSUs) moored alongside the near-shore FLNG vessel but adds complexity to the mooring and LNG transfer systems. Similarly for FLNG facilities, storage is hull-size dependent. Of the offshore alternatives, the PLNG solution is the most flexible. Storage platforms can be added and tailored for project volume requirements using the FSP tank design.
Near-shore FLNG and FSRU terminals mostly rely on conventional LNG containment systems including self-supportive types, prismatic, spherical and membrane type used commonly in LNG carriers. The enhanced Braemar-TankTek FSP tank design incorporates several elements that differentiate it from its competition. These include a lower construction cost than for equivalent tanks for most size ranges, and the ability to be fabricated in parallel with the platform in order to shorten the critical path. The FSP volumetric efficiency is flexible to meet specific requirements, and the FSP is more robust with less construction tolerance than other tank types.
Water depth is very relevant to all offshore solutions. The deepwater FLNG solution is designed with full navigation autonomy offshore for water depths beyond 60 m. Near-shore FLNG developments can either be ship-shaped or barge-shaped, and can either process (pre-treat) and convert natural gas into LNG or simply convert it directly into LNG. Near-shore FLNG shares, to some degree, several interface aspects with onshore components. Therefore, water depths are dictated by design draft requirements of the LNG carrier fleet visiting the near-shore FLNG unit for loading. Some near-shore FLNGs are not necessarily designed for full navigation capabilities so site weather conditions (i.e. storms, hurricanes) can play a decisive role in the configuration and even the applicability of FLNG to certain sites prone to extreme weather conditions. The FLNG solution in shallower waters may involve significantly higher loads on mooring systems than deepwater locations, making the mooring system more complicated. The PLNG solution overcomes limitations of near-shore and deepwater FLNGs for the 60 m or less of water depths and site conditions. The PLNG’s structural construction, with deep support piles penetrating into the seabed, allows for greater stability and simpler mooring, and increased terminal utilization rates.
Typical LNG terminal solutions near-shore (above) and FLNG (below).
Onshore LNG terminals, especially greenfield projects in remote locations and difficult terrains, can involve extremely high costs, long schedules and complex execution plans, making them cost prohibitive, particularly for small and mid-scale LNG projects. FLNG solutions requiring sophisticated, high-technology designs of offshore components tend to involve similar drawbacks. Both the near-shore FLNG and PLNG terminals are being developed as alternatives to offset these cost impacts with various drivers including broader infrastructure options, relatively simpler system components, and parallel construction strategies for faster and less expensive mid-scale developments.
The PLNG concept utilizes proven topsides equipment, processes and components, including liquefaction technologies, gas turbines, brazed aluminum heat exchangers, LNG storage tanks and standard LNG loading arms that have been widely used in onshore, near-shore FLNG, and FSRU applications. In fact, there are fewer marinization requirements to mitigate motion effects on the processing equipment, as the fixed platform PLNG solution eliminates them. Relying on less sophisticated equipment helps reduce the overall project costs. As an example, the use of fixed platforms for LNG loading, similar to an onshore LNG terminal, greatly simplifies the configuration of the loading system as compared to side-by-side or tandem loading for near-shore FLNG and FSRU using sophisticated arms or cryogenic hosing. Vendors can maintain the arms’ operating envelopes and support systems (i.e. hydraulic unit) without major modification.
An important design objective of the PLNG concept was to achieve a standardized, modular approach, in particular for the liquefaction and storage platforms, in order to provide scalability, flexibility, and applicability worldwide. Each of these objectives is intended to control costs and increase the operating lifecycle. Additionally, should economics require, the PLNG platforms can be relocated to alternative sites requiring only new jackets and piles built for site-specific criteria.
The fabrication yard requirements were an important consideration in the development of the PLNG design, in particular regarding crane size and quayside dimensions to accommodate heavy and large decks. While only a small selection of global shipyards can provide FLNG hull construction, the HongHua Offshore yard, equipped with a 22,000 tonne crane and deepwater access (adjoining two major shipyards and conveniently located by the Shanghai shipbuilding and industrial center), provides the ideal solution for the global execution plan.
This wide range of platform designs and configurations utilized in the PLNG concept allows for precision strategic planning for the large variety of available marine transportation and installation vessels in the marketplace. Liquefaction and storage platforms’ greater topsides weight and dimensions exploit transportation and installation float-over methodology using self-propelled, self-ballasting vessels. This methodology removes derrick barge uncertainty, schedule constraints, and increased costly offshore installation and hookup while greatly enhancing project success by minimizing the single largest risk component for marine installations.
Braemar-TankTek FSP LNG containment system.
The PLNG solution is presented as a less complex solution because separating each terminal component into its own fixed platform allows for “plug-and-play” flexibility and scalability. PLNG jacket and deck components vary in size, ranging from small tripods decks to larger multi-pile jacket, multi-deck structures. The benefits of segregating hazardous from non-hazardous and critical areas contribute to the overall safety performance, so risk hazards are manageable. This inherently safer design eliminates risks through spacing that is highly restricted to FLNG hull facilities.
Following the HAZID sessions, and to promote positive safety assurance, the PLNG layout configurations were developed and modified such that:
1. High-pressure/high-temperature gas processing systems were segregated onto their own platform(s) and set in the proximity of the liquefaction trains, allowing for a logical process flow, temperature and pressure profile. High-energy density refrigerants are segregated onto such a platform(s) and apart from other critical platforms to keep these volatile materials isolated and downwind of the terminal.
2. Likewise, flare and venting systems are segregated onto their own platform, located away and downwind from the terminal, but in proximity to the LNG production platforms to minimize the flare piping network.
3. LNG production platforms are grouped and segregated so high-pressure systems, rejected heat and noise are drifted away from accommodations and other areas. Similarly, storage platforms are also grouped and segregated to maintain design and safety criteria.
4. Separation distances between LNG production and storage provides a safeguard from potential leakages and dispersion to live equipment (sources of ignition).
5. The marine docking arrangements are set downstream of the prevailing wind and current so LNG carriers are able to drift away in case of mooring failure.
6. For personnel safety, the accommodations platform is installed at a safe distance away from other platforms. It will, however, be at a reasonable distance to allow personnel direct bridge-connected access to other platform work areas.
7. The PLNG terminal orientation, loading and berthing arrangements are configured to facilitate the approach of LNG carriers relative to established shipping routes or anchorages, requiring the least amount of maneuvering while maximizing distances to other terminal platforms.
The innovative execution approach by Wood-EGT-HongHua for the PLNG concept leverages global market opportunities, and proprietary resources and services. The numerous underlying factors considered in the PLNG development included:
1. Repeatability – The use of standardization where possible to design one facility and build many
2. Fully outfitted and pre-commissioned topsides decks in the fabrication yard, greatly reducing offshore work
3. Use of state-of-the-art HongHua fabrication facilities equipped with the world’s largest gantry crane for loading onto heavy-lift vessels
4. Ease of deck transportation for reduced transport fatigue, faster delivery and installation, using float-over vessels
5. Ease of deck installation for minimum installation time
6. Ability to relocate facilities if necessary – Topsides re-deployment
7. Facility life-cycle extension using modularization packages that can be upgraded as necessary.
The PLNG concept excels as an alternative for mid-scale LNG developments in 60 m or less water depth near-shore locations. The PLNG concept is flexible and scalable, accommodating additional capacity as necessary. The PLNG concept is suitable for locations between near-shore FLNG and FSRU solutions, or as an alternative where an onshore LNG facility may be cost prohibitive or site constrained. The PLNG is cost effective, fast to construct and less complex to execute than other LNG infrastructure solutions. Conventional LNG processing and handling equipment, reliably used in the industry today, is utilized on a PLNG terminal without the need for major modifications. The pre-FEED results show the Wood-EGT-HongHua PLNG concept presents a cost-effective, efficient and robust alternative for an onshore LNG or near-shore FLNG development in which natural gas is processed into LNG for export.