Moving liquefied natural gas (LNG) production offshore certainly presents its challenges for the offshore oil and gas industry, particularly when it comes to the design and construction of a floating offshore gas liquefaction plant (FLNG). This is because FLNG facilities need to maintain the utmost levels of safety and give increased flexibility to LNG production while withstanding the effects of winds, waves and currents in the open seas.
However, with prospective new FLNG locations moving away from ‘mild’ areas to sites where sea states, wind and currents can be much more severe, is standard equipment enough? Where conditions are much more demanding, conventional marine loading arms will simply not suffice, as they can result in the shut-down of the liquefaction plant in bad weather conditions. Vincent Lagarrigue, Marketing and Project Manager from Trelleborg Industrial Solutions explains why tandem offloading solutions, which rely on the use of flexible hoses are a viable alternative for the industry – not only limiting downtime, but also improving safety.
Many solutions which reduce the effect of motion and weather have been considered for FLNG transfer. Primarily, traditional LNG loading arms have been adapted to enable LNG ship-to-ship transfers in open water through side-by-side configuration. While loading arms can handle both liquids and gases, environmental constraints such as tide and wind conditions as well as earthquake tolerances, can have a significant effect on performance; compared to hoses, loading arms lack flexibility.
Tandem offloading, where vessels line up stern to bow, would allow vessels to keep more distance between them (328 feet / 100 meters distance between FLNG and LNG carriers or more) and cope more easily with greater wave heights. This significantly limits the risk of collision between the two vessels, enhancing safety, but also greatly simplifying naval operations in approach, berthing and residence.
A Question of Safety
The main objective of tandem offloading systems is to be able to transfer LNG at a similar flow rate compared to traditional ship-to-shore offloading operations performed along jetties equipped with LNG loading arms, and to enable this transfer in difficult environmental conditions.
Systems and their associated floating hoses, must be designed to operate in sea states with significant wave heights of up to 13 feet / 4 meters at connection and 15feet / 4.5 meters during transfer and disconnection, even with non collinear wind or current directions. These figures guarantee very good offloading availability in almost any location in the world.
Safety requirements have put the design of the tandem offloading system through a number of considerations, all aimed at limiting risk of personnel exposure and damage to equipment and facilities. One example is the use of floating hoses in tandem configuration, which enables the LNG carrier (LNGC) to be almost as far away from the FLNG as requested by the operator; an arrangement which brings a clear safety benefit compared to side-by-side loading or tandem loading with aerial hoses. In addition, in exposed conditions the less time the LNGC stays connected, the safer the transfer operation will be. It also improves offloading availability; degradation in weather conditions is less likely to occur if the offloading window is shortened.
So, in the absence of any mature tandem offloading solutions using floating hoses, leading manufacturers have initiated the development of their own. For example, leading manufacturers Trelleborg and Saipem have teamed up to develop a new LNG tandem offloading system which utilizes three Cryoline LNG floating hoses, as well as a hose storage system, a connection head with a dedicated storage platform on the LNG terminal and bow loading platform on the LNG carrier.
However, to meet the challenging demands placed on these new tandem systems, this technology has been further enhanced, with new parameters being put forward for the development of these hoses. For example, the choice of a 20 inch / 50 cm inner diameter LNG hose was required as this enables operators to transfer LNG at least as fast as standard LNG loading arms on traditional jetties, i.e. up to 423,776 ft3/h / 12,000 m3/h.
The end result was an LNG floating hose based on a hose-in-hose concept that consists of a field-proven outer rubber marine hose with an inner LNG composite hose, which is already well established, in particular for use in LNG ship-to-ship transfer.
A Dedicated Design
This new floating cryogenic hose consists of an inner cryogenic hose, an outer protective hose, an efficient insulation layer and an integrated leak monitoring system. Composite LNG hoses have already proven their suitability for such an application as this technology has been validated through many full scale static and dynamic tests, and many offshore ship-to-ship LNG transfers.
The annular space between the inner and outer hose is filled with insulation materials which have excellent properties over the full range of temperatures (from ambient to cryogenic temperatures). As long as external environmental conditions are above +41 °F/ +5° C, the insulation layer is designed so that no ice will form on the outer cover of the cryogenic hose.
These materials have been designed to reduce heat loss within the structure, to protect the outer rubber-bonded hose from cryogenic temperatures and to ensure LNG hose buoyancy. In addition, an integrated leak monitoring system based on optical fiber technology for gas leak detection has been included in the design in the annular space between the inner and outer hoses.
A compact and specific connection system has been designed for the application. This new technology will typically consist of 39 feet / 12 meter long sections, which will be connected together – either onshore or offshore – with threaded rods and nuts, in the same way as conventional flexible bonded hoses for oil applications.
A new concept of end fitting has also been developed in order to ensure load transfer and leak tightness, and to minimize heat loss within the offloading lines. The design of the connection system includes dedicated seals for cryogenic application which are used for static and dynamic applications, exhibit excellent sealing integrity in gas and fluid applications, and withstand rapid changes in temperature.
Developed using Finite Element Analysis (FEA), the end fittings allow for coupled thermal and mechanical loads at the very first steps of the design process. In a second step, the calculations have been validated through full scale tests performed on a dedicated test bench so as to validate the design of the connection system, to demonstrate the tightness of the connection design at room and cryogenic temperatures, and to endorse the choice of the cryogenic sealing technology. For example, cyclic compression loads up to 200 tons have been applied on a full scale connection, highlighting a safety factor of 10 in service conditions on the key components.
The main challenge for this LNG tandem offloading system qualification was to qualify the floating hose according to the EN1474-2 standard, which requires a complete set of full-scale tests.
Based on flexible bonded hose technology, which is suitable as an external hose for the floating LNG hose-in-hose concept, the cryogenic hose development program has been focused at an early stage on key elements such as composite hose suitable for transfer in cryogenic conditions and dedicated end fittings for such application. Subsequently, those elements have to be integrated within the flexible bonded hose in order to design a homogeneous, safe and reliable cryogenic hose able to meet the operators’ offloading requirements.
Several reduced scale prototypes have been manufactured and tested since 2009 at ambient and cryogenic conditions, in order to validate theories and demonstrate feasibilities. Expected for completion in 2015, additional 20 inch / 50 cm Cryoline LNG prototypes will be tested within the Qualification Test Program in both static and dynamic conditions to demonstrate the suitability of a flexible hose for LNG transfer applications including mechanical, thermal and flow tests. In particular, a fatigue test will be completed on full scale prototypes – including a complete connection system – to prove that the Cryoline LNG withstands recurrent dynamic loads for long service life.
Typically, the qualification test program includes a cryogenic bending cyclic fatigue test that will reproduce the dynamic load conditions to which the cryogenic hose will be submitted in service conditions. More than 20 tests will be performed, either destructive or non-destructive, at ambient and cryogenic temperature in order to qualify the technology, under survey of Bureau Veritas.
Derived from existing and proven technologies, the latest development in cryogenic LNG floating hoses will become a key component in offloading systems for future offshore FLNG projects. By enabling offshore transfer of LNG in tandem configuration, the cryogenic floating hose will pioneer a step change in the safety of this critical operation. This innovative system will also allow FLNG projects to be considered for harsher conditions, without excessive downtime due to offloading system availability, and with significantly reduced risk.
For more information, go to www.trelleborg.com