The paper describes the background and design approach used to develop and qualify an LNG transfer system to be used for tandem loading between an LNG producer (FLNG) and an LNG carrier (LNGC), specifically in harsh environments. Although tandem loading of crude oil is being performed worldwide every day, such operations are always challenging in harsh environment. Transfer of LNG between two large vessels is even more challenging because the standard rubber hoses used in crude oil transfer operations can no longer be used owing to the product's low temperature. The core issues in LNG tandem loading are

• the suitability and qualification of the hose or flexible pipe
• the marine operations and vessel motions in relation to dynamic positioning systems
• the mechanical design of the loading system.

The development of the Offshore Cryogenic Transfer (OCT) system has been based on proven technology wherever possible, not only for components but also for procedures. The OCT system can be an enabler for the development of large offshore gas fields or stranded gas using FLNGs in combination with LNGCs, specifically for harsh environment applications. Several LNG transfer system solutions have been put forward over the past decades, either based on specific transfer ideas or individual product proposals. Of these, the OCT system is the only tandem LNG transfer system that has been awarded a system approval by DnV according to RP A 203/EN 1474–3, as far as is known. The system approval depends on the approval of the main subsystems according to EN 1474–2: the pull-in and connection, the connectors, and the transfer hose or pipe itself.

There are significant challenges involved in offshore LNG transfer between floating units. The following five critical technology areas all need to be successfully addressed and solved in the development and qualification of an LNG loading system:

• safe operation of two large vessels during the final approach, connection, loading, and disconnection phases
• the performance, operating life, and safety features of the flexible cryogenic pipes for LNG transfer
• the design of the flexible pipe manipulating arm and A-frame storage system on the FLNG
• the design of the pull-in and connect system in the bow of the LNG carrier
• qualification of the subsystems and overall solutions according to EN 1474–2 and EN 1474–3.

An LNG transfer system is inherently more complex than crude oil loading due to the requirement of a return gas path. That means that a minimum of two, possibly three, hoses or pipes need to be connected to the bow of the LNGC instead of the single rubber hose used in crude oil transfer systems. These subjects are elaborated in more detail in the following sections.