Offshore Tandem Loading of LNG – from Idea to System Approval

Abstract

The paper describes the background and design approach used to develop and qualify a LNG transfer system to be used for tandem loading between a 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, as the standard rubber hoses used in Crude Oil transfer operations no longer can be used due to the product's low temperature. The core issues in LNG tandem loading are;The suitability and qualification of the hose/flexible pipeThe marine operations and vessel motions in relation to Dynamic Positioning systemsThe mechanical design of the loading system The development of the Offshore Cryogenic Transfer system (OCT) 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 LNGC's, 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 sub-systems according to EN 1474–2; the pull-in and connection, the connectors and the transfer hose/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, connect, loading and disconnection phasesThe performance / operating life / safety features of the flexible cryogenic pipes for LNG transferThe design of the flexible pipe manipulating arm / A-frame storage system on the FLNGDesign of the pull-in and connect system in the bow of the LNG carrierQualification of the sub-systems 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 minimum 2, possibly 3 hoses/pipes needs to be connected to the bow of the LNGC instead of the single rubber hose as used in crude oil transfer systems. These subjects are elaborated in more detail in the following sections.