For more than 30 years LNG ship to ship loading has been addressed by several Gas operators. One of the main technical challenges that have been identified has been how to extend and combine the proven technologies of static small bore diameter cryogenic piping and offshore ship to ship transfer of oil to large diameter offshore LNG transfer. In the last years the market has pushed for flexible piping suitable for LNG offshore loading systems, and the industry has responded with different technical solutions based on very different design criteria. In this paper the basic requirements for a LNG offshore loading system are presented. As a minimum the requirement of the European code prEN 1474 - II is quoted. Design Classification of LNG Flexible Pipes Today there are two completely different flexible pipes designs existing on the market, both are proven in their applicationsComposite hosesFlexible metal pipes based on corrugated stainless steel pipes (sometimes called " bellows?? although not necessarily based on the bellows technology, which is in principle limited in length) The composite hose is a proven technology for a wide range of applications, amongst others the offshore loading of all kind of ambient temperature liquids. In the LNG business they are available as emergency unloading hoses. The flexible metal pipe has been used in smaller diameters for more than 30 years for all kind of cryogenic applications, transfer lines for Liquid Nitrogen, Helium and even Hydrogen and Oxygen. So for both design options, the LNG ship to ship loading is a new application of a well known technology
Flexible risers are being deployed in more and more demanding applications in terms of water depth, remote locations, temperature, pressure and corrosive fluids. Focus has been put on long term riser integrity in general, and on fatigue performance in particular, as knowledge of pipe behavior and properties has been advanced over the last decade. In this context, accurate and consistent estimation of riser global and local response to external loading is essential. A methodology has been developed to efficiently calculate irregular wave stress time histories of tensile armour wires for flexible risers. The stress time histories are calculated directly from the global loads which are usually generated by using commercially available well proven global analysis tools. The methodology elevates the dynamic analysis of flexible risers from the conventional regular-wave approach to irregular-wave time-domain approach. This in turn allows a better assessment of the fatigue performance and provides a better fit-for-service assessment or an opportunity to reduce design conservatism. This methodology also allows for consistent stochastic fatigue evaluations to be performed in time domain simulations using the well established stochastic analysis approach. All flexible riser non-linear hysteretic effects are included and phase shift between tension and curvature is also fully accounted for. The key ingredient lies in the generation of transfer functions of all stress components using a validated local analysis (LA) tool based on finite element method. This is done because direct use of the LA tool for long time domain simulations is very computationally intensive and impractical. The stress transfer functions allow direct mapping of the tension and curvature readings to individual stress components, which are combined in a phase consistent manner to obtain the total stress-time histories. This methodology should also work well for other systems having complicated cross sections such as dynamic umbilicals and integrated production bundle, etc. Accuracy of the proposed methodology should be equivalent to that of using the LA tool directly provided that the stress transfer functions are constructed appropriately. In comparison with the traditional regular-wave methodology, this irregular wave approach has been shown to provide a significant fatigue-life improvement for the flexible riser tensile-wire in a deep water West Africa application.
The Norwegian operator Norsk Hydro has more than 80 flexible dynamic risers and service lines in operation at different platforms. Riser integrity monitoring programs have been established for the flexible risers in order to ensure safe and reliable operation. SeaFlex has performed annulus testing on a large number of these risers as a part of the programs. The free annulus volume of a flexible pipe is defined as the volume between the extruded internal pressure barrier layer and the extruded external sheath subtracted the volume occupied by pressure- and tension armor, tape and eventual other layers. Two methods are presently used by the industry for annulus free volume testing of flexible pipes, namely nitrogen pressure testing and vacuum testing. Both methods identify trends of volume reduction with time and to detect annulus flooding. Annulus testing has proven to be an efficient and reliable tool for detecting annulus flooding, blocked vent ports and outer sheet damages. This paper address the challenges related to annulus testing of flexible pipes, advantages, experiences and how such tests and the results are used for condition assessment and monitoring of the risers.
Operational experience has shown that flexible risers producing different combinations of oil, gas and water can be subjected to increased dynamic motions due to slugging — a cyclic accumulation of finite volumes of liquids at a low point of the riser (e.g. sag point of a lazy-S riser) until sufficient pressure is built up behind the slug to push the liquids up through the riser. It has been observed that the slug induced dynamic riser motions can cause riser displacements larger than those generated by moderate and some extreme waves in the absence of slugging. A major impact of the slug induced riser motions is the increased fatigue damage of the tensile wires — the cross-sectional component that most frequently defines the fatigue resistance of flexible riser systems. While international standards like ISO 13628-2 & -11 require and recommend that the effects of slug flow on riser response are considered, they provide no guidance on how to practically incorporate potential slugging effects in pipe design or analysis. A methodology has been developed to determine the remnant fatigue life of a riser subjected to slug induced motions combined with the normally considered vessel motions and wave loading. The methodology is based on using commercially available global and local riser analysis tools. The global analysis tool is used to determine the riser response induced by continuous and regular slug loading combined with loading from different irregular waves, vessel offsets and motions. The slug loading parameters are determined through an iterative process calibrating riser displacements and frequencies with those observed in the field. The local analysis tool is used to determine wire stress transfer functions, which in turn are used to derive wire stress time series from the riser tension and curvature time histories. Stress ranges are identified through rain-flow counting applied on all the calculated stress time series and fatigue performance is estimated using the Palmgren Miner summation of damage using an appropriate wire S-N curve. In a case study, the combined slug and first order wave induced fatigue damage increased by a factor of approximately two compared to the wave induced damage alone. This methodology can be used for: a) riser fitness for service assessments by bounding the impact of slug-induced riser motions observed in the field, and b) new riser design when slugging parameters are adequately bounded by flow assurance calculations.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.