QUALIFICATION OF MULTI-COMPOSITE HOSES FOR STS LNG TRANSFER

Transcription

1 Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE QUALIFICATION OF MULTI-COMPOSITE HOSES FOR STS LNG TRANSFER Gerard van der Weijde, Niels Mallon TNO - Centre of Mechanical and Maritime Structures; Van Mourik Broekmanweg 6, Delft, The Netherlands. Gerard.vanderWeijde@tno.nl ABSTRACT Reliable transfer systems are a key element in developing floating LNG technology. Multi-composite hoses may prove to be a reliable and cost effective solution. TNO, the Dutch contract research organisation, has executed an extensive test program on the multi-composite hose of Gutteling B.V. for Ship-to-Ship (STS) LNG transfers. It has resulted in qualification of the hose in accordance to EN1474-part 2 [2]. This hose is the first product that has been qualified in accordance with this new internationally accepted standard. The test program was performed in close cooperation with Gutteling B.V., EXMAR and DNV. It focused on the mechanical and flow behaviour at ambient and cryogenic operating conditions. In excess of the EN1474-requirements, a multi-level test programme is performed, more samples are tested and more extreme load combinations are applied. The purpose was to provide a data set that enables transfer system qualification in accordance with EN1474-part 3. The hose appears to have a complex mechanical behaviour: elastic non-linear, coupling of deformation modes, hysteric behaviour and large damping. Damage tolerance tests, such as impact and crushing, show that the composite hose performs exceptionally. This paper summarises the test program and describes some of the tests performed in excess of the requirements. In particular, testing for off-spec operation conditions, fatigue and creep is addressed. Absences of engineering models to predict fatigue and creep performance are future obstacles for realising the interesting proposition of multi-composite hoses. Figure 1, Ship to Ship LNG transfer Exmar Gutteling B.V. INTRODUCTION Transfer systems are a crucial enabler for floating LNG [4], [5]. To offload LNG offshore, various aerial and floating transfer systems are currently under development [6], [7]. Some are based on loading arm technology, others on metal flexible pipe or multi-composite hoses. The multi-composite hose consists of helical wires, thin liquid tight films of polymeric materials and layers of fabric like polymeric materials. For floating hoses the composite hose is covered with insulation and a polymeric outer hose. Similar composite hoses are frequently used in the oil and chemical industry for transfer. As such, the composite hose technology is not new. However, reliability and safety requirements and the cryogenic 1 Copyright 2011 by ASME

2 temperature during operation makes the LNG hose a class on its own. In 2007 the Belgian based shipping company Exmar, and US based gas trader Excelerate, developed a near-shore transfer system utilising 8 multi-composite hose of Gutteling, see Figure 1. Through trials the performance of the system was proved and subsequently the system was used for commercial transfers. In parallel, the hose was extensively tested by the Dutch contract research institute TNO. For the qualification a newly developed international standard EN1474-part 2 was used as reference. The Gutteling hose was successfully qualified by DNV in 2010 as the first qualified product under this standard. Qualification of the complete transfer system requires a review of all system components and operating procedures. Ideally the hose qualification program provides sufficient information to feed into the system qualification. In this respect EN seems to contain some flaws. This paper briefly reviews the qualification program and addresses some peculiarities of the composite hose performance. In excess of the qualification program, other testing was performed to provide more input to the qualification of transfer systems. This paper highlights some of these tests. One of the main areas lacking is the substantiation of the fatigue and creep performance of multi-composite hoses. A further analysis of this issue identifies that commonly accepted engineering tools to predict hose performance do not exist. This paper highlights an approach to fill this gap. EN1474 At the end of 2009, the European standard EN1474 entitled Installation and equipment for liquefied natural gas - Design and testing of marine transfer systems [2] was approved. This standard gives general guidelines for the design, material selection, qualification, certification, and testing details for Liquefied Natural Gas (LNG) transfer hoses for offshore transfer or on coastal weather-exposed facilities for aerial, floating and submerged configurations or a combination of these. The standard, recognized by the offshore LNG community, consists of 3 parts [1]- [3]. Part 3 covers the safety of the complete system, which includes for example ERS, QCDC and mooring systems, and handling and operation procedures. This part employs an overall safety philosophy without specifying particular acceptance criteria. Part 3 references parts 1 and 2 that cover the design, qualification and certification of loading arms and transfer hoses, respectively. As such the qualification program of part 2 provides input to the qualification of the system according to part 3. Qualification of the transfer system is to be substantiated by the system user. Qualification of the hose can be substantiated by the hose manufacturer. Note that qualification of the hose includes its end fittings. A hose may be applied in various system configurations. In qualifying a hose it is worthwhile to consider those various configurations as it provides input to the qualification requirements. Part 2 provides guidance. It specifies various tests to be performed. The test methods themselves and the requirements to be fulfilled are open for interpretation. The standard does specify the maximum load requirements, such as proof pressure, burst pressure, maximum working loads. On LNG vessels the typical operating pressure is 3 [barg] and the systems are designed for a maximum working pressure of 10.5 [barg]. This results in proof pressure requirements of 15.8 [barg] and a minimum burst pressure of 53 [barg]. Loads due weight and system constraints need to be multiplied by a factor to derive maximum working loads. For each test required, it is sufficient to test one sample. That is, no multiple samples are required and scatter is not considered. Further, the standard allows using a new sample for each test. There is no requirement to execute various tests subsequently on the same sample. Both testing at ambient temperature and cryogenic temperature is required. Liquid nitrogen can be used for tests at cryogenic temperature. Liquid nitrogen is at ambient pressure colder than LNG (-196 [ C] versus -168 [ C]). The programme heavily concentrates on testing of the prototype hose. Very limited testing on the material level is required. Rather than having very specific requirements, it should be demonstrated that the materials are suitable for their intended purpose. It is recognized by the standardization committee that this new standard needs some time to become mature. OVERVIEW HOSE QUALIFICATION PROGRAM This section provides a brief overview of the qualification program for hoses and addresses some of the peculiarities of these hoses. The stiffness of the hose has to be characterised under expected and maximum operation conditions. For the maximum operating conditions the strength also needs to be proven. The sample does not have to be tested until failure. Figure 2 shows the test set-up used for tension, twist and bending performance of the hose. Figure 2, test setup for tensile, bending, twist and bend radius performance Gutteling B.V.. 2 Copyright 2011 by ASME

3 In tension the hose appears to have a highly non-linear and hysteric behaviour. Due to the hysteric behaviour, the hose stiffness depends on its load history. In addition, the hose length at zero pressure and external force shows a moderate variation. EN1474-part 2 does not provide guidelines on how to deal with the history dependency. For example, for fibre ropes and mooring lines, which exhibit a similar hysteric and nonlinear behaviour under tension, much more details are provided in the guidelines about how to deal with length and stiffness determination [8], [9]. Possibly, these guidelines may be adopted to obtain a better definition of composite hose stiffness and length. Figure 3 shows a typical example of a tensile force-deflection curve at ambient and cryogenic condition. Note that temperature has a considerable effect on the stiffness. When the pressure is increased, or a tensile load is applied, the hose not only elongates, but also twists. When twist is applied, the hose also elongates or becomes shorter. Obviously this behaviour should be taken into account when designing tests. As one may expect, the hose stiffness is very much influenced by the internal pressure. EN1474-part 2 requires quantifying the hose stiffness at its extreme temperatures. Effects of pressure and the coupling of elongation and twist are not addressed by the standard. Hoses returned from service are more flexible than new hoses. This effect was also observed in testing. The stiffness changes after the sample has experienced a certain load history. This is attributed to some forms of (temporary and/or permanent) creep of the hose ply construction. In several tests also the dynamic behaviour of the hose is characterised in order to be able to assess the dynamic system behaviour. The hose exhibits a damping ratio that is an order of magnitude higher than metal structure [10]. It is concluded that determining the stiffness of the hose is much more complex than EN1474-part 2 anticipated. Considering the system configurations in which the composite hose are used, knowing the tensile, torsion and bend stiffness at minimum bend radius has limited value. EI m [Nm 2 ] measured linear straight bending stiffness ambient hose y = 9.6e+002*x + 7.8e P [barg] Figure 4, bending stiffness as a function of pressure Gutteling B.V.. A series of pressure testing is to be performed. It shall be proven that the hose is leak-tight at proof pressure. Also hoses with some light forms of impact and crush damage need to be leak-tight. Unlike for loading arms in part 1, EN1474-part 2 does not specify the acceptable leak rate and the amount of damage to be applied is subject for debate. Figure 5 shows examples of the light impact and crush damage applied to the hose. To further substantiate the hose strength, pressure testing up until burst has been performed at ambient and cryogenic temperatures. Figure 3, Typical force-deflection curve in an ambient and cryogenic tensile stiffness test Gutteling B.V. In bending and torsion the hose exhibits a non-linear and hysteric behaviour as well. The hysteric behaviour should be considered in designing test setups and procedures for executing the test [10]. The hose bending stiffness is also dependent on the internal pressure. Figure 4 shows the bending stiffness of the hose for large bend radii as a function of the internal pressure (varying between 0 and MAWP). Flow testing up to the maximum flow rate is required to substantiate the hose integrity under this condition. Though only ambient flow test is required, also a full scale LNG flow test is performed for providing additional justification. This LNG flow test provided valuable input with respect to flow resistance, hose dynamics and leak tightness. Figure 6 shows the ambient and cryogenic flow test set-up. 3 Copyright 2011 by ASME

4 Figure 7, cryogenic bending fatigue test setup Gutteling B.V.. The execution of such a test requires 6 weeks continuous cryogenic operation. The 8 Gutteling hose successfully passed all EN 1474-part 2 requirements. As explained above, hose qualification data is to be used to substantiate the transfer systems. In Gutteling s program higher maximum loads were applied to cover potential future systems. More load conditions were tested to further characterise the hose performance. For several tests more than one sample per test was tested. Also reliability and safety requirements in EN1474-part 3 were carefully considered. This resulted in a significant extension of the test program. The next two sections address some of the extensions to the qualification test program. Figure 5, light impact and crush damage applied to the hose Gutteling B.V.. PERFORMANCE UNDER OFF SPEC CONDITIONS EN1474-part 2 requires testing leak tightness for light damage only. In the extended program, extreme impact damage and crushing were applied at both ambient and cryogenic conditions. Series of damages were applied to determine what damage could be detected in service. With respect to damage tolerance, there appeared not to be much difference between testing under ambient or cryogenic conditions. Figure 8 shows the extreme damage inflicted to the parts at ambient conditions. Similar damage has been applied in cryogenic operating conditions. Even after this extreme damage no leakage occurred. Figure 6, flow test setup ambient (left) and cryogenic LNG (right) Gutteling B.V.. With the so-called thermal fatigue test it is proved that repeated cool down - heat up cycles do not result in leakage. Finally a bending fatigue test is to be performed. Under operating conditions (cryogenic and pressure) a minimum of 400,000 bending cycles shall not result in leakage or damage. EN1474 part 2 specifies neither the load cycle nor what should be considered to be damage or leakage. For the Gutteling hose, a large test set-up was built to simulate the transfer system accurately. The dominant bending fatigue in the system is induced by the roll of the LNG vessels. Figure 7 shows the hose bending fatigue set-up. The similarity with the system configuration in figure 1 is obvious. In the test set-up this bending and straightening is simulated by cyclic moving the right-end upwards and downwards. Figure 8, impact and crush loads applied to the hose Gutteling B.V.. 4 Copyright 2011 by ASME

5 The damaged hose samples were subsequently tested until they burst. The surprising result was that even with extreme damage, the hose burst pressure still well exceeded the requirement. A further positive result was that in cryogenic and ambient burst testing no jetting of liquid occurred. A kind of diffuse leaking process was observed instead. Jetting of LNG is a risk since the cold fluid can potentially reach the steel ship structure. That structure subsequently can become extremely brittle and fail. Obviously countermeasures are required to prevent such an event. With this respect the multi-composite hose has the preferred failure mode. It was assumed that the outer stainless steel wire has an important role on the pressure carrying ability of the hose. Thus, the effect of failing of the outer wire was assessed, see Figure 9. Also in this case no large spill of LNG is anticipated under its normal operating conditions. It is believed that this excellent damage tolerance performance is inherent to the multi-composite hose technology. Figure 10, test setup for hose emergency release. To that end, a system may be installed that releases the hose and couplings in a controlled manner. TNO performed tests on such system at ambient operating condition and at elevated pressures to simulate the higher stiffness at cryogenic condition. Part of the test program was also a release without such system in place, see Figure 10. In this test it was proven that the pressurised hose will very likely not fail and leak. This test program was supported by numerical dynamic analyses using the hose stiffness characteristics determined in qualification testing. The hose dynamics during the release event appeared to be predictable. In a cryogenic axial tensile test it was proven that the hose axial strength exceeds the 600 [kn]. All these additional tests for off spec operation conditions provide valuable input to the risk and safety assessment for qualification of the system. The risk of a large spill due to the direct failure of the 8 Gutteling hose is considered to be very small. This is a major result since up until these tests the hoses were pictured as suitable for emergency transfer only. Figure 9, simulation of failure of the hose outer wire Gutteling B.V.. Transfer systems have to be fitted with a system that allows instant release of the hoses and separation of the vessels in case of emergency. To that end transfer systems are fitted with an emergency release coupling (ERC). That coupling first closes both sides of the system and subsequently separates. The rapid closing operation can result in significant pressure peaks in the transfer and vessel system. The high pressure capability of the Gutteling hose (>150 [barg]) proven in the burst testing covers this effect adequately. In releasing the hoses, in emergency situations, both personnel and the vessel shall be protected adequately to prevent further damage. VALIDATING HOSE FATIGUE PERFORMANCE Particularly for continuous operating transfer systems, fatigue performance is one of the important parameters of the hose performance. EN1474-part 2 requires performing a cryogenic bending fatigue test and to prove that 400,000 cycles can be passed without leakage. EN1474-part 3 supposes that an engineering model is available allowing to translate load spectra into a damage accumulation model to assess fatigue. That load spectra may consist of high frequency, flow induced vibration resulting in small cyclic stresses and low frequency high bending stresses due to the relative vessel motions. Clearly the part 2 testing (one fatigue test only) will provide insufficient data to provide input to these part 3 requirements. In addition, unlike for metal designs, no engineering models exist for multicomposite hoses in the public domain that translates global hose 5 Copyright 2011 by ASME

6 deformations to strains in film and fabric plies. The consequence is that the hose fatigue performance can only be tested on the full-scale level. This is, particularly for large diameter hoses, extremely expensive and time consuming considering the full spectra of loadings. In addition, when a small change in materials or the production process is made, such an extensive program needs at least partly to be repeated. Further, the load spectra of hoses in service will deviate from that of the test program. Without an engineering model one cannot estimate the effect of the deviating service load spectra on the service life. As such the safety and reliability of the transfer system over service life cannot be substantiated adequately without such a model. In the Gutteling qualification program this is partly addressed by doing additional fatigue tests. Such an engineering model consists of two parts. The first part links the global hose deformation to local deformations of the films and plies. The second part involves the material fatigue failure model. The material fatigue failure model can likely be developed on the material level. Figure 11 depicts the anticipated process of predicting the fatigue performance of hoses. For the multi-layer composite hoses such a failure model will be complex since a local small failure in one film will not result in leakage or gross degradation of the hose. A damage accumulation model has to be developed in addition. It is believed that the industry will benefit from such a model. Hose manufactures can validate their products and alter their products at much lower costs and shorter lead times. End users benefit from being able to predict the service life dependent on the application and assess the safety over the service life. The class organisations have a tool to judge the reliability and safety of a hose in a particular system. Therefore TNO took the lead to set up a Joint Industry Project. CREEP PHENOMENA As mentioned in the previous chapter, some form of creep or relaxation makes the hose more flexible over its service life. EN1474-part 2 mentions that for a pressure test the hoses have to be stabilized for some hours. Other than that it does not require any testing for creep. When hoses are used in a system that exposes the hose continuously to a load, it is anticipated that creep performance becomes a significant parameter for safety and reliability. Creep is a function of load level and time. As for fatigue, this can best be treated by an engineering model. That model consists of similar steps as the fatigue model. CONCLUSIONS The recently released EN1474 standard provides guidance to qualifying hoses, loading arms and transfer systems. Aiming to cover all kinds of transfer systems using different kind of technologies, it is very general. When reviewing the Gutteling qualification program on a multi-composite hose for aerial transfer systems, and assuming that hose qualification programs should provide sufficient information for system qualification, the required program seems not to be balanced. The purpose of characterising the hose stiffness is in particular limited considering the non-linearity, hysteric behaviour and creep. Too little emphasis is placed on damage tolerance and fatigue performance. These properties are essential for the reliability and safety over service life. Depending on the system creep would also need to be reviewed in more depth. Multicomposite hoses pose some very interesting performance characteristics such as light-weight, high flexibility and a gentle failure mode which certainly adds to the system safety. The LNG industry will benefit if engineering models are available in order to predict the fatigue and creep similarly as for metal based structure. ACKNOWLEDGMENTS The authors thank Gutteling B.V., Exmar and Exellerate for allowing TNO to use the information from the 8 multi composite hose qualification program. REFERENCES [1] EN (December 2008)- Installation and equipment for liquefied natural gas Design and testing of marine transfer systems Part 1: Design and testing of transfer arms. [2] EN (December 2008)- Installation and equipment for liquefied natural gas Design and testing of marine transfer systems Part 2: Design and testing of transfer hoses. [3] EN (December 2008)- Installation and equipment for liquefied natural gas Design and testing of marine transfer systems Part 3: Offshore transfer systems. [4] Foss, M.M., 2006, Offshore LNG receiving terminals, Center for Energy Economics. [5] McDonalds, D., Chiu, CH, Adkins, D, 2004, Comprehensive evaluations of LNG transfer technology for offshore LNG development, Proc. of LNG14. [6] Rombout G., Peigne, A, Loisel, P., Le Cloirec, A.., Machouat, F. Maocec, D., 2008, LNG trials of a new 16 flexible hose based LNG transfer system, OTC [7] Witz, J.A., Ridolfi, M, Hall, G., 2004, Offshore LNG transfer an flexible cryogenic hose for dynamic service, OTC [8] Offshore mooring fibre ropes, offshore standard DNV-OS- E303, October [9] Cordage Institute standard CI-1500, Test Methods for Fiber Rope, May [10] Mallon, N. and van der Weijde, G., Influence of hysteresis on the dynamics of cryogenic LNG composite hoses, In Proc. of ASME-OMAE 2011, OMAE Copyright 2011 by ASME

7 Evolution of risk Degradation/aeging of plies Film properties Fabric properties Strains, thermal load and aeging in plies Strains, thermal load and aeging in stacks Mechanical- thermal hose model Degradation/aeging of hose Hose production process Hose properties Production parameters Mechanical load spectrum (frequency/time, strain levels) Mechanical- thermal Thermal loads, Aeging hose model Probability of failure: evolution of properties Consequences of failure: failure mode Condition monitoring predicting fatigue performance through an engineering model. 7 Copyright 2011 by ASME