OTC 16503 How to Provide Relevant Data for the Prediction of Long Term Behavior of Insulation Materials Under Hot/Wet Conditions ? Dominique Choqueuse (IFREMER), Angèle Chomard (IFP), Pierre Chauchot (IFREMER) Copyright 2004, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, U.S.A., 3–6 May 2004. This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract The thermal behaviour of fluid transport pipelines must be increasingly taken into account in new field developments, in order to prevent production of waxes or hydrates. The deepest developments (water depth> 1500m) are particularly sensitive to these problems and solutions are needed for such depths. The currents solutions, coated pipe, pipe-in-pipe, syntactic foam, have to be adapted to ultra deep sea, as there are limitations in terms of thermal conductivity, hydrostatic pressure, and installation possibilities. One of these solutions considers the insulation material to be in contact with the surrounding environment, which means high pressure, sea water and, in the case of modules, high temperatures (up to 130°C). In the framework of a national project a solid experience has been developed in the evaluation of insulation materials applicable to ultra deep sea flow lines. Given the long service life of these structures (20-30 years) particular attention has been focussed on the long term evaluation of the properties. This paper will present a methodology for the definition of a proper ageing test programme in order to provide relevant data for the prediction of the insulation material long term properties. Ageing test conditions must couple the pressure with the temperature in natural sea water. Accelerating ageing factors such as the temperature and the sample dimensions are also considered. This method is now applied in an on-going JIP. Tests on samples under representative conditions provide relevant data on different types of insulation materials. These will be used as input data in a model to estimate the long term behaviour of the materials under service conditions, in terms of both thermal and mechanical properties. Presentation This paper presents a test methodology for predicting the long term behaviour of insulation materials under hot/wet conditions. It is a one of the results from a three year program carried out in France which involved the main actors of the offshore industry (ref 1-3) and which mainly addressed syntactic foams. The characterisation of such materials is discussed here and results led to a recommended practice and a specification which have recently been proposed for the materials concerned (ref 4-5) The first part consists of a brief reminder and discussion of the tests performed to evaluate the essential initial properties of such materials. In the second part, taking into account the long lifetime requested (20-30 years) special interest is focussed on the long term evolution of properties. Considering a significant thickness of an insulation material and the temperature gradient through the insulation layers, the way to access the variation of properties of the materials in relation to temperature is highlighted. The basis of a model is proposed in order to estimate the long term behaviour of an actual insulation system. Based on the lessons learnt in the previous program, a JIP has been proposed to the major actors of the offshore industry and is now underway. The program of the JIP is described in the third part of the paper. Initial properties From the large list of tests used to verify the initial properties of insulation materials, some have been judged to be of primary importance. The way to verify the capability of a manufacturer to provide material specimens of large size has already been discussed and the control of large blocks by X ray tomography has showed its interest (ref 1-3). For the hydrostatic behaviour of the material, a specific test has been developed and has been described in previous papers. Special effort has concentrated on the way to determine the thermal conductivity. Based on the results of a round robin carried out between different laboratories, the way to conduct tests which can guarantee confidence in the results has already been presented. Determination of the heat capacity is now considered. For the case of the behaviour of an insulation system in a transient 2 D. CHOQUEUSE, A. CHOMARD, P.CHAUCHOT. mode (shut down), the heat capacity of the material is a parameter which will strongly govern the thermal behaviour of the insulation concept, in particular when a large volume of insulation material is used (ref 6-7). Measurements using a standard calorimeter or a Differential Scanning Calorimeter could be used in accordance with standards (ref 8). However a significant improvement in terms of accuracy, repeatability and speed of measurement seems to be obtained by the use of a Modulated Differential Scanning Calorimeter. The modulation of the temperature increase allows direct information on the heat capacity of the material to be obtained after signal analysis. An example of results is presented in figure 1. made to access the long term behaviour of the material. An ageing program was initially built on a 18 month test period in order to determine the water uptake kinetics of the material under hydrostatic pressure at four temperatures (4°C, 40°C, 80°C, 150°C) and then provide the reference data for these materials. These temperatures were chosen to cover the range of use of the material. - to evaluate the evolution of the thermal and mechanical properties induced by the ageing - to propose a model allowing extrapolation of the behavior of the material used in passive insulation systems to a long duration (up to 25 years). This initial base program has been completed by : - an aging test campaign at atmospheric pressure, at 3 temperatures 4, 40 and 80°C , in fresh water and sea water to improve the understanding of the degradation mechanism induced by the hydrolytic aging, - the development of a model allowing for classic water uptake kinetics (Fickian diffusion) to estimate the long term behavior of a system subjected to a gradient of temperature. 2.4 2.2 Cp (mJ/°C.mg) OTC 16503 2 1.8 1.6 1.4 The main lessons from this program are described hereafter. 1.2 1 30 60 90 120 150 T (°C) Figure 1 : Evolution of the heat capacity versus temperature for five insulation materials (measurements using MDSC) The heat capacity of five insulation materials has been determined from 30°C to 150°C. A significant difference of behaviour could be noted in terms of both level and evolution with respect to the temperature. An increase up to 30% of the heat capacity can be observed between 30°C and 150°C and should be taken into account in the calculation procedure. Nevertheless the determination of the heat capacity still needs some further improvements to guarantee the accuracy of the measurements. Such measurements, even using standard DSC equipment, are performed on a very small quantity of material (about 10 mg) which can appear not really compatible with the scale of the components of such materials (for example in syntactic foams). A partial round robin carried out between 2 laboratories, using the same measurement technique, has shown a significant scatter in the results (about 15%) which highlights the need to increase the accuracy in the determination of the heat capacity. Long term behaviour Taking into account the long lifetime requested for the deep offshore underwater applications a special effort has been Concerning the kinetics of the water uptake, some general conclusions have already been published (ref 3). They can be summarized in three main points. The very poor behavior of materials in contact with water at 150°C. The degradation process initiated is generally severe (complete hydrolysis for some materials) and can be initiated from 80°C if the temperature is coupled with the pressure. It is of primary importance to couple temperature and pressure to access the real behavior of the material. A significant difference has been observed between the behavior of materials immersed in sea water and in fresh water. Evolution of the thermal and mechanical properties due to aging. Concerning the degradation of the material properties it can be concluded that the water uptake will generate a significant loss of mechanical properties. However no relation has yet been established between the water uptake and a corresponding loss of the mechanical properties. The degradation mechanism induced by the water uptake is very complex (resin plastizisation, hydrolysis, interface degradation, degradation of fillers…) (ref 9-10). In addition as the degradation is not homogeneous through the thickness of the sample, the quantitative assessment of the mechanical damage is very difficult. Concerning the evolution of the thermal properties of the materials tested, a model has been developed in order to link the water uptake with the evolution of the thermal conductivity. This model is based on the rule of mixtures and can be expressed as follows : OTC 16503 How to provide relevant data for the prediction of long term behavior of insulation materials under hot/wet conditions ? ∆λ = vfw * λw 3 classical Fickian curves. It can be noted that the values of the shift of the curves which were used are in direct relation with the Surface/Volume ratio. with vfw = water volume fraction λw = water thermal conductivity 3.5 0.400 3.0 water uptake(%) For large water uptakes, the correlation between ∆λ calculated from the model and ∆λ measured on aged samples is quite good and is reported on figure 2. Knowing the evolution of the water concentration level through a thickness of a coating this can allow us to determine the evolution of the thermal conductivity. 2.5 2.0 1.5 1.0 0.5 0.300 ∆λ calculé (W/mK) 0.0 0 20 0.200 40 1/2 60 2 80 100 3 √t(h )*S(mm )/V(mm ) Figure 3. Raw data of the absorption curve (syntactic foam T° = 40°C P = 0.1 MPa) 0.100 0.000 -0.100 0.000 3.5 0.100 0.200 0.300 0.400 3.0 ∆λ mesuré (W/mK) Figure 2. Relation model/measurement of the evolution of the thermal conductivity Determination of the water uptake kinetic. A general model of the water uptake kinetics has been established. It is based on the analysis of the water uptake of samples of different sizes. It takes into account the size of the samples, and in particular the surface/volume ratio for samples not being considered as plates of infinite length and width. The sizes of the samples used for these tests are shown in table 1. Type A B C D E Size (mm) 3*10*60 10*10*10 20*20*20 50*50*20 50*50*50 water uptake(%) -0.100 2.0 1.5 1.0 0.5 0.0 0 20 40 60 80 100 √t (h1/2)*S(mm2)/V(mm3) Figure 4. Absorption curve (syntactic foam T° = 40°C P = 0.1 MPa) - Table 1 : size of samples It has been established that : - the initial water uptake is related to a surface phenomenon. The water uptake of 11 samples has been followed during 14 months and the results are reported on Figure 3. The initial water uptake has been subtracted in order to obtain (figure 4) a 2.5 - For most of the materials, in the low temperature range (T<40°C), the water absorption kinetics can be modelled by the Fickian diffusion process. The quantity of water absorbed is in the range of a few percent. The diffusion coefficient and the saturation level can then be determined. For more severe environmental conditions (temperature and pressure) the shift versus the Fickian diffusion inducing more important quantities of water absorbed could be modelled by a penetration speed reported in mm/year. The quantities of water absorbed are here in the range of some tens of %. 4 D. CHOQUEUSE, A. CHOMARD, P.CHAUCHOT. This phenomenon is reported on figure 5. It must be noted that on this curve, the x axis scale is in time*S/V and not in √time *S/V. The linear part of the curve justifies the modelling of the water uptake by a penetration speed which could be attributed to a transport phenomena in porous media in relation with Darcy’s Law (ref 11). 25.0 water uptake(%) 20.0 15.0 10.0 5.0 0.0 0 5000 10000 t(h)*S(mm2)/V(mm3) 15000 Figure 5. Absorption curve (syntactic foam T° = 80°C P = 0.1 MPa) A global model of water uptake kinetics can then be established with the following input parameters: - initial water uptake by surface unit (%) - the Fickian diffusion coefficient - saturation level for the diffusion process (%) - the penetration speed (mm/h) - saturation level in the porous media The accelerating factor generally retained for the prediction of the long term properties of polymer materials, is the temperature. The base principle used is the Arrhénuis law. The analysis of the results obtained during the program clearly shows the limits of this approach for composite materials such as syntactic foams used in the range of high temperatures (up to 130°C). Considering : - the applications where the thickness of the coating is in the range of a centimetre or even the tens of centimetres, - the possibility to perform tests on samples within the range of millimetres of thickness, - the fact that in the range of the temperatures considered, an increase of temperature can notably modify the mechanisms of water uptake kinetic, the attractiveness of using the size of the samples as an accelerating factor is clearly established. OTC 16503 The model allows the test results obtained on samples of small size to be used to predict the long term behaviour of samples of bigger sizes. Considering a material for which the absorption kinetics are governed by the diffusion and Fick’s law, the accelerating factor, in terms of time for samples of thickness ten times smaller, is 100. Considering a material for which the kinetics are governed by the transport phenomena in porous media, the accelerating factor is then only 10. Modelling of the system behaviour Several configurations for the insulation systems can be used : coated pipe, with or without protective layer, insulation module, ..; These different concepts result in the material being subjected to a temperature gradient through the thicknessso the absorption kinetics will also vary through the thickness. It is then necessary to consider a water uptake model taking into account a temperature gradient through the thickness. Moreover the main following assumptions are used: - infinite plate length - material behaviour in terms of water diffusion following Fick’s law - diffusion coefficient in accordance with Arrhenius’s law D =D0 exp (-C/T) - the saturation level does not depend on the temperature - in the case of a multilayer system the chemical potential of the two adjacent materials are maintained at the interface (ref 8-9) - limit condition on the face in contact with water is c = saturation level - limit condition on the face with no contact with water is dC = 1 (concentration gradient perpendicular to the surface) - temperature at the surface and in the material are governed by the thermal conduction. The model allows the water profile through the thickness of the insulation system to be visualized and quantified. Three cases have been studied which are reported on table 2. Temperature face 1 Temperature face 2 Face 1 in contact with water Face 2 in contact with water Total thickness (mm) Protecting coating thickness 1 2 casing coated pipe 4°C 4°C 80°C 130°C yes yes yes no 50 25 Table 2 : model parameters 3 coated pipe 4°C 130°C yes no 25 2 How to provide relevant data for the prediction of long term behavior of insulation materials under hot/wet conditions ? 3 2.5 2 1.5 1 0.5 0 80 60 40 20 0 0 10 20 30 40 temperature (°C) % water uptake casing (2 faces in contact with water) 5 years 10 years 15 years 20 years 25 years Figure 6. Water profile at different times Case1 % water uptake 2 1.5 1 0.5 0 0 5 10 15 20 temperature (°C) unprotected coated pipe 2.5 140 120 100 80 60 40 20 0 3 2.5 2 1.5 1 0.5 0 0 5 10 15 20 5 years 10 years 15 years 20 years 25 years temperature 25 thickness (mm) Figure 8. Water profile at different times Case3 50 140 120 100 80 60 40 20 0 protected coated pipe temperature thickness (mm) 3 5 humidity of the environment. % water uptake The evolution of the water profile through the material thickness has been determined from the model. The results are reported on figures 6 to 8. It can be noted that in all cases the evolution of the profile results in the presence of water in all parts of the material. This means that even in the case of a coated pipe, the part of the material at high temperature will be in contact with water. temperature (°C) OTC 16503 5 years 10 years 15 years 20 years 25 years JIP program Based on the experience gained during the five past years on characterizing insulation materials for deep sea applications, and on the demand of several petroleum companies to participate in the ongoing program, a JIP has been proposed by Ifremer, IFP and Bureau Veritas, to the offshore industry. The JIP TIDEEP (Thermal Insulation of Deep sea flow lines) was launched in April 2003. The aim of this JIP is to offer a common industry-wide understanding of the characteristics of the available thermal insulation materials which can be used in thermal insulation systems for deepwater applications (ref 13). This Joint Industry Project aims to address specific characterisation of integral external coating materials. temperature 25 thickness (mm) Figure 7. Water profile at different times Case2 The main conclusion which could be made from these results is that it is important even in the case of a coated pipe to perform aging tests under hot wet conditions within the range of service temperature. In this case, one of the key points for long term prediction could be to estimate the long term behaviour of a material placed at hot temperature at a partial saturation level. Up to now no data are available on this point. This means the development of a specific test which considers aging under controlled humidity conditions. It has been established (ref 12) that the saturation level is directly related to the relative The tests are performed in renewed sea water conditions which correspond to two service pressures (200 bars and 300 bars) and a maximum temperature of 130 °C. TIDEEP includes : - a preliminary screening phase of potential materials according to their expected performance in order to select 5 materials to be tested, - on the 5 materials retained: - a complete qualification of their behaviour in order to determine their thermo-mechanical properties, - a determination of the ageing effect due to a combination of the effects of temperature, sea water (plastizisation, hydrolysis, …) and hydrostatic pressure, - a proposal of models to extrapolate the long term properties (up to 20 years) which will be based on the analysis of potential degradation mechanisms and the use of currently approved models. A “degradation allowance” could be defined to be included in the specified coating thickness at the design stage. 6 D. CHOQUEUSE, A. CHOMARD, P.CHAUCHOT. The program of the JIP is reported on table 3 Investigation of potential materials Collection of manufacturer data sheets Selection by the SC of 5 materials 2. Initial properties Materials supply Homogeneity control of blocks characterisation (on 5 materials) Characterisation of materials Analysis Quality control 3. Properties after Ageing at atmospheric pressure Ageing under hydrostatic pressure ageing (on 5 materials) Properties after ageing on 3 materials Analysis Quality control 4. Modelling of long Modelling of the kinetics of water absorption term properties Modelling of a representative volume, at service pressure and temperature Qualification procedure proposal 5. Synthesis Complementary conclusions to existing specifications or RP 1. Screening of potential materials Table 3 : TIDEEP JIP program tasks Six companies have joined the JIP. Five selected materials have been chosen by the Steering Committee (fig 9) and the aging program is now underway. Figure 9. Samples of the Tideep program Conclusion To define a correct design of a particular insulation system, it is necessary to access data such as the evolution of the OTC 16503 insulation material properties versus temperature and time (up to 20 years). Some first results have been obtained during a five year programme focussed on that complex problem. Considering the long term properties, it has been shown that : - aging tests have to be performed in hot and wet conditions within the service range of temperature and pressure , - the aging media (natural sea water or tap water) can have a strong influence on the degradation mechanisms and rate. Natural sea water must be used in aging cells, - it is more accurate to use the size of the samples as accelerating factor than the temperature , - Fick’s law can only be used to predict long term behaviour of the material when no degradation occurs. Based on these conclusions, a thorough ageing test program is now ongoing within a JIP on five selected insulation materials dedicated to deep sea applications. First results and experience will be used to propose a global adapted model that will properly describe the thermomechanical behaviour of insulation materials under service conditions and for a long term use. Reference : 1 – “Insulation material for ultradeepsea pipe line”, D.Choqueuse et al, DOT2000 2 – “Insulation material for ultradeepsea flow assurance : how to predict 20 years life time”, D.Choqueuse et al, DOT2001 3 – “Insulation material for ultradeepsea flow assurance : evaluation of material properties”, D.Choqueuse et al, OTC2002 4 – “Recommended practice for Insulation and buoyancy systems” doc MCS ref 1-1-4-140/RP01 5 – “Specification for Insulation and buoyancy materials”, doc MCS ref 1-1-4-140/SP01 6 – “A new transient thermal model for subsea pipeline cooldown”, Y.D Chin, OMAE2001/Pipe 4003 7 – “Simulation of transient heat transfer in multilayered composite pipeline”, J Su et al, OMAE2001/Pipe 4126 8 - ASTM C518-91 "Standard test method for steady-state heat flux measurements and thermal transmission properties by means of the heat flow meter apparatus" 9 – “Vieillissement physique des plastiques”, J.Verdu, Techniques de l’ingénieur A3 1990 10 – “Action de l’eau sur les plastiques ”, J.Verdu, Techniques de l’ingénieur AM3 2000 11 - "Transport phenomena in porous media II”, edited by D.B.Ingham and I.Pop (2002) 12 – “A comparative study of water absorption theories applied to glass epoxy composites”, P.Bonniau et A Bunsell, Journal of Composite material (1981) 13 – TIDEEP proposal, doc Ifremer (2003)