Experimental analysis of the thermo mechanical behaviour of GSPP mater ial

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Moulding of Field Joint

There are two common solutions to mould a Field Joint depending on the injected material: Injected moulded polypropylene (IMPP) and injected moulded polyurethane (IMPU). The preparation for IMPP (Fig 1.22) requires that the anti-corrosion layer is in good condition and the faces of the parent coating (usually PP) are cleaned, abraded and then pre-heated to build bond strength between the PP and IMPP. Once this preparation is completed a mould is placed over the area to be treated and Solid Polypropylene is injected into the annulus. The following Field Joint coating process was proposed by Thermotite® (Fig. 1.23):
• The residual dust from the blasting operation is removed. The temperature of the steel is raised to coating temperature by use of an induction heating coil.
• Using specially developed applicators the bare steel is then coated with Fusion Bonded Epoxy Powder and a sintered layer of adhesive. The adhesive is applied as a powder onto the gelling FBE and forms an interlinking bond to the FBE and the subsequent injected PP.
• The chamfer areas of the parent coating are heated to the softening point of the material by use of radiant heaters.
• The joint area is then enclosed within a mould, and the mould locked in position. The annulus is then injected with molten PP.
• Upon the initial cooling of the surface of the PP, the mould is opened and the joint is quenched with cold water.
• The field joints are then visually inspected for defects or disbonding.
The preparation for IMPU (Fig. 1.24) requires the same preparation. Once this preparation is completed a mould is placed over the area to be treated and Solid Polyurethane is injected into the annulus, often overlapping the parent coating bevel faces and onto the OD surface of the PP.

Constraints during installation of Field Joint

A residual constraint phenomenon is one of the important problems in the field joint installation. Upon the cooling of the field joint material, because of different coefficients of thermal expansion of the materials and the heterogeneity of thermal distribution of this structure, residual constraints may lead to the appearance of a critical zone (with high stress concentrations, even with cracks). Moreover, depending on the installation’s operation, unexpected stress could be introduced when the Field Joint is set up.

Phase change of field joint material

A polymer is composed of long molecules which contain chains of atoms held together by covalent bonds. It is produced through a process known as polymerisation whereby monomer molecules react together chemically to form either linear chains or a three-dimensional network of polymer chains. The main characteristic of the chain is that the chemical bonding is strong and directional along the chains, but they are only bonded sideways by weak secondary van der Waals bonding or occasionally by hydrogen-bonding [Young, 1981]. In offshore exploitation, the most used Field joint’s materials are polypropylene (semi-crystalline) and polyurethane (thermosets or elastomer). The thermal response of their thermo-mechanical properties is different, particularly during the phase change. So, to estimate the residual constraints, the stress state and temperature evolution in the field joint area it is primordial to known the thermal behaviour of the different materials. Depending on the material, the behaviour is different: during the cooling of polypropylene, a crystallization phenomenon that takes place; but in the case of polyurethane, it is a chemical reaction that happens. In addition, for each polymer type, its properties evolve differently at its transition temperature (Fig. 1.25). In the case of a semi-crystalline polymer, there is a transition zone which starts from the glass transition temperature up to the melting temperature, the thermal mechanical properties change less suddenly than those of 100% crystalline polymer. Concerning the crystallization and melting of semi-crystalline polymers like a solid polypropylene, the degree of crystallinity and the size and arrangement of the crystallites have a profound effect upon the physical and mechanical properties. There are many factors which can affect the rate and extent to which crystallisation occurs for a particular polymer: rate of cooling, the presence of orientation in the melt and the melt temperature… The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10 and 80%, thus crystallized polymers are often called « semi-crystalline ». The properties of semi-crystalline polymers are related not only to chemical nature of polymer, but also to degree of crystallinity, distribution of polymer chain lengths and nature of additives [Seymour & Carraher, 2003]. The use of a temperature modulated differential scanning calorimetry (TMDSC) can be useful to study the transitions in semi-crystalline polymers [Genovese & Shanks, 2004; Privalko et al, 2005]. The enthalpy of melting can be calculated depending on the cooling rate of material. The degree of crystallinity of the material can be estimated by the ratio between the measured enthalpy (∆H m ) and the enthalpy in case of 100% crystallinity (∆H 0 ). There are two main TMDSC cooling profiles: linear and modulated cooling. A series of temperature profiles obtained by changing the period of modulation (per), modulation temperature amplitude (Ta), cooling rate (β 0) gave the different specific heat during the change in phase (Fig. 1.26). It is interesting to note that the different cooling profiles result in different released enthalpy and melting point.

Constraints due to installation process

Depending on the installation operation, the critical zones are different. In S-lay, during the field joint moulding, because the pipelines are assembled in a horizontal position, the inhomogeneous cooling rate in field joint material causes inhomogeneous mechanical properties and also creation of air bubbles in the material at the top of field joint.. Another disadvantage of the S-lay method is that the field joint area must be lifted onto the roller system of the stinger (Fig. 1.28). At a short time after moulding, its stiffness may not be enough to support the complex load created by pipeline weight. So a longer cooling time is recommended.
The field joints are always the most critical zones in the structure [Dixon & Jackson, 2003]. During J-lay, the stress concentration occurs in the flowline at the toe of the fillet weld due to the rotation caused through application of tension to the carrier which is transferred to the flowline at the field joint via the swaged section. During S-lay, the stress concentrations are seen to occur in the carrier
pipe at locations where the external sleeve for the field joint ends (Fig 1.29). This is due to the significant discontinuity in bending stiffness between the main body of the system and the field joint with external sleeve. A secondary concentration can also be found at the toe of the fillet weld connecting the carrier and flowline but this is significantly less than those at the end of the external sleeve. Because of discontinuity of material geometry and the residual stress during the Field Joint moulding, the interfacial area between the main coating material and field joint material is a high area for damage.

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Interface between main coating material and field joint material

The field joint coating material used must be compatible with the existing multilayer thermal insulation parent coating systems. To prepare the field joint moulding, the interfaces of parent coating material are heated to the softening point of the material by use of radiant heaters. So the polymer materials are intended to become continuous and not be susceptible to cracking or delamination during installation and operation. In fact, during the field joint material injection, because of the pressure in the mould, then in the material, the microspheres at the interface are brought close together. Thus, an interphase is formed. The higher the pressure and the temperature the easier it is for this interphase to appear. The high concentration of microspheres can lead to a weak link between materials.

Study position

As we have seen in the bibliographic study above, the behaviour of isolated multi-layer materials under the installation and service conditions is complex.
– the strong influence of temperature during the injection moulding of the polymer the evolution in temperature and in time of the thermo mechanical parameters, especially close to the changing liquid-solid phase of the material.
– As the GSPP is a composite material with glass hollow microspheres embedded in a resin matrix it could lead to a specific behaviour under hydrostatic pressures. Our aim is to propose a representative model of such material allowing the behaviour of industrial application under service conditions to be numerically analysed. This model has to be able to take into account the development of residual stress during the Field Joint moulding and the influence of the installation process. The issue of this work is thus oriented around four main axes corresponding to the next four chapters.
First, the thermal analysis is studied. It focuses on the temperature evolution of thermal parameter of main coating material and injection material. For such thermal analysis a correct model of the thermal boundary conditions, especially for convection (heat transfer coefficient, temperature or a fluid created by buoyancy force…) is required. In order to propose a validation of this approach, an instrumented experimental test on a representative application has been developed in collaboration with EUPEC. The aim is to propose a simplified approach in order to numerically analyse the influence of the main parameters of the moulding process of a field joint.
Secondly, specific tests, using a hyperbaric chamber, developed at Ifremer have been used to characterize the mechanical behaviour of GSPP under hydrostatic pressure at various temperatures. The different loading types (monotonic, cyclic and creep) give a global response of material. The tests are completed using experimental analyses with a pycnometer, in order to analyse the evolution of the failure of microspheres with respect to the applied hydrostatic pressure.
Thirdly, a thermo-mechanical model of GSPP allowing hydrostatic creep deformation is developed and implemented in COMSOL Multiphysic® software. For the IMPP material a viscoelastic model based on the generalised Maxwell model is considered to be suitable for a simplified analysis. Creep tests in tension on GSPP and solid PP have been performed on DMA (Dynamic Mechanical Analysis) with respect to temperature to determine the parameters of the model. We focus on the validation of the implemented model and on the identification of its parameters with respect to the experimental tests using an inverse identification type procedure.
The final stage of this study is to build a complete numerical model of the Field Joint, in order to allow an optimisation of industrial pipelines. As an example, a Five Layer Syntactic Polypropylene system using the injection moulding of the Polypropylene is analysed. This model follows the simplified process of an industrial Field Joint coating used with a J-lay installation type method for deep water applications (Fig 1.30). The model takes into account the influence of the manufacturing process and the influence of the service conditions (difference of external temperature and pressure). For each stage of the model, the evolutions of the thermo mechanical state are analysed. The von Mises stress – hydrostatic pressure envelopes allow us to analyse the stress state. The stress at the GSPP – Field Joint coating material and also steel-Field Joint coating material interfaces, which are known as the weakness parts of pipelines, are presented. The encouraging results enable us to define the critical zones. The proposed numerical tool allows an optimisation of the geometrical parameters of the Field Joint to be developed in order to reduce the stress state in the critical zones. Moreover a complementary experimental analysis has been proposed in order to identify the behaviour and the strength of the interfaces.

Table of contents :

List of figur es
List of tables
Résumé en français
Etude bibliographique
Modélisation thermique
Comportement mécanique
Modélisation du comportement mécanique du matériau
Modélisation du comportement de la structure
Conclusion et perspectives
Chapter 1. Pr esentation of the pr oblem
1.1 Overview
1.2 Insulation material for offshore applications
1.2.1 Mainline coating
1.2.2 Field joint coating
1.3 Pipeline installation
1.3.1 S-Lay method
1.3.2 J-Lay method
1.3.3 Moulding of Field Joint
1.4 Constraints during installation of Field Joint
1.4.1 Thermal expansion
1.4.2 Phase change of field joint material
1.4.3 Constraints due to installation process
1.5 Interface between main coating material and field joint material
1.6 Study position
Chapter 2. Thermal modelling
2.1 Heat transfer fundamentals
2.2 Heat transfer coefficient
2.3 Convective modelling by internal or external flow
2.4 Thermal modelling of Field joint
2.4.1 Thermal modelling during the phase change of field joint material
2.4.2 Cooling of field joint in moulding
2.5 Influence of crack on thermal distribution of Field joint
2.6 Proposition of validation thermal tests by injection moulded Polypropylene
2.6.1 Objective of tests
2.6.2 Description of test process
Chapter 3. Experimental analysis of the thermo mechanical behaviour of GSPP mater ial
3.1 Hydrostatic pressure test
3.1.1 Material
3.1.2 Test description
3.1.3 Behaviour under monotonic loading
3.1.4 Influence of loading type
3.2 Evolution of the failure of microspheres with respect to the applied hydrostatic pressure
3.2.1 Objectives
3.2.2 Descriptions
3.2.3 Results
Chapter 4. Thermo-mechanical modelling of offshor e polymer ic mater ial 
4.1 Overview of viscoelastic behaviour of polymeric material
4.2 Viscoelastic model of syntactic foam
4.2.1 Rheology model
4.2.2 Application of rheology model in simple loading cases
4.3 Viscoelastic model for solid polymer
4.3.1 Rheology model
4.3.2 Model implementation in a finite element analysis
Chapter 5. Thermo viscoelastic analysis of a field joint under ser vice conditions ..
5.1 Data for modelling
5.2 Thermo mechanical parameter evolution during the moulding
5.2.1 Material properties and temperature evolution
5.2.2 Stress and strain during moulding process
5.2.3 Stress evolution at the interfaces
5.3 Thermo-mechanical parameters’ evolution during lay down phase
5.3.1 The thermal mechanical variables
5.3.2 Stress evolution at the interface
5.4 Thermo mechanical parameter evolution in service
5.4.1 The thermo mechanical variables
5.4.2 Stress evolution at the interfaces
5.5 Improvement of Field Joint form
5.6 Experimental study on the Arcan assembly
Chapter 6. Conclusion and per spectives
References

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