This study of the behaviour of high-density polyethylene umbilical hoses subjected to constant and cyclic variation of pressure and temperatures attempts to simulate the temperatures and stresses experienced in offshore operations. The hoses are used to carry fluids to the top of the wellhead and provide protection for the electrical and optical controls systems. Measurements are reported for exposures at 40 C, 70 C and 100 C in water, methanol and xylene, using an applied pressure of 200 bar. The changes in the physical properties of the hose were monitored by measurement of the tensile properties, dynamic mechanical thermal analysis, differential scanning calorimetry and gravimetric uptake of the fluids. Significant changes occur immediately on application of pressure and reflect changes in crystallinity. The rates and extent of the modifications depend primarily on the ageing temperature but are also influenced by the fluid. Water has little effect on the rate at which ageing occurs, whereas xylene, which is a potential solvent for high-density polyethylene, exhibits characteristics of plasticization. Methanol behaves as a weak solvent and shows characteristics intermediate between xylene and water. Burst tests carried out on the aged material show that significant loss in strength is only observed with the highest temperatures and most aggressive solvent systems. The study indicates that engineers should use pressure-aged rather than initial materials data when designing umbilical hose systems.
In this study, measurements of nylon 11 pipes subjected to ageing at 40 C, 70 C and 100 C in water, methanol and xylene using both a constant pressure of 200 bar and a cyclic pressure regime are reported. Gravimetric measurements indicate the rate at which the solvent is absorbed by the polymer, and differential scanning calorimetry follows the changes in the crystallinity as the materials are aged. Dramatic changes in the tensile properties are observed when the pipes are subjected to high pressure and reflect a relaxation of the stress in the matrix introduced by the quenching process when the pipes were extruded. The magnitude of the change varies with the fluid and reflects their relative abilities to be absorbed by the polymer. Measurements of the relative viscosity for the polymer indicated that in the case of water and methanol, hydrolytic degradation of the polymer is time-dependent. The impact of the morphological changes in the dynamic mechanical properties revealed the movement of elasticity transition to higher temperatures and reduction in the chain mobility with time. Changes in the mechanical properties are a function of the initial stress relaxation and chain scission as a consequence of degradation and thermodynamically driven morphological changes increasing the crystallinity and embrittling the polymer. As the pipes aged the burst pressures progressively decreased. Examination of the failure surfaces indicated brittle failure and clear evidence of environmental stress cracking. Whilst the data indicate that the pipes might be used effectively for hydrophobic fluids, hydrophilic fluids and in particular methanol can significantly shorten their effective life.
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