A prograrnme of experimental work has been carrled out under the "Umbilicals - the Future" joint Industry umbilical research project, In which the behavior of thermoplastic hoses In response to rapid pressure changes has been examined in detail The hose characteristics which have been investigated include steady state volumetric expansion, ageing, visco-elasticity and dynamic volumetric expansion Simulation results are presented which show the importance of correctly accounting for these effects when designing subsea well control systems, particularly for systems where there is no subsea hydraulic accumulation
Hydraulic umbilical hoses used for direct hydraulic and pilot hydraulic operation of subsea production control systems have traditionally been required to exhibit minimal volumetric expansion(VE) under pressure to minimize the system response time. This rapid operation is now a feature required for direct hydraulically-operated pipeline safety valves The growth in popularity of electro hydraulic subsea production control systems over recent years has, however, provoked fresh Interest in the behavior of the umbilical hydraulic hoses when under pressure In such systems the volumetric expansion of hoses can be utilised to supply the peak flow requirements of individual Christrnas tree valve actuator openings Using the hose as a storage element can eliminate the need for local subsea accumulation, which is an attractive option for reducing costs and improving system reliability
Thermoplastic hoses exhibit a visco-elasticity time-related response to changes In pressure, whereby after initial pressurization of a hose, the pressure decays to a stabilized level over a period of time as the hose gradually dilates A similar time-related contraction of the hoses occurs during de-pressurisation Thus the hydraulic control hoses within the umbilical make relatively poor accumulators, unlike the conventional bladder type accumulator, the hose wall movement, is both damped and hysteretic
Where hoses represent the only hydraulic accumulation with the system, there is the danger that the supply pressure to the subsea control module directional valves will momentarily collapse during the opening of large capacity tree valve actuators System designers, therefore, need to know both the static volumetric expansion of the hoses, and their dynamic expansion relaxation characteristics In response to pressure changes
The system designer's task is complicated further by the phenomenon of hose ageing, which is well known but little understood Over the period of a few weeks at pressure the volumetric expansion of thermoplastic hoses has been found to decrease, indicating an increase In the stiffness of the hose construction Thus, a control system which utilizes hose volumetric expansion to supply the flow requirements of individual tree valve openings may perform satisfactorily during commissioning, yet be found wanting after a period In service Because of this uncertainty, control system suppliers often feel compelled to fit subsea accumulators, even though they may not in practice be necessary
This paper describes the work carried out under Work Package 12 of the "Umbilicals - the Future" joint Industry funded programme
