Research On Optimization Design And Hydrodynamic Characteristics Of Vertical Long-shaft Fire Pump For Offshore Platform

With the continuous improvement of mining technology,the global offshore oil and gas industry is experiencing rapid,sustained and stable development,which poses a rare opportunity for China due to its huge potential of offshore oil and gas resources.Every year,a considerable number of large oil and gas offshore platform projects start construction in China.Offshore platforms are the main carriers for offshore oil and gas exploitation.However,because it is limited in space,far away from the coastline,and has to be in a state of uninterrupted oil and gas exploration for a long time,great difficulties have arised in the deployment of fire-fighting equipment on offshore platforms.Using seawater as the fire-fighting water source,the vertical long-shaft fire pump（VLSFP）is featured in small footprint,large flow rate,high head,rapid start-up and reliable performance.It is widely used in large-scale wharves,offshore platforms,etc.,which are far away from land and lack a large amount of land-based water supply.The efficiency improvement and stable operation based on optimization design have become an important subject in the development and design of VLSFPs.The present research is supported by the the National Key Research and Development Project"The refined analysis and diagnosis method of the complex fluid mechanics behavior of fluid machinery（No.2018YFB0606101）",the National Key Research and Development Project "Key equipment technology for water supply and smoke exhaust of large fire scenes and integrated fire individual equipment（No.2016YFC0800608）",the National Natural Science Foundation of China "Investigation of the evolution and control of Dean vortex flow in elbow outlet conduit of low-head pumping stations（No.51809120）"and Science and Technology Plan of the Fire and Rescue Bureau "R&D and industrialization of complete sets of fire emergency drain/supply water equipment in complex environments（No.2019XFCX28）".The main research work and innovation results are as follows:（1）The VLSFP（model:XBC18/178-300LC3）driven by a diesel engine is studied.By analyzing the requirements of a number of national standards for this type of pump,a single-stage pump hydraulic model scheme is initially designed.By analyzing the test data,the multi-condition performance parameters of the single-stage model pump are determined.At the same time,the overall structure of the VLSFP driven by a diesel engine and the main components of the pump are summarized,and key application technologies of this type of pump are analyzed.（2）Under the condition that the basic design parameters of the VLSFP impeller remain unchanged,only the inclined design schemes are made for the impeller outlet（including the inclination angle of the impeller outlet θ=0°,5°,10°,15°,20° and 25°）,and the influence of the inclination angle of the impeller outlet on the hydraulic performance of the VLSFP is studied.The results show that there is an optimal inclination angle scheme of the impeller outlet.When the inclination angle of the impeller outlet θ=15°,the head and efficiency of the pump are the best under the design operating conditions,which are increased by 5.95%and 1.19%,respectively,as compared to the original model.Meanwhile,based on the optimal inclination angle of the impeller outlet,the orthogonal experiment method is used to optimize the space guide vane,and the influence of the geometric parameters of the space guide vane on the pump head and efficiency index is analyzed.The results show that the factors affecting the head index are in the following order:the wrap angle of the guide vane φ₂,the distance from blade outlet to guide vane outlet l,the outlet angle of the guide vaneα4,and the inlet inclination angle of the guide vane θ’.On the other hand,the factors affecting the efficiency index are in the order of φ₂,l,θ’,and a4.（3）Based on the no-overload theory of centrifugal pumps,the Kriging approximation model of the impeller parameters（totally 6 optimization parameters are used,including the blade inlet angle of the three streamlines of the impeller,the blade outlet angle,the blade wrap angle and the blade thickness）of the VLSFP is optimized by the improved Particle Swarm Optimization algorithm.The results show that the optimized impeller achieves no-overload at 1.5Qd,and compared with the original model,the efficiency and the head at 1.OQd are increased by 1.63%and by 0.39 m,respectively.The original impeller and the optimized no-overload impeller are combined into new VLSFPs by comparing the hydraulic performance.A two-stage optimal no-overload combination scheme is proposed:the first-stage impeller is the optimized no-overload impeller,and the secondary impeller is the original impeller.Also,a three-stage optimal no-overload combination scheme is proposed,in which the first-stage impeller is the optimized no-overload impeller,and the secondary and laststage impellers are both the original impellers.（4）Based on the inclination angle schemes of the impeller outlet,three pump models,namely,A（inclination angle of the impeller outlet θ=0°+original space guide vane）,B（inclination angle of the impeller outlet θ=15°+original space guide vane）and C（inclination angle of the impeller outlet θ=15°+optimized space guide vane）are carried out the unsteady simulation calculation.The unsteady pressure characteristics at different monitoring points of each component and the distribution of radial force on the impeller under different operating conditions are obtained.The results show that a suitable inclination angle scheme of the impeller outlet can optimize the pressure pulsation characteristics inside the pump.As the flow rate increases,the pressure pulsation at the monitoring point is significantly improved,whereas the radial force of the pump is gradually decreased.After the final optimization,the radial force of the pump model C is better than that of the pump models A and B.（5）The modal analysis of the shafting rotor of the VLSFP connected with different shaft lengths is carried out,and the dry mode and wet mode of the impeller are compared and studied.The results show that the water medium has little effect on the modes of the impeller,but the natural frequency and amplitude of the impeller reduce in the water.The change of the length and number of the single-stage intermediate shaft has a greater impact on the natural frequency and modes of the shafting rotor.In addition,the critical speed of the shafting rotor of the VLSFP connected with different shaft lengths is compared and analyzed.The results show that the orders of the critical speed of the shafting rotors vary with the shaft length.Under the condition that the total length of the intermediate shaft remains unchanged,the critical speed of the shafting rotor increases with the decrease in the single-stage intermediate shaft length.At the same time,the dimensionless parameter "length to diameter ratio δ’ of the intermediate shaft section" is defined,and based on statistical data,the guiding principle for the design of the intermediate shaft section is given,that is,the value range of the length to diameter ratio δ’ of the intermediate shaft section is 25～35.（6）The overall structure of the VLSFP is a typical long cylindrical cantilever beam layout,which is subject to effects of ocean currents.In order to investigate the hydrodynamic characteristics of this type of structure,experimental studies have been conducted in laboratory setting.The effects of varying spacing ratio（Do/d=3.2～6,or P/d=2.8～5.2）and incidence angle（α=0°～30°）on the flow around a three-cylinder array arranged in equilateral-triangular configuration under a sub-critical Reynolds number Re=8000 are investigated.The hydrodynamic forces（drag and lift）on each cylinder were directly measured using piezoelectric load cells,together with concurrent flow measurements using Particle Image Velocimetry（PIV）technique to quantitatively visualize the shear layer development,vortex formation and interaction process.The results show that the flow interference among the cylinders,which shares some similarities with that of two cylinders arranged in side-by-side,tandem or staggered configuration,is further complicated due to the presence of the third cylinder and exhibits many unique features.A number of different flow patterns,including shear layer reattachment,induced separation,synchronized or independent vortex shedding,etc.,are identified over the（P/d,α）parameter space.The different flow patterns over the（Do/d,α）parameter space are also evident in the power spectra of the fluctuating lift coefficient on each cylinder.Moreover,the effects of varying spacing ratio and incidence angle are discernible in the mean and RMS drag/lift coefficients on the three cylinders either individually or as a whole group.（7）Based on the numerical simulation of the flow interference and forces on a typical three-cylinder array in equilateral-triangular configuration that is commonly found in engineering,a method for estimating and predicting the maximum loads（including drag,lift and bending moment）of the VLSFP casing is proposed.The main conclusions are as follows:the numerical calculation method and turbulence model used can accurately predict the complex flow details of the three-cylinder array;both the cylinder spacing ratio and the incidence angle significantly affect the turbulence between the cylinders,the hydrodynamic force coefficient of each cylinder in the cylinder array,and the averaged hydrodynamic force coefficient of the cylinder array.For the full-scale vertical long-shaft pumps arranged in equilateral-triangular arrangement,the maximum drag or lift always occurs at the downstream casing Cy-3 when the incidence flow angle is =30°.This is because Cy-3 is subject to the combined effect of the dual wakes of Cy-1 and Cy-2.