填埋式柔性液體管道的三維有限元分析案例研究外文翻譯（中文）

1、Zhang, Dr. W., Tuohy, J. 1 Copyright © 2002 by ASMEProceedings of OMAE’02: 21st International Conference on Offshore Mechanics and Arctic Engineering June 23-28, 2002, Oslo, NorwayOMAE2002-28568A THREE-DIMENSIONAL F

2、INITE ELEMENT ANALYSIS OF UNBURIED FLEXIBLE FLOWLINE – A CASE STUDYDr. Wenchao Zhang, Justin TuohyHALLIBURTON WELLSTREAMNewcastle Upon TyneGreat BritainABSTRACTPipelines in the service of conveying hot fluid will tend to

3、 expand due to pressure and differential temperature. However, since the flowline is generally fixed at the end terminations to rigid structures or equipment, such an expansion will be restricted in longitudinal directio

4、n. This is particularly the case for the section remote from the pipe ends, and results in an axial compression in the pipe section. In many cases, a subsea flowline has to be trenched or buried for the purposes of prote

5、ction and thermal insulation. Consequently, the lateral movement of a flexible flowline is greatly limited, and an upward displacement is encouraged that may become excessive. Eventually, the flowline may lift out of the

6、 trench when the uplift resistance provided by the backfill cover and self-weight of the flowline is gradually overcome by the strain energy built up in the flowline. For flexible pipe, it is this excessive upward deform

7、ation being termed as the Upheaval Buckling, which can be prevented by employing adequate downward restraint, such as sand bag / rock dump or by designing a subsea pipe route to overcome this phenomenon.In this paper a c

8、ase study of the full three-dimensional finite element analysis of a trenched but unburied 6.0-inch production flowline is presented following a description of Wellstream Finite Element Method (FEM) based methodology for

9、 Upheaval Buckling analysis of flexible pipes. The effect Bending Stiffness Hysteresis and Upheaval Creep – unique to flexible pipe characteristics, is considered in addition to the general loads such as the flowline sel

10、f-weight and backfill,pretension, pressure, temperature distribution and prescribed forces (either concentrated or distributed) and displacements. The effects of environmental loads, such as the action of currents that w

11、ould result in scouring off the backfill, can also be addressed.The finite element analysis program package ANSYS was chosen for this case study due to its special feature of ANSYS Parametric Design Language (APDL) and c

12、ontact/target elements; and the general three-dimensional shell and solid elements were used to represent the flexible pipe and trench soil respectively.INTRODUCTIONWhen a flexible flowline is in service conveying hotter

13、 fluid content, it will tend to expand due to pressure and differential temperature. However, since the flowline is fixed at its ends to the subsea termination equipment, such an expansion will be restricted in longitudi

14、nal direction. This is particularly the case for the section remote from the pipe ends, and results in an axial compression in the pipe section.If the pipe is lying on the seabed this compression will cause the pipe to m

15、ove sideways to relieve its strain energy and result in a “snaked“ but stable configuration. If the pipe is trenched with a sufficient trench width but uncovered the compression may subject the pipe to deform across the

16、trench and result in a “narrowly snaked“ configuration with a limited upward displacement in the trench wall. Again, this is still a stable condition. However, on the one hand, if the trench width is too1 Copyright 

17、9; 2002 by ASME??????????????????????? ???????????????????????????????????????????????????? ?????????????????????? ?????????????????????????????? ? ???????????5681 Copyright © 2002 by ASME Downloaded From: http://p

18、roceedings.asmedigitalcollection.asme.org/ on 04/22/2015 Terms of Use: http://asme.org/termsZhang, Dr. W., Tuohy, J. 3 Copyright © 2002 by ASMEand a gradual uplifting of the flowline – a phenomenon that may be terme

19、d as Upheaval Creep.Analysis ScopeFollowing the previous discussion, the Upheaval Buckling analysis at Wellstream generally involves the four types of analyses depending upon the project requirements and the availability

20、 of input data and service condition information.? ? ? ? PRELIMINARY UPHEAVAL BUCKLING ANALYSIS. To assess the upheaval buckling potential of a flowline system and to recommend the appropriate mitigation measures, such

21、as the deployment of backfill cover. In the analysis only a representative pipe of partial length is considered for an assumed profile of geometry imperfection due to great uncertainties associated with the availability

22、of the required input information. The most important assumption made at this stage of analysis is the imperfection profile of the flowline, i.e. the Out-Of-Straightness (OOS) profile.? ? ? ? DETAILED UPHEAVAL BUCKLING

23、ANALYSIS. This is to verify the adequacy of the backfill cover estimated from the preliminary upheaval buckling analysis by analysing “actual“ upheaval behaviour of the flowline and to determine if additional cover, such

24、 as rock dumping is required. At this stage the full-length flowline is analysed based on the as built surveyed data of the flowline and the seabed/trench. Note that, to ensure a more effective analysis, the OOS analysis

25、 of the as-built flowline is usually carried out first. The raw survey data has, inevitably, various anomalies regardless the type of the survey equipment used due to the accuracy tolerance of equipment. It is therefore

26、necessary and important to remove these anomalies by using analytical tools of data filtering or smoothing.? ? ? ? UPHEAVAL CREEP ANALYSIS. This application of where the flowline would experience frequent operational sh

27、utdown cycles is to optimise the amount of backfill cover required. The depth of backfill cover determines the maximum number of shutdown cycles permitted throughout the design life. Alternatively, the required minimum d

28、epth of backfill cover is derived for the number of shutdown cycles anticipated throughout the design life. Note that the pretension results from flowline pressurisation during trenching or post-laying is considered in t

29、he upheaval creep analysis. The purpose of pressurising the pipe during trenching/post-laying is to pre-stretch the pipe so as to minimise the subsequent expansion (uplift in particular) due to operational pressure/tempe

30、rature increase. Ideally, the entire pretension pressure would result in the pipe being “l(fā)ocked in” by thetrench/backfill soil following its release; i.e. 100% effectiveness of pressurisation is achieved. However to vari

31、ous extents the surrounding soil fails to lock in the pipe fully and the non-linearity of the flexible pipe also reduces this effectiveness further. Therefore, the concept of effective pretension pressure is introduced i

32、n the upheaval creep analysis to compensate the loss of effectiveness on retaining pretension in the flexible flowline.? ? ? ? UPHEAVAL BUCKLING ANALYSIS OF FLEXIBLE FLOWLINE CONNECTED AT SUBSEA TERMINATION. The interfa

33、ce between the flexible flowline and subsea structure can also be sensitive to upheaval buckling. This type of analysis involves modelling a sufficient length of flexible flowline connected to subsea termination structur

34、e. The aim of the analysis is the optimisation of the requirements to mitigate any upheaval buckling risks to the part of the flowline that is connected to the subsea structure.Finite Element ModellingThe flexible flowli

35、ne is modelled by either a pipe or shell element, depending upon the level of analysis required. Both the pipe and shell elements have the geometry, non-linearity and large deflection capability. The effects of surroundi

36、ng media to the flexible pipe, such as trench soil, trench geometry and backfill soil, are taken into account by using either non-linear spring elements or contact elements. These elements have the generalised non-linear

37、 force-deflection capability to simulate the interaction between the flexible pipe and surrounding media. Note that only in the case of the as-built upheaval buckling analyses, is the three-dimensional solid element cons

38、idered for modelling the seabed/trench.In the analysis the ends of the modelled flexible pipe are generally considered fixed. This is conservative because all the elongation due to loading has to take place in the interv

39、al between these fixed end points. Thus, the calculated uplift will be greater than that derived from more relaxed end conditions.Upheaval Creep AnalysisThe first step in the simulation of upheaval creep is to establish

40、the relationship between the flexible pipe uplift and the backfill cover depth. This involves a number of upheaval buckling analyses for a realistic range of backfill cover depths. The required number of upheaval analyse

41、s depends upon the number of the parameters that define an uplift-backfill cover depth relationship. With the uplift-backfill cover depth relationship established, the upheaval creep calculation can then be carried out t

42、o evaluate the minimum backfill cover depth or the maximum number of loading/unloading (shut-sown) cycles for a given service life.Copyright © 2002 by ASME3 Copyright © 2002 by ASME Downloaded From: http://pro