Технология освоения морских месторождений полезных ископаемых
Староконь И.В., кандидат технических наук, доцент Овсянников Ю.М.
(Российский государственный университет нефти и газа им. И.М. Губкина)
Иванов П.П., ведущий инженер ЗАО НПЦ «Молния»
ПРОБЛЕМЫ ПЕРЕНОСА РЕЗУЛЬТАТОВ УСТАЛОСТНЫХ ИСПЫТАНИЙ ЭКСПЕРИМЕНТАЛЬНЫХ ОБРАЗЦОВ НА РЕАЛЬНЫЕ КОНСТРУКТИВНЫЕ ЭЛЕМЕНТЫ ОПОРНОГО БЛОКА МОРСКОЙ СТАЦИОНАРНОЙ ПЛАТФОРМЫ
В статье рассматриваются проблемы переноса результатов усталостных экспериментальных испытаний разрушение на реальные объекты. В качестве объекта исследования выбран опорный блок морской стационарной платформы. В результате исследования предложен механизм такого переноса и построены кривые усталости для основных конструктивных элементов опорного блока морской стационарной платформы.
PROBLEMS OF RESULT COMPARISON OF EXPERIMENTAL FATIGUE TESTS WITH ACTUAL STRUCTURAL ELEMENTS OF OFFSHORE FIXED PLATFORM JACKETS
Task scaling of results obtained during laboratory sample testing to actual structural elements (SE) of jackets (J) of offshore fixed platforms (OFP) is important from a practical standpoint. Recent researches propose different estimation theories of scaling factor impact depending on surface roughness of an actual structural element of an OFP and a weld joint as compared to a smooth sample (roughness within 0.16-0.32um), actual geometrical dimensions of a SE and weld joints of a jacket of an offshore fixed platform as compared to laboratory samples, stress concentration due to a weld joint type and possible defects, residual stresses in a weld and a heat affected zone depending on stress ratio, possible availability of metal zones with different mechanical properties, parameters of cross-sections etc. The purpose of this paragraph is scaling of fatigue endurance obtained as the result of experimental investigations to actual structural elements and weld joints of the OFP jacket. It should be noted that while procedures of different factor interaction have been developed and obtained practical confirmation for consideration of main factors affecting the scaling of experimental results of metal samples of structural elements of the OFP then such procedures have not been developed sufficiently for weld joints. Let's consider the procedure of mutual interaction consideration of the main operating and process factors affecting the fatigue endurance value change and change of other parameters of the S-N curve for main structural elements of the offshore fixed platform jacket. In many authors' opinion the ration of dimensions of a laboratory sample and elements of structural elements of the OFP jacket, asymmetry of stress cycles, surface roughness and impact of surface hardening procedures should be considered as such parameters. The net effect of these parameters on fatigue endurance is considered by introduction of the coefficient of impact according to the following formula:
-
a-i кэ МСП — Z-1
, (1)
Where, cr_lK3MCII - fatigue strength of a structural element of the OFP jacket; fatigue strength of a laboratory material sample; Ka K3Mcn -coefficient of impact of different factors. Ka K3Mcn coefficient in its turn is calculated using the following formula:
К
а КЭМСП
Where: Ka - effective stress intensification factor; Kda - scaling factor; KF - coefficient accounting roughness impact; KV- process hardening factor; Ka value is determined by the following formula:
K, = 1 + qa(aa - 1), (3)
Where qG- a coefficient of material sensitivity to stress concentration, aG-theoretical stress concentration coefficient. Based on available reference data let's take a0=2, and ^b- breaking strength equal to 490 MPa. Analyzing the diagram on Fig.1. we will obtain value q0 equal to 0.5, and value K in this case will be equal to 1.5
Value Kda can be determined using different methods. But the work shows that upon increase of tested samples areas above 4000mm2, the reduction of fatigue endurance is almost absent and Kda value can be taken approximately equal to 0,6. KF value can be calculated according to the following formula:
Kf = 1-0,22 lg(fl2)-g-l),(4)
/¿-surface roughness of actual structural element of the OFP, 17 b -breaking strength. Standards and codes specify that upon available corrosion impact KF in calculations shall be replaced withKkop which value is determined by diagram 2
Values KV for steels are taken according to data of table 1 depending on a type of strengthening treatments.
O.S
0,6
0,4
0,2
a}=l,4
'4 "„=1,3
"¿=1.2
400 600 800 1000 aB,MI7a Fig.1. Determination of material sensitivity coefficient value to stress concentration
Fig. 2. Determination of corrosion impact coefficient value on roughness under offshore field conditions
Table 1.
Dependence of KV coefficient on strengthening treatment type
Strengthening type Kv
High frequency current hardening 1,2...1,6
Nitride hardening to 0.1.. .0.4 mm depth 1,10.1,15
Case hardening to 0.2.0.6 mm depth 1,10.1,15
Surface rolling 1,10.1,25
Surface peen hardening 1,10.1,20
KV value is taken as equal to 1 due to data absence on strengthening treatments.
Let's calculate the coefficient value considering main operating and process factors for elements made of 09G2S steel:
^ = 01 + ^-1) = 4,36 (5)
So the fatigue endurance for structural elements of the jacket made of 09G2S steel (braces and horizontal elements) is 55 MPa under offshore field conditions. Applying the above principles let's make calculation for elements made of VSt3Sp5 steel (columns), which fatigue endurance for pipes made in accordance with GOST 8696-74 is 372 MPa. The fatigue endurance of smooth samples is 200 MPa. As the performed computation result, value K3Mcn- equal to 3.91 was established. Having performed corresponding computations we receive 51 MPa for structural elements of the jacket made of VSt3Sp5 steel (columns) under offshore field conditions. Let's plot S-N curves for structural elements of the OFP for which purpose it is required to determine the S-N curve slope index, m. Let's calculate m value according to the following formula:
As the calculation result it is established that the S-N curve slope index is 2.46 for structural elements of the OFP made of VSt3Sp5 steel. And the S-N curve slope index equals to 2.55 for structural elements of the OFP made of 09G2S steel.
Let's perform calculations for horizontal elements and braces made of 09G2S steel. The stress amplitude is determined by the following formula:
_ _ ajnax~GmLtt
" -—:—-(7)
Based on the stress condition analyses of the OFP jacket performed in chapter 3 let's calculate maximum and minimum stresses in a cycle as well as their amplitude for elements of the structural elements of the OFP made of 09G2S steel (table 2):
Table 2
Characteristics of stress cycles of braces and horizontal flanges of the OFP jacket at 13.9 m
wave height and 49 m/s wind speed
Stress conditions Horizontal flanges Braces
Element stresses in absence of EWL, MPa 6 7 32 17 20 77 50 104 112 57
Maximum stresses upon direct impact of wave load, MPa 71 75 92 250 182 209 198 267 344 120
Minimum stresses upon direct impact of wave load, MPa -59 -61 -28 -216 -142 -55 -98 -59 -120 -6
Stress amplitude, MPa 65 68 60 233 162 132 148 163 232 63
It follows from the table that maximum stresses are achieved both in braces and horizontal elements, minimum cycle value upon 13.9 m wave height is 216 MPa and maximum 250 MPa. Consequently oa will be equal to 233 MPa. Let's calculate parameters of S-N curve for elements of structural elements of the OFP made of 09G2S steel. We will determine number of cycles to failure upon stress amplitude of 233 MPa by the formula:
(8)
Where, Ng- number of cycles in breaking point of S-N curve at stress level o_i accepted in accordance with data [1-5] equal to 6 ■ 1Q*1 ; g_i- fatigue endurance; o_i- stress cycle amplitude Having inserted the previously obtained values we will obtain:
N,
09Г2С
/ it ч 2,55
= 6106(^-) = 1,51 10s,
V233/
(9)
Following the same considerations let's calculate parameters of the S-N curve for elements made of St3Sp5 steel based on data given in work [1-5], let's take NG=2-106. Let's analyze data on maximum and minimum cycle values (table 4.3.):
Table 4.3.
Characteristics of stress cycles of OFP jacket columns upon wave height of
13.9 m and 49 m/s wind speed
Stress conditions Columns
Element stresses in absence of EWL, MPa 45 43 18 32 51
Maximum stresses upon direct impact of wave load, MPa 250 242 115 197 173
Minimum stresses upon reverse impact of wave load, MPa -160 -156 -79 -133 -71
Stress amplitude, MPa 205 199 97 165 122
As a result, we will obtain:
(10)
Thus stress diagrams for structural elements of the OFP jacket will take the following form:
с "4031; 14s1430 222244- 2943240 370407î 4444s93 51s5~05 5524:20 44s7335 740s150 s14s945 sss97s0 ?4."0;9;
- 1 щгорп/яя pytrj яжяшш
\
\
\
\
\
\
\
\
1
650c 0 2 00000 3 kora ачество циклов,p
0 "40s1: 1431 s3c' 2222445 2943240 37040t5 4444ss0 51s5705 5524520 4ss7335 "40s150 s14ss45 ш9780 9430595
Fig. 4.3. Diagram of fatigue failure of structural Fig. 4.4. Diagram of fatigue failure of structural elements of the OFP made of 09G2S steel under elements of the OFP made of VSt3Sp5 steel under offshore field conditions offshore field conditions
Cycle asymmetry coefficient 9 has important practical value using which equivalent stress amplitudes are established and which is calculated by the formula:
'!.i:-K.:Mi::- K.::-.K.1 r." (11)
In accordance with recommendations specified in [1-5], the cycle asymmetry coefficient is selected depending on ultimate strength by data of the
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.