Technical Highlight Vol.4

Meeting the requirements of offshore structures that operate in ever deeper and colder water

Meeting the requirements of offshore structures that operate in ever deeper and colder water

1 Trends in global demand for offshore structures

Figure 1: World oil production Figure 2: World gas production

Figure 1: World oil production Figure 2: World gas production

Global energy consumption in 2035 is forecast to expand by 1.8 times that of 2010, while global GDP from 2010 to 2035 is estimated to grow an average 2.8% annually. Consumption of crude oil is expected to increase just over 30% between 2010 and 2035 mainly because of motor vehicle demand in developing countries. Consumption of natural gas, which is seen as the only energy source that will see an exponential increase in demand, is forecast to rise greatly, by 50% or more in 2035, as compared with 2010.

Figure 3: Estimated investment in offshore structures

Figure 3: Estimated investment in offshore structures

As demand for oil and natural gas increases, the decline rates of existing fields are intensifying; accordingly, new offshore drilling sites must be sought out and developed to secure oil and gas reserves. It is predicted that investment in offshore structures and related facilities between 2010 and 2020 will increase by a factor of three. Figures 1 and 2 show the trends in world oil and gas production and Figure 3, the estimated investment in offshore structures.

About 50% of oil fields discovered in the last five years were mainly located in the deep waters off the coasts of Brazil and Africa. Because oil and gas prices have remained high, even deepwater drilling has become profitable and is likely to be developed further. Demand will increase for such offshore floating structures as semi-submergible rigs (SSRs), tension leg platforms (TLPs), floating production, storage and off-loading systems (FPSOs) and spars. Even demand for drillships, the vessel most often used for scientific or exploratory drilling of new deepwater oil or gas wells, is expected to increase. Figure 4 shows common offshore structures. In addition to drilling in deep waters, exploration in extremely cold areas, such as the polar regions, where high costs have kept them untouched will likely to begin in the near future.

New gas fields, mostly small- or medium-sized and offshore, will be depleted in 10 to 20 years. Exploration or production in such fields has been limited because the profits were too low to cover the high investment costs in onshore storage tanks and subsea pipelines. However, a new concept, Floating-LNG (F-LNG) is being researched as a way of dealing with distant gas fields where liquefaction facilities cannot be constructed for geographical or political reasons. F-LNG involves a floating ship able to carry out every operation in the production of Liquefied Natural Gas (LNG), including liquefaction, storage and off-loading to LNG ships. The first such developments in the world have been initiated in Malaysia and Australia.

2 Requirements for weld metals

As offshore drilling moves into increasingly colder and deeper regions, steels and weld metals are required to be tougher to be able to withstand harsh environments. On the upper structure (above sea level), such as a jacket,notch toughness of vE-40°C ≥42J is required, and weld joints for YP420MPa class high tensile strength (HT) steels, require fracture toughness of δc-20°C ≥0.25mm.

On the other hand, the lower part (below sea level) of a jack-up-rig, which must bear the load of ocean waves and tidal flows, requires the application of YP690MPa class steels in order to lessen a rig’s total weight and improve the load bearing capacity as well. In addition, reliable welding consumables and procedures are required in order to secure appropriate notch toughness and cold crack resistance.

All-position welding consumables are essential for jackets in particular because they have multiple large diameter pipes for TKY joints as seen in Figure 5.

Figure 5: TKY joints

Figure 5: TKY joints

Carrying on-board liquefaction facilities, F-LNGs will be larger in size than FPSOs for oil; hence, F-LNG hull and tank structures are required to be unprecedentedly strong. Another requirement for F-LNG will be to deal with the wave motions of LNG as it sloshes around inside partially-filled tanks, heavily pressing against hull or tank walls. In this regard, extremely heavy plates (about 50 mm thick) will add strength, and weld joint notch toughness will be required at -40°C or -50°C as well as fracture toughness at -10°C.

Figure 4: Typical offshore structures

Figure 4: Typical offshore structures


Welding consumables for offshore structures

Table 1(page 5) shows typical welding consumables for offshore structures operating in low temperatures. Given that more offshore structures are being built in ever more challenging environments, the welding consumables developed for such offshore structures, in particular those developed for HT780MPa as well as HT520/HT550MPa class steels, are the subject of this article.

3-1. Welding consumables for HT780MPa class or YP690MPa class steels

For offshore structure fabrications using HT780MPa or YP690MPa class steels, SMAW, FCAW and SAW welding consumables have already been developed and marketed.

3-1-1. TRUSTARC™ LB-80L

Figure 6: Example of welding processes for a rack portion of a jack-up-rig

Figure 6: Example of welding processes for a rack portion of a jack-up-rig

For welding YP690MPa class steels requiring high notch toughness as well as cold crack resistance at low temperatures, ultra-low hydrogen covered electrodes (low in oxygen as well) still play a major roll. LB-80L (AWS A5.5 E11018-G H4), which was designed for DC welding, satisfies all of these requirements as shown below. Figure 6 shows an example of welding processes for rack portions of jack-up-rigs, where YP690MPa class steels are mainly used.

Table 2: Diffusible hydrogen content (ml/100g)
N=1 N=2 N=3 N=4 Average
1.9 1.5 1.3 1.7 1.6
Note: Test method: According to AWS A4.3 (Gas chromatography)
Welding current: 150 A (4.0mm dia.; DCEP)

Table 2 shows that diffusible hydrogen in test results of LB-80L is as low as 2.0ml/100g and stable. It is, therefore, regarded as the most reliable welding consumable for cold crack resistance.

Table 3: Test conditions of butt joint (LB-80L: 4.0mm dia.)
Test plate HT780MPa class steel;
50mm thick
Groove preparation Double V (50°and 70°)
Welding position Vertical upward (3G)
Welding parameters 120 A-22 V (DCEP)
Heat input 2.0 kJ/mm
Preheating & interpass temperature 150°C

The test conditions and the tensile properties of a butt joint of HT780MPa class steel welded by LB-80L are shown in Tables 3 and 4, and the macrostructure and the notch toughness transition curve, in Figures 7 and 8 respectively.


Table 1: Typical welding consumables for offshore structures in low temperature services
Welding
process
Welding
consumables
Min. applicable
strength*1(MPa)
Applicable
temperature*1(°C)
Chemical compositions of
weld metal (mass %)
Polarity or
shielding
gas
vE CTOD (δ)
0.2%YP
(MPa)
TS
(MPa)
≥47J ≥0.25mm or
≥0.10mm*4
C Si Mn Ni Mo Ti B
SMAW LB-7018-1 400/390*2 520/490*2 -40 0 0.06 0.4 1.5 - - 0.03 0.004 AC/DCEP
*3
LB-52NS 400/390*2 520/490*2 -60 -30 0.08 0.4 1.4 0.5 - 0.02 0.002
NB-1SJ 460/400*2 550/520*2 -60 -40 0.08 0.3 1.3 1.3 - 0.02 0.002
LB-55NS 470/460*2 570/550*2 -60 - 0.06 0.3 1.5 0.9 0.1 0.01 0.003
LB-62L 530/460*2 620/550*2 -60 -10 0.07 0.3 1.0 2.1 0.1 0.02 0.002
LB-67L 530 620 -60 -20 0.06 0.3 1.1 2.6 - 0.01 0.002 DCEP
LB-67LJ 530 620 -60 -40*4 0.07 0.4 1.1 2.6 - 0.02 0.002
LB-88LT 690 770 -60 - 0.04 0.6 1.8 2.6 0.7 - - AC
LB-80L 690 770 -60 - 0.04 0.5 1.4 3.0 0.8 - - DCEP
SAW PF-H55LT/US-36 400 520 -60 -50 0.08 0.2 1.4 - - 0.02 0.004 AC
PF-H55LT/US-36J 465 550 -60 -20 0.09 0.3 1.7 - - 0.02 0.004
PF-H55S/US-2N 530 620 -60 -20 0.08 0.3 1.3 2.3 0.2 - -
PF-H80AK/US-80LT 690 770 -60 - 0.08 0.3 1.7 2.5 0.7 - -
PF-H55AS/US-36J 400 520 -60 -20 0.07 0.2 1.4 - - 0.02 0.004 DCEP
PF-H62AS/US-2N 530 620 -60 -20 0.05 0.3 1.3 2.5 0.2 0.01 -
PF-H80AS/US-80LT 690 770 -60 - 0.06 0.5 1.6 2.4 0.7 - -
GMAW
(Solid)
MG-S50LT 400 520 -60 -30 0.09 0.4 1.9 - - 0.08 0.006 80%Ar-
20%CO2
MG-S88A 690 770 -60 - 0.06 0.5 1.6 3.6 0.8 - -
GMAW
(FCW)
DW-55L 400 520 -60 0 0.04 0.4 1.3 1.4 - 0.05 0.003 CO2
DW-55SH 400 520 -60 -10 0.05 0.3 1.4 1.6 - 0.04 0.003
DW-55LSR 420 550 -60 -10 0.06 0.3 1.2 1.5 - 0.05 0.004
DW-62L 500 610 -60 -40*4 0.06 0.3 1.2 2.5 - 0.06 0.004
DW-A81Ni1 420 550 -60 -10 0.05 0.3 1.3 0.9 - 0.04 0.005 80%Ar-
20%CO2
DW-A55L 460 550 -60 -20 0.06 0.3 1.2 1.4 - 0.06 0.003
DW-A55LSR 420 550 -60 -20 0.05 0.3 1.3 0.9 - 0.04 0.003
DW-A62L 500 610 -60 -40*4 0.07 0.3 1.3 2.1 - 0.04 0.003
DW-A80L 690 770 -40 - 0.07 0.3 1.9 2.5 0.2 0.07 -
Note:
*1: It is in as-welded condition but not under post weld heat treatment.
*2: The left value is applicable to AC (alternate current) welding and the right, to DCEP (direct current, electrode positive) welding.
*3: Chemical compositions of LB-52NS, NB-1SJ and LB-62L weld metals are obtained by AC welding and the others, by DCEP welding.
*4: CTOD value at -40°C is 0.10mm.
Table 4: Tensile properties of butt joint weld metal
Location Tensile properties
0.2%PS (MPa) TS (MPa) El (%)
Final 773 865 19
Center 807 864 17
Back 753 832 17
Figure 7: Macrostructure of butt joint weld metal Figure 8: Notch toughness transition curves

3-1-2. TRUSTARC™ DW-A80L

Because SMAW is inefficient and requires a rather high level of skill, the development of all-position rutile type flux cored wires (FCWs) has been desired. However, rutile type FCWs deposited weld metals with higher oxygen content and more oxide inclusions than those of SMAW in general, resulted in poor notch toughness. DW-A80L (AWS A5.29 E111T1-GM-H4) provides a solution by controlling the oxygen content in the flux while maintaining high notch toughness. The diffusible hydrogen content with DW-A80L is around 2.5ml/100g, as shown in Table 5, an extremely low level for a rutile type FCW.

Table 5: Diffusible hydrogen content (ml/100g)
N=1 N=2 N=3 N=4 Average
2.5 2.3 2.3 2.7 2.4
Note: Test method: According to AWS A4.3 (Gas chromatography)
Parâmetros de solda: 265A‒28V‒300mm/min
Wire stick out: 20mm; Shielding gas: 80%Ar-20%CO2
Table 6: Test conditions of butt joint welding (DW-A80L:1.2mm dia.)
Test plate HT780MPa class steel; 50mm thick
Welding position Vertical upward (3G) Horizontal (2G)
Groove preparation Double V (40° & 60°) Double bevel
(50° & 60 °)
Welding parameters 180-200A, 23-24V 220-260A, 25-28V
Heat input 1.7 kJ/mm 1.0 kJ/mm
Shielding gas 80%Ar-20%CO2, 25 l/min
Preheating
temperature
100 °C
Interpass temperature 100-150 °C
PWHT As-welded

Butt joint welding with DW-A80L on HT780MPa class steel was conducted in the vertical upward position (3G) and horizontal position (2G). Tables 6 and 7 show the test conditions and the tensile properties; Figures 9 and 11, the macrostructures; and Figures 10 and 12, the notch toughness transition curves in 3G and 2G positions respectively.

Table 7: Tensile properties of butt joint weld metals
Welding
position
Location Tensile properties
0.2%PS (MPa) TS (MPa) El (%)
3G Final 736 811 23
Center 807 856 23
Back 738 817 24
2G Final 776 814 19
Center 833 863 18
Back 808 843 20

3-1-3. TRUSTARC™ PF-H80AS/TRUSTARC™ US-80LT

Table 8: Diffusible hydrogen content (ml/100g)
N=1 N=2 N=3 N=4 Average
1.2 1.3 1.6 1.4 1.4
Note: Test method: According to AWS A4.3 (Gas chromatography)
Welding parameters: 500A‒30V‒300mm/min; DCEP
Table 9: Test conditions of butt joint welding with PF-H80AS/US-80LT
Test plate HT780MPa class steel; 50mm thick
Welding position Flat (1G)
Restraint
condition
Welding parameters 600A-30V-300mm/min
Heat input 3.6 kJ/mm
Preheating
temperature
75 °C 100 °C
Interpass
temperature

Developed by Kobe Steel, PF-H80AS is a bonded type SAW flux with high basicity that allows very low oxygen content in weld metals. In combination with PF-H80AS flux and US-80LT wire (AWS A5.23 F11A10-EG-G), it offers excellent notch toughness even at low temperatures. The diffusible hydrogen content in the weld metals is reduced to as extremely low as 1.5ml/100g (Table 8) by the effect of the flux on the arc. Also shown are the test conditions, the multi-layer cracking test results and the mechanical properties in Tables 9, 10 and 11 respectively. This combination obtains very high quality weld metals.

Table 10: Multi-layer weld cracking test results
Preheating&
interpass temperature (°C)
Ultrasonic test result
75 No defect
100 No defect
Table 11: Mechanical properties of weld metal
0.2%PS
(MPa)
TS
(MPa)
El
(%)
Absorbed energy (J)
-80 °C -60 °C -40 °C
768 895 23 88, 88, 90
Avg 88
101, 93, 93
Avg 96
101, 105, 106
Avg 104
Figure 9: Macrostructure of butt joint weld metals in 3G position Figure 10: Notch toughness transition curve in 3G position
Figure 11: Macrostructure of butt joint weld metals in 2G position Figure 12: Notch toughness transition curve in 2G position

3-2. FCWs for HT520MPa and HT550MPa class steels

A range of FCWs for HT520MPa and HT550MPa class steels exist in world markets, rutile type FCWs that are easy to operate and satisfy notch toughness at -60 °C as well as CTOD at -10 ° C. But, rutile type FCWs with such toughness after PWHT do not exist. However, DW-55SH (not for PWHT), DW-55LSR and DW-A55LSR (both for PWHT) are FCWs that meet these requirements.

3-2-1. TRUSTARC™ DW-55SH

Construction of the world’s largest F-LNG (the Shell Prelude F-LNG project) has started in Korea. It is more than 450 min length, over 70 m in height, and its LNG storage capacity exceeds 200,000m3. Welding such an enormous floating structure requires strict controls as well as high welding efficiency. DW-55SH (AWS A5.29 E81T1-K2C) has been developed at the client’s request. A rutile type FCW for all position welding, it offers superior notch toughness as low as -60 °C and CTOD as low as -10 °C.

Tables 12 and 13 show the respective test conditions and the mechanical properties, including CTOD at -10 °C. Figures 13 and 14 show the macrostructure and the notch toughness transition curve in butt joint weld metals with DW-55SH, respectively.

Table 12: Test conditions of butt joint welding (DW-55SH: 1.2mm dia.)
Test plate JIS G3106 SM400B; 40mm thick
Groove preparation Double V ( 45° & 60° )
Welding position Vertical upward ( 3G )
Welding parameters 200A - 26V
Shielding gas 100%CO2, 25 l/min
Preheating and interpass temp. 130 -150 °C
Table 13: Tensile and CTOD test results
Location Tensile properties Critical CTOD
(mm at -10 °C)
0.2%PS (MPa) TS (MPa) El (%)
Final 536 613 29 0.95; 0.91; 0.88
Back 541 621 30

3-2-2. TRUSTARC™ DW-55LSR &TRUSTARC™ DW-A55LSR

DW-55 LSR and DW-A55LSR (SR series FCWs for Stress Relief) (AWS A5.29 E81T1-K2C, -Ni1M) were developed for PWHT applications in the mid 1990s and have been in use ever since for offshore structure fabrications. They are rutile type FCWs for all positions,offering excellent usability as well as extremely low levels of such impurities as Nb and V. Figure 15 shows that reduced Nb and V can raise notch toughness at -60°C in the as-welded condition as well as minimize the deterioration of notch toughness after PWHT.

SR series FCWs are highly reputed by many offshore structure fabricators for their stability, high notch toughness and superb CTOD properties. Singular products manufactured exclusively by Kobe Steel, these FCWs have been reaching over 300 tons a year in sales.

Figure 15: Relationship between Nb, V and notch toughness

Butt joint weld metals deposited by DW-A55LSR were tested. Tables 14 and 15 show the test conditions and the CTOD property in the as-welded condition and after PWHT. Figure 16, the macrostructures in 3G and 2G positions, and Figures 17 and 18, the notch toughness transition curves in the as-welded and after PWHT conditions in 3G and 2G positions respectively.

Table 14: Test conditions of butt joint welding (DW-A55LSR: 1.2mm dia.)
Test plate NK KF36; 50mm thick
Groove preparation Double bevel (50° & 60 °)
Welding position Vertical upward (3G) Horizontal (2G)
Welding parameters 220A - 24V 260A - 28V
Shielding gas 80%Ar-20%CO2, 25 l/min
Heat input 1.9 kJ/mm 0.8 kJ/mm
Preheating
temperature
100 °C
Interpass
temperature
100 -150 °C
PWHT As-welded & PWHT (623 °C x 2h)

Figure 16: Macrostructures of butt joint weld metals

Table 15: CTOD test results
PWHT Welding
position
Test temp.
(°C)
Critical CTOD
(mm)
As welded 3G -35 0.75, 0.75
2G 0.62, 0.63
623°C x 2h 3G -20 0.89, 0.98
2G 0.86, 0.85

4 Postscript

Even though the shale energy revolution in the USA is impacting the supply and demand of energy worldwide, dependence on crude oil or natural gas will continue as global energy demand continues to increase. Drilling will continue to move offshore to deeper and colder waters. Accordingly, as offshore structures operate under more extreme conditions, standards and requirements, particularly for weld metals, will become more severe. For instance, a drilling project currently planned for the arctic is rumored to require CTOD properties at -60°C. However, we are ready to develop the total welding solutions required for any welding need.

Although specifications and requirements for offshore structures will vary according to client needs, ship classification societies as well as the particular dimensions, operational conditions and weather conditions, we must maintain strict welding controls and procedures. For more details about purchasing and using our products, please contact the nearest Kobelco office or sales representatives.

References
【1】 Ministry of Land, Infrastructure, Transport and Tourism of Japan, Maritime Bureau Document
【2】 Photographs by Japan Drilling Co., Ltd.


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