Technical Highlight Vol.4

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

1 Trends in global demand for offshore structures

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.

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.

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.

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

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

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

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.

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)

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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