Welding considerations for offshore structures

While deposition rate is often used to quantify welding productivity, one must also consider the softer factors that influence arc on-time. Preheat time, bead shape, slag removal and defect repair are often overlooked when describing productivity, but are critical to the overall project cost. The effect of diffusible hydrogen, the selection of alloy systems and the choice of welding procedures all play a crucial role in maintaining high quality and consistency in offshore weldments.

Welders and welding engineers face distinct challenges in an era of increased scrutiny by governmental regulations, owner’s representatives and agency classification societies. This requires that the design engineering, fabrication and construction firms re-think the margin of safety in their welding processes. To control costs, designs incorporating higher strength materials are common, as are higher productivity welding procedures. Both the higher strength metals and higher productivity procedures increase the risk of delayed cracking. Hydrogen-related cracking is known to be a function of the quantity of hydrogen, the susceptibility of the microstructure and the residual stresses on the metal. Increased productivity in welding almost always results in thicker individual weld passes and more weld passes per hour (for less time between passes). The result is larger diffusion distances, shorter periods of time for hydrogen to escape and an increased risk of hydrogen related cracking. These factors, especially when combined with the microstructures of higher-strength steels, require a new look at hydrogen control in every aspect of welding.

In addition to an increased risk of hydrogen cracking, mechanical properties such as crack tip opening displacement (CTOD) often suffer when higher heat inputs are used to increase productivity. This article quantifies the risk factors beyond those explored in standardised diffusible hydrogen, Charpy V-notch and CTOD testing. Welding procedure controls in combination with proper selection of welding consumables are key factors for risk reduction.

Improving weld quality, costs and schedules

Fabrication of offshore platforms and components not only requires an unwavering commitment to the quality of welding but also a commitment to carefully managed costs and production schedules. Costs can be controlled by using 100% CO₂ shielding gas with one electrode for all welding positions. Higher heat input welding procedures are often used to increase deposition rates and reduce welding time. However, these do not come without a price. That price is risk; specifically, risk of failing mechanical properties and risk of hydrogen related cracking.

CVN Toughness in high heat input applications

The productivity increases delivered through higher deposition rates are often accompanied by thicker individual weld passes and slower cooling rates. Both the alloy and the slag systems must be designed to develop toughness when as-deposited weld metal is reheated and refined by subsequent weld passes. Alloy systems for 70 ksi, 80 ksi and 90 ksi strength flux cored deposits are shown in Table 1. With each step up in strength the amount of alloy increases, but so does the interaction of those alloys with the slag system.

When combined with appropriate slag systems, these alloys will develop high toughness weld metal. Thicker weld beads resulting from higher deposition rates can increase the percentage of as-deposited (unrefined) weld metal. Unrefined weld metal is associated with coarser grain microstructure, more grain boundary ferrite and lower toughness in Mn-Si, Mn-Si-Ni and Mn-Si-Ni-Cr alloy systems. Proper tuning of the weld metal composition includes control of trace and tramp elements and the influence of the slag system on factors such as nitrogen, oxygen and inclusion control. When appropriately tuned and welded, high CVN toughness can be achieved at heat inputs over 55 KJ/in, as shown in Figures 1, 2 and 3. The interval represents the 95% confidence range at each CVN test temperature.

                                        Figure 1. Charpy V-Notch toughness data for 70 ksi UTS weld metal (UltraCore 712C-H Plus)

Figure 2. Charpy V-Notch toughness data for 80 ksi UTS weld metal (UltraCore 81K2C-H Plus)

Figure 3. Charpy V-Notch toughness data for ksi UTS weld metal (UltraCore 91K2C-H Plus)

CTOD Toughness in thick 70 and 80 ksi weld metal

Alloy additions and inclusion control promote the formation of finer grain microstructure and improved CTOD toughness of as-deposited weld metal. Smaller weld beads and more passes are required when such alloy and slag system controls are not engaged, significantly limiting productivity. Technology advances in CO2 shielded flux-cored electrode designs allow for better toughness in the unrefined weld metal of mild steel and low alloy systems without making traditional sacrifices to diffusible hydrogen or ease of welding at higher deposition rates. Figure 4 shows a 3.5 in. weld metal macro produced at 47 to 55 kJ/in. heat input and subsequently CTOD tested per BS7448 for through-thickness toughness, with results as shown in Table 2.

Consistent CTOD values for a weld deposit may also serve as its own productivity improvement. CTOD values quantify a weld’s resistance to ductile crack propagation. During the multiple-pass welding of thick sections, residual stresses accumulate which deteriorate a weld’s ability to resist ductile crack propagation (and reduce CTOD values). Post Weld Heat Treatment (PWHT) is sometimes required to be used by fabricators to relieve residual weld stresses. According to Offshore Standard DNV-OS-F101:

“Post weld heat treatment shall be performed for welded joints of C-Mn and low alloy steel having a nominal wall thickness above 50 mm, unless fracture toughness testing shows acceptable values in the as welded condition.”

Consistent CTOD values in the as-welded condition eliminate the need for PWHT, significantly reducing cost and improving productivity.

Figure 4. Weld macro for CTOD testing, showing pass sequence. See Table 2 for results. 

Diffusible hydrogen and consumable selection

Proper selection of welding consumables is also vital at higher deposition rates to resist a common failure – delayed hydrogen cracking in the thicker weld beads. This is especially critical when welding higher strength weld metals. Not only can low hydrogen levels reduce the risk of delayed cracking, they can also reduce the cost and complexity associated with very high preheat temperatures.

Failure mode rankings use frequency, severity and detectability of defects in determining a rating of risk. Diligent fabricators may consider the frequency of hydrogen cracking to be low. Cracking in the welds of any subsea pipeline or component is a severe defect, with increased risk of in-service failure due to the difficulty associated with detecting hydrogen cracks. Hydrogen cracks may initiate immediately or days after welding. Figure 5 shows that although the measured volume of hydrogen increases with subsequent weld passes, weld metal hydrogen has been measured to be most concentrated (i.e. hydrogen per 100 g weld deposit) in the first weld passes of a multiple pass joint. In this case, hydrogen cracks may be located deep within a weld joint (common offshore joints are as deep as 3 or 4 in.) requiring sophisticated NDT techniques for detection, and great expense to repair.

Figure 5. Diffusible hydrogen in a multi-pass well.

The cross-section of a hydrogen crack (Figure 6a) from a high strength weld indicates that such cracks may also initiate immediately. This crack was shown to originate in the first pass of a multiple pass weld. The exposed crack surface (Figure 6b) confirmed immediate cracking as heat tint was found in the first pass region. The factors of severity and detectability yield a high risk associated with hydrogen cracks in structural offshore welds.

Figure 6. Example of hydrogen cracking in a multiple pass weld. 

As shown in Figure 7, increasing time between weld passes (especially at interpass temperature) is effective for reducing diffusible hydrogen content. Although increasing preheat and interpass temperatures helps to mitigate the risk of hydrogen cracking, such practices may also reduce weld metal strength. Other methods of lowering diffusible hydrogen (interpass hold times and dehydrogenation soaks) reduce productivity. Again, avoiding such practices makes the selection and handling of consumables especially important.

Figure 7.Diffusible hydrogen time-decay for a single pass weld: More than one hour was necessary for a specimen (held at 250°F) to halve its initial diffusible hydrogen content. 

Further complicating the situation is the fact that most offshore yards are in high humidity areas near the sea. Electrodes are sometimes left out in high humidity environments, so consideration should be for not only the initial diffusible hydrogen value but also the diffusible hydrogen after exposure in these humid conditions. Before selection of high-strength offshore welding consumables, data should indicate that they are robust in their ability to deposit low hydrogen weld metal – both directly out of the box and after exposure to humid environments. An example of this can be seen in Figure 8.

Figure 8.Diffusible hydrogen of electrodes upon humidification at 80% relative humidity. The hydrogen potential for As-Received electrodes as well as for electrode exposed to high humidity should be minimal to reduce the risk of hydrogen related cracking.

Conclusion

Specifying gas-shielded flux cored electrodes for offshore applications requires forethought and diligence in order to produce robust welds that not only stand up to harsh environments, but also meet increasingly stringent industry quality and testing standards, such as CTOD values.

Risk of mechanical property failure can be reduced by understanding the role of the alloy and slag systems in developing toughness. Risk of delayed cracking can be reduced by knowing not only the diffusible hydrogen level of the electrode as it was manufactured, but also after exposure under high humidity conditions.

Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/15112016/welding-considerations-for-offshore-structures/