Ensuring Structural Integrity: The Critical Role of Welding in Offshore Platform Construction

November 21, 2023 · Leave a comment · Red-D-Arc
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Offshore platforms are highly complex structures exposed to severe forces and weather conditions. From design to commissioning, welding plays a critical role in ensuring the structural integrity and safety of offshore rigs. These massive structures rely on welding technologies to produce safe joints in conditions where one bad weld can cause devastating consequences, depending on the criticality of the weld. Therefore, welding process selection, weld application, and inspection quality are the key aspects of ensuring structural integrity in offshore platforms.

Environmental Conditions and Forces In Offshore Platforms

Offshore platforms can weigh tens of thousands of metric tons and have a reach of thousands of feet beneath the water. These structures are exposed to brutal waves carrying enormous energy that can hit the platform in unpredictable patterns across its entire body. Likewise, offshore rigs are subjected to immense seismic forces, severe storms, hurricanes, and rouge waves. Combine the platform’s weight, the diverse shape of various structures on it, and immense, unpredictable forces, and you get conditions that require every critical weld to be defect-free and up to highly stringent standards. 

To make matters worse, seawater is highly corrosive due to the presence of chlorides and microorganisms. Offshore structures are in constant danger of pitting, crevice, microbial-induced, intergranular, and galvanic corrosion. Not even the corrosion-resistant materials are safe. For example, dried-off seawater can leave chlorides on the surface of metals like stainless steel, leading to protective film breakdown and pitting corrosion that can spread unpredictably. In addition, the corrosive nature of extracted resources like crude oil creates a corrosion problem for the pipelines and vessels exposed to it. 

Welding in Offshore Platform Construction

“It’s more important than ever to fabricate offshore structures with maximum productivity to stay competitive and take on more jobs.”

Offshore oil platforms and wind farms are made from critical structures like monopiles, jackets, riser pipelines, jack-up rigs, and wind towers. These and similar components are crucial for the structural integrity of offshore structures. So, they must be welded with maximum joint quality.

Offshore structures are made from various materials like quench-tempered steels, high strength low alloy steels (HSLA), high carbon steels, stainless steels, nickel alloys, and several other exotic alloys. Welding these materials is challenging and often requires specialized equipment.

Not only is high-end equipment needed to achieve inspection-passing welds on these critical components, but it’s vital to achieve sufficient productivity. Offshore platforms are built at a high rate, and with wind towers in the race, the demand for offshore production will likely remain high, as the demand for oil and gas and renewable energy grows globally. With offshore wind farms exhibiting rapid growth and receiving institutional and private investor backing, the offshore industry will need a more productive fabrication approach. Therefore, it’s more important than ever to fabricate offshore structures with maximum productivity to stay competitive and take on more jobs.

Welding Offshore Monopiles And Wind Turbine Towers

Monopiles are an essential part of foundation systems used for many offshore structures. These massive tubular piles can have wall thicknesses of up to 150mm, making them highly challenging for welding. Monopiles are composed of multiple large-diameter tubular sections (cans) welded to one another using advanced submerged arc welding equipment (SAW). Similarly to monopiles, wind turbine towers are welded from many large diameter cans, with a similar fabrication process.

First, thick metal plates are rolled into cans using specialized equipment. Once rolled and tack welded, seam welding equipment produces a longitudinal weld, forming a single can.

Due to the sheer size and weight of the individual cans, they must be rotated and fit up using advanced growing line fit-up systems. The fit-up bed station rotates and translates the tubular sections, aligning them correctly for an appropriate joint fit-up. Next, automated welding manipulators apply the SAW welding process to join the tubes. A particular advantage of the growing line systems is that the fit-up station can grow in size and allow additional tubular elements to be added, making the monopile production process seamless.

Such massive tubular pieces need powerful, and above all, reliable SAW welding power sources. For example, the Miller SubArc AC/DC 1000 Digital can output up to 1250A at 60% duty cycle and 1000A at 100% duty cycle and possess a highly reliable Thyristor power regulation. Since these machines operate on extremely high currents, it’s highly recommended to use reputable brands. Production delays could incur penalties and prolong the project timeline. You don’t want your SAW machines to be a bottleneck of the operation when they perform the most critical duty — forming the entire length of the monopiles and towers.

Welding Offshore Rig Jackets

Some offshore oil rigs and wind turbines use jacket structures instead of monopiles. Unlike monopiles, a jacket structure has three or four legs diagonally and horizontally braced against a transition centerpiece and each other for maximum stiffness. Welding jacket structures is very challenging due to complex joint geometries (T-K-Y joints). 

Similarly to how the before mentioned cans are formed, the offshore jacket structures require cold forming of thick section pipes. Pipes are seam welded using a longitudinal seam welder with an SAW system (usually a multi-wire system), while girth welding usually requires a single-wire SAW welding. Motorized pipe rollers allow rotation and girth welding of heavy pipes to each other safely and efficiently. While a pipe roller rotates the pipe, a welding manipulator can produce an automated weld with maximum precision.

Welding pipes that meet the surface of another pipe in T-K-Y join geometries poses a significant challenge. Unlike girth welding joints, these joints meet at an angle with little room for electrode manipulation and a high possibility for errors due to distortion and poor fit-up. In addition, heat affected zones (HAZ) often overlap with these joint geometries, which makes a significant engineering challenge. These pipes are made from high-strength steel that can experience negative changes when overheated, making the problem of overlapping HAZ more problematic. In addition, these joints are usually welded in outdoor conditions in the yard where the entire jacket structure is fabricated. Contractors often choose a stick welding process or self-shielded flux-cored arc welding (FCAW-S) to weld these joints. 

Due to the complexity of the T-K-Y joints and the need for accurate pre-heating and post-welding heat treatment, it’s paramount to use induction heating equipment, like the Miller Pro-Heat 35™. These joints can’t be uniformly and accurately heated with open flame welding due to their complex geometries. On the other hand, induction heating equipment can be “molded” around the joints to fit snuggly and provide a uniform thermal gradient. In addition, induction heating equipment is significantly less dangerous and doesn’t require fuel tanks, unlike open-flame solutions.

Welding Offshore Jack-up Rigs

Jack-up rigs are offshore structures that can be towed or self-propelled to the offshore oil well. These structures have three or four structural legs that can lower or retract for transport. Once the rig arrives at the oil well, the structural legs are lowered to the ocean floor to support the rig. Since the structural legs are the most important structural element of these self-elevating platforms, they are made from high-strength steel and must be fabricated with absolute weld quality. 

Jack-up rig legs are typically designed with an open-truss system with tubular steel sections that are crisscrossed for support. The jack-up rig legs have a dedicated rack and pinion gear system to manipulate the platform height. This system requires welding heavy-thickness half pipes (chords) to an extremely thick steel rack plate (with teeth on both sides). Together, the load-bearing elements of the legs look like a rack plate sandwiched between two chords. The pinion gear system manipulates the height of the platform by traveling with gears over the steel rack teeth on the structural legs. 

Welding chords to racks and racks to racks requires extreme power input and deposition rate and is usually performed with the SAW welding process. Making a solid rack-to-rack weld can be time-consuming and lead to costly repairs if done incorrectly. Since racks can be more than 200mm thick and are often specified with tight included angles to reduce the amount of welding, they require using advanced SAW power sources, like the Lincoln PowerWave AC/DC 1000.

Welding Offshore Pipelines

Offshore riser pipelines connect the wellbore and the drilling facilities above water, making them crucial for successful extraction operations. These pipelines are exposed to hydrodynamic forces, buoyant forces, various loads, high pressures and temperatures, saltwater, and corrosive agents flowing through them. Like pipes themselves, welds must endure the combination of these forces without failure, or the drilling operations could come to a halt.

Cladding these pipelines with corrosion resistant alloys (CRA) using automated TIG cladding equipment can significantly prolong their lifespan. CRAs like stainless steel and nickel alloys are expensive, so it’s not economical to produce offshore pipelines from CRAs. However, cladding carbon steel pipes from the inside with CRAs improves their corrosion resistance and provides a cost-effective solution. 

Offshore Processing Equipment

While we focused on welding critical structural elements of offshore platforms in this article, offshore rigs also facilitate oil and gas processing equipment. Offshore topside processing systems are far more compact than those in onshore oil and gas downstream processing facilities, like petrochemical plants and oil refineries. Still, offshore platforms have heat exchangers, crude oil separation units, gas compressors, crude oil stabilization systems, and other equipment that relies on complex pipes and tubes and pressure vessel equipment.

Therefore, welding is not only critical for the structural and subsea pipeline aspects of the offshore facilities but also for topside oil and gas processing. 

While structural welding is mainly performed using SAW and large weld automation equipment like weld manipulators, fit-up systems, and pipe rollers, a more sophisticated approach is necessary for the processing equipment. For example, our orbital welder rentals are a perfect choice for process piping and welding heat exchangers. They provide exceptional weld quality because the TIG weld head rotates around the pipe joint with maximum consistency and accuracy. It’s very challenging to achieve the same weld quality with manual TIG, especially after the operator becomes fatigued. 

Rent Or Lease Your Welding Equipment From Red-D-Arc

The offshore industry is highly diverse, but so are the welding solutions. We discussed some critical welding applications in this article. However, the offshore industry has many more areas where welding plays a major role, and our experts are well-versed in them. Red-D-Arc can help you scale your operations, take on more offshore jobs, and play your role in the next chapter of the oil and gas and wind farm offshore revolutions.

Contact us today, and our team of experts will help you decide which welding and cutting equipment you need for your next offshore challenge. Whatever the problem you are facing, welding technologies are often a solution. By applying weld automation, advanced power sources, and innovative techniques, it’s often possible to break the boundaries of what is possible, meet strict deadlines, and satisfy client’s expectations.

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