Ocean Sea Offshore Platform Structural Steel Fabrication Station Hydropower Tunnel Frames

Ocean Sea Offshore Platform Structural Steel Fabrication and Hydropower Station Tunnel Steel Frames

As one of most pioneering light& heavy steel framing fabricators in China, we have engaged in quite a lot of Ocean Offshore Platform and Hydraulic Power Station structural steel fabrications for years.  Except the China domestic market activities, we also have the capacity in many overseas offshore platform and hydropower station field structural steel fabrications based on America US standard, Europe standard, UK Britain and Australia standard and so on. 

The large-scale structure of the offshore platform has a significant trend, the structure is complex, the welding workload is large, and the welding of joints is inherently difficult. In addition, the high degree of stress concentration makes the structure in a more dangerous state; while the large-scale structure of the offshore platform forces the thickness of the components As it increases, the danger of its destruction increases. Therefore, in the construction and repair of offshore platforms, the requirements for structural welding are getting higher and higher. The author of this paper discusses the welding requirements and welding methods by taking the successful experience in the construction of offshore platform steel structures such as "Panyu 34-1 drilling rig module" and "Kenli 3-2 WHPA platform workover rig" as examples.

Basic requirements for welding
Welding shall be constructed in accordance with the requirements of relevant structural construction drawings or technical specifications, and the platform structure shall also be welded in strict accordance with the approved welding procedures.
Welding process characteristics

In order to prevent cold cracks and improve the toughness of the heat affected zone, low hydrogen type and ultra-low hydrogen type alkaline electrodes are mostly used.

Welding heat input is usually limited to the range of 40-50kJ/cm to ensure the toughness and brittle fracture resistance of the welded joint.
Thick plate weldments generally need to be preheated before welding, and the allowable range of preheating temperature is between +50°C and 0°C. The maximum interpass temperature does not exceed 250°C, and electric heaters or gas heaters will be used for preheating at specified temperatures of 50°C or higher to prevent cracking.

After welding, pipe joints and key components also need to be heat treated to eliminate welding residual stress.
The truss structure of pipe fittings mostly adopts all-position, full penetration, multi-layer and multi-pass manual welding.
Fillet welds of pipe joints and key components subjected to alternating stress also need to be ground or repaired to trim welds and eliminate surface defects, reduce stress concentration and improve fatigue life.

Common Welding Forms
There are two types of grooves, X-shaped and V-shaped. When the thickness is less than 20mm, the V-shaped groove is mostly used, and when the thickness is greater than 20mm, the X-shaped groove is mostly used. However, in the case of a small pipe diameter, in order to facilitate the welding operation of the welder in the pipe, although the plate is thicker, it is also necessary to use V-shaped groove. type groove. The form of slope opening is shown in Figure 2 below:

There are three types of grooves: double slope, single slope and fillet welding. In case of joints, double-slope and single-slope welding are mostly used, and fillet welding is mostly used at non-nodes. The form of slope opening is shown in Figure 3 below:

Common welding methods
The commonly used welding methods for steel structures are: automatic submerged arc welding, manual arc welding and CO2 gas shielded welding. The specific welding method used depends on the characteristics of the components.
Preparation work before welding
The edge preparation of the components to be welded should be precise and uniform, and should be free of moisture, grease, paint, rust and other oxides.

Assembly shall conform to an approved welding procedure specification and any action taken to correct improper assembly shall be to the satisfaction of the inspector.
If the root opening of the butt welding is too large, the edge repair welding shall be approved by the inspector. The length of each edge repair welding shall not exceed t/2 or 12.5mm (whichever is smaller), and t is the thickness of the thinner member. thickness.
The cut should have a suitable radius at the corner to keep the local stress concentration to a minimum.
Measures should be taken, such as welding codes or tack welding, to keep the components to be welded in the correct position and alignment.
For the butt welded joints of plates with a bearing load thickness difference exceeding 4mm, the thicker plate should be chamfered, and the chamfering length generally does not exceed 4 times the thickness difference.

The joints formed by the tubular members should have precise alignment and the bevel angle on the circumference should be continuous from the largest transition to the smallest.
Whether preheating before welding is required should be determined according to the thickness of the workpiece and the ambient temperature, or according to the regulations in the process specification.

Welding process
According to the steel grade of the platform components and the platform design requirements, in order to avoid the corrosion of the weld metal, in the selection of welding materials, the electrode potential of the weld metal should be positive to the base metal and the overheating zone.
During welding, the weld metal and welded joint at the fracture must maintain the same properties as the non-fracture, including toughness, hardness, fatigue and corrosion. The main index to measure the toughness of the material is the Charpy V-notch impact toughness value.
Welding work should be carried out in sheltered conditions with wind, rain and snow, especially when gas shielded welding is performed in the open air. There must be wind protection.

The ambient temperature during welding shall not be lower than the minimum temperature specified in the approved welding procedure approval test. When welding under severe weather conditions, appropriate measures shall be taken. Both sides of the groove shall be kept dry and the temperature shall be at least 5°C. .

The butt joints of offshore structures should generally be fully penetrated. For butt full penetration welds with a thickness of more than 6mm, in order to ensure full penetration of the entire thickness, grooves should be opened and single-sided or double-sided welding should be performed.
The stress concentration caused by structural characteristics and welding forming is an important factor affecting the fatigue life of welded joints. The shape and defects of the fillet welds of special components must be adjusted and the surface of the base metal should be smoothly transitioned. And the throat size of the fillet weld should not be less than 0.7 times the height of the welding leg. If the gap between the mating surfaces of the components exceeds 2mm, but not more than 5mm, the height of the welding leg should be increased by a gap amount.
Hammering should not be used to correct the deformation of single-layer welds or root and cap welds of multi-layer welds. In the interlayer welding of multi-layer welding, after the deposited metal is solidified, the welding slag is eliminated and the method of hammering can be used to correct the deformation or reduce the residual stress.

Post-weld heat treatment
After the joint is welded, if the post-weld heat treatment is not carried out immediately, the post-weld heat treatment can be carried out. The common method is: use a heating pad to heat the weld bead and both sides to 250 ~ 350 ° C, and then use a thermal insulation tape to keep the heat for 0.5 ~ 1h, then Cool in air.

For post-weld heat treatment, first heat the weld bead and both sides to about 310°C, then heat to 590-650°C at a heating rate of 100-200°C per hour, hold for 1.5h, and finally cool in air.

Weld Repair
Weld repair shall be carried out according to the approved repair process, and in the same area, welding repair shall not exceed two times. In order to ensure the repair quality of the weld, the repair length of a single weld should not be less than 50mm, and the steel should not be damaged when removing defects in the weld.
Welding Inspection

Full-time quality monitoring should be implemented for the structural welding construction of offshore facilities, and full-time inspectors should be required to inspect the structural welding quality according to the approved construction inspection process, and strictly implement the three-level inspection system. That is, self-inspection, mutual inspection and special inspection. The inspection scope includes in-process inspection and completion inspection. After all the inspections are qualified, they will be reported to the third-party inspection agency and the owner for approval.
(1) Process inspection and control
1) When burning down the hydrogen electrode, the welder must bring a heat preservation cylinder before receiving the electrode, and the heat preservation cylinder must be connected to the power supply when in use.
2) Each welder should bring necessary tools such as hammer, wire brush or wind shovel before welding, and check whether the connection of the power cord is firm, check the environmental sanitation and wind force and direction of the construction area, and take certain preventive measures , which is a prerequisite for ensuring the quality of the weld.
3) When performing bottom welding, the construction personnel should be very careful, and should not be careless because the reverse side needs to be gouged. In fact, the quality of the bottom welding plays a very important role in the quality of the weld.
4) For butt welding and full penetration fillet welds, preheating should be performed at 50°C to 100°C before welding, and in multi-layer multi-pass welding, the interlayer temperature of each weld should be controlled at 100°C to 150°C. Between ℃, each welding seam should be connected continuously as far as possible at one time, and it should not be operated intermittently due to the influence of external factors, so as to ensure the interlayer temperature.
5) In the case of multi-layer multi-pass welding, before the next welding seam is welded, slag knocking and appropriate grinding should be carried out on the previous welding seam, and a 100% visual inspection should be carried out. Welding work, otherwise the defects must be eliminated in time to continue welding.
6) Carbon planing should be carried out on the back of all butt welds and full penetration fillet welds. The depth of carbon planing is generally about 5mm. For the defective parts, it can be thoroughly planed until the defects are eliminated.

(2) Completion inspection
1) Appearance inspection 48 hours after the completion of welding, in the state where the surface welding slag is removed and no anti-rust paint is applied, report to the third-party inspection agency and the owner for inspection.
2) Non-destructive testing of welds The inspectors should pass a valid visual inspection (such as meeting the relevant requirements in AWS), and should be able to perform the work within the three-year validity period of the inspection. Non-destructive testing should generally be carried out 48 hours after welding. When the weldment is required to be subjected to post-weld heat treatment, non-destructive testing should be carried out after heat treatment. It mainly adopts ray, ultrasonic, magnetic powder or coloring detection method or a combination of several detection methods.
To sum up, compared with land steel structures, due to the more complex environment of marine steel structures and the gradual progress towards the deep sea, the welding process and quality control of marine steel structures have become particularly important, so the structural design and construction documents are strictly followed. The technical requirements for each link of welding can ensure the safe use of the steel structure of the offshore platform within the design period.

Specification: