1 Overview
The Offshore Oil 981 deepwater semi-submersible drilling platform was started in April 2008 at Shanghai Waigaoqiao Shipbuilding Co., Ltd. It is the first 6th generation 3000m deepwater semi-submersible drilling platform independently designed and constructed in China. Ocean Oil 981 integrates the world's first-class design concept and first-class equipment. It is the world's first platform designed according to the harsh sea conditions in the South China Sea. It can withstand the typhoon once in 200 years and choose DP3 dynamic positioning system. The platform is 114m long, 90m wide and 132m high. The maximum working depth is 3000m and the drilling depth can reach 12000m. Graded ccs (China Classification Society), ABS (American Bureau of Shipping) double class. The completion of the platform indicates that China has already possessed automatic research and development capabilities and international competitiveness in the field of marine engineering equipment.
The semi-submersible drilling platform system is huge, and the design, construction and operation are very difficult. The “Ocean Oil 981” is the most advanced platform in the world at the design stage, and it is the most complicated one in the global floating drilling platform. Kind. The platform uses a large number of different types of high-strength steel, the technical steel volume is about 21000t, plus no domestic construction experience can be found, so the welding work of the platform faces great challenges. How to successfully complete the welding work of a large number of high-strength steel is urgent solved problem. This paper introduces the key welding processes and equipment applied in the platform construction process, and analyzes the important position of welding work in the construction of marine engineering.
2. Key welding processes and technologies
(1) Welding process of large thickness high strength steel plate
The platform is all high-strength steel of grade 360 or above, of which EQ56 copper plate is used for 350t and EQ70 copper plate is 50t. There are many reports and applications on EQ56 and EQ70 high-strength steel. This type of low-carbon enamel steel has high yield strength, satisfactory plastic toughness, good weldability and relatively low crack sensitivity. It is a typical type. Mature material. However, due to the high material strength, the weld heat affected zone, especially the coarse grain zone, tends to have cold cracks and toughness.
EQ56 and EQ70 are mainly used in the key strength component area of the platform's column and upper hull connection. Although the commonly used orientations such as SMAW, GMAW and FCAW are suitable for the welding of EQ70 high-strength steel, the sensitivity from crack and safety of platform use. Considering and referring to the experience of offshore engineering construction at home and abroad, the Conarc80 welding rod produced by Lincoln Electric was used for electrode arc welding. Compared with general strength and low alloy high strength steel, EQ56 and EQ70 high strength steel are sensitive to the preheating temperature, interlayer temperature control, welding heat input and post-weld residual stress control. EQ70 high-strength quenched and tempered copper processing and welding have many limitations, such as the process evaluation is not exempted: post-weld flame correction is not recommended; heat input is strictly controlled; preheating and inter-layer temperature have upper limit requirements, more stringent electrode baking and The electrode allows exposure time; the positioning welding has strict requirements; the delay is more than 72h.
According to the weldability of steel, ABS and ccs were applied to EQ56 and EQ70 respectively, and several welding procedure qualifications were implemented to verify; the suitability of welding consumables and the applicability of welding process. It has been proved by the double-class classification society and on-site practice that the relevant process parameters are reasonable and effective, and have good operability and implementability. The recommended welding parameters for EQ56 and EQ70 are shown in Table 1 and Table 2.
The EQ56 thickness specification of the offshore oil 981 platform is 32mm, and the thickness specification of the EQ70 is 38mm, 70mm. With the increase of the thickness of the plate and the complexity of the structure, the work difficulty of the field welding increases, and measures should be taken in advance to cope with it.
Mainly reflected in the following aspects:
First, because the shipyard is currently pushing the CO2 gas protection welding, there is a general lack of high-skilled three-class welding arc welding welders, often through the short-term training to obtain the three types of completion qualifications of the classification society, so the level of welding personnel is not level, the actual welding Insufficient inspections often lead to defects in the welding process. In addition, compared with the 507 electrode, the Conarc80 electrode shows that the molten subway is poorly flowable and the edge of the weld is prone to slag inclusion. Therefore, the EQ series copper plate welding training should be prepared in advance for the relevant personnel. Skill welders demonstrate and explain operational skills and announcing matters. It is necessary for trainers to simulate the on-site environment, such as high temperature, small space, etc., and people are lying half-lying for welding.
Second, the on-site operation belongs to the outdoor cross-three-dimensional operation, and there is a problem that the position of the weld bead is small and the operator is inconvenient. In particular, the weld seam of the EQ board needs to be preheated to 80 °C before welding, and continuous operation in a high temperature environment is a great test for the welder's physical strength. Therefore, we must do a good job in production preparation and logistics support.
Third, the cold natural environment in winter has a great impact on the quality of welding. Strong wind speed will affect the protection of the welding rod, causing arc blow, splashing, and generating defects such as pores and slag inclusions. In addition, the low temperature state will accelerate the cooling rate of the weld, and the tendency of hardening will increase. The copper plate should be preheated by the electric ceramic heating sheet before the construction, and the interlayer temperature is maintained between 80 and 210 °C. After welding, the asbestos is used for heat preservation and post-heat treatment, so that the diffused hydrogen has sufficient time to escape, which plays a good role in hydrogen elimination and reduces the incidence of cold cracks.
(2) High-strength steel welding process with local complex structure
The hull structure of this platform is relatively complicated, and there is a local welding work at the 6GR position. Complex structures are prone to stress concentration and excessive deformation. The welding technology of the joints of the column and the upper outer hull, the cross bracing and the column, the sling base, the guide wheel base, the propeller base and the paver base should be studied, including the processing method of the steel plate and the slope. Techniques such as mouth form and gauge, welding sequence.
(3) Welding residual stress control of local complex structures
Residual stress is the stress that exists when the object is balanced by an external factor. The complex structure tends to generate stress inside the component under the action of the welding thermal stress field. When the excessive residual tensile stress is not eliminated, there is a tendency to generate delayed cracks. Therefore, how to reduce the influence of residual stress on weld quality is also the key point.
The preliminary study on the welding residual stress control of important local structures can be carried out by numerical calculation and experimental verification. Residual stress is reduced by welding heat input, solder remelting, weld grinding, pre-weld preheating and post-weld heat treatment. Studies have shown that grinding the welded joint can reduce stress concentration, smooth the weld and the welded joint, improve the distribution of welding residual stress, and operate easily during the actual construction process. Pre-weld preheating and post-weld heat treatment play a good role in reducing the residual stress of the weld. Although it is not easy to realize due to the complicated heating device, it is required to be welded from the EQ56/EQ70 steel plate. The actual construction process plays a key role in ensuring the quality of the weld and reducing the residual force.
(4) Welding technology of high pressure mud pipeline 4130 steel
The ASTM4130 high pressure mud pipe has a high working pressure, high strength level in the pipeline, complicated chemical composition and thick pipe wall. Therefore, research on the weldability of 4130 pipe, the mechanical properties of welded joints and heat-affected zones, the toughness of welded joints and welding parameters are also key. Finally, the on-site construction process of the 4130 pipe is to be determined.
3. Key welding equipment and methods
The deepwater semi-submersible drilling rig is constructed by segmenting the land. The first stage is the assembly of the components; the second stage is the staged construction; the third stage is the final assembly of the dock. Increasing production efficiency is an eternal topic in the manufacturing industry. It is especially important in the shipbuilding and marine engineering manufacturing process. It even takes the length of the production cycle as an important indicator to consider the overall level of the enterprise. At the same time, the welding man-hours generally account for 30%~40% of the hull construction time. How to improve the welding production efficiency is the highlight of all shipbuilding enterprises. Therefore, all kinds of automatic welding, high-efficiency and large heat input welding methods are also born.
(1) Component assembly and stage construction stage
Component assembly construction is also called group or small closure, and segment assembly construction is also called middle group master or middle closure. First of all, all the components need to be assembled. At this stage, the submerged arc welding operation, the double wire submerged arc welding operation and the FCB method (flux copper pad single-sided submerged arc welding) are usually used for large-area assembly line welding.
After the slab, various hull parts are combined into hull parts, such as T-shaped members (T-row), I-shaped members, floor longitudinal frame splicing and rib frame, etc., respectively, HS-MAG multi-electrode CO2 automatic welding, CO2 automatic fillet welding Machine, CO2 automatic fillet welder, etc.
The establishment and application of the flat DC waterline is one of the important ways to realize the mechanization and automation of flexible manufacturing technology by using the principle of grouping technology in shipbuilding and shifting the single production to the same batch production line. In modern shipbuilding, in order to improve the speed of segmentation, the flat panel and the vertical and horizontal members of the plane segment are manufactured by the production line, and the FCB welding process is used to realize the large-area panel, which accelerates the straight segmentation. The manufacturing schedule also ensures the quality of the flat panels. The principle of the FCB method is to lay a layer of uniform thickness of the pad solder (heat curing flux) on the copper plate, use compressed air to open the hose, raise the copper pad of the flux, and hold the copper plate with a certain pressure. At the back joint, the front side is subjected to multi-wire submerged arc welding, and the root of the groove of the steel plate is melted by an electric arc, and the weld of the double-sided forming of the single-sided welding is obtained by the action of the flux and the copper liner. Due to the direct contact between the flux and the back side of the steel plate, the weld is formed well. The copper gasket under the flux is not in contact with the molten metal, and has good thermal conductivity. It can effectively control the residual height of the weld and allow the use. The larger welding current is used for welding, so the FCB method has become one of the main welding processes in the flat panel welding of various shipyards.
(2) Total group and loading stage
In the main group loading phase, each structural segment and component segment is carried in the dock platform and the dock to carry the total group, such as the bottom segment, the side segment, the arm segment. The side vertical butt joints are welded by EGW vertical gas welding; some side butt welds are welded by CS-100C hard track all-position welding frustum; the deck butt welds are made of CO2 gas shielded welding, submerged arc welding The cover is welded.
4. Conclusion
The offshore oil 981 deepwater semi-submersible drilling platform is a process of exploration and progress from the initial stage of construction to delivery. Faced with complicated welding technology difficulties and urgent construction cycle, all welding technicians have overcome difficulties and accumulated rich experience and technical reserves in actual production and construction, laying a solid foundation for the future development of marine engineering.
