Q1: What are the key points of field welding of API pipeline steel pipes?
Strict control is required for field welding of API steel pipes: pre-welding preparation includes groove processing (30°±5°V-shaped), preheating (100-150℃) and cleaning; welding materials should match the parent material (such as E8018-G welding rod for X80 steel); multi-layer and multi-pass welding is adopted, and the interlayer temperature is controlled to be ≤250℃; root welding recommends STT (surface tension transition) or RMD (regulated metal deposition) process; filling welds use swing welding, and the heat input is controlled at 15-25kJ/cm; cover welds need to pay attention to the excess height (0-3mm). Immediately after welding, 200-250℃ post-heating is carried out, and RT or UT testing is completed within 24 hours. Important welds also require hardness testing (HV10≤248).
Q2: What are the key points to pay attention to when going downhole with API casing?
Casing downhole is a high-risk operation. The key points include: checking the steel grade, wall thickness and thread type (such as BTC, LTC) before entering the well; using special thread compound (in accordance with API RP 5A3); controlling the lowering speed (≤0.5m/s) to avoid collision with the well wall; filling drilling fluid with every 10 casings to balance the pressure; using a torque monitor to ensure that the threaded connection torque meets the standard (such as 7" 29# L80 requires 12,500-15,500ft-lb); forcible pressure is prohibited when encountering resistance, and the well should be drilled through. H₂S wells also need to monitor the casing potential to prevent sulfide stress cracking. After completion, the casing pressure test (1.5 times the working pressure, stabilizing the pressure for 30 minutes) is required.
Q3: Special installation requirements for submarine API pipelines?
Submarine pipeline installation needs to consider: using S-type or J-type pipe laying method, controlling the bending radius ≥ 25 times the pipe diameter; using a counterweight layer (concrete thickness ≥ 40mm) to prevent floating; the welding cabin needs to maintain positive pressure to prevent tide; anode blocks are set every kilometer (output current ≥1.5A/m²); ROV monitors the bottom status of the pipeline in real time; cross sections need to be protected with sandbags or saddles. Deepwater pipelines (＞500m) also need to undergo buckling analysis, and AUV inspections are carried out after installation (to detect overhangs, corrosion, etc.). Compared with onshore pipelines, the cost of submarine installation is 3-5 times higher and the construction period is 2-3 times longer.
Q4: What are the corrosion monitoring technologies for API steel pipes?
Mainstream monitoring methods include: ultrasonic thickness measurement (measurement of corrosion rate every month); intelligent pipe cleaning (PIG) for Internal inspection (accuracy ±0.1mm); ER probe (real-time monitoring by resistance method); FSM (field fingerprint method) to measure wall thickness loss; online corrosion monitoring system (such as LPR, ENM electrochemical method). For buried pipelines, pipe-to-ground potential (CIPS) and DC voltage gradient (DCVG) need to be measured regularly. The data should be entered into the integrity management system, and the repair procedure should be initiated when the corrosion rate is greater than 0.4mm/year or the residual strength is less than 80% of the design pressure.
Q5: What are the common methods for API steel pipe repair?
According to the type of damage, select Optional repair methods: Pitting (depth < 20% wall thickness) is reinforced with polymer composite materials; local thinning (20-50% wall thickness) uses B-type sleeves or fixtures; circumferential cracks require replacement of pipe sections (length ≥ 3 times the diameter); internal corrosion can use the inversion lining (CIPP) technology. Welding repairs must be preheated (150-200℃) and low-hydrogen electrodes must be used. Pressure testing (1.25 times the working pressure) and non-destructive testing are required after repair. Pipeline repairs in acidic environments require special control of hardness (HB≤22). All repairs should comply with ASME PCC-2 or GB/T 30579 standards.
