In the ever-evolving landscape of offshore oil and gas extraction, the push into deeper waters has necessitated the development of robust and reliable mooring systems. Suction anchors have emerged as a preferred solution due to their precise positioning capabilities and relatively short construction timelines. A recent project involved the fabrication of twelve large-diameter suction anchors for a 60,000-ton cylindrical FPSO’s multi-point mooring system. The primary challenge, and the focal point of this discussion, centered on the welding of the main lifting points—fabricated from high-strength steel castings—to the surrounding carbon steel structure. The design incorporated these steel castings for their superior strength and stability, but to control costs, their dimensions were minimized. This necessitated the addition of a compensating framework made from carbon steel grade E36 transition plates welded to the steel castings. This created a dissimilar metal welding scenario with significant inherent difficulties, primarily due to the high carbon content and consequent poor weldability of the steel castings, making the joint prone to cracking, especially at the root, where repairs are particularly challenging. This article, from our first-hand experience in project execution, details the comprehensive strategy we developed to ensure welding quality and efficiency for these critical joints.
The structure of the suction anchor is complex, comprising several key components. The main body is a cylindrical shell constructed by joining multiple single sections. Internally, strengthening frames are installed. The critical element is the main lifting point, which consists of the steel casting itself, welded to a surrounding framework of E36 carbon steel transition plates. This assembly is then integrated into the suction anchor shell. Other components include top structures and chain hawse arrangements. The fundamental manufacturing sequence involves preparing the steel castings, assembling them with the transition plates, welding this sub-assembly, and finally integrating it into the larger cylindrical structure. The integrity of the weld between the steel casting and the carbon steel transition plate is paramount for the anchor’s lifting and operational safety.
Our initial review of common practices for welding steel castings to carbon steel in suction anchors, both domestically and internationally, revealed a predominant reliance on Gas Metal Arc Welding (GMAW), specifically using carbon dioxide as a shielding gas (often termed FCAW-G or MAG). While this method offers flexibility for manual welding in various positions, its drawbacks include relatively low deposition rates, a high number of weld starts and stops requiring extensive grinding, and consequently, high labor costs. To enhance efficiency and quality for this project, we embarked on developing multiple welding procedures tailored to different aspects of the joint geometry and production environments. We formulated three primary Procedure Welding Preparation Sheets (PWPS) supported by corresponding Procedure Qualification Records (PQR).
| PWPS ID | PQR ID | Base Materials | Joint Penetration | Welding Process Sequence | Welding Position | Thickness (mm) | Post-Weld State | Purpose/Application |
|---|---|---|---|---|---|---|---|---|
| S-A001 | S-DNV001 | EH36-Z35 to A148 Gr. 90-60 | Complete Joint Penetration (CJP) | SMAW Root + SAW Fill/Cap | 2G (Horizontal) | 50 | Post-Weld Heat Treated (PWHT) | Straight and circumferential seams for primary joint. |
| S-A002 | S-DNV002 | EH36-Z35 to A148 Gr. 90-60 | CJP | SMAW Root + FCAW-G Fill/Cap | 3G (Vertical) | 50 | PWHT | General manual welding, especially for fit-up and root. |
| S-A003 | S-DNV003 | EH36-Z35 to A148 Gr. 90-60 | CJP | SMAW Root + FCAW-G Fill/Cap / SAW | 1G (Flat) | 50 | PWHT | Repair welding procedure and submerged-arc welding application. |
The selection of S-A001, which introduced Submerged Arc Welding (SAW) for fill and cap passes on these steel castings joints, was a significant innovation. SAW promised much higher deposition rates and superior weld quality for the long, straight, and circumferential seams. The total weld length for each steel casting-to-transition plate joint was calculated to be 8,976 mm. This was strategically divided into regions suitable for different processes. The straight and circumferential sections accounted for approximately 6,000 mm (about 67% of the total length), and the curved fillet (R-angle) sections accounted for 2,976 mm. The plan was to use SAW for the 6,000 mm of straight and circumferential welds, and FCAW-G for the R-angle sections. This division of labor was crucial for efficiency. The successful application of SAW on steel castings in this context is, to our knowledge, a first in suction anchor construction, and its development was prioritized after the initial FCAW-G procedures to meet site needs.
The choice of SAW equipment was critical. Standard fixed-boom SAW systems are limited to linear travel, making them suitable only for straight seams. To weld the circumferential seams on the steel castings assembly, a movable SAW tractor or buggy system was essential. This equipment can follow curved paths, allowing it to weld the entire circular joint provided the rotation speed of the workpiece (the steel casting assembly) is synchronized with the welding travel speed. The deposition rate for SAW can be modeled to estimate productivity gain. The theoretical volume of weld metal deposited per unit time is given by:
$$ Q = \frac{\pi d_w^2}{4} \cdot v_w \cdot \eta $$
Where \( Q \) is the deposition rate (mm³/s), \( d_w \) is the wire diameter (mm), \( v_w \) is the wire feed speed (mm/s), and \( \eta \) is the transfer efficiency (typically > 0.95 for SAW). Compared to FCAW-G, which has a lower deposition rate due to shorter arc time and spatter loss, SAW offered a potential efficiency increase exceeding 300% for the applicable sections, justifying the development effort.
Early production involved welding the first two steel castings in an outdoor assembly area, where the entire suction anchor cylinder was already assembled, and the steel casting was fitted inside. The welding sequence was to first complete the joint between the steel casting and the transition plate (the “casting weld”), and then weld the outer perimeter of the transition plate to the main shell. This approach presented several immediate and significant constraints, which we analyzed in depth, leading to a pivotal change in strategy for the remaining ten steel castings.
| Constraint (Outdoor Site) | Root Cause Analysis | Impact | Solution (Indoor Workshop) | Resulting Advantage |
|---|---|---|---|---|
| Fixed Tack Weld Cracking | High restraint stress from the primary casting weld concentrated on lower-strength tack welds holding the transition plate to the shell. | Safety risk, repair delays, quality concerns. | Weld the steel casting joint separately, eliminating external restraint during this critical phase. | Zero tack weld failures; reduced stress concentration. |
| Inefficient Preheating | Large thermal mass, cold outdoor environment, and use of resistance heating pads led to slow heat-up and significant heat loss. | Preheat to 154°C took ~48 hours; frequent welding interruptions. | Controlled indoor environment with induction heating. | Preheat achieved in ~10 hours (400% faster); stable interpass temperature. |
| Inefficient Position Rotation for SAW | Rotating the entire massive suction anchor cylinder (6.5m diameter) for each SAW pass on the circumferential seam was mechanically demanding and time-consuming. | Slow cycle time per weld layer, excessive wear on rotation equipment. | Rotating only the much lighter steel casting sub-assembly. | Faster rotation, less equipment stress, simpler logistics. |
The analysis clearly favored moving the steel castings welding operation to an indoor workshop. This decision fundamentally improved our control over the welding process for these sensitive steel castings joints.
With the optimal location determined, we implemented a rigorous, step-by-step control protocol for every steel castings weld. This began long before the arc was struck, with stringent incoming material control. We mandated that all steel castings undergo full Ultrasonic Testing (UT) by the supplier to eliminate inherent material defects. Furthermore, the welding bevels were machined at the foundry to our specified profile to save time, though we had to meticulously remove any anti-corrosion coating applied for shipping. The joint design was a double-V (X) groove with a 50° ±5° included angle, with the internal side representing about two-thirds of the 50mm thickness. Dimensional inaccuracies in the steel castings were accommodated by custom-fitting the E36 transition plates, maintaining a root gap between 1-4 mm. Misalignment was kept below 4 mm, and critically, no hammering or temporary attachments (like strong-backs) were allowed directly on the steel castings to prevent localized stress and potential material damage.
Preheating was non-negotiable. The minimum interpass temperature of 154°C was required to slow the cooling rate, reduce the risk of hydrogen-induced cracking in the heat-affected zone (HAZ) of the steel castings, and minimize residual stresses. We employed medium-frequency induction heating for its efficiency and controllability in the workshop. The heating bands covered a zone at least equal to the material thickness on both sides of the joint. Temperature was monitored continuously with calibrated thermocouples. The heat input \( H \) for each weld pass was carefully controlled and calculated as:
$$ H = \frac{60 \cdot V \cdot I}{1000 \cdot S} $$
where \( H \) is heat input (kJ/mm), \( V \) is voltage (V), \( I \) is current (A), and \( S \) is travel speed (mm/min). For the steel castings, we maintained a relatively low heat input, typically between 1.0 and 1.5 kJ/mm for the initial passes, to manage microstructural changes.
| Welding Phase | Process | Electrode/Wire | Current (A) | Voltage (V) | Travel Speed (mm/min) | Heat Input (kJ/mm) | Shielding Gas (if applicable) |
|---|---|---|---|---|---|---|---|
| Root Pass | SMAW (E7018) | 4.0 mm | 120-140 | 22-24 | 100-150 | ~1.1 – 1.3 | N/A |
| Hot/Initial Fill Passes | FCAW-G | 1.2 mm (E71T-1C) | 180-220 | 24-28 | 250-350 | ~0.9 – 1.2 | 100% CO₂ |
| SAW Fill/Cap Passes | SAW | 4.0 mm (S3 Ni1) | 500-600 | 28-32 | 400-500 | ~1.7 – 2.3 | Flux |
The root pass was executed using Shielded Metal Arc Welding (SMAW) with low-hydrogen electrodes. Tack welds were either incorporated into the root pass or removed and repaired properly. After root completion and back-gouging, the welding sequence proceeded. For the straight seams, after sufficient fill using FCAW-G, we switched to SAW. A critical practice was the welding sequence within each layer: we always deposited the weld bead on the carbon steel (E36) side first before welding on the steel castings side. This helped manage dilution and residual stress distribution. For the circumferential SAW seams, precise coordination between the turning gear rotation and the SAW tractor speed was vital. The relationship is defined by:
$$ v_{rotation} = \frac{v_{weld}}{\pi \cdot D} $$
Where \( v_{rotation} \) is the rotational speed (revolutions per minute), \( v_{weld} \) is the welding speed (mm/min), and \( D \) is the diameter of the joint at the weld centerline (mm). For the R-angle sections, which were geometrically complex, manual FCAW-G was used with strict control over gun angle and weave technique to ensure proper fusion to both the steel castings and the transition plate.
Despite meticulous controls, a few defects were detected via Non-Destructive Testing (NDT), primarily in the root and hot pass regions. The defects were localized lack-of-fusion and slag inclusions, but notably, no hydrogen cracks were found in the steel castings HAZ, validating our preheat and low-hydrogen process controls. The root cause analysis pointed to two main factors: minor inconsistencies in the joint gap along the circumference of the steel castings, and insufficient depth during back-gouging after the root pass, which left minute defects from the initial hot pass unremoved. The repair procedure was stringent. Defect removal was done using air carbon arc gouging, but only after preheating the area to a temperature 20-30°C higher than the original interpass temperature. The gouging was performed carefully until all defective material was removed, creating a smooth, U-shaped groove with a minimum radius of 5mm. The repair weld was then carried out using the qualified repair procedure (S-A003), with even stricter control over preheat and interpass temperature. We limited the number of repairs on any single section of the steel castings joint to a maximum of two to avoid excessive thermal cycling.
The comprehensive strategy yielded excellent results. The welding procedure qualification records (PQRs) provided a solid technical foundation. The decision to weld the steel castings sub-assemblies in a controlled workshop environment eliminated major external variables and restraint issues. The innovative use of SAW for the majority of the weld length on these steel castings joints provided a dramatic increase in productivity and consistent quality. Rigorous procedural controls from material preparation through to final inspection ensured that the inherent challenges of welding high-carbon steel castings to lower-carbon structural steel were systematically managed. The project successfully delivered all twelve suction anchors with the critical steel castings welds meeting all quality standards. This experience demonstrates that with careful planning, appropriate process development, and disciplined execution, the welding of large, high-strength steel castings to carbon steel in critical offshore applications can be achieved with high reliability and efficiency, setting a valuable precedent for future similar structures.