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Thicker and heavier, and with sharper tolerances than ever before – this was in essence the challenge Heerema Vlissingen faced in the construction of the template for the Tombua Landana oil and gas platform. The answer was found in smart logistics and precision work, supported by proven welding solutions. (See page 14 for a description of the Tombua Landana project).
Heerema Fabrication Group (HFG)
Heerema is a name that requires little explanation in the oil and gas industry. It is one of the bigger, globally operating players in the engineering and fabrication of large and complex structures for the oil and gas industry. It has been active in the offshore industry ever since oil and gas were discovered in the North Sea in the early 1960's and enjoys a reputation for state-of-the-art engineering, fabrication and project management.
HFG has yards in the Netherlands (Vlissingen and Zwijndrecht) and in the United Kingdom (Hartlepool). All are equipped with large prefabrication and assembly halls for indoor construction and are capable of handling large projects, simultaneously.
HFG is part of the Heerema Group, together with Heerema Marine Contractors (HMC) which transports, installs and removes offshore facilities, and INTEC engineering, which provides engineering services to the energy industry. HFG Engineering, a subsidiary of HFG with offices in New Orleans and Houston, specialises in on- and offshore facility designs.
All Heerema companies operate an integrated management system that complies with ISO 9001: 2000 (Quality Management Systems), ISO 14001: 2004 (Environmental Management System) and OHSAS 18001: 1999 (Occupational Health and Safety Management System) standards.
The Tombua Landana template
The tower base template (TBT) has a surface area of forty by forty metres, is 24 metres high and weighs 3,000 tons. It includes 12 main foundation piles with a total weight of 9,500 tons. It was completed and shipped to Angola in December 2007.
Figure 1 shows a sketch of the TBT. The principal components are the pile sleeve clusters, the rows and the leveling jacks. Not indicated, but discussed later in this article, are the lifting trunnions – used for dual crane lifting with one of HMC's specialised barges.
The pile sleeve clusters form the cornerstones of the TBT. They guide the twelve foundation piles which are driven through them into the sea bed. Crucial during installation of the 190 m long piles in nearly 400 m deep waters, are the open cones on top of the pile sleeves. They catch the foundation piles hanging from the crane and guide them into the sleeves, after which pile driving commences.
Part of the length of all foundation piles remains extended above the pile sleeves. The Tower Base Section –the lower part of the tower –is placed over them and secured to the template.
The rows are a network of heavy pipes connecting the four pile sleeve clusters to form a rigid construction. Four leveling jacks, devices to position the template horizontally with great accuracy, are attached to the central columns of the rows. The shim piles on the leveling jacks rest on leveling piles in the sea bed. Leveling is performed by jacking the template up or down relative to the shim piles.
The entire Tombua Landana project is characterised by very narrow construction tolerances, the substructures being placed on top of each other, in nearly 400 m deep waters - a particularly unforgiving environment for any misalignment.
Also, the TBT was subject to strict dimensional tolerances –up to three times more precise than normally required in offshore fabrication. Moreover, it was the first part of the tower and all eyes were focused on Heerema. Two Daewoo representatives and two representatives of Chevron supervised the project and carried out regular inspections.
Steel grades, mechanical requirements and preheat temperatures
Steel grades were purchased according to the "General Specification 1.14 Structural steels and other materials", issued by the Cabinda Gulf Oil Company for the projects in block 14. In this specification, material types are ranked I and I-X, II and II-X, III, IV and V. Material types I are for structural members and tubular joint cans which are fracture critical and material types II are for structural members and cans where failure would pose a threat to the structure. Material types III, IV and V are for non-critical components. A list of valid steel classifications is given for each material type. Heerema Vlissingen purchased various types of plate according to EN 10225 Grade 355 (thermomechanically controlled rolled) and API 2MT1 as rolled, covering the demands of material types I and II, and meeting special constructional requirements such as "through thickness properties". All main steel was purchased from Dillinger Huttewerke in Germany.
Mechanical weld requirements are established by Cabinda's General Specification 1.15 –Structural welding and inspection. Charpy V-notch impact testing of both weld metal and heat affected zone was required on all welding procedure qualifications, with notch locations at the weld centre line, fusion line and FL+2mm. CTOD testing of the WM and HAZ was required for type I and II steels with a thickness greater than or equal to 63 mm (2.5"), to be performed on the thickest steel to be welded while using the highest preheat and interpass temperature permitted by the welding procedure to be qualified.
Table 1 gives an overview of CVN and CTOD requirements. An additional cross weld zone hardness requirement was set at HV10 325 maximum.
In constructions such as these, involving thick material, the prevention of hydrogen induced cracking (cold cracking) is essential. This starts with the purchase of steels with limited hardenability. Cabinda's General Specification 1.14 for structural steels and other materials therefore specifies a maximum Pcm value of 0.23 (extended CE formula).
In welding, preheating, along with the use of low-hydrogen consumables, is essential in the prevention of cold cracking. Cabinda General Specification 1.15 refers to AWS D1.1, for preheat and interpass temperatures to be applied. For type I and II steel, this resulted in the preheat and interpass procedure of Table 2, according to Table 3.2 of the AWS.
Welding processes and consumables
Offshore fabrication, in general, is characterised by the use of heavy plates and pipes, in all welding positions. A certain number of joints can be brought in the downhand position, but the bulk remains manual work on often complex structures such as the nodes in platform jackets. Heerema Vlissingen applies three principal welding processes:
The selection of consumables for these processes was guided by mechanical properties, productivity and, very importantly, low-hydrogen characteristics.
Flux-cored arc welding
For manual positional welding, Heerema Vlissingen uses, almost exclusively, FILARC PZ6138-ø1.2mm, an all-position rutile flux-cored wire welded in Ar/ CO2 mixed gas (FILARC is a product name in the ESAB consumables range). It is alloyed with 0.9% nickel and micro-alloyed with titanium-boron for good CVN toughness down to -60°C and good CTOD properties at -15°C.
These all weld metal properties have been sufficient for virtually all the projects Heerema has handled over the many years they have been using this wire. In addition, it gives a deposition rate in positional welding that cannot be met by any other manual process (3-4 kg/h at 100% DC). It is extremely welder friendly, operating in the spatter-free spray arc mode over the full range of applicable welding parameters. Root passes can be produced on ceramic weld metal support.
Crucial for Heerema Vlissingen, when selecting FILARC PZ6138, was the fact that ESAB was able to present low-hydrogen test results, consistently within EN hydrogen class H5, for every individual batch of wire produced over many years. This, in combination with a practically unlimited shelf life and minimal moisture pick-up on the shop floor, convinced them this was the best wire for their fabrication.
Heerema Vlissingen also uses FILARC PZ6125 – a basic wire used for the welding of roots without ceramic backing.
Submerged arc welding
For SAW, Heerema Vlissingen selected an unconventional flux/wire combination: ESAB OK Flux 10.47/ OK Tubrod 15.24S- ø4.0 mm - a combination of a fused flux and a basic cored wire.
Fused flux, used with solid wires, has never been popular for demanding fabrication applications, due to poor mechanical properties. They hold certain advantages, though, the most important for offshore fabrication being that they are completely non-hygroscopic and can be used without re-baking. By engineering the mechanical properties through the basic cored wire, rather than through a high basic agglomerated flux, this combination reaches the following typical mechanical properties:
Sufficient for most applications, this allows Heerema Vlissingen to use the flux without re-baking, directly from the packaging, which significantly simplifies the flux handling at the yard. For the Tombua Landana project, welding procedure qualifications were performed at the required temperature and mechanical demands were met with a comfortable margin.
Another huge advantage is that the deposition rate from the cored wire is substantially higher than from a basic flux with solid wire, resulting in fewer layers and a productivity that is ~30% higher, which is a welcome improvement for any fabrication.
Due to the glass nature of the flux, the grain strength is significantly higher than that of the fully basic agglomerated fluxes. This results in less break-down and hence no problems with "dusting" and, therefore, all round improved recycling.
SMAW (MMA) welding
FILARC 76S is universally applied for tack welding and for the first passes in joints with difficult access. It is a basic AC/DC MMA electrode with good CVN properties down to -60°C and is CTOD tested in the AW and SR condition. It has a vast track record that dates back to the years when MMA was the standard for manual welding in offshore fabrication. FILARC 76S is low-hydrogen with low moisture absorption properties. It is supplied to Heerema Vlissingen in VacPac vacuum packaging for ultimate protection.
Major welding applications in the pile sleeve cluster
Figure 2 (click image to enlarge) shows the fabrication of a pile sleeve cluster. Its main components are indicated. The pile sleeve - the part which guides the foundation piles - has been pre-fabricated by Sif Group bv, in Roermond, along with the foundation piles themselves. Also the parts of the conical "pile catcher" allowing the use of SAW - mostly circumferential welds - are already attached during pre-fabrication Heerema Vlissingen completes the catchers with stiffener plates. This involves a vast amount of full penetration butt welds, as well as fillet welds, all performed with manual FCAW, using FILARC PZ6138. Where possible, root passes are deposited on ceramic weld metal support.
The welds connecting the pile sleeve to the shear plate involve a symmetrical double-sided K-joint in 51 mm thickness (openings angle 40 degrees, root gap 5mm, root face 1mm), welded with the SAW process, using the OK Flux 10.47/OK Tubrod 15.24S flux/wire combination and ESAB A2 welding tractors. The root pass of these full-penetration welds is done with PZ6125, on ceramic backing, whereas sufficient thickness for the SAW process is obtained by a hot pass with PZ6138. In this stage, the construction can still be turned on roller beds, aided by contra weights, in order to use the productive SAW process on both sides.
Turning the construction is no longer possible when two pile sleeve-shear plate pairs – grit blasted and painted - are connected to the main leg. Here a combination of SAW for the downhand side and FCAW for the overhead side is used (root FCAW on backing). The preparation is a 2/3 –1/3 K-joint, so that the larger part of the joint volume can be welded in the downhand position with the productive SAW process. The 1/3 side is completed in the overhead position, using FILARC PZ6138 rutile cored-wire.
When the shear plate of the third, and last pile sleeve in a cluster, is connected to the main leg, the joint position is horizontal-vertical. The joint preparation is again a symmetrical K-joint welded completely with the FCAW process, using FILARC PZ6138.
The upper and lower yoke plates are connected by means of manual FCAW. It concerns full penetration X- and K-welds with all welding positions occurring. Again PZ6138 is the main consumable (roots on ceramic backing).
TKY-joints in rows
The rows - a network of heavy pipes connecting the four pile sleeve clusters – are pre-fabricated both indoors and outdoors and require great dimensional precision.
The two columns on the left and right are not part of the structure. They have the same dimensions as the main legs of the pile sleeve clusters and precisely set the dimensions of the row, before (tack) welding. Permittable deviations here are as narrow as ± 1/4" (6 mm) horizontally and ± 1/8" (3 mm) vertically, requiring extreme accuracy. It is a procedure of virtually endless dimensional control. The same procedure is repeated during erection of the template, before rows are finally connected to the legs in the pile sleeve clusters.
All nodes (TKY-joints) are welded in the positions as they occur in figure 7 with FCAW using PZ6125 for the root and PZ6138 for filling. Figure 8 shows the FCAW welding on a special TKY-joint –the lifting trunniuns. These are used to attach the lifting cables onto the template during installation. Part of it is welded with SAW with OK Flux 10.47/OK Tubrod 15.24S.
The foundation piles are pre-fabricated by Sif Group bv, arriving in 83-93 m lengths. To achieve their final length of 170 –190 m, they need to be welded together. This is again done by FCAW with PZ6138, but here it is mechanised welding with ESAB Railtrac equipment. The joint configuration is adapted to this method -an unsymmetrical X-joint with most of the weld volume on the outside. The inside part is welded manually, vertically-up. The root pass is deposited on ceramic backing. After removal of the fit-up plates, the majority of the weld volume is mechanised welded from the outside, verticallyup from 6 to 12 'o clock, mostly with a slight weaving motion. Welding parameters are adapted to the several clock positions by the operators.
Dimensional control and weld finish
Normal offshore fabrication, eg, a jacket on top of which the deck and operational facilities are placed, is naturally, subject to strict dimensions, but it is more forgiving than in the case of the Tombua Landana project. The fact is that three substructures, fabricated by three different yards, are stacked on top of each other in almost 400 m deep waters - and simply have to fit. This places extremely high demands on the dimensional control – roughly 3 times as high as normally required.
This is as equally valid for the fabrication of the TBT's components –the pile sleeve clusters and the rows –as it is for the assembly of the super structure. To exemplify the dimensional control and its implications for welding, we return to the fabrication of the pile sleeve clusters.
This image shows the completed pile sleeve cluster and the nominal distances between the centre lines of the pile sleeves and the centre line of the main leg (5172.5 and 5173 mm). The maximum acceptable tolerance on these distances is 3 mm. A similar small tolerance is valid on the distances between the pile sleeves themselves, in the X and Y directions and on the mutual distances into the Z direction. This dimensional control is the key requirement, and everything else is subject to it.
Ideally, the pre-fabricated yoke plates, including K-bevel, fit exactly, so that there is a constant root gap between the pile sleeves/main leg and the yoke plate's K-bevel. In practice, this is extremely difficult to achieve. Practically always, the root gap appears to be more or less eccentric. This must be corrected by grinding on the narrow side and buttering & grinding on the wider side –an extremely time consuming exercise.
Measuring was performed by three parties; Heerema Vlissingen, Passe-Partout (independent contractor) and Chevron, who worked independently according to agreed measuring principles. Chevron were responsible for final measuring and reporting.
Another time-consuming aspect was Class C and Class A grinding of weld surfaces. Grinding is done with an aluminium oxide based disc.
Class C grinding is required for the TKY joints between the braces and the dummy leg (middle of the row) and between the braces and the main leg of the pile sleeve cluster. It is performed to correct excessive convexity, notches or undercut at the toes of the weld. The grinding of the toes of the cap must be performed to the point where a 1 mm diameter wire cannot pass between the disc and the plate.
Class A grinding is performed on the welds connecting the lower yoke plates to the main legs of the pile sleeve clusters –at both sides of the K-joint. Class A means that weld profile is ground back to the theoretical radius. This is checked by using a template with a 45 mm radius, with a gap the size of a paperclip not being allowed. The total length to be ground per weld was 2 x 11.5 m for each of the four main legs.
The Tombua Landana template was one of the most challenging projects ever undertaken by Heerema Vlissingen. It required a carefully planned factory lay-out and a level of precision not before experienced. The company finished the project within the agreed delivery term and its sister company, HMC, will be involved in sea transportation and installation of the 474 m tall Tombua Landana oil and gas platform.
Safety was essential. To step up its performance beyond already tough levels, Heerema Vlissingen took part in Chevron's safety programme –Incident and Injury Free (IIF) –in which Chevron gave workshops and training to the yard personnel aiming at individual development. For its welding solutions, Heerema Vlissingen relied on low-hydrogen consumable technology from ESAB –a supplier that meets Heerema Vlissingen's demands in any respect, including quality management systems, environmental management systems and occupational management systems. Like Heerema, ESAB is SO 9001 and ISO 14001 approved world-wide.OHSAS 18001 is the latest approval obtained byESAB.
The final words of this article should beaddressed to the welders of Heerema Vlissingenwho did such a tremendous job, notwithstandingthe high preheat and interpass temperatures andoverall tough working conditions.
Acknowledgement We thank Heerema Production Manager, Harm Sanstra, for facilitating our visits to the Vlissingen yard.
By Alfred Van Aartsen, Heerema Vlissingen B.V., The Netherlands And Eric De Man, Esab Nederland B.V., Amersfoort, The Netherlands.