Fabricating an 8-leg launch jacket in record time

Using multiple work centers and close project management, L&T delivered the MNP jacket on schedule.

Larsen & Toubro (L&T), India was awarded the Mumbai High North (MHN) process platform, living quarters, and process gas compressor module project by ONGC, on 31 July 2009. The demanding, 33-month schedule included EPCI of an 8-leg launch jacket weighing in excess of 13,500 metric tonnes (mt) with jacket delivery in 12 months.

The project management team decided to execute the MNP Jacket project from multiple work centers with independent task forces: engineering at L&T Valdel (LTV) Bangalore, fabrication at L&T Modular Fabrication Yard (MFY) Sohar, Oman, and project management and installation at Mumbai.

The MNP jacket (60m x 82m x 81m) and its piles were completed ahead of schedule, in ten months. This was achieved by meticulous planning and coordination between teams, and “imagineering” unique fabrication sequences. The MNP Jacket sailed away in November 2010.

Preliminary calculations indicated a substantial increase in jacket weight when compared to the client’s FEED calculations, due to an increase in topsides area and load. Vendor inputs for equipment, packages, and installation constraints had cascading effects on jacket weight.

Due to time constraints, the project team decided to start fabrication based on drawings submitted to the client for approval. Construction engineers were available for real-time inputs on drawings.

The steel required was ordered from Korean mills, based on the preliminary material take-off (MTO). Since the dimensions were preliminary, tubulars with standard length of 11.8m and can length with extra 0.3 m were procured.

Schedule optimization:

The jacket was intricate, including 20 skirt legs with skirt pile guides, and 23 internal risers (over top and bottom row instead of side panel). Jacket dimensions required the outside panels to be fabricated elsewhere, and to be moved to the final roll-up location after roll-up of the inside panels. Self-propelled, modular trailers were used for transportation of panels. The fabrication sequence was altered four times to optimize activities and timing.

Fabrication

Client representatives at LTV during engineering reduced cycle time and speeded document approval. Client’s consultants were authorized to expedite the work. Difference in time zone and work week between Oman and India was effectively utilized for query resolution. The fabrication sequence optimized use of resources during fabrication cycles. Its flexibility motivated the construction team to innovate and try possible options. Special tools and welding processes (automatic FCAW) were used for launch cradle welding and internal stiffener insertion inside the main leg. The jacket’s outside legs had double batter, whereas the inside (launch frame) had single batter. The main jacket was to be supported on the seabed with 20 skirt piles and guides. The dimension between the two launch legs was 20m.

This jacket was divided into multiple components including: launch cradle (two per launch leg), launch truss (Row 2 & 3), side panels (Row 1 & 4), Row A and B side panels, Five horizontal frames, mud-mat (steel), pile guide and sleeves, buoyancy tanks (on Rows 1 & 4 for controlled upending), and 23 risers (10 on top, 13 on bottom); sump caisson, fire, water, and utility caisson.

The launch cradle had transverse and longitudinal webs that were welded to the launch leg. The cradle was supported by timber along the bottom and the mating surface was timber over a stainless steel (SS) skid beam with wax and grease in between. The launch cradle was fabricated in 3 or 4 pieces, kept on the skid beam, and joined. Care was taken during welding to minimize distortion and to provide saddle supports. Launch trusses were fabricated on the saddle supports, which were designed for jacking down.

The launch truss members were designed to withstand normal wave forces as well as load-out forces during skidding. The launch leg had internal/external stiffener rings at nodal locations to resist pinching stresses. The launch leg was made of 63mm-thick, 2z (50ksi) material. Launch trusses were fabricated in parallel position 85mm higher than required to avoid contact between rolling members, during the roll-up process. Legs were placed on cup/ saddle support and cross bracings were added.

Once the panels were fabricated, the horizontal framings were erected. Cross bracings of Row A were installed. Row 2 was rolled-up first against the horizontal framings, guyed in position, and horizontal framings fitup with the leg was completed. After welding, the cranes were positioned to roll-up Row 3. The panels, weighing around 1450mt, were rolled-up using four cranes. Once Row 2 & 3 panels were in position, Row B cross bracings were installed and welded.

Jacking down

It was decided to jack down the entire jacket instead of individual panels of Rows 2 & 3, considering safety and the following advantages, the inside jacket structure as a whole is sturdier, connected panels eliminate misalignment, and the members between Legs 2 & 3 could be erected and welded.

The roll-up saddles were inserted between the launch cradle and launch panel; Rows 2 & 3 were supported on these saddles. Individual panels were rolled up; the inside jacket was completed and jacked down to the required height.

Side panels

Leg-pots were designed to accommodate ground elevation change from initial to final side panel location. Side panels were transported with leg-pots and placed on final location, after which skirt sleeves and guides were completed. Plates provided easy maneuverability for offloading the panels. The flooding and grouting line for leg was completed prior to roll-up. The panel was then rolled-up against the inside jacket using four cranes.

Horizontal panels

The inside horizontal panels were fabricated as “K” and positioned using temporary steel before roll-up of Rows 2 & 3. These panels were guyed with wires and roll-up was done against them. This avoided the necessity of a pup piece and girth weld, for which 100% radiography is required, thereby saving time. Erecting 13 risers (top row B) with synchronized cranes was time consuming and had an adverse impact on the project schedule. During brainstorming sessions several options were evaluated. The project management team decided to build temporary “riser racks” and erect them after completing the frame of Row B.

This procedure had cost and schedule advantages, was safe, fast, and required much less steel. Care was taken to avoid damage to concrete-coated risers.

The horizontal panels between Rows 1 & 2 and Rows 3 & 4 were used as “riser racks” to temporarily keep the Row B risers. These inclined braces were checked in STAAD Pro for deflection against the risers’ load. Temporary stoppers were provided to prevent risers from slipping. Riser clamps were pre-assembled and welding was completed after alignment of all riser clamps. Once the risers were stored in racks, the top panel with riser clamps was erected; welded, and then individual risers were lifted and clamped.

Mud mat, pile guide, and sleeves

The mud mat was made of 4mm corrugated steel plate, seam-welded over the rolled part of the frame. The mud mat with frames was lifted in sections and installed.

The piles (20) were parallel to the legs. Each corner leg had three piles and the central leg had two piles. The main leg did not have any piles, and internal stiffeners were provided to resist against bending load and prepunching load. Piles were provided with pile guide to facilitate welding during driving. Pile sleeve was joined to the leg with a combination of tubular section and yoke plate.

Buoyancy tanks

Buoyancy tank design and connection details were very complicated and required approval from the installation contractor. The two buoyancy tanks weighed 600mt each, including 150mt connection details for each tank.

The required resources were mobilized and three separate teams were formed for fabrication and installation. Buoyancy tanks were positioned using four cranes and rolled on inclined support using two cranes. The roll-up was more critical due to inclined cup supports and probability of slippage of the whole structure.

Load-out

A special skid-way was created at MFY on which the MNP was assembled. Load-bearing capacity of the skid beam was calculated at 150mt/m and involved 600mt of fabrication. Due to time constraints, outsourcing fabrication or bought-out skid beam was feasible, however, considering quality, it was decided to procure a suitable skid beam.

The ground was leveled and compacted, and workers placed sleepers to prepare the skidding track. The vertical load on skid beam / sleepers was 1500 kN/m. Traction load along skid beam and sway load perpendicular to the track were max 15% and 5%, respectively.

The skid beam top surface was leveled within 6mm using shim plates between the skid beam and sleeper surface and then welded together. This weld provided locking against the traction pull required during skidding operation. The skid beam top was SS plate, stitch-welded over CS material and the mating surface was SS plates with timber. FC grease with slip coat was used as lubricant. During loadout, break-out was achieved with only 270mt (2% of jacket weight).

Load-out operation required fourstrand jacks (900mt each), 34 strand wires (18mm) with SWL 15mt in each strand jack, power packs for strandjack operation, motorized ballast pumps, 24 No’s (1000mt/hr) and 16 No’s (250mt/hr), ballast line pipes, mooring wires, mooring winches, etc.

The barge was made stern on, moored by lines and winches. Strand wires were then pulled from strand jack to anchor frame mounted on the barge. A temporary platform was fabricated on the skid beam to facilitate strand-wire pulling operation. These innovations saved two days. The jacket was ready for load-out, once all the strand wires were laid down. A pushing bracket with 450mt push jack was welded to the skid beam. Strand wire pre-tensioning began jacket movement. The jacket was moved at 10m/hr and the barge level was monitored continuously. Barge level was within 25mm throughout the operation.

The jacket reached the final loadout location as it touched the stopper plate. Once sea fastening activities were completed, the jacket was ready for sail out.

The MNP jacket was successfully launched in Mumbai High field on 20 January 2011. Successful completion of the launch jacket in record time of 10 months demonstrated that new benchmarks can be established with proper coordination among teams, client’s support, calculated risks, proper risk mitigation plans, and out-of-the box thinking. Some innovative concepts that could not be implemented in this project can be implemented in future projects. OE

Link to video illustrating the project: MNP jacket project: http://www.youtube.com/watch?v=aq2W-girP_g

Acknowledgement

The authors wish to acknowledge the contribution of co-authors Mssrs. S. P. Kulkarni, Dibyendu Das, Prabhas Tripathi, Sandeep Badhe, Shardul Samant, and Devendra Awadhiya during execution of the project and their assistance in the development of this article.

Mr. C.S. Kole is Chief Executive of L&T Modular Fabrication Yard, Sohar, Oman and has been responsible for planning and execution of fabrication projects for L&T’s Hydrocarbon Business at Sohar yard. He has 32 years of experience and has successfully completed more than 30 major onshore and offshore projects. Kole is a Civil Engineering graduate and gold medalist from University of North Bengal.
Mr. Kumar Rudra is Head – Projects and is responsible for L&T’s Upstream Domestic Oil & Gas Operations. He has more than 22 years of experience and has successfully completed more than 12 major EPC projects. Rudra is a Chemical Engineering graduate from Jadavpur University, Kolkata, West Bengal.