A Mega Lift - Lifting of Prill Tower Trusses

Quote - "I'd rather do something and fail than do nothing and succeed!" (Unknown)

“Company will be out of business!” This was the failure consequence recorded by a senior team member during “what if” analyses conducted for a planned activity to be carried out on world’s tallest Prill Tower. The probability of this consequence, at that point in time, was recorded as ‘High’. This was March 2009.

On August 29^th, 2009, Prill Tower 3 execution team showed that handwork, dedication, planning and team work can turn a seemingly impossible task into an accomplishment.

The activity was the lifting of Prill Tower bucket room structure, approx 450 Tons in weight, to a height of 109 meters. By completing the lift safely, this team created history in Pakistan. This was the day when total load was transferred from the lifting arrangement to the structure itself thus completing the activity.

Prill Tower Bucket Room

Prilling tower is the final piece in a complex maze of equipment, piping, and processes, which are required to produce Urea fertilizer. This landmark cylindrical tall structure houses the equipment necessary to produce a shower of molten Urea from a height which then meets the air draft coming from the bottom windows. This results in solidification of urea into round Prills. The main equipment is called the Prill Bucket which is housed, along with its support equipment in ‘Prill Bucket Room’. For Prill Tower 3, this room is constructed in the middle of a steel support structure (an arrangement of trusses) at a height of 109 meters. Total load of this arrangement, including the exhaust structure, is about 900 Tons.(See Fig 1)

A CONSTRUCTION CHALLANGE

Construction at an elevation of 109 meters by any means was a tough task and involved many safety risks, almost all with a fatality potential. Building the world’s largest ammonia / urea complex and the tallest Prill Tower also meant that whatever construction methods will be adopted will be unique and mostly with no parallels in the world to learn from. The planning work therefore started earlier in February, 2009. Joint teams of owner company execution and Safety and sub-contractors put hundreds of man-hours debating and discussing each and every aspect of the Bucket room construction. The designer was also involved in these discussions and helped improve the constructability through improvisations in their design. Consultants, specializing in heavy lifts, were involved and various mechanisms of constructing and rigging this room were discussed. While the design aspects were agreed and work started on the fabrication of steel Trusses, the debate continued on whether to assemble these trusses piece by piece at the top and then complete the concreting work (Option 1) or to assemble the trusses at bottom, do the concrete works and lift the assembly as a complete unit (Option 2). Both options had their merits and demerits. Option 1 was conventional and rigging of small members which make up a truss was conceivable. However assembly at 109 meter height and accessibility for construction activities made it highly risky and unsafe. Option 2 seemed to be much safer as all critical jobs of truss assembly and concreting will be done on ground very safely and the final completed structure will be lifted in one go. The problem with option 2 was “Who had done this before and how?”

Option two was ultimately chosen. The probability of failure, deemed to be “high” in March 2009, was brought down to a level of “very low” by employing a lifting technique very little known in this part of the world. The technique is known as ‘Strand jacking’

STRANDS, JACKS AND SPECIALISED HEAVY LIFTING

The decision to assemble trusses on ground concluded that around 450Tonnes of weight has to be lifted from 0 to 109 meters height. Only a specialized lift contractor could do that. The contractor was found after an international hunt. Some of their famous lifts, also featured in the ‘Mega Structures’ TV series include lifting of the connection bridge between Petronas Towers, Pinnacles of Burj Dubai and Taipei 101. Their lifting experience goes beyond thousands of tons and hundreds of meters of heights. The technique they use for such mega lifts is called Strand Jacking. In this technique, high pressure hydraulic jacks (SLU) pull or push the load by means of metallic ropes or ‘strands’. Multiple strands are attached to one SLU. Number of SLUs can be increased to accommodate the weight, shape and size of the load being lifted. Action of these units is synchronized and with each stroke they raise or push the load until the desired position is achieved. (Fig 3)

After going through the details of this technique, it was concluded that if this huge structure is to be lifted in one go, it has to be done by strand jacking and by a company specializing in this field. We finally signed a contract for this mega lift in July 2009.

PRILL TOWER 3 TRUSS LIFT MEGA LIFT

The methodology adopted by lift contractor to lift Prill Tower 3 bucket room, a structure of about 450 Ton weight was based on 8 SLUs. Each SLU was capable of safely lifting 80 Tons of load and had 7 strands each. These strands, thru special end pieces were attached to specially designed ‘eyes’. Jacks themselves were mounted on specially designed temporary steel structures. (Fig 2). A lot of effort went into the design and review of these temporary structures as the success or failure of this lift was totally dependent on them. This effort was led by our in-house structural engineers and involved reviews by 3^rd party designers also.

The lifting equipment reached site on August 12th. By then owner company and contractors’s teams were working round the clock to prepare and erect the temporary arrangements and complete the bucket room structure on time. The team was confronted by two totally opposite situations; (1) They had to be very thorough and cannot afford to miss on the slightest of construction activities. If they missed, there will be no way to go back and redo once the bucket room is lifted to its final position. (2) They had to be very fast so as to complete the structure and lifting arrangements on time. If they don’t, they may loose the window available with lift contractor for their special equipment and end up taking the whole project to an unrecoverable delay. A balancing act between safety, quality and progress was to be maintained. The pressures were extreme and time ticking by quickly. Site conditions, extreme hot weather and month of Ramadan (Muslim month of fasting), all added to the already full platter of Prill Tower team.

As the saying goes ‘when the going gets tough, the tough gets going’ the team found their energies and focus strengthening day by day. As the day of actual lift off approached, support from all interfaces poured in, resulting in a highly organized and motivated construction ‘machinery’ working for one common goal.

HOUSTON, WE HAVE A LIFT OFF!

26 August approx 1000 Hrs, the truss lifted completely off the ground, the team took a sigh of relief and celebrated, for a short duration. The task was not yet over, in fact the critical phase had just started. 450 Tons of load was now hanging in the air, its weight supported on the in house designed arrangements. Will the lift be completed without any unforeseen issues? Will the lifting arrangements bear the dynamic forces and will not deflect beyond the design limits? Will the holes on the truss ends fit properly over the ones in the mating parts at 109 meter elevation? These were some of the many questions which loomed over the heads of Prill Tower execution team for the next few days, eating away their sleep cycles and motivating them to stand at site for 14 ~ 16 hours daily at a stretch in sweltering heat in the month of Ramadan. Goats were sacrificed, Quran recited, onsite meetings and debates held on the smallest of expected deviation from plan and visitors managed by this team with support from everybody from company executives to trainee engineer during the next couple of days.

FEAT ACCOMPLISHED

Phone’s started buzzing and ‘Mubarik ho’ txt messages started making rounds at Ifftar time on August 29. The jacks have been released! The load now rests at its correct elevation! Feat accomplished!

This feat carries a background of smaller feats that if were not accomplished would not have resulted in this achievement.

The hero’s of this accomplishment were;

The designers and fabricators; Who, by their technical expertise and workmanship, ensured all pieces of this complex structure fit together. The surveyors; who’s attention to detail and finesse resulted in perfect bolt to bolt alignment. The Safety Facilitators: Who ensured the safety and well being of their execution teams. The Executors; Who, despite all odds, put in their very best to achieve this milestone. The Leaders / Decision makers: Who kept their teams motivated and provided constant guidance. And in the end; The Core Prill Tower Team; who, dedicated their days and nights to achieve this feat.

The record made by this team may be broken in future but the precedence they have set will live for ever with us.

‘Impossible is Nothing!’