Site visit: District Cooling Plant 3, Qatar
The skyscraper business district of Doha’s West Bay is home to Qatar Cool’s newest addition to its growing portfolio of district cooling plants
An EPC contract, District Cooling Plant 3 is an industrial project, and Al Jaber Engineering has been tasked with the engineering/design, procurement and construction side of the contract.
A turnkey job, Mohamed Saleem, project manager, Aljaber Engineering explains: “We are building the district cooling plant including all the set-out work and process work for Qatar Cool, which is a specialist in chilled water.”
Construction started in May 2014 and the project is due for completion in May 2016. Offering 35,000 refrigeration (RT) tonnes of mechanical cooling, the plant will also feature a thermal energy storage (TES) tank, offering an additional 5,000 RT.
“Qatar Cool has three existing plants, numbers 1 and 2, ours is number three,” he notes and adds: “The company also has an integrated district cooling plant (IDCP) at the Pearl, serving the entire area. This is considered one of the biggest DC plants in the world.
“Before bidding on the District Cooling Plant 3, the consultant was Hyder Consulting, who undertook the concept design and all the bidding documentation, including contractual and technical specs. The sub-consultant we appointed afterwards was Sheker, to develop the detail design and the client appointed Dar Al Handassa to act as a supervising consultant,” Saleem explains.
The very core of what makes this project unusual is its specific post tensioning (PT) slab work, which was awarded to CCL, which provides specialised engineering services for concrete structures; buildings, bridges, tanks etc. using post-tensioning and pre-tensioning techniques. It also provides concrete strengthening solutions – the CCL repair and strengthening unit. As well as structural strengthening CCL’s repair services include concrete repair, specialist grouting, corrosion prevention and bearing inspection and maintenance.
Saleem outlines why PT was chosen for the project: “There are several advantages with PT: First, as an industrial job, with loads anticipated to be huge, and slabs having to support heavy equipment.
“Second: The levels for the slabs are relatively high, reaching 10 metres between each level which, for traditional concrete, would require back-propping. We decided to go for PT to eliminate the need for back-propping. This is a tremendous advantage for us on this job in particular, because we cannot reach 10 metres in height to make back-propping for the slabs.”
John Habib CCL, general manager for Qatar and Kuwait details the PT technique: “For the slabs you have a different design technique. You can design the slabs for normal reinforced concrete slabs (RC) or for PT, or for a steel structure and more, but let’s focus on three types: RC, PT and steel.
“No one system is better than the other, each has its merits and applications,” he stresses. Each system is developed for a need, in terms of open space and spans between the vertical, supporting elements.
He continues: “Originally structural engineers designed ‘traditionally’, using an RC slab. Now, with architects having different requirements, like more open space with fewer vertical elements, the space between the verticals is between eight and 15 metres.
“Formerly, the vertical supporting elements were placed at four to 7m centres; in such cases RC slabs are the best fit. To offer more flexibility for the end-user and the client, the space was increased to between eight and 15m; in such cases PT slabs are the best fit as the normal RC will be thicker, heavier and will require drop beams,” Habib explains.
With spans beyond the traditional span spacing of 8m, using PT helps reduce the thickness of the slab and the amount of reinforcement required, replacing some of this with high tensile steel.
“For this particular building, the main advantage of PT is the speed of construction. By using PT, the technique will allow the formwork to be removed within five to six days as opposed to 10 to 14 days for RC.”
With a double height between the slabs a special type of formwork is required.
CCL designed the PT slabs to carry the construction load of the upper slab during casting which allows the contractor to use only one set of formwork, doing away with any back propping. Traditionally with RC, the formwork will be spread over multiple levels, with all the traditional back-propping in place. “So, for double formwork, there is double time and double problems,” Habib observes.
“The important thing on this job was that there was only one set of formwork during casting.
“One level below is supporting the casting load. What we did for this job is, we designed the slab to carry the casting load, because one level cannot support the other level by itself. We designed the slab with additional PT to allow the removal of the formwork in five days and to allow it to carry the additional load of the casting slab above.”
According to Saleem, the original design did not consider PT. He elaborates on how the decision was reached to use this technique: “A contractor had done a feasibility study to utilise PT in lieu of normal/traditional reinforced concrete, for a number of reasons, including ease of construction and detailing, cost saving and fast redeployment of shutters.
“Because of the 10-metre height, a special shutter has to be used. The PT technique reduced the need for additional sets which, overall, will lead to saving in cost and time. We decided to go for PT after the feasibility study was concluded.”
Habib adds: “The PM on this job is familiar with CCL Systems as he had used them on other jobs. While he tried to introduce this process early in the job, the approval process takes time; from the contracting firm’s management, the tendering process, technical enquiries, review from the procurement department and the company’s head office etc.
“Thereafter, the consultant required proof that the PT process would in no way jeopardise the quality or efficiency of the slab compared to what was originally provided.
“We had to make sure that whatever was originally provided to the client was the same, plus. They would not be allowed to provide less.
“While this process takes place, the contractor cannot wait, he has labour on site and he is paying salaries etc. We had to act fast, while considering and securing all necessary approvals and prove to the third party consultant and to the project main consultant that you are giving additional value,” he explains.
Another advantage of using PT is savings in material, the quantity of concrete and passive reinforcement.
Habib expounds: “With a longer span, PT will also reduce concrete and steel usage. If you compare original cost of the traditional RC with the new cost of the PT, you have value engineering in terms of speed.
“Having less reinforcement to install and less material/concrete, as well as faster installation methods, savings on manpower with faster construction, ensures deadlines are met timeously. You also have less congestion of reinforcement on-site, fewer difficulties, and fewer drop-beams with a flatter element,” he adds.
“As an EPC we obviously wanted to save money as contractors, and this is a value engineered solution, but the main objective was to eliminate the need for back-propping,” Saleem emphasises.
“We did a comparison between designing with PT or RC, and we revisited the figures with a comparison between estimated and actuals – and the difference was small. However, the difference between assumed and actual figures now, since the slabs are completed, shows that the saving is relatively high,” he contests.
The biggest challenge to the project was the congested building site; in the heart of West Bay. “Also, because it requires many utility connections, approvals are needed from a number of authorities and the coordination of logistics between them all.
“For example, we need a water connection as well as treated sewage effluent (TSE) connections as well as power, with a dedicated substation to make it a stand-alone entity. While the substation is under Kahramaa jurisdiction, we are responsible for the construction of this sub-station; the plant has to be connected to the existing network of chilled water pipes. It therefore has to be fitted with an electricity power supply and potable water and drainage for the reject water,” Saleem details.
The project will be the first plant in Qatar to use TSE for make-up water, reducing reliance on potable water which satisfies the environmental requirements of the Qatari water and electricity authority Kahramaa.
It is also the first district cooling plant in Qatar to achieve Leadership in Energy and Environmental Design (LEED ) certification, having been designed to LEED Silver standard.
Habib adds: “It is worth noting that PT can contribute to at least four points toward the LEED certification by using innovation in construction and reduction of passive reinforcement and concrete, further reason to employ this value-add technique,” he concludes.