Pulling power

This is a city abuzz with activity and there are trenches and diggers at every turn. But one project by its sheer magnitude stands out from the rest: the 400 kv power link being installed by Nexans in consortium with Prysmian.

“This 400 kv project is a 16 km turnkey project for Kahramaa [Qatar General Electricity and Water Corp.] and it is the first time a 400 kv cable is being installed in Qatar,” explains project manager Laure Tavernier.

The installation forms part of phase 7 of Kahramaa’s power transmission system expansion programme. The transmission line will link super substations at Dukhan Road and Al Wajbah and will replace pylons that already line much of the route.

Three cable circuits each comprising three phases are being installed along the 16 km route between the two substations. Nexans is responsible for laying two of the three circuits and is leading the project. The total contract value is Qatar riyal 641 million (US $176  million).

Planning & preparation

In November 2006, the letter of award was signed by all parties involved and the project kicked off, with Nexans and Prysmian agreeing to complete the installation with 18 months.

“Immediately after a contract is signed we mobilise our team to start the engineering and design. We have two main tasks: the design of the cables and accessories and the planning of the route,” says Nexans’ Hubert Lerebours. The engineering and design phase can take up to six months, but it is this attention to detail that Nexans says lies behind the success of a project.

“We place a lot of emphasis on engineering phase so there will be no surprises or hold-ups later on, such as a cable being too short or too long,” he continues.

“In the high-voltage business you manufacture the cables according to the client’s specification for each section of project. If we have 100 m more than what we need on the cable that is pure loss for us and we cannot do afford that with the price of these cables. The specification has to be determined but also the requirements in terms of the length.

When we give the factory the order to start manufacturing the engineering onsite has already been done and we know the route, where the cable will be placed, where the joint bay will be, and the length of each section. Only once this is done can we order the parts from the factory. Each client has his own specifications in terms of design and testing and all this has to be customised for each project and client.”

The tender documents always outline a theoretical route, but it is down to the company installing the cables to confirm its feasibility and to obtain all necessary permits.

“All countries have planning authorities to define planning the route and the different corridors for the services that have to be installed. So the first step is to receive from the client the tentative cable route, to look at the routing they propose and to submit this routing to the different authorities and this takes around 6 months,” says Lerebours.

“At the same time that we apply for authorisation, they tell us where other services are so we can avoid them, for example all the services running parallel or crossing our cable route.”

For this project permits had to be obtained from the water, telecoms, roads authorities, the urban planning department and from Qatar petroleum.

Pit stop

Once all the authorisation and documentation has been received the project team draws up a preliminary route, which is then verified using trial pits.

“We perform trial pits on each location where we cross existing services and also at the theoretical joint bay positions to cross check that it is really free of services,” says Lerebours. A trial pit typically measures 3 m by 50 cm.

The number of trial excavations depends on where the cable is being routed: in a congested area one might be dug every, whereas in the open desert one pit every 100-150 m might suffice.

“We use the trial pits to confirm the services that are supposed to be there are there and in the right position, as per the drawing and also that where is supposed to be nothing there really is nothing and we can put our cable in.

If there is any mismatch, say if a service that is supposed to be in the position of our trial pit is not there, then we extend it further to find where it really has been laid,” he continues.
 

Only after these investigations have been completed can the route be fixed and marked out both on paper and on the ground and the heavy machinery can be brought in.

Digging deep

For this 400 kv project, the cable is being installed outside Doha in an area with few existing services. The first two circuits mostly run parallel to a road and are 3.8 m apart from centre to centre in a 5 m corridor granted by the authorities. The third circuit is 100 m away on the other side of the road.

Nexans subcontracted its civil works for this project to Heliopolis and ETA. The three firms have a long history of working together, including the installation of a 400 kv cable in Abu Dhabi for ADWEA. That project was also run in consortium with Prysmian.

Here in Qatar, Heliopolis is responsible for digging out the trenches and joint bays. “They use a trenching machine, which goes fast and straight. Qatar is very rocky so it was good to have this machine instead of rock breakers and so on. Around 100-200 m of trench is dug each day,” comments Tavernier.

The trenches are 1.3m deep and 1.2m wide. Where the route crosses a road or other obstacle horizontal drilling using plant machinery is carried out to tunnel under it. First a small pilot bore is made then a reamer is pulled through which gradually enlarges the bore wall to accommodate the welded pipes that will house the cable. Later when the cable is pulled through it is wrapped in gauze for protection.

“We are also doing horizontal drilling beneath Qatar Petroleum pipes,” she reveals. “It took a lot of time to get authorisation of course and now it is taking a lot of time to do the drilling because we are going 7 m depth in order to be 5 m away from their pipes.”

The 16 km route is divided into 33 sections of around 480 m in length so there are 32 joint bays per circuit to be dug out. A typical joint bay is 12 m in length.

ETA is responsible for the other civil works on this project. Before the cable laying begins the newly dug trenches have to be prepared as Tavernier explains: “First we do soil investigation to know the thermal resistivity of the soil, and then we make layers to channel the heat away. We add a layer of cement which is very fluidised, this is called fluidised thermal backfilling (FTB).” The FTB assists the dissipation of the heat generated as the electricity flows through the power cables.

Made in Japan

While the ground works are carried out in Qatar, elsewhere the cable sections are being manufactured.

“It will take up to three months before completion to make all the sections, so we make a batch of production at a time according to site requirements. A batch of about 18 drums will take about three months to get ready and tested by client, then about one and a half months to deliver.

The shipping might take only three weeks but the mobilisation from factory to ports and from the Port of Doha to clear customs and get to it to site for laying and inspection by client means it takes about 1.5 months,” says Lerebours.

“The idea is to get a batch coming when we need it, so it is not waiting to be installed and no trench is open without cable being ready. Immediately after the trench is ready cable should be laid and the joints should also have been manufactured and transported to the site. On top of that, the teams and manpower have to be in place and with the required materials,” he continues.

For this project, the power link was made at Nexans’ joint venture factory in Tokyo, Japan, the Nippon High Voltage Cable Corp. (NVC). It was then shipped to Qatar.

The transportation of these drums is very costly, for one drum which is 4.5 m high it costs Nexans NVC to Doha around 20,000. Special ships and shipments are very difficult to find as the drums are above 4 m and take up a lot of space, we cannot put them inside a container,” notes Tavernier, adding: “Each drum weighs 5 tonnes, plus the 25-30 tonnes of the cable.”

The transmission line being installed by Nexans is a dry cross-linked polyethylene design cable, with a 2500 m2 section copper conductor. The weight of the cable is 54 kg per metre with the copper accounting for 25 kg and the lead for 14 kg per metre.

Drum roll

Nexans prides itself on having a fast and efficient team on the ground and Tavernier says one phase of a cable circuit can be fitted in about one hour and the three phases that form one section finished in a day.

Cable laying rollers are laid out along the course to enable the transmission line to move smoothly. A trailer then raises the drum to enable it to turn and the hook on the end of the cable is clipped onto a winch which then pulls the cable through the trench into position.

At each side of the joint bays longer lengths of cable are installed, using a technique known as snaking, as Tavernier explains: “We have to snake a cable in order to have some spare metres of cable in case there is a problem. Kahramaa asks for 6 m per section spare.

If we have a problem inside a joint bay in 10 years time we would open the joint bay, see what the problem is and then to avoid replacing the cable we pull in extra cable from each side of the joint bay. Snaking is manual work, but it is technically very precise with everything marked out in advance. You have to know exactly how to bend the cable in order to get the three metres extra.”

The snaking is carried out in a 5 m-wide dug out. Excavators are used for earthworks here, rather than a trencher. Up to 50 ETA workers are involved in each snaking session due to the immense weight of the cable.

Despite the huge effort on their part, Tavernier is secretly confident the spare lengths will never be needed: “We have been installing cables for Kahramaa since 1976 and we have done this snaking since then but we have never had to use it.”

Jointing sections

With the snaking complete, jointing work can commence. Nexans has designed a special metal hut that fit over the joint bays and provides a clean, dust-free and air-conditioned space for jointers to work in. Here the cutting, warming and welding of the cables can take place in a controlled environment.
 

“Jointing is really the heart of high-voltage installations and that is Nexans’ expertise,” comments Tavernier, outlining the procedure: “We clean and install the cable on the strong support, after that we cut the cable and we heat the cable to straighten it.

Then we remove the outer sheath, open the insulation and clean the conductor. We remove the waterproof paper and then we prepare everything in order to do the jointing; we have to polish the cable.

“The sheath is removed millimeter by millimeter and the cable is coned to fit the silicon pre-mould. The main part of the job is the welding of the conductor. They crimp the conductor with 200 tonne pressure. The cable needs to be kept in a straight line in order to maintain the conductivity. They then fit the casing, which they fill with silicon gel.”

She adds: “Sometimes we install straight joints, but on this project there are only cross bonding joints.” Jointing a three phase cable generally takes 12-14 days to complete, with four days needed to prepare the cable and one day to do the jointing. It is a meticulous process with detailed records and measurements kept throughout, providing the client with complete traceability. Thereafter the corresponding connections need to be made, including the earthing.

The joints are connected to two access vaults fitted near the join bays; one for earthing and one for partial discharge testing. “Some times every 4 km we are putting an additional one for fibre optic splicing, because in addition to these three cables there will be a fibre optic line installed, which we are splicing every 2.5-3 km,” Tavernier comments. These vaults are protected with steel covers.

After the power link and fibre optic cable have been fitted and jointed, more FTB is laid then warning cements blocks are placed on top, clearly labeled Kahramaa project 400 kv. The trench is then backfilled with sand. A compacting machine is brought in to smooth the surface and plot markers are put in place to prevent third party damage.

Project progess

Installation of the first two circuits is expected to be completed by the end of September with the third being finished in February.

“The contractual date was the 30 June but we are waiting for the availability of the substations,” says Tavernier, referring to the first two circuits. The completion date for the third is later, as it was a variation order to the original tender.

As project manager Tavernier meets with ABB, who is building the substations, on a monthly basis to synchronise the works. “We have to carry out terminating activity to connect our cable to their transformers,” she says. “We use a sealing end filled with oil, which takes 10-12 days to install onsite so I need to coordinate it with my colleagues from ABB to be sure my equipment is compatible with their equipment and to be sure that the bushing is compatible with mine.”

The cable will then be commissioned before being energised. Throughout the project the cable sections undergo rigorous testing by both the manufacturer and the client or a representative of the client. On top of that each stage of the installation is inspected and signed before work can move onto the next stage. Everything is checked including the thickness of the cement layers, the cable pulling, snaking, jointing and so on.

The testing begins when the cable is still at the production site. “Once a length has been manufactured it is tested internally, then we test with the client or with a third party to witness the test, this is called a factory acceptance test (FAT).

Once the FAT test is signed by either the client or the third party, we can send the length to the country after which testing is then done on site,” says Tavernier. “In parallel we perform two type tests which the client demands. Then each time we lay a section of cable we are testing the outer sheath – we put 10 kv on for one minute just to check that the sheath is not damaged.”

Commissioning phase

Nexans has its own commissioning equipment, which it transports around the world and rents out to other cable installing companies as well. “We are commissioning this at 260kv for one hour each phase – this is huge because we are not connected to the substation yet so we have to create our own power to test it, says Tavernier.

We are measuring the partial discharge in every joint pit and the cable has not to break down for one hour at this voltage.” The energisation dates have not been finalised due to the delays with the substations. But the project is expected to be wrapped up by April 2009.

Yet Nexans’ work in Qatar will be far from over. The French firm has so far been involved in all bar one of the phases of Kahramaa’s transmission system expansion project. And Nexans and Prysmian have already begun working on phase 8, which involves the installation of 220 kv and 132 kv power links.

Tavernier is not the project manager for this new project, instead she will be overseeing the installation of Qatar’s second 400 kv transmission line in Ras Laffan.