Breaking the conventions of offshore oil and gas production, in 2009, Zakum Development Company (ZADCO) decided to construct a collection of artificial islands in the Arabian Sea, with the aim of increasing production at its Upper Zackum field offshore Abu Dhabi.
ZADCO, working on behalf of Abu Dhabi National Oil Company and in conjunction with ExxonMobil, abandoned its original plan for 25 wellhead platform towers at the field, together with hundreds of kilometres of new flowlines, and opted for the artificial islands strategy instead.
The company wants to increase field production from 550,000 to 750,000bpd by 2015. It hopes to sustain this for 25 years.
Though not an entirely new concept to the industry – they have been used off the coast of California, and in Alaska utilising ice – artificial islands constructed for drilling purposes remain extremely rare and have never before been executed on such as scale as that projected here.
Switching strategies is a bold move, but what benefits can ZADCO hope to reap from using man-made island structures instead of conventional steel rigs?
Constructing artificial islands depends on a number of factors, but, most notably, location. The Arabian Sea is already famed for a number of artificial islands, such as Dubai’s famous Palm Islands and The World, proving the sea has good geology for building these structures.
To form islands, sand must be pumped from the bottom of the seabed, a process known as dredging. It is then used in constructing the bases of the islands.
For Upper Zakum, the seabed off Abu Dhabi provides a "unique" situation, whereby building islands is cost-effective, says Frank G Adams, president of Interface Consulting, an offshore engineering consultancy firm. "The richness of the sea bottoms for establishing islands is particularly unusual, and it just so happens the technology to develop islands in that area is fairly robust," he says. "It’s a unique concept for Abu Dhabi."
With localised materials and expertise – the dredging contract for Upper Zakum went to Abu Dhabi’s National Marine Dredging Company – islands are a most cost-effective alternative to steel rigs that have to be imported and constructed with outside expertise.
Islands may also make operational costs cheaper, suggests Jean-Paul Boyer from Pentair. Commenting on an online discussion, he says: "Running an installation on ‘firm land’ is less costly than running an offshore array of several platforms."
Sigbjørn Sangesland, a professor at the Department of Petroleum Engineering and Applied Geophysics at the Norwegian University of Science and Technology, cautions, however, that islands may only be beneficial for large reservoirs. "I would assume, if you have a small reservoir, it may be more economic to put another platform on the location," he says.
Water depth is another key issue when considering artificial islands as opposed to steel rigs. The Upper Zakum field area is in shallow water with a depth ranging from 6-13m, ideal for ease and reasonable cost of construction.
"To build an artificial island in deep water will require a lot of material," says Sangesland. "I would assume it would be very expensive and impractical."
This is a feeling echoed by Adams, who says: "You would have to have too much soil. In deep water, the use of different types of steel platforms and concrete platforms is more appropriate."
Maximising the reservoir
A key reason ZADCO has opted for artificial islands is to "improve long-term reservoir recovery", according the company, and to extend its drilling reach from 10,000-35,000ft using Exxon Mobil’s extended-reach drilling technology, as well as maximum reservoir contact technology. Drilling from artificial islands offers many advantages for this, as well as some technical challenges.
Adams says that, with a cluster of steel structures, you are limited to the amount of wells you can have, whereas with a land base you are able to create a lot more wells – "as many as 200" – which would never be possible with steel rig structures.
ZADCO says: "Many of the wells on the islands will be complex and expensive, but their contribution will be much higher than historic Upper Zakum wells. This will improve production performance, optimise the well count and minimise capital investment."
Technically, Sangesland agrees, he says drilling from an island looks like "a good alternative".
"If you have a reservoir to be produced, it will be beneficial to produce the reservoir at a high angle or horizontally, because then you will increase the drainage," he explains. "That is an advantage."
He likens what ZADCO is doing to what is taking place at Wytch Farm in Devon, UK, where extended-reach drilling is being used to drill offshore from the land. It is home to one of the longest-producing wells, which has a horizontal displacement distance of 10,728m and a total length of 1,278m.
Drilling from land suits this type of drilling strategy in particular, but Sangesland warns that, whether from a rig or an island, it is "time-consuming and there are always problems".
Adams adds: "I understand they have some problems in the drilling process, managing the loads on the drill stems, because the islands are a fairly good distance from where the reservoirs are, and as they are going deep in the reservoir in a multiple manner, there are some concerns about mechanical loads on the drill stem."
But he adds: "There are so many positives that, whatever drawbacks there are to the technical sides, those can be easily dealt with."
Environment and safety advantages
"From an environmental standpoint, the impact is less," says Adams, categorically.
This is because building artificial islands involves using natural elements like sand, and not forcing huge steel structures into the seabed, which have to bear the weight of drilling equipment and other facilities, he explains.
Consultancy engineer Ramboll, which carried out an assessment of the environmental impact of the artificial islands, concluded that construction of the islands would not significantly affect the environment and the existing structures.
One hazard the company did identify was that some pipelines in the vicinity of the islands were found to require additional stabilisation in terms of concrete mattresses.
However, another environmental and cost bonus of artificial islands is that they do not need to be deconstructed; they can be left to erode naturally or turned into something that supports local wildlife. But the key positive of producing from an artificial island is space. Platforms can only be so big, whereas, on an island, facilities can be spread further out, reducing the fire risk.
"From a safety standpoint, it’s far superior to steel structures," says Adams. "If something happens at a facility, people have to get off the steel structures, but on an island, there are areas they can go to without having to risk life and limb."
In the event of a fire, it is also much easier to use boats to evacuate a land-based structure rather than a steel rig, where workers tend to rely on helicopters, which are more limited in space.
An interesting, but limited, future
Artificial islands, which have been constructed in abundance around the Persian Gulf, make an attractive option for a strategy for offshore drilling for a number of reasons, including space, environmental impact, safety, cost and number of wellheads.
However, these advantages are limited by a number of factors, such as water depth, sand quality and reservoir size, which have limited their use and will no doubt continue to do so.
Adams believes it is unlikely that this strategy will take off in other parts of the world on the scale seen at Upper Zakum.
"There are certain areas in the world where there is shallow water production but you may not be able to dredge the soils to create islands," he says.
Though more positive, Sangesland agrees the use of artificial islands is dependent on many factors. Therefore it seems, for now, the countries of the Arabian Sea will continue to enjoy an offshore drilling situation that is, as yet, unique.
