The world's biggest Christmas tree – under water
The first of a total of 14 Christmas trees for the Ormen Lange project was positioned last week at a depth of 850 meters in the Norwegian Sea. Hydro's Thomas Bernt, who is responsible for subsea operations, says that it is the world's biggest Christmas tree, or wellhead valve and piping assembly, ever used in an offshore project.
An oil industry "Christmas tree" consists of valves, safety systems, fittings and requisite measurement instruments connected to the templates.
Considerable dimensions are required on Ormen Lange, and the first Christmas tree weighs as much as 65 metric tons and is measures 4.0m x 4.5m x 3.9m.
It was installed on Template A and connected to the manifold. The tree was pressure tested against the manifold and everything was found to be in order.
The next step is to finalize drilling of the well and prepare it for gas production (completion). Remaining drilling will take place by passing the drill string through the Christmas tree and down into the reservoir.
"All in all some 14 Christmas trees have been ordered from the supplier, FMC Kongsberg. There is a total of almost 500 Christmas trees on the Norwegian continental shelf, though most of them are a lot smaller than those on Ormen Lange," states Bernt.
Postponed due to weather
"The Christmas tree was loaded on board the drill ship West Navigator in Kristiansund, and installation was completed without any great difficulties," says drilling manager Robin Hartmann of Shell, who reveals that the greatest concern had been the prevailing weather conditions out on the field.
The weather in north-western Norway in November and December is not to be taken lightly, and Hartmann reckons that it will only be from February, March and onwards that the main bulk of Christmas trees will be installed. With waves greater than four to five meters high the operation cannot be carried out in the field.
"Nevertheless, the positioning of this first tree in good time before Christmas, and the tests that show the systems working as planned, represent quite a milestone, " adds Hartmann.
The Christmas trees have undergone an extensive test programme at Grenland Offshore in Tønsberg (in 2005 and 2006) and at Vestbase in Kristiansund (2006).