Statfjord "B" is an integrated drilling, producing and accommodation platform 1n the Norwegian sector of the North Sea with advanced safety and loss prevention design features. Prior to preparing a specification for the wellhead equipment for this facility a review of advances in wellhead technology was undertaken with particular reference to fire resistance and its application on Statfjord "B". It was subsequently decided to take a fire resistant approach in preparing the' specification.
While wellhead fire resistance has been of interest to the industry for many years the application of the technology has been limited and the basic concepts not widely known. This paper reviews existing knowledge on the subject of wellhead fire resistance and the rationalization for taking the fire resistant approach for Statfjord "B". Particular reference is made to the design features of the Statfjord "B" wellhead and Xmas tree assembly, which we believe to be the "state of the art" in wellhead fire resistance.
Fire resistant wellheads of various degrees of sophistication are in service throughout the world. However, the Statfjord "B" design will be the first installation which retains fire resistance when there is pressure on the tubing/production casing annulus. This is possible as the design incorporates a tubing head spool with integral fire resistant valves and fire resistant seals for the tubing and production casing annuli.
The main application of this technology is for offshore production platforms, where wellhead/Xmas tree assemblies are in close proximity and could be subject to direct flame impingement in the event of a burning, uncontrolled well. We consider that the Statfjord "B" fire resistant wellhead equipment will represent the basis for future standards on offshore insta11ations.
The Statfjord Field is located on the Nsrwegian6 United Kingdom boundary between latitudes 61 and 61 3D' north and was discovered in 1974. It is the largest oilfield in the North Sea extending over 15 miles from north-east to southwest and averaging 2 1/2 miles in width. The field will be developed from three fixed platforms of the "Condeep" design.
Statfjord "B" is the second platform in the development of the Statfjord field. This platform combines in a single structure the production process, utility and auxiliary functions, along with the drilling facilities and the domestic accommodations.
This type of combined activity, production, drilling and quarters platform is known as a PDQ platform. The normal production rate will be 150,000 STB/day of crude oil with individual well rates as high as 30,000 STB/day. The platform is equipped with a single drilling rig operating between two well shafts each containing twenty-one slots. 'Wells will be completed with 7" production tubing and 6 3/8" nominal bore Xmas trees.
During the design phase, concern was expressed with regard to the overall risk level of the platform as it was then envisaged. The principal point of concern was that the facility would combine drilling facilities and complex process plant, handling large volumes of hydrocarbons, on one installation with the living quarters.