ABSTRACT

The ATAS, Atmospheric Transportation and Access System, is a concept for producing Troll Field gas to landfall Norway utilizing one atmospheric Subsea Chambers to process the produced gas from wet subsea completed wells.

The base case system uses only reservoir energy, however electric powered gas compression can be used to extend field life.

Access to the subsea chambers is via a lift inside a non articulated slender Monopile which also gives a continuous atmospheric connection to the subsea chambers.

The Monopile houses a lift shaft capable of transporting 4 men and 1 ton of equipment from the surface to the sea-floor chamber. The chamber is designed to handle 26.25 MMSCM/D and manifold 18 wells into a 30" export pipeline 65 km to shore.

BACKGROUND

In the search for cost-effective deepwater offshore oil and gas exploitation concepts for the Troll field, subsea completed wells were considered to be one of the building blocks. Restrictions in the performance of well-control, -killing and testing due to the long distances to shore and other offshore platform called for some kind of field support facility. Also pipeline pigging was considered necessary.

At this time Aker-Technomare presented a new loading tower concept, the Monopile, which consists of a slender buoyant steel tower rigidly fixed to the base structure. The Monopile also included a small topside unit with a Helideck.

This non-articulated tower was considered to be a possible solution to the access of subsea chambers and in addition a continuous connection between the subsea chambers and the atmosphere.

Based on this idea a study was performed with the objective to demonstrate the feasibility of a redesigned dry Monopile, with a central lift from topside to the subsea chambers. And to investigate the safety aspects for personnel using such a "liftshaft" for access to subsea chambers.

The conclusions given in this study were positive and Statoil decided to go one step further and defined the scope of work for the ATAS study. For major design criteria see table 1

OBJECTIVES OF THE ATAS STUDY

The objective of the study was to determine the feasibility of a subsea one-atmospheric system for transportation of produced gas from the Troll Field to landfall Norway.

The Concept is based upon the use of dry one-atmospheric chambers located on the seabed, accommodating all the necessary gas production and subsea well control equipment. Access to and from the chambers is via a slender steel buoyant, non-articulated structure fixed to the chamber base and supporting an unmanned topside service facility.

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