Post Operational Investigation of the Recovered North East Frigg Subsea Production Equipment After 10 Year's Service
Authors: Worley L.J. , Fjaertoft L. 
Published: October 2008

Elf Petroleum Norge had for 10 years, been operating the North East Frigg field. This gas field was the first subsea field on the Norwegian Continental shelf, It was shut down on the 8th May 1993.

Elf Petroleum Norge used the shut down as an ideal opportunity to review the performance of the subsea equipment, An investigation was initiated, its purpose, to gather information regarding the history, wear, effect f cathodic protection, corrosion etc from the X-mas tree components.

SCOPE OF WORKExternal inspectionPressure testingInspection of internal partsanalysis of components and fluidsReport findings

This paper is in two parts, The first part presents the working condition of the wells, environment, fluid, production etc. The second part presents details of the in-depth investigations of the production equipment.

INTRODUCTION

The North East Frigg (NEF) gas field was discovered in the Norwegian North Sea on block 25/1 and 30/1 O in 1974, NEF is located some 250 km north west of Stavanger and 19 km north east of the Frigg Field central complex, see figure 1. Authorization to develop the NEF field was granted in March 1980. lf, the operator, had an 11.096% share in the field, whilst their partners share was as follows, Statoil 8.4%, Norsk Hydro 13.806%, Total 8.698% and Esso 58%.

SUBSEA EQUIPMENT

Six identical subsea well-heads and x-mas trees were clustered into a landing and protective template with eight slots, Ievelled and pile anchored to the bottom. Two groups of three wells each were drilled in line on each side of the template. Each wellhead received the x-mas tree, topped by a protecting cap.

Each x-mas tree produced gas through a production spool (facing towards centre line of the template) connected by a spool-piece to the flowline. Both the manifold and the flowlines were integrated into the template. Diver assistance was required to measure and install the six flowline spool-pieces, which ties the xmas trees to the manifold/header and the 16" pipeline. The flow rate was adjusted only from the Frigg Platform, No chokes were required on the subsea trees.

At the level of the wellhead connector, the x-mas tree was connected to its individual umbilical providing means for the remote control functions, the injection of methanol and for killing or bleeding off the well, All completion or workover operations were to be performed with a special workover blow-out preventer (BOP) stack, equipped with an orienting spool. Most of the subsea installation problems were found in the mis-orientiion of the different components installed on wellhead (guide base, BOP stack, tubing hanger,(x-mas tree). The maintenance of the wells, like wireline operations, were performed through a dual completion riser carrying a lower riser package landed on top of the x-mas tree in place of the x-mas tree cap. A lower safety block integrated into the lower riser packageallowed wireline to be sheared and the pressure to be contained in the unlikely event that the disconnection of the riser was required above the safety block.