UNIQUE PIPELINE REPAIR CONDUCTED IN NORTH SEA

J.L. Daeschler
Mobil North Sea Ltd.
Aberdeen, U.K.

An innovative method for repairing a severely damaged subsea pipeline flanged connection has been completed by Mobil North Sea Ltd., U.K. The repair, at 140 m (460 ft) water depth in the Beryl field, took only 14 hr and required minimum diver intervention and no diver habitat.

CONNECTION FAILURE

An inspection in January of this year revealed that a 6-in., 5,000-psi (API) flange between a steel rigid pipeline and flexible flow line had been leaking extensively enough to wash out both RX46 ring grooves and a section of the ring gasket. A survey of the damage indicated that an emergency replacement of the ring gasket would be ineffective.

This left operator Mobil North Sea with the options of cutting and replacing the flange on the rigid pipeline by hyperbaric welding or using a mechanical flange to replace completely the 450 ft, 6-in. flexible flow line.

Unfortunately, no equipment for either repair was immediately available. Earliest possible delivery was 2-3 months; installation would require 5-8 days.

Mobil conceived a solution that used field-proven technology found in drilling risers and subsea tree connections. It consisted of a male-male stab assembly equipped with Cooper Oil Tool's "S" seal rings.

SEALING RINGS, SUB

Introduced in 1977, these rings are one-piece elastomer seals fitted with molded anti-extrusion steel coil springs. In subsea tree systems, the seal is widely used as either the primary seal or as a back up to the metal seal or stab mandrel on seal subs.

Cooper Oil Tool, Aberdeen, designed and manufactured the stab mandrel and tested the dummy flanges (Fig. 1).

According to Cooper, the sealing sub was designed for diver installation and removal. It was manufactured from materials compatible with the existing flanges with the advantage of a phosphate primer and molybdenum disulfide epoxy coating.

The sub seals inside a 6-in. bore which effectively reduces the flange separating force, when compared with an RX46 gasket. Since gasket preload is also no longer required, it is sufficient to make up the fasteners to a reduced torque of 150 ft-lb to prevent any flange movement. A flange which captivates the sub reduces the flange bending moment and provides a means of installation and removal.

The sealing sub has a minimum bore of 4.865 in. which has a flow capacity of approximately 20,000 b/d and offers no restriction.

UNDERWATER BORING

To facilitate alignment of the stab assembly by the diver, a dedicated diver-operated stab assembly insertion tool (Fig. 2) was also provided and tested ashore in March.

In fact, two complete underwater hydraulic boring tools were manufactured and fully tested by the diving and installation contractor, Stena Offshore, Aberdeen.

To facilitate alignment and rigidity, the tool face was located on the remaining RX46 groove profile and secured in place by bolts at fixed through the flange holes.

The most critical part of the repair was the accuracy of underwater in situ boring of a 4-in. long section of each flange, removing -/8 in. on the internal diameter, in order to obtain both nominal tolerances on finished diameter and surfaces sufficiently smooth for the "S" seal to perform.

Hydratight Ltd., Aberdeen, was called upon to operate the underwater hydraulic equipment, combined with Robert Bros. Engr. Ltd., Dumbarton, for design and fabrication of purpose-made machine tools.

The boring tool was set with such accuracy for radial depth and axial penetration that the dive work was minimal. To verify the performance of both the sealing sub and the boring equipment, a complete dry run test was performed underwater at Stena Offshore's facility in Aberdeen.

Being fully prepared by shore rehearsal testing of both personnel and machines, with a full 100% backup on sub-seals and boring tools, was a prerequisite for being ready for the offshore operation.

Stena Offshore of Aberdeen was the installation and diving contractor, using the d.s.y. Stena Constructor. The detailed program, conducted in only 14 hr on June 9, involved the following: