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Multiwall system used to repair subsea line

Reprinted from the May 4, 1981 edition of Oil & Gas Journal Copyright 1981 by PennWell Publishing Company Multiwall system used to repair subsea line .

Used with permission:
B.E. Fruck, Unisert Multiwall Systems, Inc., Conroe, TX
Durward Archer, Burmah Oil (Bahamas) Ltd., Freeport, Grand Bahamas

A subsea ballast line located at South Riding Point, Grand Bahama Island, was recently rejuvenated for Burmah Oil (Bahamas) Ltd., Bahamas Development Corp., using an innovative multiwall repair system.

The oil storage and transshipment terminal provides the necessary facilities to transfer oil from oceanfaring super tankers to shuttle tankers able to dock stateside ports.

The 5,000-ft line stretches from a tank farm offshore to an island, and is submerged in 110 ft of water at the deepest point. The 36 in. OD steel pipe was installed 7 years ago with a concrete coating on the external surface. Since ballast fluids carry some hydrocarbons with them when ballast waters and oil stored in the same tanks onboard a ship, Burmah had kept a watchful eye on the integrity of the line with respect to the potential harm of corrosion and environmental pollution. The line transports 40,000 bbl/hr, at an approximate fluid velocity of 12fps, and remains full in between loading periods.

The line was also to be used as a standby oil unloading line where a flow rate of 100,000 bbl/hr and an approximate velocity of 28 frps would be experienced for short periods.

After a full investigation at the close of 1979, it nevertheless became apparent that the steel line had been severely pitted internally. Since Land & Marine from England had installed the initial system, Burmah Oil contracted them as agents to coordinate the repair program.

Line repair system.. Plastics offer the most optimal corrosion-protective qualities in many situations. But the problem equally as often becomes that of minimizing the mechanical force parameters so that the plastic materials can be effectively used.

Unisert Systems Inc., headquartered in Houston, was contacted to provide the design basis and their developed expertise in installing an IT3 multiwall system. The method of manufacture for the IT3 system entails sliding a corrosion-resistant plastic pipeline into a metallic pipeline and locking the two in place by pumping cement grout into the annular space.

This procedure can be accomplished in lengths of several thousand feet and can, therefore, be used to repair existing pipelines. The initial capital cost comparison also became very favorable, the rejuvenation cost being 50% that of replacement cost for this type of situation.

The multiwall system differs from a barrier-type coating in that each layer retains a degree of pressure-handling capability and structural strength. The cement and plastic together bridge over the localized areas where the steel pipe suffered from pitting corrosion.

In this case, the plastic pipe was constructed as a fiber-reinforced thermoset, so that it's 0.76-in. wall alone was sufficient to withstand the internal pressure of 100 psi and design pressure of 150 psi.

A second major aspect considered was the external collapse force from the seawater hydrostatic head in the event of a steel-wall failure during installation. The FRP wall was, therefore, designed to resist 50 psi external pressure.

The internal-diameter reduction to 30.0 in. was not predicted to significantly affect flow rates through the line primarily because of the smooth surface of the new plastic pipe. Generally, the cement phase served to:

  • Pre-stress the plastic pipe in compression during installation phase.
  • Transfer pressure tangential stresses from the plastic to the steel wall. In this case, since the condition of the steel wall was to a large degree uncertain, the plastic pipe was provided with the capability of carrying the entire pressure load.
  • Anchor the plastic into the steel pipe longitudinally such that the temperature expansion coefficient of the total system became that of the steel.
  • Increase the negative buoyancy through its weight, in this case by 150 lb. / ft.
  • Serve as a secondary corrosion defense system by passivating the steel through its CA(OH)2

Repair Procedure. The repair procedure followed five basic phases: inspection and adaptation of the existing line, joining of the FRP pipe, insertion of the plastic pipe, cementing of the annular space and reconnection of the total system.

Inspection of the existing line revealed random pitting on the bottom of the pipe. Some of these sites had completely corroded through the steel wall.

A sizing ring was pulled through the pipe to ensure passage of the maximum FRP diameter of 32.66 in. which occurred at the joints. The minimum internal diameter of the steel line was established at 34.5 in.

A 32-in. nominal OD Owens Corning fiberglass reinforced isophthalic polyester pipe was constructed in 60 ft. lengths and was joined with the standard butt and wrap technique.

Three joints produced a stick; various sticks being manufactured on site simultaneously.

The existing steel line was ascertained to have been laid with a minimum bending radius of 2,000 ft. The FRP pipe was, therefore, designed to flex or bend to that limit without microcracking while it was being pushed into the existing steel case.

The plastic pipe was pushed into the existing pipe in sticks of 180 ft. with tugger winches of 40-ton capacity each, but only 60% of that potential force was actually used.

The agenda for the day involved a stick, aligning a stick into the mainstream process, connecting that stick to the rest of the pipeline, and then allowing that buttwrap joint to cure overnight.

The entire procedure was repeated for each day.

The length of the sticks was restricted purely by the physical area of the working site. He underwater portion of the project required 63 joints scheduled for a 26-day completion.

The cementing process differentiates the IT3 installation from what is commonly termed a "slip lining" operation. That design of the installation procedure had to account for the axial strain in the plastic pipe incurred from both the internal and external pressures during the cementing process.

The injected mix consisted of neat cement grout, Portland cement, and water, conforming to a density of 16 lb./ U.S. gal and a water cement ration of 0.43. Various additives were blended in with the cement/water mix to control homogeneity, viscosity, and pumpability.

Laboratory tests predicted a viscosity of 100 cp. The injection pressure for a cement grout was limited by the external collapse pressure of 225 psi in the plastic pipe, derived from both the structural strength in the wall and the internal pressure within the plastic pipe.

Cement entry and exit ports were welded onto the steel line at specific intervals, the length between them so set to avoid excessive pressure drops with cement-grout flow. The injection pressures were thus maintained at a lower level than either the collapse pressure of the plastic pipe or the internal-pressure capacity of the steel pipe.

As shown in Fig. 4, the cementing procedure commenced from the elbow upwards to the onshore position. The annular space at the elbow end was sealed by an inflatable tube seal and a small initial plug of thixotropic cement.

When encountering an inaccessible end point as with the elbow, it becomes imperative to correctly design a sealing mechanism since it separates the annular space from the internal fluids.

The ports were distanced 500 to 1,300 ft. apart, and cementing pressures ranging from 30 to 60 psi were experienced at a sea level at an average flow rate of 150 gpm.

A skid-pumping unit mounted on a barge was positioned over each port in succession and was connected to the pipeline with a flexible hose. Fresh water was used to mix the cement grout.

All precautions were taken by Burmah Oil to contain any minor hydrocarbon residue purged from the annular area during the cementing process with clean-up crews and oil-spill booms.

The riser was likewise fitted with same FRP pipe and grouted in place. Reconnecting the steel-cement-FRP system back to a bare steel pipe consisted of installing an FRP packer at a flange connection to separate the annular space from the internal fluids.

In these larger sizes of FRP pipe, resin and chopped mat are laid from across the outer packer face to the internal surface of the pipe, thereby providing the fluid seal.

The elbow posed a special consideration since it was not practically feasible to remove it and install flanged connections. The line, however, was of a large enough diameter to fit a man inside the elbow and coat the steel elbow directly with fiberglass mat and resin. In the same action, the annular space was further sealed off at the elbow end.

The various phases were roughly completed within the following time table, in order of event: Phase 1 - 3 weeks, Phases 2 and 3 - 4 weeks combined, Phase 4 - 2 weeks and Phase 5 - 6 weeks.

The time to completely dewater the pipeline to coat the elbow extended the project to schedule considerably. In total, the system was decommissioned for 7 months which included the time for another separate phase of the project-that of further anchoring the line externally with rock anchors, grout bags, and shutter boxes.

The system successfully withstood a hydrostatic pressure test of 225 psi and has operated effectively since October 1980.

One advantage of the rejuvenation process is that both strength and corrosion resistance are provided by the plastic pipe and cementatious structure.


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