Post-construction infield lining rehabs subsea pipelines

Aug. 4, 2014
Petronas Carigali (PCSB) owns and operates an extensive network of subsea pipelines off Malaysia in the South China Sea.

Robert Walters
Anticorrosion Protective Systems
Dubai

Petronas Carigali (PCSB) owns and operates an extensive network of subsea pipelines off Malaysia in the South China Sea. These pipelines can be several kilometers long, in varying water depths and from platform-to-platform and platform-to-shore. Internal corrosion, due in large part to sulfate-reducing bacteria (SRB), can shorten pipelines' life cycle. SRB-based internal corrosion has forced PCSB to replace pipelines in as little as 4 years.

In April 2011 PCSB began a joint project with Anticorrosion Protective Systems LLC to develop materials and technologies necessary to implement installation of plastic liners into existing and new subsea carbon steel hydrocarbon pipelines in which SRB is one of the principal sources of corrosion. The infield liner (IFL) protects the internal pipe bore from corrosion and offers secondary containment in the event of rupture or damage to the outer steel pipeline.

Most of PCSB's subsea pipelines are carbon steel and were installed by lay barge, with single or double random joints of steel pipe welded together on the deck of the barge and laid onto the seabed. After welding, crews on the barge could address any flaws in or damage to the external corrosion protection. Crews could not, however, address damage caused to internal coating by the welding process. As a result, the lines were laid without internal coating and instead used additional wall thickness of sacrificial steel, compensating for the calculated rate of corrosion for the pipe's design life.

Corrosion is rarely a linear phenomenon and certain types of corrosion can damage the pipe wall much more quickly than allowed for during design. Pitting, grooving, cracking, or crevicing to the interior pipeline wall can occur rapidly, with instances of pipeline installed with a 20-year or longer design life failing in as few as 4 years, as PCSB had already experienced.

IFL can rehabilitate an existing subsea pipeline when:

• It is desirable to extend its service life beyond the period of operation for which it was originally designed.

• Unforeseen operational parameters, such as carbon monoxide or SRB corrosion, have caused the pipeline to reach the end of its useful life ahead of schedule.

• Routine inspection has shown that greater than anticipated corrosion is taking place.

• Pipelines have been decommissioned or abandoned due to integrity-related issues.

In such circumstances, an IFL liner can provide a less expensive and quicker solution than relaying new pipe, particularly if lay barges are not readily available in the region.

Development, qualification

The project began by testing an existing nominal 8-in. diameter Kevlar reinforced plastic liner, produced by Primusline in Germany for the utility market. Although this material had not previously been used for the purpose of lining subsea pipelines, the general liner matrix had many of the necessary physical attributes:

• High tensile strength.

• Moderate chemical resistance.

• High flexibility.

• The ability to be manufactured and spooled in long lengths.

The qualification of the liner was undertaken in accordance with the API RP 15S "Qualification of Spoolable Reinforced Plastic Line Pipe" (March 2006), with further reference to applicable ASTM test standards, the API 17 series, and NACE standards. Testing and qualification occurred in Germany, Norway, and the UAE.

The final IFL liner consisted of a polyvinylidene fluoride (PVDF) inner liner, developed and produced by Solvay Specialty Polymers, a tightly woven aramid core, using Dupont's Kevlar fabric, and an outer layer of abrasive-resistant thermoplastic polyurethane from BASF. The finished liner tested as suitable for hot, sour hydrocarbon service at temperatures up to 110° C. and with a standalone burst capacity of 120 bar. Other versions of the liner were developed for less aggressive service such as water reinjection and gas transmission.

PVDF had the highest hydrocarbon chemical resistance at elevated temperatures, performing better than either thermoplastic polyurethane (TPU) or high-density polyethylene (Figs. 1-3).

In addition to PVDF's high chemical resistance, the material had extremely low permeability (see table).

The IFL liner matrix's Kevlar core gave it the tensile strength required for installing the liner into existing carbon steel subsea pipelines up to 5 km long (about 3 miles) in a single pull.

Usable IFL lengths depend on pipeline diameter, configuration, and the number of short-radius bends. Trials, however, indicated that the rehabilitation of a typical 6 or 8- in. OD hydrocarbon flow line could be feasible for up to 10 km.

The IFL material, though manufactured in a circular profile, can be temporarily flattened for transportation and reeled onto a transportation drum sized to fit conventional shipping containers. Each drum can hold up to 5 km of IFL liner, depending on liner diameter.

Installation

APS in summer 2013 installed IFL in a 6-in. OD natural gas pipeline and 8-in. OD crude oil pipeline in Petronas's Samarang oil field, offshore Sabah, East Malaysia in the South China Sea, about 72 km northwest of the Labuan gas terminal. The lines were operating at 60 and 7 bar, respectively. Both pipelines, each slightly shorter than 2 km, were nearing the end of their natural service life, after operating for more than 35 years.

Samarang was initially developed by Sabah Shell in 1975, with PCSB becoming operator in 1995 and both reducing production declines on Samarang itself and turning it into a hub for production of adjacent fields (OGJ, May 1, 2000, p. 40).

A thorough inspection of the existing subsea pipeline preceded detailed planning of the Samarang IFL rehabilitation project. This process included collation of all data relative to prevailing operating parameters and conditions. Inspections were carried out by intelligent pigs and other external remote inspection tools.

These data helped engineers assess the general condition and remaining wall thickness of the existing pipeline and verify the IFL liner size requirements in the event that an enhanced tight-fit high pressure liner was required. Decommissioning, cleaning, and gauging preceded offshore deployment of the IFL-installation marine spread, as did shipment of the liner drums to Asian Supply Base Sdn. Bhd.'s Labuan marine supply base, where the liners were further processed into a folded format for shipment to Samarang field.

Workboats installed the IFL at speeds up to 10 m/min. The crew pulled back a feeder cable fired through the pipeline during the final cleaning and gauging procedure using the liner installation winch cable and connected it to a towing head on the leading end of the liner.

Engineering determined the specific winching loads necessary for the liner insertion using predictive IFL software. The winch packs used for installation came equipped with load cells and override devices to interrupt operations in the event of greater than predicted loads during the winching. The operator could also set the devices to cut out automatically at a given load if the engineered safety factor relative to the liner yield strength were approached. In this instance, however, as with most 0.5-5 km liner insertions, forces were no more than one-tenth the liner's tensile yield strength.

Installation of IFL end termination coupling devices at riser flange locations preceded liner installation. After the IFL was drawn through the entire pipeline length, crews used air to re-round it. The liner, manufactured to the same diameter as the host bore, then expanded to form an intimate fit with the inner wall of the pipe.

Installation of the end termination inserts, which ensure reliable compression seals and restraint at the liner ends, followed re-rounding and fitting. Hydrotesting the relined pipe demonstrated it to be ready for recommissioning and return to operation.

Petronas estimates the service life of these lines to have been extended by at least 30 years.

The author

Robert Walters is founder and chair of Anticorrosion Protective Systems, specializing in pipeline corrosion engineering services, particularly utility and hydrocarbon pipeline coatings and rehabilitation systems. He is a corporate member of several regional Societies of Trenchless Technology, a founding board member of the UAE-based Gulf Plastic Pipe Academy, and member of the Malaysian-based Petronas Steering committee for development of subsea pipeline rehabilitation methods.