PRESSURE-TESTING METHOD PERMITS LINE-SEGMENT ISOLATION

J. M. Lowes
Hydra-Lok Ltd.
Barrow-in-Furness, U.K.

Pressure or leak testing a segment of a pipeline is now possible by use of a pressure-testing tool that isolates a locally disturbed area of the line.

The system avoids the potentially expensive alternative of decommissioning an entire length of line for pressure testing.

High-pressure pipes and pipelines are essential for hydrocarbon production, processing, and transportation. For safety reasons, procedures need to be established and implemented during construction and maintenance which ensure the continued operation of a plant or pipeline.

These procedures often call for high-pressure hydrostatic or leak testing.

Although only small elements of a pressure system may be disturbed, it has been necessary on occasion to carry out extensive pressure testing for an entire system.

This can, for example, involve the testing of many miles of pipe in a pipeline system or the total flooding of pipework which has previously been maintained dry.

The present alternative to a total-system test has been developed from the Hydra-Lok pile-connection system. The new system can be used to test only a locally disturbed area and provides an efficient and cost-effective means of either hydrostatic or leak testing welds, spurs, flanges, and valves.

The pressure-test media can include any of the following: water, glycol, nitrogen, or nitrogen/helium mix.

THE SYSTEM

The elements of the system consist of a steel mandrel with two rubber seals disposed along its axis (Fig. 1). This tool is inserted into the area of the line to be tested and pressure is applied to the seals and the annular space created between the seals, body, and pipe.

Pressure is supplied by a purpose-built power pack. Tools can be supplied which allow pressures up to 11,000 psi to be applied at pipe diameters from 4 to 48 in.

Operation of the tool depends upon provision of an accurate pressure differential between the seals and the annulus; this differential is maintained regardless of pressure. The facility providing the differential is built into the power pack.

The seals can be inflated simultaneously or operated independently, a facility which can simplify filling the annulus with fluid during vertical operations.

The key to the entire system is the Hydra-Lok seals. This patented system has been developed over several years and used extensively for the Hydra-Lok pile/structure connection system.

The seals are manufactured from various grades of rubber and, so that the seals can withstand the high extrusion force applied to them, are backed with steel segments.

This particular design of seal allows very large clearances between the plug and the pipe bore, enabling the tool to be inserted into tubes with a variety of wall thicknesses and around moderate-up to five pipe diameters (5D)-bends.

When the tool is operated, the seals and steel segments expand to contact the pipe wall. Typical diametrical clearances on a 16-in. tube, for example, would be 1-15 in.

OPERATION

The system has application, to a large number of situations for which access is available, and it is possible with plugs as described to isolate small sections of the pipe. Figs. 2-5 indicate typical examples.

Access through pig traps is an obvious way of installing the tool, although major pipework from riser to production equipment has been reconstructed with an "add-on" technique. The final closure must be tested by other means,

The most common application is at welds or flange connections. The system, however, has found use in the testing of spurs without recourse to pressurizing the major pipe (Fig. 4) and testing reinstated valve bodies (Fig. 5).

Leak tests can be undertaken using liquid with fluorescent indicators or gas/helium mixtures using mass spectrographic techniques as a means of detection.

Conventional hydrostatic pressure tests can be undertaken and provide an extremely sensitive means of monitoring any leakage due to the very small volumes of liquid involved.

EXPERIENCE

Considerable experience in the application of the system has been gained over the last few years.

Applications have been as diverse as leak testing subsea-valve connections to full pressure testing of a small 2-in. spur on a 30-in. pipeline. Tests have been undertaken for a number of clients covering the range of pipes from 6 to 34 in. diameter and at pressure from 100 to 250 bar.

The following examples indicate the sort of testing that can be undertaken.

VALVE PRESSURE TESTING

The operation was carried out on a dry-gas line where it became necessary to change a 26-in. valve at the head of a riser. The valve in question was one of several in-line adjacent to a pig trap, all of which needed to be refurbished.

The alternative to using the new system would have been to hydrotest the entire line and then start a totally new drying process before putting it back into service with gas.

The operation was used first to seal off the 24-in. line while the valve was removed for repair (a plugging operation).

When the valve was replaced, the same tool was then to be used to pressure test the new valve and its mating flanges.

Because the line in question was a dried gas line, the pressurizing medium for the seals was glycol, and nitrogen for the annulus.

The arrangement is shown in Fig. 6.

The operation was performed as follows:

Ball valve to be changed was closed and adjacent valve removed to allow the plugging tool access to spacer section.
Tool was pushed into spacer section; wheels were attached as tool entered.
Blanking flange with glands for push rod and hoses were attached to spacer section thus sealing tool inside.
Ball valve was opened and push rod used to position tool beyond valve.
The tool seals were inflated with glycol to seal off pipeline (annulus left at ambient pressure). Pressure was held without loss or necessity for topping up for 135 hr.
Space behind tool including valve was vented to atmosphere until pressure became ambient.
Valve was removed and replaced with new one. The spacer section was replaced and cavity pressurized to same pressure as pipeline; checked for leaks.
Seals were depressurized and tool withdrawn to a position so that seals could straddle the valve and flange faces.
Seals were inflated with glycol and the annulus with nitrogen. Test pressure of 3,000 psi (204 bar) was held for 7 1/2 hr without loss.
Seals and annulus were depressurized and tool withdrawn into spacer section.
Ball valve was closed to isolate gas line.
Spacer section was vented to ambient; blanking flange was removed to allow tool to be removed, and work commenced on bringing line back into service.

It is worth noting that during the line-plugging operation the seals were inflated for a period of 135 hr during which time the umbilicals were disconnected.

The system was backed up by a precharged accumulator on the tool. But during this time, negligible pressure drop occurred, showing good seal integrity. The use of the new system in this situation preserved the desiccated atmosphere within the gas line thus obviating the necessity of redrying the line.

SUBSEA TESTING

Another operation was performed subsea to confirm the integrity of a flange joint made to attach a spur control valve to an existing flange.

An existing 36-in. oil trunk line had been laid some years previously with a 16-in. blanked off tee with a view to the future attachment of a 16-in. line. The 16-in. line was to be connected to a ball valve which was flanged to the tee piece on the trunk line.

In order to attach the valve it was necessary to depressurize the trunk line and remove the blank flange from the tee. The new ball valve was then mated to the tee and the made-up flanged connection was leak tested with the plugging tool.

The tool was removed by withdrawal through the valve, the valve was closed, and the main trunk line brought back into service.

The operational sequence was as follows:

Main trunk line was depressurized and divers removed blank flange from tee piece.
New 16-in. valve with plugging tool preinstalled was lowered to seabed; umbilicals to surface.
Plugging tool was inserted into tee, ball valve flange mated to tee flange and bolted up.
Plugging tool was operated to pressure test the flanged connection pressurizing medium water with added vivid red fluorescent dye Rodamin B.
Divers checked for leaks at flange using ultra violet light. Pressure was also monitored at test gauge.
Pressure was held for 3 1/2 hr, after which time the tool was depressurized and pulled to open end of valve for recovery to surface.
Ball valve was closed to allow the main trunk line to be brought back into service.

The use of the system in the operation just described enabled the disturbed area only to be pressure tested, the ball valve having previously been tested after manufacture.

The operation saved a considerable amount of time because the alternative was to carry out a test by pressurizing the full line which was several miles long. Such a pressure test could require the subsequent pigging and drying of the line before reintroduction of the product.