Ice plug employed on subsea pipeline bend during repair

The first controlled-temperature ice plug in the bend of an offshore gas trunkline has been carried out for Phillips Petroleum Co. Norway on its Norpipe A.S. platform in the German sector of the North Sea. The procedure was part of a subsea valve repair operation.

BJ Process & Pipeline Services Co. (BJ Services), Aberdeen, completed the 5-day operation which it said is significant because it allowed gas to flow continuously through a bypass line while valves were being repaired.

The ice plug was successfully formed offshore and tested to a differential pressure of 1,450 psi. Repair of two valves required only 5 days during which time gas production was operating at close to 50-60% via the platform bypass, says the service company.

Import pipeline

The 36-in. Emden pipeline is approximately 440 km long and includes two gas-compressor platforms. It is the export trunkline for gas produced from the Ekofisk field, as well as for other major operators in the Norwegian North Sea. The pipeline runs between Ekofisk 2/4R platform and a gas-processing plant in Emden, Germany.

BJ Services says that it became apparent that three main line isolation valves on the Emden line were unable to meet the criteria for maximum leak rate. Before any remedial work could be carried out, two isolations were required for safety purposes with a facility to bleed off any minor leakage: a "double block and bleed" arrangement.

Before repair of emergency shutdown valves ESDV2 and ESDV3 (Fig. 1 [122,019 bytes]), ESDV1 was closed and used as a temporary isolation. ESDV1 would be repaired later during a scheduled shutdown.

To allow installation of the mechanical plug, an additional isolation was required. The second isolation needed to be located on a tight bend with a 3D curvature between ESDV1 and the valves in need of repair (ESDV2 and ESDV3).

Installing a mechanical plug inside of this bend would have been extremely difficult, says BJ Services. An ice plug could be formed by directly applying liquid nitrogen to the pipewall at -196° C., but investigation of the metallurgy of the pipeline material revealed that this low temperature was unacceptable.

Controlled freeze

As an alternative, Phillips Petroleum Norway asked BJ Services to form an ice plug using a controlled-freeze method the service company had developed.

This technique uses a heat exchanger to control the coolant temperature at higher temperatures than liquid nitrogen. The coolant is pumped through a sealed jacket which is clamped around the pipeline. BJ Services provided gel to fill the bend for a much quicker freeze.

Using computational fluid dynamics (CFD), the service company's engineers showed that a plug could be formed in 52 hr with coolant at -50° C. After formation, the temperature could then be raised to -30° C. during repair operations to take advantage of the pipeline's improved mechanical properties.

The company employed structural finite-element analysis (FEA) to check the effects of pipe contraction at low temperature. Inspection showed that stresses caused by temperature gradients were acceptable, as was the differential expansion of the ice plug and pipe material.

Surveys of the bend were carried out to give precise jacket sizes, and CFD results were used to formulate the most-efficient coolant flow. BJ Services also checked the structure independently using FEA.

In addition, the jacket was constructed and fitted to a full-size test bend. Extensive onshore testing of the freeze plug was completed before mobilization, says the company.