Full Scale Crack Arrest Test And Arrestor Device Performance For The Flags Gasline
Authors: Hayes David J. , Lux Manfred D. 
Published: December 2007

Damage to large diameter, high pressure gaslines, Transporting rich natural gas, can cause initiation and propagation of ductile fractures. Recognizing this potential hazard, a full scale burst test was carried out to establish the crack arresting capability of arrestor devices fitted to the FLAGS Gasline. During the test, the crack initiated at 14 480 kPa (2 100 psi), developed into a fully ductile propagating fracture, and was positively arrested by the arrester devices. The test demonstrated the arrestors to be an economic and effective means of minimising damage to the FLAGS Gasline due to a propagating ductile fracture. propagating ductile fracture

Introduction

The Far North Liquids and Associated Gas System (FLAGS) is a joint venture of Shell U.K. Ltd. and Esso Petroleum Company Ltd. The offshore pipeline will carry natural gas and hydrocarbon condensate at a maximum operating pressure of about 13 790 kPa (2 000 psi) from the Brent Field in the North Sea to the gas giant at St. Fergus, Scotland. After extraction of the liquids, the lean natural gas will he received by the British Gas Corporation, for transmission throughout the U.K. Construction of the gasline was started in dune 1976, pipe laying was finished in June 1978. Final underwater tie-in and commissioning are scheduled for 1979.

The pipeline is approximately 448 km (278.5 miles) long of 914 mm 36 inch diameter pipe with 22 mm (0.867 inch, wall thickness, of steel API 5LX 60. 80% of the gasline lies in water over 100 metres (328 feet) deep, with the deepest part being 165 metros (541 feet). Maximum operating pressure is 14 480 kPa (2 100 psi). The outside of the gasline is covered with a glass fibre-wrap reinforced asphalt corrosion coating and a minimum of 57.15 mm (2 1/4 inch) thick unslotted concrete jacket with heavy steel cage or spiral wrap reinforcement. The concrete has a minimum crushing strength of 34 475 to 41 370 kPa (5 000 to 6 000 psi).

Although the high strength concrete coating gives sufficient protection against impact from fishing gear, there is a possibility of external damage to the pipeline caused by the dragging of large anchors by pipeline caused by the dragging of large anchors by surface vessels such an tankers, work vessels, etc. if such damage exceeds a critical dimension (dependent upon pipe toughness), a propagating ductile fracture may result. It was therefore of particular concern during the early design phases of the FLAGS Gasline to provide adequate safeguard against such a serious eventuality.

Consideration of the current state of understanding of the phenomenon of ductile crack propagation in gas pipelines and the absence of sufficient propagation in gas pipelines and the absence of sufficient full scale burst test data applicable to the FLAGE configuration with its rich gas mixture led to the decision to carry out such a test. in this paper we discuss the phenomenon of ductile crack propagation in gas pipelines and the probable influence of the submarine environment. The details of the full scale test which was carried out are then discussed. Since the role of gas dynamics is fundamental to the fracture behaviour of gas pipelines the test was carried out using a gas mixture closely simulating that to be expected under operating conditions. The results are reviewed and their implication as to the integrity of the FLAGS gasline are summarized.

DUCTILE FRACTURE OF GAS PIPELINE

In discussing the phenomenon of ductile fractures in gas pipelines it must be emphasised that its occurrence depends on an outside cause. Normal material selection, construction and testing procedures ensure integrity under operating conditions. It is only when an outside action causes sufficient damage to the line that the potential for propagation exists.

For the FLAGS Gasline essentially only damage from large anchors can be envisaged.