During the past 15 to 20 years, duplex stainless steels have gained wide acceptance in the oil, gas, chemical, and refining industries. Because of their desirable mechanical properties of corrosion resistance, high strength, and light weight, the offshore oil and gas industry has been quick to recognize their usefulness. Engineering design firms have been using duplex stainless steels to greater advantage to further reduce topside weight.
When using duplex stainless piping systems, thin-wall sections can pose serious dynamic problems. Even though their fatigue properties appear to be superior to most carbon steels, many fatigue failures involving duplex steels have occurred in North Sea operations. In systems experiencing little or no mechanical dynamic excitation, fatigue failures will not be a problem. Conversely, thin-walled systems exposed to flow-induced excitation due to low frequency random turbulence or high frequency sonic, or near sonic, flow conditions involving pressure control, recycle, or safety relief valves, can be highly susceptible to fatigue failures. These failures can occur at vibration amplitudes previously considered safe in thicker wall piping systems.
Failures of this kind were experienced during the start-up of the Tiffany Platform in small diameter branch connections attached to thin-wall duplex steel gas compression piping and, although affecting only a very limited number of connections, they were symptomatic of the problems recently experienced by a number of North Sea operators.
This paper describes the problems experienced and the approach used by AGIP (UK) Limited, together with Southwest Research Institute to investigate the causes of the failures and to identify simple remedies to allow quick restoration of safe production operations. The methodology used in obtaining data such as modal impact, vibration, noise, pressure pulsation, and dynamic strain are presented. Analysis to define the nature of the problems is also discussed. The solution of the fatigue failures is set forth along with suggestions for additional research that should be performed to develop guidelines and procedures for existing and new installations.
The Tiffany platform for oil and gas production and processing is located approximately 250 kilometers northeast of Aberdeen, Scotland in the UK sector of the North Sea. The platform was designed to initially produce and process oil and gas from the Tiffany and Toni reservoirs with the Thelma reservoir to be developed in the future.
The platform design consists of an eight-leg jacket secured to the sea bed by 16 vertical piles grouted into pile sleeves; a group of four piles at each corner leg. The topside modules consist of:
a process module
a utilities module
an accommodation module with helideck
an angled flare boom (supported from the process module)
a drilling deck and associated drilling services.
Capacities of the facilities are:
Gas: 3.26 × 106Nm3/d (115 mmscfd)
Injection Water: 125,000 bpd
A 304.8-mm (12-inch) oil export line runs about 5 km to the Brae A to Forties oil pipeline. Similarly, a 254-mm (10-inch) gas export line runs nearly 34 km from the platform to the Brae A to Brae B gas pipeline.
Installation of the platform is over a steel well guide template through which four predrilled production wells were drilled and completed before the platform was set. The total number of planned production wells is seven with four water injection wells.