Loss Control by Subsea Barrier Valve for Offshore Gas Pipeline

Abstract

Abstract. Platform-mounted shut-down valves for offshore pipeline systems, are vulnerable to fire and explosion hazards. Should this primary barrier fail, the full inventory of the pipeline could be consumed in the fire. A secondary subsea barrier would reduce substantially the loss of pipeline inventory, but also expose the platform to a new hazard. Valve failure could lead to the formation of gas or oil plumes which could be carried towards the platform. The choice of subsea barrier location, requires a trade-off between these safety concerns. The paper presents one prerequisite for such a trade-off; an analysis of the potential gas loss for different secondary-barrier positions and valve-closing rates when a guillotine break occurs at platform deck level. The paper gives a detailed study of the flow characteristics close to the valve and over the length of the segment between the barrier valve and the rupture. Particular attention is paid to the boundary conditions at the valve. These have been formulated based on the method of characteristics. Computer simulations using the simplifying assumption of isothermal flow have been compared to the adiabatic case. The differences between these cases have been found to be small. Problem Discussion. We consider the situation depicted in Fig. 1. The subsea pipeline is connected to the process equipment on the platform through a vertical segment. In the case of a fire or explosion on the platform, a possible pipeline failure would be at deck level, as indicated by the symbol R. At the broken end a counterpressure of about one bar exists. Two subsea barrier valves, one upstream at location A, the other downstream at location B, are installed to prevent a major loss of gas in case of flowline rupture. It is assumed that the barrier valves start to close at the same time as the guillotine rupture occurs. The aim of the present investigations is to determine the mass loss and flow variables in the pipeline. A good physical discussion of the pipeline break problem without valve is given by [1],[2] and a more detailed mathematical treatment is developed in [3],[4]. The length L of the pipeline segment between the valve and the broken end as well as the valve closing time TClose are considered to be two dominant parameters influencing the rate of outflow. The range of lengths L considered varies from an upper limit of 2000m to a lower limit imposed by the riser height (or ocean depth). The closing time is chosen in the technically realizable range of 40 s to 100 s. Two cases are of interest, one representing flow towards the platform (A) and one away from it (B). The pressure level will in general be higher with flow away from the platform. More important is the pressure gradient in the steady state. For Case B, the break leads to a flow reversal from the steady state direction.