TS-133
Waterhammer Prevention in Emergency Valve Closing of Crude Oil Unloading Equipment
Koki SHIOZAKI* and Izuo AYA*
ABSTRACT
There was a crude oil spill accident in the bay of Tokyo by the rupture of a dolphin type oil unloading equipment between a crude oil tanker and a sea berth. While transmitting oil, heavy wind blew the tanker away off the berth and broke the loading arms. In order to avoid this type of accident a new type loading arm with the function of emergency coupler detachment was developed later. However, because the undersea pipeline between the sea berth and the oil tank on the land is very long, the waterhammer followed by the emergency valve closing of the new system could become a problem.
In this study, the pressure transients followed by a waterhammer in the system was studied with the method of characteristic. The operational factors which have influences on the pressure rise were discussed. The pressure transient when air is trapped in the pipeline after the coupler of loading arm is detached at an emergency was also studied. Considering this effect of air, a new device to suppress the pressure rise in the oil loading system was proposed.
Key Words: Oil spill accident, Oil loading system, Waterhammer, The method of characteristics, Column separation
1. INTRODUCTION
There was a crude oil spill accident in the bay of Tokyo in 1990 caused by the rupture of an oil unloading equipment between a crude oil tanker and a sea berth which was located 7 km off a shore of Chiba prefecture. While transmitting oil, heavy wind of over 20 m/s blew the tanker away off the berth. Due to the defect of mooring system, the loading arms were stretched beyond the limit. Couplers at the end of the arms were all broken and crude oil of over 20 m3 was spilled on the sea.
In order to avoid this type of accident a new type loading arm with the function of emergency coupler detachment was developed. In the system, two hydraulically-operated valves are equipped on both sides of the coupler and they are closed before the coupler is detached.
In this study, fluid transients of this system are estimated by the method of characteristics [1] - [5]. As the berth is distant from the shore and the pipeline is considerably long, large scale column separations (the formation of vaporous cavities in the pipe caused by pressure drop) are estimated to occur mainly at the downstream of the valve, and bring about the high pressure rise subsequently. The influence of operational parameters on the magnitude of pressure rise is predicted.
On the fluid transients, the effects of free gas existing in the oil of gas evolution caused by the waterhammer event are not negligible especially in the case of oil [6] - [9].
In order to consider these effect, Discrete Gas Model [6] is used.
In the oil loading system, it is possible that the movement of the moored ship is so fast that the couplers are unexpectedly cut off before the isolation valves close. In these cases, oil flows out or air flows in through the unclosed isolation valves depending on the pressure of the pipe. The pressure transients of them and the amount of oil spill from the valves are estimated.
Finally, a waterhammer suppression device for this particular system is proposed and its performance is discussed.
2. DESCRIPTION OF MODEL
2.1 Schematic figure of the oil unloading system
Figure 1 shows a schematic figure of the system. The centrifugal pumps at the bottom of the tanker unload oil through three loading arms installed on the berth. Two butterfly valves (Inside diameter=0.40 m) for the isolation at an emergency are equipped on both sides of the coupler. Three pipelines from the loading arms are united to a bigger undersea pipeline. The line is buried at the bottom of the sea. The depth of the sea is about 20 m. The line leads to oil tanks on the shore. There are several undersea lines corresponding to different oil refineries, and the length of them are 8 to 14 km. The pipelines are all made of carbon steel.
2.2 Computational model
The pipeline length between the coupler and the point where the three pipelines are united to a big pipeline is about 50 m. This section is considerably short compared with the undersea line. When a large scale column occurs at the downstream of an isolation valve, a vaporous cavity grows over that section.
