Type selection design and application of submarine pipeline ball valve

1 Overview

With the rapid development of offshore oil and gas industry technology and people's demand for energy, the development of deep-water oil and gas fields will become the focus of the development of offshore oil industry in the future. The underwater oil and gas production system is less affected by the disaster weather and has strong reliability, so it becomes one of the key facilities for the exploitation of deep-water oil and gas fields. Subsea pipeline valve is an important part of subsea oil and gas production system. Taking the practical application of two subsea pipeline ball valves (hereinafter referred to as ball valves) in an offshore oilfield production and development project as an example, this paper introduces the type selection design, land test and subsea operation of subsea pipeline ball valves.

2 common standards

Subsea pipeline valve is the general name of all valves used in subsea pipeline. It is mainly used in subsea manifold, subsea pipeline terminal and oil and gas transmission pipeline of offshore oil and gas engineering. Submarine pipeline valves are mainly ball valves, flat gate valves and check valves.

The main standards used for submarine pipeline valves are api6dss, api6a, api17d and iso13628-1.


3 operation mode

The operation modes of submarine pipeline valves mainly include hydraulic control, ROV (remote operated vehicle) and diver operation.

3.1 hydraulic control

Subsea pipeline valve is integrated with hydraulic source, hydraulic control unit and umbilical cable to form a system. The control unit with hydraulic source supplies pressure to subsea pipeline valve to open and close the valve. The advantage of this operation mode is that it can automatically open and close the valve, monitor the status of the submarine pipeline valve in real time on the offshore oil platform, and use ROV to operate the valve when the hydraulic control system fails. However, the hydraulic control is complex, there are many potential leakage points, and the investment cost is high.

3.2 ROV

ROV interface is set on the valve of submarine pipeline, which is divided into top mounted and side mounted. The valve is opened and closed by ROV equipped with torque tool. The advantages of this operation mode are that the valve structure is relatively simple compared with the hydraulic control, the cost is relatively low, and the cost of the project construction stage can be saved. However, ROV operation can not achieve emergency shutdown when the offshore oil platform needs to close the submarine pipeline valve in case of emergency, and the offshore oil platform can not monitor the status of the submarine pipeline valve.

3.3 diver operation

When the water depth is shallow, the valve of submarine pipeline is usually equipped with hand wheel, which can be operated by divers by air diving. In addition, for the submarine pipeline valve with ROV interface, if conditions permit, divers can also use special operation tools to operate the submarine pipeline valve by means of saturated diving.


4 type selection design

4.1 main technical indexes

The design service life, design temperature, design pressure, ball valve form, design standard, valve size, pressure grade, valve body material, ball material, ROV interface grade, ROV interface mode, ROV interface standard and valve manufacturing grade are mainly considered in the selection design of ball valve. In addition, attention should be paid to the form of valve base, ROV handle and its form, the distance from ROV centerline to valve centerline, the marine biological protection device at ROV interface, the scope and content of third-party inspection, and the high-pressure tank test of valve and actuator (affecting cost and delivery time). The purchaser shall clarify the technical details when conducting technical clarification with the supplier to prevent unnecessary contract changes.


4.2 end connection

There are flange connection and butt welding connection in the end connection of ball valve (Fig. 1).

(a) Flange connection (b) butt welding connection

Fig. 1 ball valves for subsea pipelines with different end connections

When the ball valve is butt welded, the potential leakage source of the valve can be reduced and the reliability of the pipeline system can be improved. In addition to the inconvenience of replacement and maintenance, the valve has little influence on operation, weight and price. In the design of underwater oil and gas production system, the reliability and maintainability of valves should be considered first. Considering comprehensively, the ball valve should be welded.


4.3 sealing form

In deep water, the ball valve not only bears the internal pressure of medium, but also hydrostatic pressure. In order to prevent medium leakage or seawater entering the valve body, two seals are generally set between the valve body and the environment, in which the metal seal is the main seal. There are SPE single piston effect and DPE double piston effect on the sealing surface of valve seat. The principle of SPE is that there is a sealing area difference between the valve chamber and the upstream sealing surface. When the pressure of the valve cavity increases to a certain value, the pressure difference between the valve cavity and the upstream is greater than the initial pressure of the spring, so that the valve seat is pushed away from the ball and the pressure of the valve cavity is released to the upstream. The principle of DPE is that there is a sealing area difference between the downstream and upstream of the valve seat. When the pressure in the valve chamber increases, under the action of the pressure difference and the initial sealing pressure of the spring, the valve seat is pushed to the ball to realize the bidirectional sealing of the valve. SPE + DPE is generally required for ball valves, and SPE is usually selected as the upstream sealing surface of the valve, so as to realize the function of automatic discharge to the upstream when the valve cavity is overpressured.


4.4 sleeves

Sleeve is a short joint welded at both ends of the valve, which is used to weld with the submarine pipeline terminal or the pipeline on the underwater manifold. In order to prevent damage to the seal of valve body caused by sleeve welding, it is generally required to complete the welding of sleeve before leaving the factory. At the same time, in order to facilitate the subsequent welding with the pipeline, the sleeve is required to use the same material as the pipeline. The length of the sleeve must ensure that the heat generated during welding with the pipe will not affect the sealing performance of the valve. The material number and requirements of the sleeve can be provided by the valve purchaser, purchased by the valve manufacturer, or provided by the valve purchaser to the valve manufacturer. After the welding of sleeve and valve is completed, it is necessary to carry out the hydrostatic test on the weld, and the third party is required to witness.



5 land function test

The ball valve must complete the factory acceptance test (fat factory acceptance test) including appearance test, pressure test and operation function test before leaving the factory. The buyer's representative and the third party's representative are usually required to witness the fat, and the valve manufacturer is required to provide the fat report signed by the third party before delivery.

The ball valve needs to be welded with the pipeline. After the construction of the whole subsea production system structure, the hydraulic and electrical continuity tests need to be carried out. Before the installation of submarine pipeline ball valve, its function should also be tested. It mainly tests the performance of opening and closing operation, including rotation direction, torque size and rotation number.



6 subsea operation of ball valve

Ball valve A is connected with the pipeline of offshore platform a, and ball valve B is connected with the pipeline of offshore platform B. the logistics is transported to FPSO for treatment after converging through submarine pipeline tee (Fig. 2).

Figure 2 layout of subsea pipeline base plate

Before launching, the two ball valves are in open state. After the installation of subsea pipeline base plate, because the whole subsea pipeline system needs ball passing and hydrostatic test, the two valves remain open. After the test, platform a was put into production first, so valve a was still open. At this time, platform B is not put into production, so valve B needs to be closed first. When B platform is put into production, open B valve. Therefore, it is necessary to close and open valve B.

The submarine pipeline base plate is installed by divers in the way of saturated diving. After the pressure test of the submarine pipeline system, divers use special tools (Fig. 3) to close the submarine pipeline valve B.

Figure 3 submarine pipeline ball valve operation tool used by divers

When platform B is put into operation, the closed ball valve B needs to be opened. At this time, because the saturation diving support ship has recovered, the ROV is equipped with torque tools to open. The ROV should be working type, and the operation tools are the same as those used in the land test. When the ball valve is closed, the pointer at the ROV interface points to the letter s. When the ball valve is opened, the pointer at the ROV interface points to the letter O (Fig. 4).

(a) Valve closed (b) valve open

Figure 4 closed and open state of ball valve in submarine pipeline



7 conclusion

Deep sea oil and gas exploitation is the development trend of resource development in the future. Submarine pipeline valve is the key component of submarine pipeline manifold and other equipment, which will be more and more widely used.