Why are offshore pipeline valves so important? Of the valves used in an offshore plant, the pipeline valves are the largest, heaviest, most expensive and complex valves, and have the longest delivery time. The aim of this article is to provide information about top-entry pipeline valves used in the offshore oil and gas industry, including their general features and some design considerations.
Figure 1 shows the export pipeline from the platform to the shore. The pipeline includes two parts, topside and subsea, subject to two different design codes, ASME and DNV. The export-line valves are installed in the pipeline on the platform upstream of the subsea section, and they are designed based on ASME code.
These pipeline valves generally are:
- top-entry design
- ball or through-conduit gate valve types
- welded to the pipeline
- large sizes, such as 38-inch, and high-pressure class
- heavy, as a result of size and pressure class
- piggable (a PIG—piping injected gadget—is run into the pipeline for purposes such as cleaning and maintenance)
These valves are normally designed according to the API 6D standard. They are exposed to pipeline loads that must be taken into consideration in calculating valve wall thickness.
TOP ENTRY FOR ACCESSIBILITY
Pipeline valves are top entry, giving access to the valve internals from the top. They have a one-piece body design that gives better mechanical resistance to pipeline loads, as compared to split-body or side-entry designs.
Top-entry valves are welded to the pipeline, offering less likelihood of leakage than flange connection. The top-entry valve’s welded connection also has the advantage of saving two pipeline flanges and two valve flanges. Figure 2 shows top-entry and side-entry valves.
The main advantage of top-entry design for pipeline valves is inline maintenance, which means there is no need to remove the valve from the pipeline during maintenance. In addition, the top-entry design gives more flexibility for stem enlargement in case of high torque requirements.
The welding of the valve to the pipeline is done through a “pup” piece (a short length of pipe required between two fittings, a fitting and a flange or between two flanges to make-up a required dimensional distance), or a pup piece plus transition piece. A transition piece may be required due to a difference in thickness between the valve and pup piece, or welding between two different materials. The welding could be more challenging if the valve and the connected pipe are made of dissimilar materials. Figure 3 illustrates welding of a 30-inch ball valve.
These top-entry valves can be ball valves or through-conduit gate valves. The selection between the ball and through-conduit gate type depends on parameters such as cost, delivery time, valve dimensions, space availability, weight, client preference, etc.
All these export-line valves are actuated. Actuator options are hydraulic, pneumatic and electrical. The choice of actuator depends on factors such as valve size and pressure class, required torque to open and close the valve, valve opening and closing time, as well as valve failure mode.
MINIMIZING THE WEIGHT OF TOP-ENTRY VALVES
There are different ways to reduce the weight of top-entry valves. The first one is to calculate the body wall thickness based on ASME Section VIII Div.02 instead of ASME B16.34. Thickness reduction decreases the end-to-end and height dimensions and the weight of top-entry ball valves. The table shows the reduction in thickness, end to end, height and weight of a 30-inch CL1500 top-entry ball valve through wall thickness calculation according to ASME Sec. VIII rather than ASME B16.34.
Another way to save weight is to use cylindrical nuts rather than hexagonal. Using cylindrical nuts saves body and bonnet area, as well as nut area, which in turn saves weight. Figure 4 shows a 38-inch valve class 1500 with electrical actuator and cylindrical nuts.
In addition, the valve needs to be designed to withstand design pressure, hydro test pressure and pipeline loads. Proper design of the pressure-containing parts such as body, bonnet, ball, seat, pup piece and bolts is especially important for withstanding the loads. Finite element analysis (Figure 5) is done on the valve parts to make sure they are strong enough.
The loads vary and could be axial, bending or torsional, as well as complex loads associated with waves. The design and weight of pipeline valves of the same size and pressure class installed on the same pipeline could be different. The reason is that valves in different locations may experience different loads.
Bending tests are performed on the top-entry valve to validate the results of finite element analysis. The bending test for design and accidental loads can be done with the body and bonnet assembled together. This test can be repeated with the maintenance load when the bonnet is off during maintenance and the valve is more fragile against the loads. Spool pieces are normally used during the bending (load) test at the ends of the pup pieces to increase the length of the valve and achieve sufficient bending moment. Installed sensors (strain gauges) on several critical areas of the valve body can show the possibility of excessive elastic deformation of the body due to the loads. Body deformation externally can cause seat leakage, and body deformation internally may damage the seat or ball and body.