System sketch shows a typical system with two valve actuators and return to sea. (Images courtesy of Agito AS)

For more than 20 years the oil industry has been using computers for the dimensioning of hydraulic lines in umbilicals. The need for accurate calculations is heightened by the fact that the umbilical is one of the most expensive individual components in a subsea installation. If it is dimensioned incorrectly, major time and cost overruns can result.

In this article we show how a model for simulating this kind of system can easily be set up and designed using SimulationX software and the SimulationX subsea hydraulic library. The model example is based on available information from the various suppliers of these kinds of components and systems. We have chosen to look at a system for the completion and workover of a subsea well. The principle is identical for traditional subsea production systems.

System specification

A typical completion or workover system for subsea wells (Figure 1) consists of four main components:
1. Hydraulic power unit (HPU);
2. Umbilical;
3. Subsea control module (SCM); and
4. Valve actuator.

These are the main components in the system and can be found as pre-made elements in the subsea hydraulic library. By using a library like this, the user will benefit in shorter time for modeling and easier re-use of previous models.

Hydraulic power unit (HPU)

The HPU for these systems is normally located on the deck of the vessel from which the operation is being controlled. There are different types of pumps, but the most common type uses accumulators that are charged by fixed pumps. These pumps, which start and stop at various pre-programmed pressures, are controlled by a PLC.
All parameters of the different components in the Surface HPU element are easily accessed from the parameter window of the Surface HPU in the software.

Umbilical

All hydraulic communications go through separate hoses or pipes that are bundled together into an umbilical. In temporary systems such as a workover and completion system, hoses are most common. These hoses have properties that must be taken into consideration in the simulation. The ability of hoses to accumulate liquid can be a disadvantage in systems that require rapid bleeding of the lines. But this property can also be turned into an advantage in systems where large actuators are to be operated.

Using hoses with high volumetric expansion can in some cases replace subsea accumulators.

The dimensioning of the umbilical is important to the performance and operation of the whole system. It is therefore important that the model of the hose is accurate and that it includes the delays that are experienced in practice.

Different umbilical elements are available in the subsea hydraulic library, depending on the configuration of the system. There are different models depending on whether the umbilical is reeled up topside (horizontal first, then vertical), or if the umbilical goes straight down to the seabed, leaving the extra umbilical lengths subsea (vertical first, then horizontal).

Both options can be parameterized as either steel tube or flexible hose. In addition, the elements can be parameterized with variable pressure depending on the water depth.
To calculate accurate time behavior in the umbilical element, a method called a “distributed line model” is used where the line element internally is split into several separate elements. In simple terms, a distributed line model is a series of line elements in which each line element calculates the restriction and inertia/acceleration of the fluid as well as the flow. Between each line element, the pressures are calculated as a function of the flow from the previous linear element, flow to the next linear element, and the line’s volumetric coefficient of expansion (Ve). By using this method in the Umbilical element, time delays in long lines can be calculated with high accuracy. This is important for systems in which the pressurization and bleeding of lines are used as methods of Emergency Shut Down (ESD). These functions often have strict time requirements, which now are possible to simulate with high accuracy.

Subsea control module

Today’s systems are usually designed with a subsea control module from which the valve is operated. There are many types of subsea control modules, but the majority include an incoming supply line, return line, and several function lines that operate various types of actuators. The number of function lines depends on the type of application that is to be operated by the subsea control module.

Subsea control systems often use water-based oils and therefore allow return oil to be released into the sea in limited amounts. The subsea hydraulic library comes with a pre-made control valve for a subsea control module. The valve element includes operational parameters such as opening and closing time behavior, internal restrictions, and a detailed description of the valve reset function.

Valve actuator

The valve actuator for a subsea gate valve is often a linear actuator. Including the valve differential pressure across the gates/seat sealing, the model will be somewhat more complicated because of the variable friction. The subsea hydraulic library contains pre-made elements for both gate valve and ball valve. These valve elements are built up of different physical sub-elements where each individual element represents a characteristic of the valve behavior.

With a differential pressure across the gates/seat, the friction will be variable. This behavior is important to include in the gate valve element since this is an influence on the opening/closing time of the valve.

System model

The subsea hydraulic library contains all main elements required for the model. The remaining elements such as restrictions in connectors, internal piping, check valves, etc., are standard elements found in the standard hydraulic library. The graphical presentation makes the models much easier to reuse and share with other engineers.
Results from the system model

When the system model has been completed and all elements are parameterized, the simulation itself can start. It is important to clearly understand what you want to demonstrate through these simulations. Often it is sufficient to demonstrate that the requirements of standards and specifications are met. We have chosen to show three such sequences in this article:
• Pressurisation of an umbilical line;
• Operation of a gate valve; and
• ESD.

Pressurization of an umbilical line

The initial status is that the HPU is ready with full accumulators and the umbilical supply line vented to return. After 1 second the HPU valve opens and starts to pressurize the umbilical line. The umbilical is 1,968.6 ft (600 m) long, and the line has an internal diameter of 3?8 in. Transaqua HT has been chosen as the hydraulic fluid.
The subsea equipment is at a depth of 1,640.5 ft (500 m), and with the chosen fluid, the initial pressure will be approximately 52 bar subsea. The HPU header valve is commanded to open at t=1 second. The valve opening time is set to 0.2 seconds. The umbilical is fully pressurized and at steady state after 5 seconds.

Operation of a gate valve

During the operation of a gate valve, it is important to check whether the operation affects other valves in the system. This is done by operating a valve and at the same time monitoring the pressure variation in the neighboring gate valve actuator. If the pressure variation is too high, opened valves might start to close, and an unwanted shut-down might be the result. The options that we then have are usually to increase the dimensions of the lines in the umbilical or to install a subsea accumulator. Both options will affect the time it takes to pressurize the umbilical and to bleed it when shutting down. The model is set up with two gate valves, and we operate one valve whilst the other is set in the open position.

The opening time is approximately 50 seconds, and the minimum pressure differential over the actuator piston is approximately 62 bar when opening and 61 bar when closing; i.e., the valve starts to close at a pressure difference of 61 bar across the piston.

The model shows a clear drop in pressure when the neighboring gate valve is opened. The pressure drop across the actuator piston is 71 bar, which is above the pressure drop as the valve starts to close. This shows that our system is appropriately designed for these two valves.

We must also investigate whether the drop in pressure between the supply line and return line to the subsea control module is higher than the control valves’ re-set pressure, which is set to 60 bar in this model. The model shows that the drop in pressure between the two lines is 69 bar, which is above the valves’ re-set pressure.

ESD

All systems of this type are built so they automatically shut down if an accident occurs. These shutdowns should not be dependent on a power supply. The normal solution is to design the system in such a way that the gate valves close if the pressure is bled off up at the HPU.

The gate valve is completely closed after approximately 159 seconds. If the system had been set up with more valves and subsea accumulators, more liquid would need to be bled off via the umbilical. Naturally, the whole shut-down sequence would then take longer time.

These results are only to be seen as examples. Acceptance levels, including safety margins, are to be decided by the system suppliers and will vary from project to project.