Shutdown Simulations

In this exercise, you will run the above model for 2 hours at steady state conditions, followed by an 8 hour shutdown. In order to modify the simulation to represent this process, follow the procedures below:

Hint: Remember to change ENDTIME = 10 h in the Model View window (Case Definition  INTEGRATION).

4.2.1 Valve Scheduling

Both valves shall be fully open for the first two hours of the simulation (Steady State). After this period, the wellhead valve and the platform valve are simultaneously shut over a period of 60 seconds to shutdown the production. This will effectively initiate a shut-in period of 8 hours.

The simple way to do the valve manipulations is to use time series for valve openings. To change the valve scheduling for each valve, follow the simple procedure below:

In the main OLGA canvas, select a valve (VALVE-WH for example). In the Properties window, click on the Timeseries icon ( ) and fill in the table according to the table below:

Units

TIME M 0 120 121 600

OPENING -- 1 1 0 0

Hint: The method was explained before in the Ramp Up exercise (Section 2.7).

The final timeseries should resemble the following picture. Note that the time is given in minutes with the corresponding valve openings specified (1 representing fully open and 0 fully closed). The valve moves linearly with time between the time points specified with the exact position determined by interpolation. You may specify any opening between 0 and 1.

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Once you have updated the VALVE window for timeseries, Close the window to update the scheduling for VALVE-WH. To implement the same timeseries for the other valve, right-click on VALVE-WH in the main OLGA window and open the Global Instances table. In the Edit Multiple keywords of type VALVE window, copy and paste (Ctrl+C, followed by Ctrl+V) the entries of TIME [M] and OPENING from VALVE-WH to the relevant fields in VALVE-P.

4.2.2 Hydratecheck

In this part, you will calculate the minimum insulation layer thickness required to prevent hydrate formation as a result of 8h shutdown of production. For this, you will have to monitor the difference between the hydrate formation temperature at the local pressure and the local fluid temperature in OLGA (variable DTHYD). To facilitate such a feature, the hydrate formation curve needs to be input in OLGA.

In the Model View window, navigate to Library, right-click and select Add  Hydratecurve as shown:

In the Properties window, click on the Property Page as shown below.

In the HYDRATECURVE window, enter the following data. Note to change the default temperature unit to ^oC and default pressure unit to bara.

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You should be able to see the Hydrate curve in the same window as you enter the data.

In the Model View window, navigate to Flow Component  FLOWPATH : PIPELINE and right-click to select Add  FA-models  Hydratecheck.

In the Properties window in the HYDRATECURVE field, click on the ellipses button and select HYD-1 as shown below. This curve will now be used by the HYDRATECHECK option in OLGA.

In the Model View window under FLOWPATH : PIPELINE, add a new branch-level PROFILEDATA entry and specify the variable DTHYD. You will use this variable to evaluate the insulation requirements later on.

Observe that since DTHYD = Thydrateform - Tfluid, DTHYD is positive when the fluid temperature is below the hydrate formation temperature, i.e. the fluid can form hydrates. It is normal to add a safety margin of about 5^oC. Therefore, for design purposes it is considered dangerous to operate when DTHYD > -5^oC. Note that in practice, the acceptable margins must be evaluated in each case.

4.2.3 Insulation requirements for hydrate prevention

Using the Parametric Study described in previous exercises, change the PIPELINE insulation thickness to determine the minimum required insulation thickness, in order to prevent hydrate formation after an 8h shut-in period. Note that you should select Library as the main parameter and then, select THICKNESS for the pipeline wall.

You should experiment with 30, 40 and 50mm insulation thicknesses to find the optimum design specifications for the pipeline wall. Note that both pipe layers must be specified as shown below:

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Hint: if in doubt, refer to Exercise 1, Section 2.4.2 to see the steps for creating Parametric Studies in OLGA.

Check profiles of DTHYD for the Pipeline to determine how much insulation is needed for the DTHYD to stay below -5^oC throughout the entire 8h shut-down period (you may disregard the riser as it will be mostly gas filled during shutdown with no free water present to form hydrates).

Question: Is the insulation sufficient to maintain the minimum pipeline temperature at least 5C above the hydrate formation temperature at the local pressure at the end of the 8 hour shutdown?

If not, determine the required insulation level to achieve this.

Remember to modify the Shutdown case with the appropriate insulation and run it.

Note that the initial steady state and shutdown operations in this exercise were performed in a single simulation case. This is because the insulation thickness (calculated based on hydrate prevention requirements) could not be changed as part of a RESTART simulation. Therefore, the total simulation time adds up to 10 hours.