**4. Data analysis**

**Figure 7.** Onshore and offshore map of oil and gas area in Indonesian regions (images courtesy theoilandgasyear).

and biases is crucial for high accuracy requirements. **Table 3** shows parameter processing and

**Figure 8.** Baseline concepts using one stable reference station for subsidence monitoring along large offshore oil and gas

Final precise orbit from International GNSS Services (IGS) can be downloaded in every 2 weeks after the observation time, while earth rotation parameter can be downloaded either daily, every 2 weeks, or on yearly basis. As for phase center parameter, we can download from UNAVCO website or Bernese website. Many mirror addresses are also available for

processing strategy that are used on data processing using scientific software.

downloading GNSS GPS parameter data processing.

area of Indonesia.

94 Multi-purposeful Application of Geospatial Data

**Table 4** shows the result simulation of baseline concept using one stable reference station, while **Table 5** shows the result simulation of baseline concept using reference station at clustered offshore regions, for subsidence monitoring around offshore oil and gas area of Indonesia. On each figures we can see the baseline length and the average for height component from simulation result. For baseline with more than 1000 km, the average height component is around 1 cm. For baseline with less than 1000 km, less than 1 cm of average can be seen.

Results from data simulation show that the scenario using reference station at clustered offshore are given better result than scenario using only one stable reference station. Nevertheless with using scenario of only one stable reference station, generally it is sufficient enough for monitoring offshore oil and gas platform subsidence in such large offshore like in Indonesia.


**Table 4.** Result simulation of baseline concepts using one stable reference station for subsidence monitoring along large offshore oil and gas area of Indonesia.


**Table 5.** Result simulation of baseline concepts using reference station around clustered offshore regions for subsidence monitoring around offshore oil and gas area of Indonesia.

For baseline with more than 1000 km when we get 1 cm of accuracy, it is very good indeed. It may give the effectiveness and efficiency to the monitoring projects by the oil and gas company and others.

**Figures 10**–**13** show graphs of repeatability on data simulation processing result for scenario using only one stable reference station for subsidence monitoring with baseline that varies more than 1000 km. A good repeatability on each graph can be discovered. A clear view of accuracy given by this scenario would be achieved. Generally it is sufficient enough for monitoring offshore oil and gas platform subsidence in such large offshore area like in Indonesia.

**Figures 14**–**17** show graphs of repeatability on data simulation processing result for scenario using reference station at clustered area for subsidence monitoring with baseline that varies less than 1000 km. There is a good repeatability on each graph. A clear view of accuracy given

by this scenario has been achieved. There is a high confident to use this scenario for monitor-

From simulation results the capabilities of GNSS GPS technology on monitoring subsidence on oil and gas platform are proven. Indeed the real data observations are available in quite numbers of offshore oil and gas platforms around Indonesia. **Figures 18** and **19** show example of subsidence signal at Platform X and Platform Y somewhere located on classified area. Very

ing offshore oil and gas platform subsidence.

**Figure 11.** Repeatability of H result simulation baseline BAKO-BTNG 2,207,624.866 m.

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**Figure 13.** Repeatability of H result simulation baseline BAKO-CMRE 1,707,364.779 m.

**Figure 14.** Repeatability of H result simulation baseline CNAB-CFAK 359,191.773 m.

**Figure 12.** Repeatability of H result simulation baseline BAKO-CNAB 3,163,103.652 m.

**Figure 10.** Repeatability of H result simulation baseline BAKO-CSAB 1,868,631.008 m.

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**Figure 11.** Repeatability of H result simulation baseline BAKO-BTNG 2,207,624.866 m.

**Figure 12.** Repeatability of H result simulation baseline BAKO-CNAB 3,163,103.652 m.

**Figure 13.** Repeatability of H result simulation baseline BAKO-CMRE 1,707,364.779 m.

**Figure 14.** Repeatability of H result simulation baseline CNAB-CFAK 359,191.773 m.

**Figure 10.** Repeatability of H result simulation baseline BAKO-CSAB 1,868,631.008 m.

pany and others.

For baseline with more than 1000 km when we get 1 cm of accuracy, it is very good indeed. It may give the effectiveness and efficiency to the monitoring projects by the oil and gas com-

**Table 5.** Result simulation of baseline concepts using reference station around clustered offshore regions for subsidence

**Baseline ID Baseline length (m) Observation session Average for height component from** 

**Table 4.** Result simulation of baseline concepts using one stable reference station for subsidence monitoring along large

**Baseline ID Baseline length (m) Observation session Average for height component from** 

1. CNAB-CFAK 359,191.773 24 h/31 days 0.007 2. CNAB-CMAN 319,811.751 24 h/31 days 0.011 3. BNOA-CBTU 934,272.232 24 h/31 days 0.005 4. BNOA-CDNP 100,619.250 24 h/31 days 0.004

1. BAKO-CSAB 1,868,631.008 24 h/31 days 0.021 2. BAKO-BTNG 2,207,624.866 24 h/31 days 0.014 3. BAKO-CNAB 3,163,103.652 24 h/31 days 0.011 4. BAKO-CMRE 1,707,364.779 24 h/31 days 0.011

monitoring around offshore oil and gas area of Indonesia.

offshore oil and gas area of Indonesia.

96 Multi-purposeful Application of Geospatial Data

**simulation (m)**

**simulation (m)**

**Figures 10**–**13** show graphs of repeatability on data simulation processing result for scenario using only one stable reference station for subsidence monitoring with baseline that varies more than 1000 km. A good repeatability on each graph can be discovered. A clear view of accuracy given by this scenario would be achieved. Generally it is sufficient enough for monitoring offshore oil and gas platform subsidence in such large offshore area like in Indonesia. **Figures 14**–**17** show graphs of repeatability on data simulation processing result for scenario using reference station at clustered area for subsidence monitoring with baseline that varies less than 1000 km. There is a good repeatability on each graph. A clear view of accuracy given

> by this scenario has been achieved. There is a high confident to use this scenario for monitoring offshore oil and gas platform subsidence.

> From simulation results the capabilities of GNSS GPS technology on monitoring subsidence on oil and gas platform are proven. Indeed the real data observations are available in quite numbers of offshore oil and gas platforms around Indonesia. **Figures 18** and **19** show example of subsidence signal at Platform X and Platform Y somewhere located on classified area. Very

**Figure 15.** Repeatability of H result simulation baseline CNAB-CMAN 319,811.751 m.

**Figure 16.** Repeatability of H result simulation baseline BNOA-CBTU 934,272.232 m.

clear subsidence with rates of about 10–20 cm per year can be seen on the graphs. Some platforms have trend of few centimeters per year; it depends on how much oil and gas are being exploited, etc. In a detailed viewing, fluctuation on the trend can be seen. Sometimes it is

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Large rates of subsidence are usually taking places in old oil and gas platform where depletion on reservoirs is more due to decrease in pore pressure, etc. There are already quite number for old oil and gas platform in Indonesia region. In this case monitoring program is quite important today. GNSS GPS is mostly the chosen technology for the programs. Nevertheless in many cases, the methodology on data acquisition and data processing is not correct. That is why it is necessary to write this kind of paper to help people on understanding more the capabilities of GNSS GPS technology on monitoring subsidence on oil and gas platform, as well as share the more correct concept and methodology. There are other methods that have been implemented together side by side with the GNSS GPS such as using altimeter and pres-

Due to oil and gas exploitation, offshore oil and gas platform may experience subsidence. It has been observed from the real measurement that the rates can be varying from 1 to 10 centimeters per year and even more for certain places. In a detailed viewing, we can see fluctuation on the trend. Sometimes it is accelerating, while in some cases, it is slowing or linier. This situation will depend on how much oil and gas are being exploited and also how much pore pressure left, the fluid injection, etc. This subsidence information is mandatory for risk assessment and safety requirement. Continuing subsidence may deform the

sure gauge. They rely on identification the changing on MSL through times.

accelerating, while in some cases, it is slowing or linier.

**Figure 19.** Example of real data of platform subsidence in platform Y offshore of Indonesia.

**5. Closing remarks**

**Figure 17.** Repeatability of H result simulation baseline BNOA-CDNP 100,619.250 m.

**Figure 18.** Example of real data of platform subsidence in platform X offshore of Indonesia.

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**Figure 19.** Example of real data of platform subsidence in platform Y offshore of Indonesia.

clear subsidence with rates of about 10–20 cm per year can be seen on the graphs. Some platforms have trend of few centimeters per year; it depends on how much oil and gas are being exploited, etc. In a detailed viewing, fluctuation on the trend can be seen. Sometimes it is accelerating, while in some cases, it is slowing or linier.

Large rates of subsidence are usually taking places in old oil and gas platform where depletion on reservoirs is more due to decrease in pore pressure, etc. There are already quite number for old oil and gas platform in Indonesia region. In this case monitoring program is quite important today. GNSS GPS is mostly the chosen technology for the programs. Nevertheless in many cases, the methodology on data acquisition and data processing is not correct. That is why it is necessary to write this kind of paper to help people on understanding more the capabilities of GNSS GPS technology on monitoring subsidence on oil and gas platform, as well as share the more correct concept and methodology. There are other methods that have been implemented together side by side with the GNSS GPS such as using altimeter and pressure gauge. They rely on identification the changing on MSL through times.
