Recently, several traditional major oil companies, such as British Petroleum (BP), Royal Dutch Shell, Exxon Mobil, and Chevron, have exploited oil from onshore, offshore, and subsea areas because the total amount of oil obtained from sources on land has not been able to meet the demand for oil, which for dozens of years has been growing in developing countries such as China, India, and others. As the technology for drilling develops and oil prices rise to more than $100 per a barrel, oil wells in onshore, offshore, and especially subsea areas are becoming more productive for oil companies. Because of the emergence of new areas for exploiting oil, new oil-producing countries such as Brazil, Malaysia, Indonesia, Nigeria, Angola, and others are playing larger roles in the global oil market. The exploitation of subsea areas for oil production is particularly challenging and interesting to scientists and engineers because of the extreme high pressure conditions. In addition, oil wells in subsea areas are attractive to oil major oil companies because the total amount of oil from oil wells in subsea areas that have already been discovered is greater than the amount of oil available from existing oil wells on land.

Since 2003, when ChevronTexaco (Chevron) succeeded in drilling oil wells located in Toledo in the Gulf of Mexico at a depth of approximately 3,052m [1], many oil wells in subsea areas have been drilled and exploited. Because of the environmental restrictions that exist in subsea areas, a subsea oil production system must be able to resist extreme high pressure and seawater. In addition, it is not easy to access most oil wells locate at depths between 1,000 and 2,000 m. Thus, many organizations, such as the American Petroleum Institute (API), the American Bureau of Shipping (ABS), and Det Norske Veritas (DNV), have established and enforced rules for relevant companies to ensure the safety of subsea oil production systems. However, a tragic disaster occurred at the Macondo well, located in the Gulf of Mexico, on April 20, 2010 [2]. This disaster brought about the deaths of 11 persons, tremendous property loss, and inestimable environmental damage by oil spills. According to a BP internal report, the causes of this disaster were complex. However, if the monitoring system for a subsea production system works well, the extent of the damage resulting from such a disaster can be reduced. Oil companies have recognized the importance of inspection, repair, and maintenance (IRM) and have invested considerable amounts of money in IRM to maintain existing subsea oil production systems.

According to [3], the market for subsea oil production systems is worth 300 billion dollars as of 2014. However, the market is very exclusive and is monopolized by a few companies [4]. However, the Korean government and many companies, including three large shipbuilding companies, have tried to manufacture relevant equipment. A subsea data logger system for IRM is one of several types of equipment that the Korean government and many companies want to put in place because this type of system is based on information technology (IT) and has fewer technical barriers than other types of equipment.

The primary objectives of this study were to (1) analyze existing subsea data logger systems, (2) establish requirements for subsea data logger systems, (3) implement a prototype subsea data logger system, (4) conduct a test of the prototype, and (5) demonstrate the feasibility of developing a subsea data logger system. The remainder of this paper is organized as follows. Section 2 describes related work. Section 3 describes the requirements for a subsea data logger system. Section 4 describes the process of implementing a prototype of a subsea data logger system. Section 5 presents conclusions.

A subsea oil production system is typically composed of an X-mas tree, flowlines, risers, an umbilical, and production facilities. If there are many wells, a manifold can be added to a subsea oil production system. Figure 1 shows an overview of a subsea oil production system.

The X-mas tree is installed on the top of the oil well, which is called the wellhead. The X-mas tree regulates the production of oil and serves as part of the interface between the oil well and other equipment by providing a blow-out preventer (BOP). Sometimes, the X-mas tree becomes a passage by which chemical compounds are injected into the oil well or the flowlines, and sometimes it is used to control downhole valves. Additionally, the X-mas tree removes the excess pressure on the annulus and maintains a constant flow of oil through a choke. For these reasons, the X-mas tree is an important piece of safety equipment [5]. Flowlines are installed between the X-mas tree and risers or between the X-mas tree and a manifold. Flowlines are passages by which crude oil is moved the production facilities. Risers are installed between the X-mas tree or manifold and the production facilities. Risers push crude oil upward at high pressure. The umbilical, which is installed between the production facilities and the X-mas tree, is composed of cables and fiber optic cables and provides electrical power and hydraulic power. The X-mas tree, flowlines, risers, and manifold can be controlled through the umbilical [6]. Figure 2 shows a typical cross section of an umbilical. Production facilities are floating mass structures that produce various oil products from crude oil and provide storage for oil products until they are transferred to chemical carriers. Production facilities are usually called floating production storage and offloading (FPSOs).

After subsea oil production systems are installed in subsea areas, these systems can be operated for 20–30 years to exploit crude oil. Thus, data loggers should be employed with subsea oil production systems. Data logger systems have been widely used to collect various types of data from the environment, sensors, and equipment [7-10]. However, data logger systems have not been widely used with subsea oil production systems because most experts have held the view that applying verified equipment and strict rules guarantees the safety of subsea production systems and that operation of subsea data logger systems and remotely operated vehicles (ROVs) for IRM result in additional operational costs. Actually, the rules related to subsea equipment for oil production systems are very strict, and only proven equipment can be applied to a subsea oil production system. For these complex reasons, the importance of IRM has been relatively little recognized, while subsea oil wells have been widely exploited.

Fortunately, many subsea data logger systems and ROVs have been installed to monitor subsea oil production systems since the disaster that happened in 2010. In addition, relevant markets are steadily growing. The main function of a subsea data logger system is to monitor abnormal vibrations and leakage under extreme conditions, such as strong seabed currents, low visibility, corrosion, and extreme high pressure. Among these environmental conditions, extreme high pressure is the most important concern in developing a subsea data logger system. A subsea data logger system is typically composed of a housing cap, a main body, a battery pack, a sensor board (or embedded system), a memory disk, and optional penetrated interfaces for the outer sensors, as in the 2H subsea data logger shown in Figure 3. Installation of a subsea data logger system on a riser using an ROV is shown in Figure 4.

We analyzed the top three existing brand products in the global markets to determine the requirements for a subsea data logger system. These products are typically installed and operated to maintain and monitor subsea production systems with ROVs. Based on this analysis, several significant requirements were established for the system to be developed.

First, a subsea data logger system needs robust material to protect an internal embedded system and connecting interfaces from extreme high pressure and corrosion by seawater for a minimum of 20–30 years. The candidate material for the case is austenitic type 316 stainless steel (SS). The properties of this material are shown in Table 1.

Austenitic 316 SS cannot completely prevent infiltration of seawater at extreme high pressure. Thus, a subsea data logger system needs O-rings and grease to seal the case, and these O-rings and grease should be tested to prove their performance. The case also needs interfaces to connect external sensors and cameras to an internal embedded system. In addition, these interfaces have to follow de facto rules. Each interface type is dependent on the types of sensors and cameras used. Table 2 shows the variable supporting interfaces to connect sensors and cameras to a subsea data logger system.

Second, an optimized embedded system has to be developed and installed inside a case with batteries. The most recently developed subsea data logger systems support wireless protocols. However, the system proposed in this paper does not support a wireless network at this time. The embedded system should have multiple analog channels to gather analog signals from the external sensors. To store these signals, the embedded system needs a converting device to transform analog signals into digital signals. A subsea data logger system has to be installed on equipment suitable for subsea production systems. Thus, the power consumption of the system has to be low so that environmental data can be gathered from subsea production system equipment and stored for long periods of time.

Third, dedicated software should be provided with a subsea data logger system to extract data from the subsea data logger system. A subsea data logger installed on production equipment should usually be retrieved every half month to one month to extract data and change the internal batteries. The software has various packages for data logger group management, configuration management, battery management system, data export and import management, backup and recovery management, user account management, and auto report management. Figure 5 shows an overview of the software and a snapshot of the software interface provided by CDL Inertial Engineering.

In general, the basic shape of a case for a subsea data logger system is a cylinder, which is stronger than a cube or a rectangular parallelepiped under extreme high pressure. In this study, an external case for a subsea data logger system was designed. The case is composed of a main body, a top housing cap, a bottom housing cap, and O-rings and is made of austenitic 316 SS. The top housing cap has four interfaces for external sensors. In addition, the interfaces have O-rings and grease to seal gaps between the top housing cap and the main body. An embedded system and battery are arranged in the main body. The top housing cap is composed of a cap and a buttress for the embedded system and battery. When the embedded system needs to be fixed or upgraded, the top housing cap is turned and pulled. Figure 6 shows the basic blueprint for the case.

The length of the main body is 266 mm, and the diameter of each housing cap is 8 mm. The total length of the external case is 282 mm. The external diameter is 100 mm, and the internal diameter is 75 mm. Thus, the thickness of main body is 12.5 mm. This case should be able to withstand at least 200 bars of pressure. According to the results of the structural analysis described below, this case can withstand up to 400 bars of pressure, but the case was tested at pressures up to 200 bars because the performance of the O-rings and grease could not be tested in this study.

Before manufacturing the external case of the prototype, the stability of the external case should be confirmed. To do this, a structural analysis was carried out using a finite element (FE). A FE model of the main body, a top housing cap, and a bottom housing cap made of austenitic 316 SS was developed using three-dimensional tetrahedral elements with four nodes. Orings for sealing the gaps around the main body, top housing cap, and bottom housing cap were not considered because it is not easy to determine the strength of O-rings. Table 3 shows the dimensions, number of nodes, and number of elements of the FE model. The physical properties of the external case are shown in Table 4.

Two boundary conditions, a vertically standing state and a laterally lying state, were assumed. In other words, the external case was modeled as standing perpendicular to and horizontal with the seabed. Pressures of 50, 100, 200, 300, and 400 bars were simulated as being gradually applied to the external case. As Table 5 shows, the external case was stable under 400 bars of pressure.

In general, a subsea data logger system does not need to gather signals from inside the case because stability is already verified before releasing. However, it was necessary in this study to gather signals from inside the case to verify the stability of the inner embedded system. Thus, a customized embedded system was developed in this study. The specifications for the embedded system are shown in Table 6. The embedded system for the prototype is shown in Figure 7.

The embedded system uses a PA-33X manufactured by Keller to gather internal and external pressure signals, and uses an SHT75 manufactured by Sensirion to gather internal humidity and temperature signals. Figure 8 shows the embedded system that connects to the top housing cap.

The housing was tested at pressures below 200 bars to verify the stability of the external case. The testing was conducted at the Korea Research Institute of Ships and Ocean Engineering (KOSORI), using an extreme-high-pressure chamber facility. The embedded system and battery were installed together inside the case. The top housing cap was sealed and was connected to the external pressure sensors as shown in Figure 9.

Table 7 summarizes the test conditions. The test duration was 2 hours. Figure 10 shows a snapshot of the testing in the extreme-high-pressure chamber facility.

After the 2-hour test was completed, the prototype was removed, and its appearance was examined. Figure 11 shows the results of the appearance examination.

When the top housing cap was removed, no leakage was found on the embedded system or inside the case. In addition, the LED light on the embedded system was still working. This means that the embedded system appeared to works properly. To verify the performance of the embedded system, data stored data on the 8-Mb flash memory were extracted and analyzed. Figure 11 shows the results. Figure 12 shows the results of a comparison of data on the pressure on the chamber and the pressure measured by an external sensor. No great difference was observed. The data from the temperature sensor and the humidity sensor were stored as well. The results indicate that the embedded system worked properly during the test. During the test, the inside temperature of the prototype remained stable at between 10.8°C and 11°C, a range of only 0.2°C. This temperature range is appropriate for the operation of the embedded system. The inside humidity of the prototype remained stable at between 10.3% and 10.55%, a range of only 0.25%, which is an appropriate humidity range for operation of the embedded system. The inner pressure of the prototype remained 6 bars during the test. The sealing process brings about this result. The inner pressure is also appropriate for operation of the embedded system.

As oil consumption increases and the price of oil rises to more than $ 100 per barrel as a result of the economic growth of emerging markets such as those of China and India and depletion of oil from land-based sources, oil wells in onshore, offshore, and especially subsea areas become increasingly productive for oil companies. Because a subsea oil production system has to be operated safely for 20–30 years, the market for IRM services and subsea data logger systems will grow steadily for some time. In addition, the safety of subsea oil production will be more strictly enforced in coming years because of the accident that occurred in the Gulf of Mexico in 2010. Therefore, a Korean company that develops a subsea data logger system can have a chance to enter another blue ocean market, even though the market for subsea equipment is very conservative as other researches already succeeded [11, 12]. Because of the extreme high pressures to which subsea equipment is subjected, many people have thought that the development of a subsea data logger system would be difficult. The results of this study demonstrate the feasibility of developing a subsea data logger system based on the prototype described in this paper.