Subsea production systems

Subsea production systems are an alternative development option for an offshore field. They are often a very cost effective means of exploiting small fields which are situated close to existing infrastructure, such as production platforms and pipelines. They may also be used in combination with floating production systems. 

Subsea production systems provide for large savings in manpower as they are unmanned facilities. However, these systems can be subject to very high opex from the well servicing and subsea intervention point of view as expensive vessels have to be mobilised to perform the work. As subsea systems become more reliable this opex will be reduced. 

Various types of subsea production systems are being used and their versatility and practicality is being demonstrated in both major and marginal fields throughout the world. 

As subsea production systems are remote from the host production facility there must be some type of system in place which allows personnel on the host facility to control and monitor the operation of the unmanned subsea system. 

Oil and gas exploration and production in the Gulf of Mexico (3,200 platforms, 75 MODUs, 33,000 miles of pipeline, subsea production systems, wide range of support equipment), offshore California (23 production platforms,... 

ISO 13628-2 2000 Petroleum and natural gas industries — Design and operation of subsea production systems — Part 2 Flexible pipe systems for subsea and marine applications... 

Offshore deep-water oil and gas fields are increasingly developed with subsea production systems. By subsea production systems, we understand production (X-mas) trees, templates, manifolds, pumps, separators, and associated equipment that are placed on the seabed. 

A subsea production system is usually tailor-made for a specific application and for a specified service life. The service life is determined from the reservoir properties and the production plans and may, for example, be io = 10 years. Experience has shown that the initial service life estimates have been too short. Improved technology, new methods for enhanced oil recovery and tie-in of neighboring wells often lead to an actual service hfe that is significantly longer than the initially planned service life. 

The following situation is typical At some time t < to, the operator has to decide whether to tie-in a neighboring well to the existing subsea production system or to install a new system. The tied-in well and possibly enhanced oil production from the old wells will increase the planned, total service life to 2. The question is then Can we, at time /], trust that the existing subsea production system will survive and function satisfactorily till time /2 Or, should the current system be discarded and replaced by a new system either now (at time ij) or in the near future (i.e., around time to)7... 

Aven, T. Pedersen, L.M., 2014. On how to understand and present the uncertainties in production assurance analyses, with a case study related to a subsea production system. Reliability Engineering and System Safety 124(2014) 165-170. 

It is usefiil to consider the case of an installation of a subsea gathering system for a natural gas production field. The pipeline design for a new gas production facility consisted of 20 cm diameter subsea gathering lines (flow lines) emptying into a 19 km, 50 cm diameter subsea transmission gas pipeline. The pipeline was to bring wet gas from an offshore producing area to a dehydration facility on shore. The internal corrosion was estimated to be 300-400 mpy. The corrosion mitigation options considered were (i) carbon steel treated with a corrosion inhibitor (ii) internally coated carbon steel with a supplemental corrosion inhibitor (iii) 22% Cr duplex stainless steel (iv) 625 corrosion-resistant alloy (CRA). The chance for success was estimated from known field histories of each technique, as well as the analysis of the corrosivity of the system and the level of sophistication required for successful implementation (Table 4.42). 

The case-study is based on an actual sand erosion issue present in an offshore oil production system. Material degradation due to processes of erosion/corrosion is the main focus. The production system is located subsea and connected to a spread-moored FPSO (Floating Production Storage and Offloading) unit, which is used as a hub, processing the fluids produced from the subsea wells. 

A simplified example of subsea HIPS is shown in Figure 8.25. It is intended to protect the production system in case of overpressure in the production pipeline. 

SubseaMaster (ExproSoft) Components in subsea oil/gas production systems... 

In 1986 when the oil price crashed to 10 a barrel, operators began to look very hard at the requirements for offshore developments and novel slimline, reduced facilities platforms began to be considered. The reduced capital outlay and early production start up capability, coupled with the added flexibility, ensured that all companies now consider subsea systems as an important field development technique. Although the interest and investment in subsea systems increased dramatically, subsea systems still had to compete with the new generation of platforms, which were becoming lighter and cheaper. 

Sensors on the tree allow the control module to transmit data such as tubing head pressure, tubing head temperature, annulus pressure and production choke setting. Data from the downhole gauge is also received by the control module. With current subsea systems more and more data is being recorded and transmitted to the host facility. This allows operations staff to continuously monitor the performance of the subsea system. 

Figure 23.37 Floating production and subsea systems for oil exploration. Source Reprinted from US DOI.

For all these phenomena, high-fidelity predictive CFD methods are sought to be applied in connection with experiments as predictive tools. This is true for flow assurance modeling, in particular as to subsea oil production and transport, and in EOR systems, including using steam or Carbone Dioxide injection in wellbores. At the downstream level, say when use is made of the treated gas for energy production, various technical issues still pertain, as in the gas turbine combustion sector, where CFD is also required to optimize fuel injection, atomization and mixing. 

More than 2 million miles of pipeline in the United States are used to transport natural gas, crude oil, petroleum products, and other petrochemicals economically and efficiently over long distances. These steel pipelines can be subjected to corrosive conditions both internally (from the aggressive fluids being transported) and externally (from the a ressive soil or subsea environments). The catastrophic failure of an oil or gas pipeline can result in loss of life and environmental disasters. In United States, more incidents/accidents in pipeline systems are due to human intervention, followed by external and internal corrosion.The U.S. Depart-... 

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