fibre optic cable: the next big thing for subsea control? · pdf filecopyright © 2010...

Copyright © 2010 SubOptic of 7

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FIBRE OPTIC CABLE:

THE NEXT BIG THING FOR SUBSEA CONTROL?

Antoine Lecroart, Ronan Michel (Alcatel-Lucent Submarine Networks)

Email: <[email protected]>

Alcatel-Lucent Submarine Networks, Route de Villejust, 91620 Nozay, France

Abstract: Is there a unique opportunity for submarine cables combining Direct Current supply and Fibre Optic (DCFO) communications as contributors to the evolution of subsea control in the oil & gas industry? This paper examines how existing technology could be leveraged into this high-value application. A number of subsea functions related to the subsea control (or subsea processing) in oil & gas offshore fields may benefit from the availability of electrical power close to the subsea wells. The fibre optic submarine cable technology initially developed for telecommunication applications can play a big role by bringing sizeable amounts of power to subsea systems while simultaneously effecting dependable communications between the templates and topside units. Based on field proven products representing hundreds of thousands of kilometres of cable deployed, this new approach will potentially allow replacement of hydraulic devices with electrical ones in subsea control systems. While this will not totally eliminate the need for pipes and umbilicals, it could reduce their number or make them simpler and safer.

1 FIBRE OPTIC CABLE: A FIELD

PROVEN PRODUCT The submarine cable industry has gathered experience in excess of 20 years for the design, manufacture and installation of optical cable and related wet plant equipment.

The industry benchmark is a 17 mm diameter cable for deep-water applications, also called lightweight (LW) cable. The deep-sea cable is built around a stainless steel tube, a wire vault, a copper tube swaged onto it and a polyethylene wall.

Steel Tube

Optical Fibres

Composite Conductor

Steel Wire Vault Filling Jelly

Insulating Sheath

Figure 1 : Lightweight (LW) Fibre Optic Cable

This LW cable can be further protected with different types of armoured packages to provide increasing levels of protection, progressing from LW, to Light Weight Protected (LWP), Single Armour (SA) and Double Armour (DA) versions.



Figure 2 : LW, LWP, SA & DA Cable Types

Different types of fibre can be used for different applications, following ITU-T recommendations G.652, G.654 or G.655. The fibres are submarine specific and are procured with a 1.8% screen test in order to guarantee excellent cable mechanical and optical characteristics in service. Fibre ageing is also a guaranteed parameter at 0.005dB/km over the 25 year design life of the system.

While this cable design represents the best mechanical protection for the fibres, the subsea control applications will be using another key feature of the cable: its ability to transport DC power using the sea water as the return path. The copper conductor in the cable is normally used to feed the wet plant equipment along the cable (repeaters) but can also be used to provide power locally at some given locations. The cable electrical insulation is qualified to sustain a 12kV DC operational voltage for 25 years, and is compatible with the use of electroding fault detection methods. Table 1 below gives the main electrical characteristics our standard telecom fibre optic cable.

Composite conductor resistance at 10°C

≤ 1.0 or 1.6 Ω/km

DC. resistance temperature coefficient

4.10-3/°C

Insulation between composite conductor and water

> 105 MΩ.km

Dielectric strength between composite conductor and water

> 45kV DC for 5 min

Nominal capacitance in sea water 0.18 µF/km

Table 1 : Fibre Optic Cable - Electrical Characteristics

2 DCFO OFFSHORE APPLICATIONS

2.1 Concept This single conductor submarine cable technology can be used to connect subsea control systems, either by tiebacks to a platform or with a step-out from land direct to a field.

Subsea control represents the system required for the safe and reliable operation of the various subsea valves to control either the production or injection wells of an offshore field. Initially this control function was performed with hydraulic systems. They have evolved firstly into electro-hydraulic systems and now AC all electric systems with fibre communications such as the Snøhvit 145km step-out in Norway. However at these distances AC transmission is becoming inefficient, as the capacitance of the cables results in significant losses in the power transport as shown in Figure 3 below.



Figure 3 : DC vs. AC Transport Efficiency

Building on the inherent advantages of the submarine cable design, a “transport” function can be defined based on this DCFO solution to provide multiple subsea users with secure optical communications and an adequate level of local power. Indeed, a number of different users can benefit from this combined power and communications solution. While subsea control comes directly to mind, the evolution of subsea instrumentation related to reservoir monitoring (such as 4D seismic), multi-phase flow metering or flow assurance systems can also potentially benefit from this technology.

As generally the case in the oil & gas industry, redundant A+B paths will be used to provide the proper level of service availability while allowing maintenance of the DCFO transport system. The subsea box acting as the communication and power source to each subsea user is called a DCFO node.

Standard repeatered submarine cable systems have proven DC powering for much longer systems up to 12,000km, and of course can offer communications for equally long distances. As the capability of subsea telecom cables is studied for offshore applications, a number of their networking features become attractive:

• Branching units can distribute power and communications from a single cable to several interfaces within a field. This introduces the possibility of conserving risers by using a single cable for the control of a number of fields;

• Duplicate cables can provide hot standby capability, ensuring high availability, ~ 99.99%, whilst still each supporting multiple nodes;

• Connectivity could be provided for additional subsea nodes on a permanent or temporary basis by provisioning extra fibres and underwater terminations.

Figure 4 : Branching Unit (BU)

The key to rapid progress and successful simple system integration is to develop open interface standards, where possible based on existing terrestrial standards and practices.

For communications it is proposed to adopt the widely used Ethernet standards. This is fairly consistent with the move towards a unified TCP/IP approach for most of the interfaces subsea for either control or instrumentation instead of the vast amount of proprietary solutions that have been used over the years.

The most suitable initial choice appears to be gigabit ethernet (GbE). This offers high capacity that will provide substantial margin for future needs and good availability of high reliability components for practical modem and router implementation. This communications interface will be offered both subsea and topside. Other options are available if more bandwidth is required, for example at the 10GbE level.

Similarly, for subsea power, the provision of a 375V DC supply allows a good balance between efficient power distribution and selection from the wide



availability of components to be used within the control system. Furthermore, provision of power levels up to 10kW will enable a range of subsea valve control systems to operate, including direct valve drive. This will be possible by converting the 10kV DC transported by the copper conductor of the cable into the 375V DC secondary voltage. A Medium Voltage Converter (MVC) will be used to perform this conversion inside the DCFO Node.

Additionally a subsea mechanical ‘interface’ is required between the DCFO transport node and the control system or instrumentation provider(s). Wet-mate connectors may be used for the communications and power, the termination assembly will be a ‘lighter’ version of existing umbilical termination assemblies.

2.2 Proposed Architectures Initially, it is expected that the first applications will be relatively short in length and will not require the use of in-line amplifiers also known as “repeaters”. Figure 5 below gives a possible architecture for such a short haul tieback using DCFO. This architecture ought to be adequate for most of the oil fields where it is not practical to have very long export lines due to the nature of the petroleum that is extracted.

Figure 5 : Unrepeatered DCFO System

Architecture

In other applications where longer tie-backs are required, for gas fields for instance, repeaters can be inserted in the solution to lengthen the optical transmission distance. Figure 7 below

gives a possible architecture for very long tie-backs using DCFO.

Figure 6 : Repeater

Figure 7 : Possible Repeatered DCFO System

Architecture

3 PROOF OF CONCEPT In the summer of 2009, Alcatel-Lucent installed the NEPTUNE Canada submarine network consisting of a cable ring connecting six undersea nodes to the Port Alberni shore station in the North East Pacific off the Vancouver Island [1].

Figure 8 : NEPTUNE Canada Layout

This 850km system was purchased by the University of Victoria (UVic) to conduct undersea science experiments over long periods of times [2]. It demonstrates many of the DCFO capabilities and shows that long distance, networked communications



and power step outs are feasible. The science nodes are gateways where the oceanographers plug their science instruments using ROV wet-mate connectors [3]. Each science port available at the node is able to provide 400V DC with up to 25A of power and an optical GbE connection. Each node is composed of a Trawl Resistant Frame (TRF) where the ROV serviceable node module can be secured.

Figure 9 : NEPTUNE Canada Node and ROV

The node module itself is a mechanical arrangement of two main pressure vessels with syntactic foam made almost neutrally buoyant. The work class ROVs currently found on the oceanographic vessels used by the scientists can handle the node module.

Figure 10 : NEPTUNE Canada Node Module

The two pressure vessels are the Medium Voltage Converter (MVC) and the Low Voltage/Comms (LV/Comms) unit.

4 OPERATIONAL ASPECTS It is paramount to evaluate the hazards that submarine cables can face in order to ensure that the proper choices are implemented for cable protection. External

aggressions can be considered as two categories: the human factors and natural factors. Human factors are mainly commercial fishing, vessel anchors and other seabed activities. Commercial fishing is responsible for over 75% of cable aggression faults and most trawler related faults occur in less than 500m water depth. Natural factors will be sea bed current, marine life, hostile sea bed terrain, earth quakes, underwater landslides, and abrasion. These natural factors usually account for less than 10% of cable faults. Cable burial is the natural remedy against bottom fishing and is a field proven and effective means of protecting the cable against bottom trawling. In stable seabed, cable burial to over 0.75 metres is deemed sufficient against fishing aggression.

Alcatel-Lucent owns and operates a fleet of cable laying vessels equipped with deep sea ploughs capable of installing the cable up to 3m below the seafloor in soft soils. ROVs are also used for trenching or to perform other remedial work when the conditions are such that ploughing cannot be efficient. These DP2 vessels can perform best in class installation and repair operations even in adverse weather conditions.

Figure 11 : Modern DP2 Cableship

The cable repair operations are based on the use of cable jointing techniques performed onboard the cable ship. Alcatel-Lucent has developed as series of jointing boxes using polyethylene over-moulding to repair the cables in any circumstances.



Figure 12 : Deep Sea Cable Jointing Box

Rapid and secure fault location is the key to be able to perform fast repairs. This can be achieved in a couple of ways depending on the nature of the fault. Most faults are shunt faults; where the cable is damaged in such a way that the copper tube is exposed to the seawater thereby creating an electrical path to earth. In this case, the fibres are not damaged and by re-balancing the system voltage at both ends to put the system zero voltage point at the shunt fault location, the system can remain in service until a repair can be scheduled. Electroding is also possible allowing a probe to locate the cable on the seabed.

5 CONCLUSIONS The benefits of moving to a combined all-electric control system combined with a DCFO system include the following:

• No hydraulic operational costs; • Reduced umbilical costs: The umbilical

can be simplified to fluid flow lines, together with this low cost step-out;

• Improved system maintainability: The flow lines and Power/Control lines are independent, the redundant Power/Control lines can be repaired within a few days of mobilisation;

• Extended length capability: Up to 600km or more from shore limited by the powering;

• Huge data capability: GbE, 10GbE are available options, enabling improved real-time applications, such as reservoir monitoring and recovery;

• Multi-field capability from one step-out: Offering potential economies for host platforms supporting many fields.

The mature telecom cable technology can provide for high reliability communication and power distribution in all offshore environments. This technology tackles all the requirements at once:

• Real-time broadband communications • Undersea networking and connection • Extended reach • Environment independency • Future proof • Proven and economic.

The competence to implement such solutions is available from the subsea communications industry, which has provided the infrastructure for the transformation of international telecommunications since the rollout of subsea fibre over 20 years ago. These achievements have been based on seamless design and qualification, comprehensive system integration testing and consistent application of standards.

The value proposition to the oil and gas industry is a new asset management approach for deployment of the fields of the future and enhancement of mature fields alike.

In arrangements similar to the NEPTUNE Canada solution, the cable associated with a subsea DC/DC converter can be the prime vector for new subsea control and instrumentation systems. For this application, specific architectures based on duplicated feeds would be used. Fail-safe mechanisms as well as full supervision of all the functions along the control chain are also mandatory. While a DCFO solution for offshore oil & gas applications would be subject to different constraints from solutions for scientific needs, the proof of concept already exists and the adaptation of the solution to the specific needs of subsea control and instrumentation appears within reach.



6 REFERENCES [1] A. Lecroart, N. Shaheen, P. Shawyer, “Cable Science Observatories Solutions: Bringing Power and Broadband Communication to the Ocean Depths”, SubOptic 2007, Baltimore, USA, Paper TuA2.4.

[2] NEPTUNE Canada Website: http://neptunecanada.ca/infrastructure

[3] P. Phibbs, R. Cook, “NEPTUNE Canada Cable Ocean Observatory - Now a Reality”, SubOptic 2010, Yokohama, Japan.

fibre optic cable: the next big thing for subsea control? · pdf filecopyright © 2010...

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