Ragnvald Graff explains the importance of umbilicals in the offshore oil and gas sector
Umbilicals are an essential item of equipment for subsea oil and gas developments. They provide the link from the host facility through which control is exercised, power transmitted and utilities such as injection chemicals supplied to the subsea wells.
An umbilical is a long, flexible construction consisting of tubes, cables, armouring, fillers and wrapping contained within a protective sheath.
Typically, umbilicals range in length from a few km, to well over 100km – but future projects are expected to see umbilicals reaching lengths of well over 200km and possibly even longer. The multiple functions it performs, the extreme reliability required and the demanding environment in which it is installed and operates, make an umbilical an extremely high-tech product.
Current market developments – the advance of exploration and production into ever deeper waters, the increasing length of step-outs and the growing number of satellite developments from mature fields – indicate that in coming years umbilicals will play an even more important role in offshore oil and gas production.
Power, control and service requirements
The most common form of umbilical contains electric cables for transmitting power and control signals, and high, medium or low-pressure tubes for carrying hydraulic liquids to control valves and chemicals for injection into the well or pipeline. It is therefore known as an electro-hydraulic umbilical. There may be additional elements – for example, fibre-optic cables for monitoring purposes are increasingly being incorporated.
The umbilical host is normally another offshore installation, a fixed platform or a floating platform or ship (FPSO). However, it may also be a shore-based facility.
One element is placed at the core of the umbilical – it may be a power cable, a tube, or a bundle of tubes. The other elements are laid up around the core, if necessary in more than one layer. Fillers are placed between the elements to provide a stable construction, and a plastic sheath may be applied as an outer layer. Once manufactured, the umbilical is wound onto a turntable or reel on the installation vessel, which then installs it by unwinding it and lowering it into the sea.
Static and dynamic umbilicals
The length of an umbilical that lies on the seabed is referred to as the static section. And the part of the umbilical running from the host facility through the water column to the seabed is known as the dynamic section - if it is free-hanging. This is the case when the host facility is a floating system. The dynamic section is subject to substantial forces that do not impinge significantly on the static section, such as waves and current, and, in the case of a floater, the motions of the platform or vessel itself. These factors have to be taken into account during design and engineering of the umbilical.
If the dynamic section hangs freely between the platform and the seabed, the configuration is known as a free-hanging catenary. Different installation configurations that provide support for the umbilical in the water column may also be used and, in general, the complexity and severity of the requirements for dynamic umbilicals increase with increasing water depth. Dynamic umbilicals used in very shallow waters are sometimes even more challenging as the movements imposed on the umbilical are usually more severe and abrupt.
In the case of a fixed platform, the umbilical is usually installed and supported in a J-tube that runs from the surface to the seabed and provides protection against sea motions. The bottom of a J-tube, as the name suggests, curves round towards the horizontal. The section of the umbilical installed in the J-tube is considered to be static. Dynamic and static sections may be manufactured as one unit, or as two sections which are joined with a transition joint.
Thermoplastic hoses and steel tube umbilicals
Originally, only thermoplastic hoses were used to carry fluids. But in some kinds of service they have specific weaknesses – for example, they can deteriorate when exposed to some types of chemical, have a limited pressure capability and can be subject to collapse when used in deep waters. In the early 1990s, therefore, Nexans pioneered the alternative use of super duplex steel tube that eliminates the potential performance problems that might be encountered with thermoplastic hoses by offering good corrosion resistance and chemical compatibility with most chemicals, as well as good collapse resistance and a high strength-to-weight ratio. Some customers specify other tube materials, for example zinc-clad, seam-welded high-strength, low-alloy steel.
Nexans umbilicals and submarine power cables are manufactured at the firm’s purpose-built Halden factory in Norway, which offers a totally integrated operation from engineering and steel tube welding to qualification, termination and testing. It can manufacture laid-up umbilicals with diameters of more than 250mm and to date has supplied well over 1,800km of umbilicals.
A special capability offered by Halden is the application of layers of steel armour for additional protection or to add weight. After passing through the armouring machine the next stage in the process is either the extrusion of an outer polyethylene sheath or application of a polypropylene wrapping with bitumen added as part of the yarn to provide good corrosion protection. All the termination work and factory acceptance testing is performed at the factory.
The finished umbilical is stored on one of a variety of turntables. From here it is transferred directly to the cable vessel docked at the factory’s deep-water quay.
A good example of umbilicals in action is the West Nile Delta Taurus Libra project in Egypt where Nexans delivered 48km of static umbilicals to BP, and partner DEA. The umbilicals consisted of electrical and fibre optic cables as well as hydraulic and chemical lines.
BP’s Shaz Deniz field in Azerbaijan also uses umbilicals supplied by Nexans. For the next stage of the project, BP Exploration (Delta) has ordered 100km of static umbilicals and associated accessories. This will help BP and partner DEA develop the Giza, Fayoum and Raven gas condensate fields. Umbilicals also play a key role in the Greater Enfield development, off the coast of Exmouth, Western Australia. The project, worth around 20 million euros, will be Nexans first to be installed in Australia.
Submarine power cables and installation
The market for submarine power cables is currently very active and Nexans serves this market with turnkey and supply contracts. For installing submarine power cables, Nexans is the owner and operator of the C/S Nexans Skagerrak - one of the world’s most advanced cable-laying vessels, with a state-of-the-art global positioning system and multiple cranes. Not only is it equipped with a 7000 tonne capacity turntable but also has the ability to repair submarine power cables. Skagerrak provides a platform for CAPJET - an in-house trenching system developed for trenching umbilicals, power cables and fibre-optic cables as well as pipelines based on the water jetting principle. The water jet system is used to create the trench and to propel the trenching machine. It is based on Newton’s third law of motion – every action has an equal and opposite reaction. The propulsive force or thrust from the reaction that is created when the stream of water is forced out through the rear side of the Capjet. A solid track record of approximately 10,000km of trenched product has so far been achieved by the three Capjet machines operated by Nexans.
Oil and gas exploration and production is moving towards harsher environments, such as deeper water further from shore with longer subsea tie-backs. This is causing new potential flow assurance challenges for operators, particularly in deep water where low temperature and high pressure at the ocean floor increases chances of hydrate and wax formation, blocking the flow of oil and gas.
It is common to prevent hydrate and wax formation by injecting chemicals into the well stream but this is not always the most beneficial method seen from a technical, cost and environmental perspective. Valuable space topside and logistics for the chemical processing equipment is very costly. To find a better solution, Nexans, Statoil and the research organization Sintef created the DEH (direct electrical heating) solution – an economically and environmentally friendly solution. By transmitting electric current through the flowline, DEH heats the flowline from ambient temperature above the temperature at which hydrate and wax formation begins. This prevents a blockage and maintains fluid temperature during shutdowns. A great example of this in action is at the Shaz Deniz field, located in the Azerbaijan sector of the Caspian Sea. This is the world’s most challenging and largest DEH project to date, with a total of 130km of the cable system supplied to provide flow assurance for 10 subsea flowlines.
The DEH solution has been applied to 30 flowlines worldwide. Nexans has supplied 24 of them to projects for Statoil, Chevron and BP in Norway, Angola/Congo Brazzaville and Azerbajan - six new systems will be installed in the next two years.
Umbilicals and submarine power cables play a key role in subsea oil and gas developments.a Nexans is committed to the future development of umbilical technology through a combination of design, engineering and manufacturing capabilities with rigorous testing, installation and commissioning.
Ragnvald Graff is with Nexans Norway