PRODUCTION • PROCESSING • HANDLING
Ragnvald Graff explains the importance of umbilicals in the offshore oil and gas sector
VITAL CONNECTIONS
U
mbilicals are an essential item of equipment for subsea oil and gas developments. Tey 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 over 200km and possibly even longer. Te 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
Te 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. Tere may be additional elements – for example, fibre- optic cables for monitoring purposes are increasingly being incorporated. Te 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. Te 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
Te 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.
Tis is the case when the host facility is a floating system. Te 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. Tese 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. Te bottom of a J-tube, as the name suggests, curves round towards the horizontal. Te 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 that are joined with a transition joint.
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