The global cable company Nexans recently announced a 10 year framework agreement with BP to supply umbilicals and other accessories and services for various oil and gas projects worldwide. The agreement will run from 2012 to 2022 and will cover the design, manufacture and supply of bespoke umbilicals that will carry fluid, power and control and telecommunication services for new subsea and oil and gas projects together with DEH systems that provide flow assurances in pipelines.
Umbilicals play an increasingly important role in offshore and subsea oil and gas developments. They provide the connection between the host facility through which control is exercised, power transmitted and utilities such as injection chemicals delivered to subsea wells. Indeed current trends in the market – the growing number of satellite developments from mature fields, the advance of exploration and production into ever deeper waters and the increasing length of step-outs – indicate that in coming years umbilicals will play an even more important role in offshore oil and gas production.
Umbilicals are long flexible constructions made up of tubes, cables, armouring, fillers and wrapping contained with a protective sheath. The most common form of umbilical often contains electric cables for transmitting control and power 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.
Given the long lengths they reach, the reliability required and the extremely tough environment in which they operate, an umbilical is a very high tech product. At present they range in length from a few kilometres to well over 100 km. However, future projects are expected to see cable lengths reach well over 200 km.
All umbilicals are custom designed to meet specific customer needs, which entails a structured approach and strict methodology, together with clearly defined design results. There are very few ‘off the shelf’ umbilicals. Nexans is able to draw on an extensive database derived from many years of umbilical and component qualification testing and field experience. The company has also developed UFLEX, a finite element based nonlinear stress analysis tool, in order to increase understanding of the behaviour of umbilicals as exploration moves into even deeper waters, approaching 3000 m.
The potential for damage on or under the seabed means that the design of umbilicals requires not only that they are fit to operate reliably over the course of their design lifetime but that they do not suffer damage from the forces they are exposed to during the installation process. Furthermore, once installed it is important that these cables are protected from other sea activities such as trawling or anchorage.
To overcome the various difficulties inherent in laying subsea umbilicals, Nexans has developed its own in-house trenching system known as Capjet. The Capjet system is used for trenching umbilicals, power cables and fibre-optic cables as well as pipelines. This ROV (remotely operated vehicle) uses a water-jet system both to create a trench and to propel the trenching machine. Capjet fluidises the seabed material to create the trench – the fluidised material then falls back on top of the umbilical as back-filling after the machine has passed. However, the umbilical does not pass through the machine and no forces are directly applied to it - there is therefore no risk of damage to it during the trenching process. The machine is able to trench sandy soil and most clay conditions. Nexans operates three Capjet systems with a total track record of 9000 km trenched to date.
As offshore oil and gas exploration heads for ever-deeper waters and demands and these projects become ever greater, umbilicals will play an increasingly important role in the extraction and transportation projects.
Written by Ragnvald Graff.