Umbilicals

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There are various types of subsea umbilicals, including for remote operated vehicles (ROVs), communications, control of and supply of chemicals to subsea production systems, and supply of power to subsea processing systems. This article discusses the last two types. ROV umbilicals are discussed in a separate article.

What is a subsea production umbilical?

An umbilical is a bundle of tubes and cables that

  • provides hydraulic power to subsea control systems
  • provides electric power and signal to subsea control systems
  • delivers chemicals for subsea injection at the subsea christmas tree or downhole
  • can provide bulk methanol
  • can provide gas for gas lift

An electrohydraulic subsea control umbilical tyically supplies electrical power and multiplexed signal, hydraulics, and chemicals to one or more subsea control modules (SCMs) controlling subsea christmas trees and/or other elements of a subsea production system; and signals from subsea instrumentation back to the control center.

A direct hydraulic control umbilical provides direct hydraulic control of each valve on a subsea christmas tree, through a bundle of tubes from the topsides hydraulic power unit (HPU) to the subsea tree. No electrical power or signal is required. Actuation of valves is by the supply of hydraulic power to the relevant tube. This is accomplished by opening the relevant valve on the manifold located on the topsided HPU. This type of umbilical is limited to subsea production systems with short offsets to the host and few trees.

An electric power umbilical supplies the large amounts of power (up to 100KV) required to operate subsea boosting and processing systems. They may or may not include wires for multiplexed control systems and instrumentation, and tubes for hydraulics and chemical injection.

Umbilicals may be classified by the materials used for the tubes, i.e. steel-tube thermoplastic.

Umbilicals or portions of umbilicals may be classified by whether the riser portion is supported for the full length (static)or is suspended in the water column from a floating structure (dynamic)

Umbilicals are typically terminated subsea by an umbilical termination assembly (UTA), which allows for the distribution of hydraulics, chemicals, and electrical signal and power through flying leads connecting the UTA to the subsea production system.

Steel Tube Electrohydraulic Umbilical

Simple, static umbilical- few wells, shallow to medium depths

SimpleSSField25%.png SimpleEHUmbilical25%.png Media:SimpleEHUmbilical.png (Click on link to see full-screen version of image)

The subsea field in the image (top left) is a small shallow water development consisting of:

  • three wells
  • a manifold with a pigging loop
  • jumpers from the wells to the manifold (ROV-installed)
  • dual pipelines terminating in pipeline end termination skids (PLETs)
  • jumpers from the PLETs to the manifold
  • an umbilical terminating in an umbilical termination assembly (UTA)
  • electrical and hydraulic flying leads from the UTA to each subsea well

The platform to which all this ties back is a bottom founded jacket-and-pile structure. The umbilical is therefore pulled from the sea bottom to the deck through a tube, or is otherwise supported throughout its entire ascent. I tis a static umbilical.

Because it is static, and because the field is relatively small, a simple umbilical like the one depicted in the image (above right) would work.

A simple electrohydraulic (EH) umbilical will have the following components:

  • steel tubes to deliver chemicals and hydraulic power to the subsea production system. There may be one or more backup tubes for use in case of tube failure. Tubes may be sheathed in LDPE to achieve the proper cross-sectional geometry.
  • one or more electric cables (usually quads of 4 to 10 square mm)to deliver power and multiplexed signal to the subsea control system, and to carry instrumentation signals back to the surface control station.
  • fillers to help create the proper cross-sectional geometry.
  • tape which is wrapped around the elements as they are bundled and twisted into an umbilical. This holds the entire assembly together until it is passed through the extrusion head to receive its HDPE sheathe.
  • HDPE Sheathe

Complex, dynamic umbilical- deepwater, multiple wells or drill centers

ComplexSystem50%.png SteelTubeUmbilical33%.png Media:SteelTubeUmbilical.png Armored40%.png

The subsea field pictured in the image (above left)is a large, complex field A larger, more complex EH umbilical may have additional features:

  • cables will be armored for dynamic applications, at least in the dynamic zone above the touchdown point
  • steel tubes may be sheathed for wear protection where they are in contact with other steel tubes in the dynamic zone
  • steel rods or cable may be added to achieve the correct weight-to-diameter ratio to avoid clashing in the dynamic zone with other risers hung off nearby
  • armor may be added for weight-to-diameter ratio, tensile strength and crush-resistance during installation (above right cross-section)
  • fiber optics may be added in place of or in addition to signal cables
  • the umbilical may be bundled in two or more passes, necessitating additional layers of tape

Special case- Kvaerner

AkerUmbilical20%.png

Kvaerner builds umbilicals slightly differently than other bundlers:

  • the pitch of the helical wind is about 2% rather than the usual 6-7%
  • the tubes and cables are held in a matrix made up of reeled elements which lock together as they are bundled into the umbilical
  • carbon rods may be included in umbilicals designed for very deep water, to provide axial stiffness without adding significant weight


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