There are various types of subsea umbilicals, including for remote operated vehicles (ROVs), communications, control of subsea production systems, supply of power to platforms and subsea processing systems, and transmission of power generated by ofshore wind turbines. This article discusses umbilicals used in subsea petroleum production systems.
What is a Subsea Production Umbilical?
An umbilical is a bundle of tubes and cables that
- provides hydraulic and electric power to subsea control systems
- carries electric signals to and from subsea instrumentation and controls
- 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 typically 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. It also carries 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 topside hydraulic power unit (HPU). This type of umbilical is limited to subsea production systems with short offsets to the host and few trees. This article focuses mostly on electrohydraulic umbilicals.
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.
The above schematic shows the topsides and subsea equipment that may comprise a subsea electrohydraulic control system, including:
MCS- master control system, including a human-machine interface
ROS- remote operating system
HPU- hydraulic power unit
EPS- electric power unit
UPS- uninterruptable power supply
TUDU- topside umbilical distribution system, alternatively
TUTU- topside umbilical termination unit
UTA- umbilical termination assembly
EFL- electric flying lead
HFL- hydraulic lying lead
SDU- subsea distribution unit (where necessary)
SCM- subsea control module
Steel Tube Electrohydraulic Umbilical
Simple, static umbilical for few wells, shallow to medium depths
Media:SimpleEHUmbilical.png (click to enlarge)
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 electrohydraulic (EH) umbilical terminating in an umbilical termination assembly (UTA)
- electrical and hydraulic flying leads from the UTA to each subsea well
The manifold is tied back to a bottom-founded jacket-and-pile platform. Production is routed to the platform from the production trees via the jumpers, manifold, and pipelines. Hydraulic power and electric power and signal are provided from the platform through the EH umbilical to the SCM. The umbilical is installed on the platform by pulling from the seabed to the deck through a tube, or is otherwise supported throughout its entire ascent. It is a static umbilical.
Because it is static, and because the field is relatively small, a simple EH umbilical like the one depicted in the image (above right) is required.
A simple 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 low density polyethylene (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 to hold the entire assembly together until it receives its high-density polyethylene (HDPE) sheathe.
Complex, dynamic umbilical for deepwater, multiple wells or drill centers
The subsea field pictured in the image (above left) is a large, complex field, with multiple wells, arranged in multiple drill centers, each with the elements described in the simple well scenario described above. In this case, the umbilical is suspended from a floating host platform, a semisubmersible. It is therefore a dynamic umbilical. A large, complex electrohydraulic (EH) umbilical may have the following features, in addition to those included in the simple umbilical:
- cables will be armored for dynamic applications, at least from the hangoff point through the wave-affected zone, and through 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 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- Aker
Aker 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 tensile strength without adding significant weight.
Steel Tube Materials
Super Duplex Stainless Steel (SDSS) is the preferred material for deepwater umbilicals because it requires no cathodic protection, is lightweight and strong, has been qualified by major operators and manufacturers for dynamic service, is is usually pilgered rather than seam-welded, and has a successful track record. The most commonly used is 2507, which contains 25% chromium and 7% nickel.
SDSS is duplex stainless steel which, combined with the high chromium and nickel content, give it a pitting resistance equivalent (PRE) of greater than 40, the threshold for use in a seawater environment of less than 60 degrees Celsius without cathodic protection. PRE is calculated for SDSS using the following formula:
PRE= %Cr + 3.3x(%Mo + 0.5x%W) + 16x%N
SDSS is generally more expensive than other materials, and in periods of high activity may add significant time to delivery of the umbilical. Seam welded tubing is available, and is gaining acceptance in some sectors of the market.
Welding of SDSS tubing is problematic, particularly in sizes between 1" and 4" diameter.
Zinc-coated Nitronic 19D is a lean duplex stainless steel, which has a lower chromium and nickel content than SDSS. It is lower in cost than SDSS, but has less tensile strength, and requires cathodic protection, provided by the zinc coating which is extruded onto the outside surface of the completed tube. It has been qualified for dynamic service. It is thicker-walled and heavier than SDSS for the same pressures and water depths, and is seam-welded, but may be used where these risks are acceptable, and lower cost drives the decision. Risks of seam welds may be mitigated using full volumetric inspection, or by adding a spare tube to the umbilical.
Fusion-bonded-epoxy-coated Coiled Tubing is used for larger tubes (typically over 1"), where cost and/or delivery are key drivers. It has a standard epoxy coating to protect from corrosion, a layer of bonding agent, and an HDPE outer coating. It is seam welded. Concerns are corrosion due to holidays in the coating, and leaks in the seam welds.
316L Stainless Steel is sometimes used where expected field life is less than 5 years and pressures required are less than 3000 psi. It is not qualified for dynamic service. Its advantages are low cost and quick or even off-the-shelf delivery.
Where mostly low pressure is required (e.g. to operate the valves on a low-pressure tree), but at least one higher pressure tube is required (e.g. to operate the SCSSV), one or more 19D tubes may be added to a mostly 316L umbilical.
Thermoplastic Electrohydraulic Umbilical
Thermoplastic umbilicals are constructed from thermoplastic hoses which are kevlar-armored and outer-sheathed by the umbilical manufacturer. The umbilical shown in the image at left does not include cables, but could. No assembly machine with counter-rotating bobbins is required to remove torsion from the tubes and cables because they are not helically wound, but oscillated as they are pulled through the closing die. Typically the umbilical must be armored to provide tensile strength, ensure on-bottom stability, and resist crushing loads from the installation tensioner.
High collapse resistance hoses may be included for for umbilicals to be deployed at deeper depths. Collapse resistance is provided by an internal carcass, much like flexible pipe.
Chemicals used for injection downhole or further downstream may be incompatible with the hose material. The umbilical manufacturers have experience with many chemical cocktails, so may have experience with the combination of chemicals the operator plans to use. If not,the operator may commission an accelerated compatibility test.
While not suitable for very deep water, and rarely used in the Gulf of Mexico, thermoplastic umbilicals are still commonly used in shallow water, and where large swells cause fatigue at the touchdown point, such as Eastern Canada, West Africa, and Brazil.
Integrated Service/Production Umbilical
Umbilicals may include large tubes for service, e.g. bulk methanol, or gas lift (Integrated Service Umbilical or ISU, right), or may include a tube for oil or gas production (Integrated production Umbilical, or IPU, left).
Service tubes are typically in the 1-4" range, which is very expensive and difficult to weld in super duplex. These have occasionally led to large cost and schedule over-runs.
Production tubes can reach 10-12" in diameter. While expensive, super duplex tubes over 4" are less problematic to weld than in the 1-4" range.
High Voltage Power Umbilical
Media:HVHybridUmb.png <--Click link to enlarge
High voltage Umbilicals are increasingly used to supply power to subsea boosting pumps, at increasing distances from the host. Typical power requirements for a subsea pump motor may be in the 1-3MW range, and offset from the host has now exceeded 100km.
These umbilicals may be dedicated to power supply, or may include hydraulic and chemical injection tubes, controls power and signal cables, and/or fiber optics.
The magnetic field generated by high voltage cables may interfere with other conductors in the umbilical, particularly where there are more than one 3-phase system, e.g., for powering multiple pumps. Interference from two sets or more of 3 cables may be mitigated by laying up the insulated cores in separate passes, and counter-rotating them.
Umbilical Termination Assembly (UTA), Subsea Distribution Unit (SDU)
A UTA or SDU distributes the hydraulic and electric power and signal from the umbilical to the various subsea control pods. A UTA is typically attached to and installed with the umbilical, while the SDU is installed separately and connected to the umbilical UTA with a flying lead. The choice depends on the complexity of the development, with the SDU being deployed where more electric and hydraulic flying leads are required. The SDU allows for a bigger structure and distribution to more control pods.
In the pictures above, a mockup of a Subsea 7 remote operated vehicle (ROV) is deployed from a crane during the system integration test (SIT) to ensure that during the installation, the actual ROV will be able to pick up, handle, and install the flying leads. While not a fully functional ROV, the mockup is dimensionally correct.
In the picture on the left, the mockup ROV is gripping the bars on the left of the UTA with its left manipulator and is opening the cover over the electrical flying leads with its right. It may now install or remove a wet-mateable electrical connector on the end of an electrical flying lead. The bars and the handle on the connectors are standard interfaces specified in API 17H.
In the picture on the right, the mockup ROV is installing a hydraulic flying lead which it has picked up from the deployment basket. The interface between the SDU and the flying lead is specified in API 17H, as is the interface between the ROV and the flying lead.
A mockup of an Oceaneering Millenium ROV (left) is used to simulate connecting the hydraulic flying lead to an umbilical termination assembly (UTA) during system integration testing (SIT). Plastic vertebrae on the ends of the umbilical and the flying lead act as bend limiters, preventing bending beyond the maximum bend radius (MBR). The hydraulic junction plate is connected to the flying lead on roughly a 45 degree angle to facilitate fly-in by the ROV. This configuration is commonly called a "cobra head."
The orange API 17H "bucket" (right) is the interface between the flying lead junction plate and the torque tool mounted to the flying lead orientation tool (FLOT) on the front of the ROV. Once the hydraulic connectors on the junction plate are aligned (in this case using the probe at the top of the junction plate), the torque tool turns an acme thread which engages with the matching thread on the UTA-mounted junction plate and pulls the hydraulic connectors together.
The image to the lower left is an electric flying lead, terminated in wet-mateable electrical connectors. The ROV grasps the paddle to insert the connector into the receptacle on the UTA, the subsea distribution unit (SDU), or subsea control module (SCM).
The image to the lower right is a photograph of an FMC subsea control module (SCM) mounted on a subsea production tree being tested prior to installation. The wet-mateable electric flying leads are installed directly into the top of the SCM. The hydraulic flying lead junction plate is mated to the corresponding junction plate mounted on the tree. From there hydraulic fluid is conducted to the SCM via tubing connected to quick disconnect connectors on the SCM mounting plate. Note that the API 17H ROV interface ("bucket") and junction plate are installed vertically rather than horizontally, which is customary.
The SCM is the component of the entire subsea production system most likely to fail, so it is set up for easy retrieval to the surface. An ROV disconnects the electric flying leads, and installs a retrieval tool on the SCM. It is not necessary to remove the hydraulic flying leads, since they are not directly connected to the SCM.
Subsea High Voltage Connectors
High voltage wet-mateable electrical connectors which can be installed by ROV have been on the critical path in the development of easily installed, easily maintained subsea boosting and processing centers. Capabilities of some connectors (by manufacturer) are listed below.
22 kV 900A Tronic
6.6 kV 1600A Deutsch
3.3 kV 100A Deutch
36 kV GE
Dynamic Umbilical Design Concerns
Fatigue(Vessel motion, waves, currents, and vortex induced vibration)
- Motions at hangoff point- bend stiffener design
- Tube friction
- Motions at touchdown point
- Vortex induced vibration (VIV)
Interference- clashing with other risers- this may be mitigated by
- weight to diameter ratio
- hang-off angle,
- bend stiffener design
Accumulated Plastic Strain (Strain Budget)- includes strain from
- initial coiling of tubes on bobbins
- bundling of umbilical
- coiling of umbilical on reel or carousel
- repair (retrieval and re-installation)
Dynamic Umbilical Design Process
The design of a dynamic umbilical is an iterative process, and is time consuming. It may even become the critical path, depending on market conditions for tubes, cables, bundling, etc. if not expedited.
The following graphic illustrates the iterative nature of umbilical design.
Manufacturing of Steel Tube Umbilicals
The manufacture of steel tube umbilicals requires that each tube and electrical cable be axially counter-rotated one full turn for each turn it is wound around the axis of the umbilical, in order to relieve the torsion introduced by the helical winding. This necessitates complicated machinery like the horizontal assembly machine shown on the left.
The bobbins containing the tubes and cables in a horizontal assembly machine are mounted on vertically rotating plates (right) lined up as in the image on the left. The central element of the umbilical is pulled through the center of the machine by a tensioner, and the cables and tubes are laid up around it. As the vertical plates rotate, the bobbins rotate in the opposite direction, one complete turn for every turn of the plate.
At the end of the process, the assembled umbilical is pulled through a closing die, taped, and coiled on a reel or carousel.
Fillers used to space out the tubes and cables to make a better geometric pattern for bundling do not need to be counter rotated.
If further tubes and cables are required, the umbilical may be pulled through the machine again, and another layer added.
During the bundling process, the assembly machine will be stopped as bobbins empty and are replaced. Tubes from replacement tubing bobbins must be welded to tubing from the empty bobbin that has already been bundled into the umbilical. Similarly, cable from the replacement cable bobbin must be spliced to cable from the emptied cable bobbin.
Once complete, the bundle is pulled through the extrusion head where the HDPE or other material is extruded onto the bundle to form the protective sheathe. This step is done separately from the bundling process because it is ideally done in one pass to avoid the necessity of "cold starts", where hot extruded sheathe material must be bonded to material which has hardened while the bundle has been stopped.
When an umbilical is bundled on a vertical machine (lower left), the bobbins holding the tubes and cables are loaded onto counter-rotating cradles on a horizontally rotating carousel. The central member is pulled by a tensioner up through the center of the carousel, and along with the tubes and cables on the bobbins, through a closing die, a taping head, and down to reel or carousel.
As with a horizontal machine, the entire bundle may be pulled back through to add further elements, or it may be pulled through the extrusion head to add the HDPE or other sheathe.
Wire armoring machines (right) are typically used on thermoplastic umbilicals to provide tensile strength, collapse resistance, and on-bottom stability. However, an armor layer may be applied to a steel tube umbilical to add tensile strength for deep installation and hangoff, or to achieve a desired weight-to-diameter ratio to avoid clashing with pipeline risers and other umbilicals hanging off the platform nearby.
FAT, Applications, SIT, Storage
As the sheathe is extruded onto the umbilical it may be stored to a reel or carousel. The picture at left shows several umbilicals stored on a carousel that has been divided by posts inserted in postholes arranged circumferentially around the deck of the carousel. The size of the storage compartments can be varied by using different sets of circumferential post holes. The picture at right shows an umbilical stored on a standard 8.6 meter reel.
At this point the umbilical is ready for factory acceptance testing (FAT). Tubes and cables are tested and accepted (or not) by the client. A lack of electrical continuity or a leaking or plugged tube can cause lengthy delays, because it is impossible to find the exact location without opening the sheathe, and spreading out the components, often multiple times. Repairs must then be done to both the defective components and the overall umbilical.
Once the umbilical passes FAT, applications, or termination activities begin. The umbilical termination head (UTH) and the mudmat, jointly known as the umbilical termination assembly (UTA), is connected to the umbilical at the subsea end. The umbilical manufacturer will weld the umbilical tubes to the tubes entering the UTH, and the contractor supplying the electrical splice enclosures will make the electrical splices. The entire umbilical assembly will now be ready for system integration testing (SIT). At this time other equipment such as the subsea distribution unit (SDU) or surface-controlled subsurface safety valve (SCSSV) may be incuded in an expanded SIT using the flying leads (picture at right).
The point of final acceptance by the client will depend on terms negotiated between the umbilical contractor and the client for loadout.
The picture upper right shows a loadout of reels lifted by crane, and a carousel which was loaded onto the installation vessel empty, with the umbilical reeled on after. If the carousel and crane are both capable, the loaded carousel may be lifted onto the installation vessel.
The center picture shows an umbilical being loaded onto an umbilical installation vessel which has a carousel permanently installed below the main deck.
The picture on the right shows the special case of the Duco plant off the Houston Ship Channel. The umbilical will be loaded onto a barge and transported out the Ship Channel and if necessary along the Inland Waterway to the port most convenient for the installation vessel. In the picture, the carousel is already loaded onto the barge. The umbilical stored on the permanent carousel (at the right of the picture) will be reeled through the plant and onto the barge.
At Duco, umbilicals on standard 8.6 or 9.2 meter reels can be lifted onto the barge with a rented mobile crane.
The vessel at the top left is the same vessel shown in the previous section on quayside loadout. It is a multi-service vessel (MSV) that has been adapted for installation of umbilicals. Note that the reels and the carousel are both lined up toward the overboarding chute on the port side. The first reel is fitted with a TR-40 winch, which has a standard interface to the 8.6 meter reels. Once that umbilical is installed, the winch will be moved back to the next reel in line.
The smaller vessel (top right) is also an MSV, loaded for installation in shallower and calmer water. Note the rudimentary overboarding chute at the stern. As the reels are emptied, thy can be moved by the onboard cranes. No winch is visible, so it is assumed that the reels are driven by underrollers.
The large vessel (left) has a j-lay tower over a moonpool, and presumably below-decks carousels. This setup is capable of installing large umbilcals in deep water.
Again, the vessel shown in the quayside loadout and installation vessels sections. The umbilical is routed overboard and down through the installation tower/chute (top left), which preserves the minimum bending radius (MBR).
The umbilical is restrained and lowered using the tensioner (top middle), which takes the full weight of the umbilical. The treads must be long enough to spread out the crushing load such that the maximum allowable crushing load of the umbilical is not exceeded.
The pressure of the umbilical tubes is monitored during installation to ensure their integrity after laydown (top right.)
To the left, an umbilical bend stiffener is the first end installed. Here it is overboarded through the chute. A cable will be lowered through the I-tube through which the umbilical is to be pulled into the platform, and attached to the umbilical pulling head at the top of the stiffener. As the umbilical is lowered over the side of the installation vessel, the cable will be pulled in until the top of the bend stiffener meets the bottom of the I-tube and connects with it. The cable will continue to be pulled in until the breakaway bolts that connect the stiffener to the pulling head break, and the umbilical slides up through the stiffener and the I-tube and is captured on deck.
The video monitor on the right is showing an ROV installing a hydraulic flying lead on an umbilical termination assembly (UTA) or subsea distribution unit ( SDU). We can see the flying lead, cobra head, ROV manipulators, and the bucket/junction plate. We can also see a dummy junction plate, used to protect the hydraulic connectors when a flying lead is not connected, and a dummy hot stab, used to protect the hot stab point when not in use.