A subsea distribution system (SDS) consists of a group of products such as umbilical and other in-line structures that provide communication from subsea controls to topside. This article describes the main components of the SDS currently used in subsea oil/gas production, and defines its design and the functional requirements of the system.

Hydraulic System

The following main parameters need to be determined for the hydraulic system:

  • Reservoir sizing;
  • The time to prime the hydraulic system from a depressurized state;
  • Opening and closing response times of the process valves under conditions of minimum and maximum process pressure;
  • The time for the pressure to recover following a process valve opening;
  • The time to carry out a sequence of valve openings, such as the opening of a tree (neglecting choke valve operation);
  • The stability of opened control and process valves to pressure transients, caused by operation of the other control and process valves (sympathetic control valve de-latching, process valve partial closing, etc.);
  • Response time to close process valves in the event of a common close command, such as an emergency shutdown at the surface, venting off hydraulic control valves via supply lines;
  • The time to vent the umbilical hydraulic supplies;
  • The impact that failure of subsea accumulation has on the safe operation and closure of the process valves;
  • The extent of control fluid leakage rate that can be accommodated by the system;
  • System response times for simultaneously opening and closing multiple

choke valves.

Hydraulic Flying Leads and Couplers

HFL Assembly (Courtesy UNITECH)

The HFLs are made up of three main components: the tubing bundle, the steel bracket assembly heads, and the MQCs. The bundle is terminated at both ends with an MQC plate, which is used to connect the HFL to the trees, or the UTA. These assemblies have padeyes for handling and deployment purposes.


Tubes are bundled and jacketed for protection and to prevent kinks in the tubes. End terminations from the tubes to the MQC assemblies and to the couplers are welded and NDE (None Destructive Examination) examined. Termination assemblies are structurally sound and capable of withstanding all transportation, installation, and operation loads.

Connection Plates

The hydraulic flying leads are supplied with MQC plates designed for a number of couplers. All tubes and coupling assignments need to match UTA and tree assignments. All couplers and the HFL are capable of withstanding full design pressure and test pressure. The design takes into consideration the fact that all couplers will be energized to a full test pressure of 1.5 design pressure during a FAT (Fabrication Assemble Testing) proof test. Design of all MQCs and couplers also considers the potential for leakage due to the external pressure being higher than the internal pressure. All seals are compatible with the chemicals, methanol, and hydraulic control fluid used. Hydraulic couplers are leak free in the unmated condition with high pressure of full working pressure or low pressure of 1 psi.

MQCplates on the HFL haveROV-operable attachment means in order to connect to the inboard MQC plates at the UTAs, trees, and manifold. Makeup at depth must be achieved with all positions pressurized or only one side pressurized. Breakout at depth must be achieved with no positions pressurized. No special tooling is required for installation or removal of the terminations. Hydraulic couplings must be qualified for the duty and be capable of mating and unmating at worst case angles after coarse alignment, without failure. The MQC must be capable of mating and unmating 50 times on land and 30 times at design water depth without seal replacement and without leakage when made up and subjected to internal pressures between 0 psi and the system working pressure, and maximum hydrostatic pressure. The following potential misalignments should be considered in MQC design:

  • Linear misalignments: 1.5 in. in each direction;
  • Axial misalignment between male and female connector limited to 3 degrees;
  • Rotational misalignment limited to 5 degrees.

MQCplates are outfittedwith an emergency release device.The design also prevents the MQC plates from engaging if they are not aligned properly. The couplers are not engaged prior to proper alignment. The emergency release mechanism makes provision for the separating forces necessary at depth. MQC plates are visible for ROV inspection. This visibility must be capable of indicating full engagement of the MQC plates and means to verify that installation was properly made. It must be possible to inspect for leaks during operation.


All circuit paths in theHFL are proof tested to 1.5maximumdesign pressure. HFLs are visible to the ROV at the design water depth and in installed configurations. HFL assembly design incorporates the means for offshore deployment. The design also incorporates the capability for HFLs to be lifted and installed by ROVs subsea. Maximum weights in air and water (for empty and filled tubing) and any buoyancy/flotation requirements must be defined. Methods for handling, installation, and retrieval and for providing any necessary permanent flotation must be specified. Offshore test/verification procedures and long-term storage procedures must be provided.

All assembly drawings, list of materials, and interface drawings to ROV operation and installation must be provided. The type of fluid that is required inside the HFL during shipping and deployment must be defined. Storage fluids must be compatible with chemicals and hydraulic fluid. HFLs are outfitted with MQC protection caps during shipping. The maximum allowable pull on a termination and the minimum bend radius for assembled HFLs must be specified. HFLs have clear permanent markings visible to an ROV during design life at service subsea.

Hydraulic Couplers

Female Hydraulic Couplers
Male Hydraulic Couplers

The hydraulic couplers for deepwater applications need a spring strong enough to seal against the external pressure head in order to prevent seawater from contaminating the hydraulic fluid, and need to be designed such that only a low-pressure force is required for makeup.Couplings are available that require a low force for makeup, and these are available with dual redundant resilient seals and with a combination of resilient and metal seals.

For deepwater applications a fully pressure-balanced coupling is preferable. These are fairly new concepts, but are available in single coupler pair design and also the four coupler hydraulic circuits design. The single pair is a resilient seal with a porting design that allows for inherent pressure balancing across the poppets, which provides pressure assistance to the spring closure force when the coupling is disconnected. The design uses a combined metal and resilient seal arrangement acting in a shear seal arrangement as the coupler mates and disconnects. Both designs use the principle that the flow path is radial and hence produces no resultant separation force. It is important for hydraulic couplers to have adequate flow paths to ensure that adequate hydraulic response times are achievable.

The poppets are balanced to prevent them being driven hydraulically from the central open position and sealing against one of the seal faces. The female couplers are usually assembled into a hydraulic jumper stab plate so that they can be retrieved and the seals replaced if necessary. The female couplers are assembled so that they are floating on the plate to allow for any manufacturing tolerances. The backs of the connectors have to be terminated by screw-on hose termination couplers or by screw seal or welded assemblies for the termination of steel tubing. Joint Industry Conference (JIC) hose terminations, which swage inside the central core tube of a standard thermoplastic hose, can only be used when the hose can be maintained full of fluid of a specific gravity equal to or similar to the specific gravity of seawater. This is to prevent collapse of the hose in deepwater applications.

Alternatively, high collapse resistance (HCR) hose with a spiral flexible metal former under the core tube must be used. The flexible inner core is designed to withstand the external seawater pressure and to prevent the hose core from collapsing. The HCR hose requires a different type of coupling that has a welded construction. The metal former inside the coupler slides inside the spiral hose support and seals by swaging onto the outside of the thermoplastic liner. When stab plates are densely populated, it can be difficult to turn and orient all of the hoses through 90 degrees and into the hose/cable restraint. Right-angled connectors are used to orient the hoses into the clamp. It may also be necessary to have these connectors of stepped heights in order to allow hose makeup and to avoid tight bends or kinking of the hoses.

  • The couplers include inboard and outboard MQC plates that provide the mechanism for mating, demating, and locking multiple coupler connections within a single assembly.
  • The hydraulic coupler system is configured to ensure that the replaceable seals are located in the hydraulic flying leads.
  • The design of the hydraulic system should consider water hammer, high-pressure pulses, and vibration on couplers. This includes external sources, for example, chokes.
  • Where high cyclic loads are identified, the design and manufacturing should be reviewed to mitigate associated risks, for example, the use of butt-weld hydraulic connections.
  • The designs minimize ingress of external fluid during running and makeup operation.
  • The couplers are designed for reliable and repeatable subsea wet mating under turbid environmental conditions.
  • The couplers have a minimum of two seal barriers to the environment unless the barrier is seal welded.
  • All chemical and hydraulic circuits within the same component are rated to the same design pressure.
  • The couplers are designed for operation and sealing under the maximum torque and bending moment applied to mated couplers through MQC engagement and misalignment.
  • The couplers have metal-to-metal seals with elastomeric backup seals.

Elastomeric seals must be compatible with the operating fluid.

  • Couplers are furnished with necessary protection equipment in order to protect the equipment when being unmated and in-service and to prevent calcareous buildup and marine growth.
  • Poppet couplers are to be used on all hydraulic and low-flow chemical services.
  • Poppetless couplers are to be used on full-bore and high-flow chemical injection lines to reduce pressure losses and eliminate trash buildup through the poppet area (i.e., methanol supply and annulus vent lines).
  • Spare umbilical tubes have popped couplers.
  • Consideration must be given to the ability to bleed trapped pressure in poppet circuits when recovered to the surface (i.e., residual operation pressure or head pressure once disconnected from the system).
  • Special consideration is given to scale buildup prior to connection.

Electrical Power System and Communication

The following main parameters need to be determined by means of a system power demand analysis:

  • Voltage at subsea electronic module (SEM) for maximum and minimum SEM power loads;
  • Voltages at each SEM at maximum and minimum numbers of subsea control modules (SCMs) on the subsea electrical distribution line;
  • Voltages at SEM at minimum and maximum designed umbilical lengths;
  • Voltages at SEM at cable parameters for dry and wet umbilical insulations;
  • Minimum and maximum subsea power requirements;
  • Maximum current load;
  • Topsides electrical power unit power factor versus SCM voltage.

A communication analysis is conducted to determine the minimum specifications of SCM and master control station (MCS) modems:

  • Modem transmit level;
  • Modem receive sensitivity;
  • Modem source/load impedance.

Electrical Flying Leads and Connectors

EFL Assembly

The EFL connects the EDU to the SCM on the tree. Each SCM utilizes two independent EFLs from the EDU for the redundant power on communication circuits.


The EFL assembly is composed of one pair of electrical wires enclosed in a thermoplastic hose, fitted at both ends with soldered electrical connectors. The assembly constitutes an oil–filled, pressure-compensated enclosure for all wires and their connections to ROV-mateable connectors.


Wires are continuous and are, at a minimum, of 16 AWG. A twisted-pair configuration is recommended. Voltage and current ratings for the wires are sized to not significantly degrade overall circuit performance based on the results from the electrical analysis. Wires are soldered to the connector pins and protected by boot seals of compatible material. Pin assignment matches the system requirements. A hose with low collapse resistance, specifically selected for subsea use, with titanium or equivalent end fittings, connects both electrical connectors of the flying lead, to ensure compatibility of materials used. The length of the wire within the hose is sized to allow for any stretching of the hose up to failure of the hose or the end fittings. Hose stretching does not allow for any pull load on the soldered connections.

Hoses are a continuous length with no splices or fittings for lengths under 300 ft (91m). Any use of splices or fittings is brought forward for approval on a case-by-case basis. Hoses are filled with Dow Corning –200 dielectric fluids. The compatibility of the hoses, boot seals, and wire insulation with the compensating fluid and seawater is confirmed. The wire insulation is a single-pass extrusion and suitable for direct exposure to seawater.

All wires are100 % tested for voids and pinholes by immersion in water hipot. Connectors are marked appropriately to simplify ROV operations. An alignment key or other device is incorporated to ensure correct orientation. The electrical connectors must be qualified for the duty and be capable of making and breaking at worst case angles, before and after course alignment, without failure. They must be capable of making and breaking 100 times under power on the female pin half without any sign of damage to pins or sockets and still remain capable of excluding seawater.


Connectors are provided with shipping protection covers. All assemblies are identified with tags on both ends of the EFL. Tags must be designed such that they do not come off under severe handling onshore, offshore, and during installation and must be visible to ROVs at working water depth. The color of the hose is visible to ROVs when in subsea use. Such colors include yellow or orange. In order to be easily visualized and identified by ROV in underwater situations, the ROV handles and the bottom plastics sleeves of flying leads should be painted with colorful marks according to standards and codes. These marks should be identified throughout the life time of the system.

All EFL assemblies are filled with compensating fluid to a slight positive pressure (10 psi) prior to deployment. Flying leads installed subsea are protected with mating connectors when not in use. These EFLs are temporarily located on parking positions on the E-UTA assembly, at the tree, or on a parking stand installed for that purpose.

Electrical Connectors

Electrical connectors have the following basic requirements:

  • An electrical connector is a termination for electrical cables used to transmit electrical power of low voltage and communication signals between subsea production control system components.
  • Electrical connectors at the very minimum meet all requirements as stated in the latest revision of ISO 13628-4 and ISO 13628-6.
  • The number of electrical connectors in series is kept to a minimum. Redundant routing follows different paths. Consideration should be given to keeping voltage levels as low as practical in order to minimize electrical stresses on conductive connectors.
  • Connectors are either Tronic or ODI.
  • The electrical connector is capable of making wet-mateable electrical connections utilizing an ROV. They are designed and constructed for normal and incidental loads imparted by ROVs during make-or-break operations.
  • It is important to confirm the type of connector halves – whether it is “cable end” type or “bulkhead connector” type.
  • The Christmas tree side has male (pin) connectors and the flying leads have female (socket) connectors.
  • Connectors are configured to ensure that no male pins are powered up while exposed. Electrical distribution systems should be designed such that “live disconnect” is not required during normal maintenance or if possible during failure mode operation or recovery periods.
  • Connectors are furnished with the necessary equipment to protect it from being unmated while in-service and to prevent calcareous buildup and marine growth.
  • Optical connectors for any fiber-optic lines are fitted with long-term protective caps.


[1] P. Collins, Subsea Production Control Umbilicals, SUT Subsea Awareness Course, Houston, 2008.

[2] T. Horn, G. Eriksen, W. Bakke, Troll Pilot - Definition, Implementation and Experience, OTC 14004, Houston, 2002.

[3] International Standards Organization, Petroleum and Natural Gas Industries – Design and Operation of Subsea Production Systems – Part 8: Remotely Operated Vehicle (ROV) Interfaces on Subsea Production Systems, ISO 13628-8/API 17F, 2002.

[4] International Standards Organization, Petroleum and Natural Gas Industries – Design and Operation of Subsea Production Systems – Part 4: Subsea Wellhead and Tree Equipment, ISO 13628-4, 1999

[5] National Aerospace Standard, Cleanliness Requirements of Parts Used in Hydraulic Systems, NAS 1638-64, Class 6, 2001.

[6] International Standards Organization, Hydraulic Fluid Power – Fluids – Method for Coding the Level of Contamination by Solid Particles, ISO 4406, 1999.

[7] Deep Down Company, Subsea Accumulator Module, <http://www.deepdowncorp. com/deepdown/products/sams>.