The subsea connections include flowline connections and umbilical control connections. The types of subsea flowline connection may be divided into four types of connections: welded, flanged, clamp hub, and mechanical connections. The welded connection is normally used for subsea connections in very shallow water, and the welding procedure is carried out in a one-atmospheric chamber to achieve a dry environment.
File:Cross Section of Bolted Flange.png
Cross Section of Bolted Flange
File:Misalingment and Swivel Flange Assemblies of Taper-Lok Type.png
Misalingment and Swivel Flange Assemblies of Taper-Lok Type

For the other three types of subsea connections, the primary purpose of the connection method is to create a pressure-tight seal that resists the loads associated with subsea environments. The connection between the sealine and the connection point is generally made after sealine end alignment is complete. The mechanical connectors use either a mandrel or hub style interface, an the actuating tools are hydraulically or mechanically actuated. For deep water, all seals experiencing hydrostatic pressure should have bidirectional capability. In the following subsections we discussed several types of sealine connectors that typify the numerous options available.

Bolted Flange

The flanged connection utilizes metal gasket designs to allow the flanges to make face-to-face contact. The most typical flanges used are either APl 6A/ 17D [7] type or ANSI/ASME B16.5 type flanges, and the design of bolted flanges is covered in ANSI/ASME B16.5 and ISO 13628-4 [9]. They are commonly used in shallow water (<1000 ft) with the aid of a diver. The designs make use of metal ring joint gaskets, which compress when the bolts are tightened. Special consideration should be given to these gaskets for underwater applications. Some gaskets tend to trap water behind the gasket when made up underwater, resulting in improper sealing of the gasket and flange connection. The gasket ring grooves should be specified with welded inlays to provide a corrosion-resistant surface finish. Welded inlays are not relevant when corrosion-resistant alloy (CRA) materials are used for flanges.

A bolted flange has the following characteristics:

  • Bolts and nuts are used to preload two flanges.
  • A metal gasket is compressed between the two flanges to create a seal.
  • It is commonly used at shallow diver depths.
  • It can be made up with specialized ROV-operated tools.
  • Flanges must be closely aligned before makeup.
  • Swivel flanges can account for rotational misalignment.

Clamp Hub

A clamp hub connector is similar in principle to the bolted flange connector. Clamp hub connectors may use the same metal ring gaskets as bolted flange connectors or use proprietary gasket designs. The clamping device forces the mating hubs together as the clamping device is tightened.

Rotational alignment is unnecessary since the mating hubs do not have bolt holes, except for multibore hubs. On the other hand, most clamped hubs do not permit the amount of initial misalignment that bolted flange connections may provide.

Clamp hubs are often used for subsea connections, because they are small and have a reduced number of bolts to handle, install, and tension. They can, however, be difficult to disconnect on occasion. Since the clamp hubs are pulled together by the tensioning of the bolts (wedging the clamp halves on the tapered hub profile), residual stress and friction resists removal of the clamp halves, even with the bolts removed. Divers often have to hammer or use a special tool, to strip the clamp halves of the hubs, particularly if they have been subsea for a considerable length of time. The characteristics of clamp hubs are summarized as follows:

  • The clamping device preloads two hubs as the device is tightened.
  • A metal gasket is compressed between the two hubs to create a seal.
  • It can be made up by divers or ROV-operated tools
  • It is faster to make up than a bolted flange because it has fewer bolts.
  • Hubs must be closely aligned before makeup. Rotational alignment is not critical.

Collet Connector

Collet connectors are the most widely used for jumper spool style connections. The connection principle is similar to that for a hub and clamp except that a series of longitudinally segmented collets replaces the clamp. The collets are activated by an annular locking cam ring. The cam ring slides axially along the collet length to either open or close the connector. The driving angle on the cam ring is typically self-locking to prevent incidental unlocking of the connector.
File:Pair of Collet Connectors.png
Pair of Collet Connectors

The engagement between the hub and collets is either mechanically or hydraulically actuated. Funnels or guide posts provide coarse alignment for the mating hubs. After the collet connector is landed, an ROV installs the hot stab and provides hydraulic power to the stroking cylinders, pulling the hubs together at a controlled rate. The open collets provide initial makeup alignment to protect the metal seal. The standard design accommodates up to 2 angular misalignment and 1.5–in. axial offset. Once the hubs are mated, hydraulic collet actuating cylinders close the collet fingers to complete the connection. The collets impart a preload that gives the connection the same strength characteristics as the pipe. The seal is pressure tested after makeup via the ROV control panel.The characteristics of collet connectors are summarized as follows:

  • Collet segments are driven around a connector body and hub by an actuator ring.
  • A metal gasket is compressed between the body and hub to create a seal.
  • It functions by means of integral hydraulics or by an ROV-operated tool.
  • It can align misaligned hubs.
  • Rotational alignment is not critical.

Dog and Window Connector

Dog and window flowline connectors are similar in principle to standard wellhead connectors where a number of “dog” segments are collapsed into a corresponding groove in the mating hub to lock the connection. The dogs are captured by “windows” within the connector assembly. The number of “dog” segments varies between manufacturers and can be as simple as a longitudinal cut in a fully circumferential ring. A locking piston is driven forward to collapse the dogs, setting the seal, and preloading the connection.

Its characteristics are summarized as follows:

  • The locking dogs held in a window of the connector body are driven inward around a hub by an actuator ring.
  • A metal gasket is compressed between the body and hub to create a seal.
  • It functions by means of integral hydraulics or by an ROV-operated tool.
  • Rotational alignment is not critical.

Connector Design

The sealine connector choice and subsequent design should consider factors such as water depth, intervention method, type of connection point, sealine installation method, and misalignment tolerance compatibility with the alignment
File:Dog and Window Connector.png
Dog and Window Connector
method. In addition, the choice and design of the connector can be influenced by the factors discussed next.

Connector Stresses

Residual stresses in the line itself and the sealine connector resulting from a particular alignment method and the additional axial movement required for end connection should be analyzed in conjunction with operating stresses to determine if the combined stress is within allowable limits.

Makeup Requirements

Makeup requirements for connectors should be reviewed to ensure that

(1) the connector will deform or deflect the gasket to result in a seal;

(2) there is enough preload in the connector to offset the installation and operating loads, which could otherwise break the gasket seal; and

(3) there is enough axial clearance and access for seal replacement. The sealine connector design should be such that after makeup it will not lose its sealing capability under cyclic pressures, temperatures, or natural vibration loading and design external loads.


The sealine connectors should be tested in plant to the hydrostatic test pressure stipulated for the sealine. In some cases, the connector may be part of and tested with the subsea facility. In such cases the connector should be tested to the hydrostatic test pressure stipulated for the subsea facility. If TFL (through flow line) is specified, each made-up connector should be drifted in accordance with API RP 17C [11]. Additional in-plant testing may be required to verify makeup preloads, fit, and functional performance of locking devices and hydraulic actuation devices.

The end-connector equipment is designed to provide some testing means to verify that the gasket has formed an adequate seal and the connector has been fully actuated or clamped together after it has been installed in subsea. In the evaluation of the connection methods, the following factors should be considered:

  • Sealing reliability;
  • Ruggedness or resistance to damage;
  • Ease of recovery;
  • Installation cost;
  • Template piping interface fabrication difficulty;
  • Resistance to operating loads;
  • Initial equipment costs.

To choose the connection method, the criterion is assigned to each factor according to its importance. Each connection method is ranked against the other methods on a scale from one to three within each criterion.


[1] FMC Technologies, Subsea Tie-In Systems,

[2] T. Oldfield, Subsea, Umbilicals, Risers and Flowlines (SURF): Performance Management of Large Contracts in an Overheated Market; Risk Management and Learning, OTC 19676, Offshore Technology Conference, Houston, Texas, 2008.

[3] G. Corbetta, D.S. Cox, Deepwater Tie-Ins of Rigid Lines: Horizontal Spools or Vertical Jumpers? 2001, SPE Production & Facilities, 2001.

[4] F.E. Roveri, A.G. Velten, V.C. Mello, L.F. Marques, The Roncador P-52 Oil Export System Hybrid Riser at an 1800m Water Depth, OTC 19336, Offshore Technology Conference, Houston, Texas, 2008.

[5] Technip, COFLEXIP Subsea and Topside Jumper Products,

[6] Cameron, Cameron Vertical Connection (CVC) System,

[7] American Petroleum Institute, Specification for Subsea Wellhead and Christmas Tree Equipment, API Spec 17D (1992).

[8] American Society of Mechanical Engineers, Pipe Flanges and Flanged Fittings, ASME/ANSI B16.5 (1996).

[9] International Organization for Standardization, Petroleum and Natural Gas Industries - Design and Operation of Subsea Production Systems - Part 4: Subsea Wellhead and Tree Equipment, ISO 13628-4, (1999).

[10] B. Rose, Flowline Tie-in Systems, SUT Subsea Awareness Course, Houston, 2008.

[11] American Petroleum Institute, TFL (Through Flowline) Systems, second ed., APIRP-17C, 2002.

[12] E. Coleman, G. Isenmann, Overview of the Gemini Subsea Development, OTC 11863, Offshore Technology Conference, Houston, Texas, 2000.