UMBILICAL DESIGN
Contents
Static and Dynamic Umbilicals
Static Umbilicals
The design of umbilicals incorporates mechanical strength to withstand crushing and tensile loads during handling, installation, and service. The umbilical is also of sufficient weight to ensure satisfactory seabed stability. The umbilical and its pulling head/termination design allow for installation into the facility approach. Wall thicknesses of steel tube elements are sized to meet requirements for allowable stresses under all installation and operational conditions. Other design analyses calculate:
- Maximum allowable tension and minimum breaking strength;
- Recommended back tension during lay;
- Strength of terminations;
- The effect of radial loads (collapse pressure);
- The effect of dropped objects and snagging (e.g., ship’s anchor);
- The effect of installation tensioning devices;
- Maximum allowable impact loads;
- Bend radius and bending stiffness;
- Torsional balance;
- Hydrodynamic stability on seabed;
- Environmental loads and hydrodynamic stability of beach approach;
- Material and outer sheathing suitability for onshore applications.
Dynamic Umbilicals
The umbilical system is expected to operate in a static mode after installation. However, the umbilical system will be subject to dynamic loading during installation and to environmental loads in the facility approach. Further, potential unsupported spans along the seabed may be subjected to fatigue owing to vortex-induced vibrations (VIVs). Dynamic and fatigue analysis should be carried out to evaluate fatigue properties of the umbilical system given the anticipated installation and environmental loads and to establish the maximum allowable span lengths. Minimum equired fatigue life may be 10 times design life.
Design
Conceptual Design
Preliminary Cross-Section Sizing
- Tube sizing;
- Possible interaction with vendors.
Preliminary Configuration Design
- Strength, interference, etc.;
- Preliminary component design;
- Early confirmation of feasibility.
Early Identification of Manufacturing Issues
- Issues affecting bid or spec requirements.
Service Environment The umbilical is designed for immersion in seawater for the specified design life. Consideration should also be given to
- Storage prior to installation;
- Exposure to service fluids;
- The seabed and topsides environment in terms of radiation, ozone, temperature, and chemicals;
- Imposed dynamic conditions within the free-hanging regions
- Protection against dropped objects.
Detailed Design
Parameters
- Temperature range;
- Maximum working load;
- Minimum breaking load;
- Minimum bend radius;
- Dynamic service life.
On-Bottom Stability Study The umbilical is designed to be sufficiently stable, when laid on the seabed, for the seabed condition and seabed current values. The behavior of the umbilical on the seabed can be characterized by friction coefficients in the axial and lateral directions
Cross-Sectional Design
One of the initial stages in the design of an umbilical is the placing of the components of the umbilical in the cross-section design. The cross section of an umbilical could include various items such as steel tubes for transporting hydraulic and other fluids, electrical cables, fiberoptic cables, steel ods or wires for strength capacity, polymer layers for insulation and protection, and polymer fillers to fill in the spaces between the components and keep them in place.
Manufacture Design
- Lay-up;
- Sub-bundles;
- Inner sheath;
- Armoring;
- Outer sheath.
An outer sheath is applied as a continuously extruded thermoplastic sheath or as a covering of helically applied textile rovings.
Manufacture
The procedures for umbilical manufacturing should be in accordance with ISO 13628-5 [2].
Lay-Up
Lay-up operations are carried out in a clean, dedicated, controlled area, which is subject to a regular cleaning schedule. Optimum fiber optic cable and hose or steel tube lay-up configurations, fillers, etc., are provided to minimize the overall diameter and weight while meeting the general performance and construction specifications and to ensure good flexibility.
Lay-Up Configuration
The minimum lay angle of the umbilical components should be confirmed. Umbilical components, steel tube, and optical fiber will be laid up in a continuous helix or planetary configuration. If hoses are used an oscillatory cabling technique will be used.
Damaging Pull
The cabling or lay-up is designed so that the individual components will not be strained, deformed, or otherwise affected when the components are subjected to a tensile pull. A table of maximum allowable tension on umbilical components throughout the manufacture process should be summarized and submitted.
Acceptance Testing
After cabling of the umbilical hoses or steel tubes, the following checks are conducted prior to extrusion of the inner/outer sheath on the umbilical. A hydraulic proof test of hoses or steel tubes is conducted after each cabling layer (1.5 times working pressure).
Inner Sheath
The operation is carried out in a clean dedicated, controlled area, which is subject to a regular cleaning routine. The total cabled assembly is protected with an inner sheath extruded tightly over a taped assembly. The extrusion of the inner sheath OD should be monitored in two planes 90 apart.
Outer Sheath
The total cabled assembly is protected with an overall jacket extruded tightly over the assembly. The extrusion of the outer jacket OD is monitored in two planes 90 apart.
Marking
The marking of subsea umbilicals should usually be done according to Section 9.14 of ISO 13628-5 [2].
Main Manufacturers
The following is the introduction of main subsea umbilical manufactures in the world and their productions:
1. DUCO DUCO is a member of Technip Group. The work scope and products of DUCO include:
- Design and verification of umbilicals and hardware;
- Manufacture of thermoplastic hoses in sizes between 1/4” -1.5” ID with working pressure between 3000-12,500 psi;
- Manufacture of steel tubes in sizes between 3/8” -3” ID, materials covers carbon steel, stainless steel, Duplex and Super Duplex;
- Manufacture of opto-electric cables, low voltage cables ranging in cross sections between 2.5-25 mm2; medium voltage cables ranging in size tween 35 mm2 to 400 mm2 with voltage atings up to 36 kV
2. Kvaerner Oilfield Products With manufacturing sites located in Norway an USA, Aker Kvaerner provides subsea umbilicals for more than 15 years and its products include:
- Steel tube umbilicals: from basic direct-hydraulic and electrohydraulic umbilicals to large, integrated service umbilicals; Long continuous manufacturing without transition joints/offshore umbilical connectors;
- Integrated production umbilicals (IPU): the IPU is designed based on the proven technology and qualified for both static and dynamic applications in shallow and deep water;
- Power distribution umbilicals: dynamic and electrical analysis completed, PVC profile and construction technique for increased impact and crush resistance; carbon fiber rod technology used to enhance axial mechanical characteristics; Improved stability compared to conventional armoured power umbilicals.
3. Nexans Since its foundation in 1915, Nexans Norway has been the main supplier of subsea umbilicals in Norway. Nexans has supplied more than 2400 km of umbilical. Some typical products of mbilical include,
- Umbilicals for Shell NaKika project;
- Umbilicals for Statoil Snohvit field, includes 45 km long umbilical from land to subsea and represents a record-breaking length;
- Umbilicals for BP’s King, Atlantis and Thunder Horse projects in GoM.
Verification Tests
Tensile Test
A representative length of the completed umbilical, which takes into account the end effects and pitch lengths of the umbilical components, should be subjected to a two-stage tensile loading program.
Bend Stiffness Test
A representative length of the completed umbilical is subjected to a bend stiffness test procedure.
Crush Test
A sample of the completed umbilical is subjected to lateral loading to allow determination of its resistance to deformation.
Fatigue Test
Mechanical testing is undertaken to determine the fatigue resistance of an umbilical. The test regime is chosen to demonstrate that a particular design or design feature is suitable to withstand the repeated flexures sustained by an umbilical during manufacture, transfer spooling, load-out, I- or J-tube pull-in, burial, and, for a dynamic installation, operational service hroughout the service life.
Factory Acceptance Tests
The following final testing is conducted on the completed umbilical:
- Visual Inspection;
- Electric cable;
- Hoses;
- Tubes.
All test results are recorded and certified by the umbilical manufacturer.
Power and Control Umbilicals
As specified above, the power umbilical is used for supplying electrical power from shore to platforms, between platforms, or from platform to subsea equipment. Supplying power to offshore installations from energy sources onshore makes for smaller and lighter offshore structures, reduced personnel requirements, and lower CO2 emission levels. With this solution any number of installations can be linked and provided with power from an onshore power source. The power supply cable system can be expanded to form a network between offshore fields, providing flexible and safe power utilization for the oil and gas industry. Two types of submarine power cables are to be distinguished: “dry” design or “wet” design, with the former being more reliable but at a higher cost.
- Communication will not be influenced by weather or surface traffic.
- A greater bandwidth is available compared to radio frequencies.
- Optical fibers provide higher data transmission rates.
IPU Umbilicals
The IPU umbilical is designed to combine the function of an umbilical with that of a production or an injection flowline, and also for supplying high voltage power to potential subsea users. The temperature in the flowline is maintained through a combination of thermal insulation and active heating. Typical IPU components and configurations include,
- A large bore central pipe, 6”-12”, to transport well fluid, water, gas or other fluid that is required in large quantities;
- Around the central pipe, an annular shaped PVC matrix to keep the position of the spirally winded umbilical tubes and cables, and provide thermal insulation to the central pipe;
- Embedded in the PVC matrix, but sliding freely within it, the various metallic tubes for heating, hydraulic and service fluids, and the electrical/ fiber-optic cables for power and signal;
- A protective sheath in polyethylene or similar material.
References
[1] R.C. Swanson, V.S. Rao, C.G. Langner, G. Venkataraman, Metal Tube Umbilicals- Deepwater and Dynamic Considerations, OTC 7713, Offshore echnology Conference, Houston, Texas, 1995.
[2] International Standards Organization, Petroleum and Natural Gas Industries, Design and Operation of Subsea Production Systems, Part 5: Subsea Umbilicals, ISO 13628-5, (2009).
[3] Technip Technology and Teamwork Achieve World Class Success for Shell Perdido, Oil & Gas Journal on line, Volume 108 (Issue 31) (April 1, 2010). http://www.ogfj.com/ index.
[4] N. Terdre, Nexans Looking beyond Na Kika to Next Generation of Ultra-deep Umbilicals, Offshore, Volume 64, Issue 3, Mar 1, 2004, ttp://www.offshore-mag. com/index.
[5] O. Heggdal, Integrated Production Umbilical (IPU for the Fram Ost (20 km Tie- Back) Qualification and Testing, Deep Offshore Technology onference and Exhibition (DOT), New Orleans, Louisiana, 2004, December.
[6] Det Norske Veritas, Submarine Pipeline Systems, DNV-OS-F101, (2007).
[7] Det Norske Veritas, Fatigue Strength Analysis of Offshore Steel Structures, DNV-RPC203 (2010).
[8] J. Hoffman, W. Dupont, B. Reynolds, A Fatigue-Life Prediction Model for Metallic Tube Umbilicals, OTC 13203 (2001).
[9] W.K. Kavanagh, K. Doynov, D. Gallagher, Y. Bai, The Effect of Tube Friction on the Fatigue Life of Steel Tube Umbilical RisersdNew Approaches to Evaluating Fatigue Life using Enhanced onlinear Time Domain
