Umbilical Technological Challenges and Analysis
Contents
Umbilical Technological Challenges and Solutions
Some of the technological challenges are discussed in the following subsections.
Deep Water
The deepest umbilical installation to date (Apr, 2010) is the Perdido 1TOB umbilical in 2,946m (9,666ft) in Gulf of Mexico[3]. Some other deepwater umbilicals in Gulf of Mexico are Shell’s Na Kika project in 2,316m (7,598ft), the Thunder Horse umbilical in 1,880m (6,168ft) of water, and the Atlantis umbilical in 2,134m (7,000ft) of water [4]. A challenge in design is that steel tubes are under high external pressure as well as high tensile loads. At the same time, the increased weight may also cause installation problems. This is particularly true for copper cables because the yield strength of copper is low. In ultra-deep water, a heavy dynamic umbilical may present a problem to installation and operation because its hang-off load is high. For design and analysis of an ultra-deepwater umbilical, it is important to correctly model the effect of stress and strain on an umbilical and the riction effect. Sometimes, bottom compression may be observed for an umbilical under a 100-year hurricane scenario. In this scenario, the design solution may be to use a lazy-wave buoyancy module or to use carbon fiber rods. The use of carbon fiber rods allows umbilicals to have a simple catenary configuration, without the need for expensive, inspection/maintenance demanding buoyancy modules. The carbon fiber rods enhance axial stiffness because they have a Young’s modulus close to the value of steel but with only a fraction of the weight. One of the concerns surrounding the use carbon fiber rods is their capacity for compressive loads. Hence, it is beneficial to conduct some tests that document the minimum bending radius and ressive strength of the umbilical. If the currents an ultra-deepwater umbilical will be subjected to are severe, it might be necessary to use strakes for VIV protection, although the use of strakes has so far not been required. The strakes may, for instance, be a 16D triple start helix with a strake height of 0.25D.
Long Distances
The length for the Na Kika, Thunder Horse, and Atlantis umbilicals is 130, 65, and 45 km, respectively. The longest yet developed is 165 km in a single length, for Statoil’s Snohvit development off northern Norway. One of the constraints on umbilical length is the capacity of the installation equipment. The Nexans-operated installation vessel Bourbon Skagerrak can carry up to 6500 tonnes of cable, which equals a length of 260 km, assuming the umbilical unit weight is 25 kg/m.
High-Voltage Power Cables
The design constraints are the low yield strength of copper, which requires an increasing amount of protection as depths increase, and the weight of steel armoring employed to provide that protection as depths increase. Fatigue of copper cables in dynamic umbilicals is another technical challenge.
Integrated Production Umbilical (IPU)
Heggadal [5] presented an integrated production umbilical (IPU) in whichthe flowline and the umbilical are combined in one single line.The IPU cross section consists of the following elements:
- A 103/4-in. flowline with a three-layer PP coating (its thickness is 4 and 14 mm for the static portion and dynamic portion, respectively). File:IPU Dynamic Cross Section, Supper Duplex Flowline (5).pngIPU Dynamic Cross Section, Supper Duplex Flowline (5)
- Around the flowline, there is an annular-shaped PVC matrix that keeps in place the spirally wound umbilical tubes and cables and provides thermal insulation to the flowline.
- Embedded in the PVC matrix, but sliding freely with it, are the various metallic tubes for heating, hydraulic, and service fluids, the electrical/fiber optic cables for power and signals, and the high-voltage cables for powering the subsea injection pump.
- It has an outer protective sheath of polyethylene 12 mm thick.
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To qualify a new design concept like this, a series of analysis and qualification tests were conducted [5]:
1. Analysis
- Global riser analysis and fatigue analysis;
- Corrosion and hydrogen-induced cracking assessment;
- Thermal analysis;
- Structural analysis (production pipe, topside and subsea termination);
- Reeling analysis;
- Electrical analysis;
- Reel/trawler interaction and on-bottom studies.
2. Basic Tests
- Mechanical material tests, fatigue, corrosion, etc.
3. Fabrication Tests
- Fabricationand closing test;
- STS injection test;
- QC tests and FAT;
- Pre/postinstallation tests.
4. Prototype Tests
- External hydrostatic test;
- Impact test;
- Model tensioner test;
- Reeling and straightening trials;
- Stinger roller trial;
- Repair trial;
- Vessel trial;
- System test;
- Dynamic riser full-scale testing.
Extreme Wave Analysis
An important aspect of the umbilical design process is an analysis of extreme wave/environmental conditions. A finite element model of the ambilical is analyzed with vessel offsets, currents and wave data expected to be prevalent at the site where the umbilical is to be installed. For example, in the Gulf of Mexico, this would include an analysis for a 100-year hurricane, 100-year loop current, and submerged current. The current and wave irections are applied in a far, near, and cross condition. This analysis is used to determine the top tension and angles that the hang-off location of the umbilical is likely to experience. These values are then used to design an adequate bend stiffener that will limit the umbilical movements and provide adequate fatigue life for the umbilical. Design analysis based on extreme wave analysis includes: 1. The touchdown zone of the umbilical is analyzed to ensure an adequate bending radius that is larger than the minimum allowable bending radius. It is also important to check that the umbilical does not suffer compression and buckling at the touchdown zone. 2. A polyurethane bend stiffener has been designed to have a base diameter of x inches, and cone length of y ft. This design is based on the aximum angle and its associated tension, and maximum tension and its associated angle from dynamic analysis results using the pinned finite element odel. 3. The maximum analyzed tension in the umbilical was found to occur at the hang-off point for the 100-year hurricane wind load case when the essel is in the far position. 4. The minimum tension in the umbilical may be found to occur in the TDP region for the 100-year hurricane wind load case when the vessel is in the near position. 5. The minimum bend radius (MBR) is estimated over the entire umbilical, over the TDP region and the bend stiffener region, respectively. They are to be larger than the allowable dynamic MBR. 6. The minimumrequired umbilical on-seabed length is estimated assuming it is subject to the maximum value of the extreme bottom tensions.
Manufacturing Fatigue Analysis
A certain amount of fatigue damage is experienced by a steel tube umbilical during manufacturing, and this needs to be evaluated during fatigue analysis. The two main aspects of umbilical manufacturing fatigue analysis that require attention are accumulated plastic strain and low cycle atigue. These are explained next.
Accumulated Plastic Strain
Accumulated plastic strain is defined as “the sum of plastic strain increments, irrespective of sign and direction” in DNV-OS-F101 and DNV-RP-C203 [6,7]. Accumulated plastic strain can occur in the steel tubes of an umbilical during fabrication and installation. The accumulated plastic strain needs to be maintained within certain limits to avoid unstable fracture or plastic collapse for a given tube material and weld procedure. Accumulated lastic
strain is the general criteria used by umbilical suppliers to determine whether the amount of plastic loading on the steel tubes is acceptable. An allowable accumulated plastic strain level of 2% is recommended for umbilical design.
Low Cycle Fatigue
The umbilical steel tubes are subject to large stress/strain reversals during fabrication and installation. Fatigue damage in this low cycle regime is calculated using a strain-based approach. For each stage of fabrication and installation, the fatigue damage is calculated by considering the contributions from both the elastic and plastic strain cycles. The damage calculated from low-frequency fatigue is added to that from in-service wave and VIV conditions to evaluate the total fatigue life of each tube of the umbilical.
In-Place Fatigue Analysis
The methodology used to assess wave–induced, in-place fatigue damage of umbilical tubes can be summarized as follows:
- Selection of sea-state data from a wave scatter diagram;
- Analysis of finite element static model;
- Umbilical fatigue analysis calculations;
- Simplified or enhanced approach;
- Generation of combined stress history;
- Rain flow cycle counting procedure or spectral fatigue damage;
- Incorporation of mean stress effects in histogram.
The first three items of fatigue analysis mentioned above are described in the following three subsections. The main difference between fatigue analysis for an umbilical and a SCR is the effect of friction when the tubes in the umbilical slide against their conduits and each other due to bending of the umbilical. The methodology discussed here for umbilical in-place fatigue analysis is based on two OTC papers [8,9]. In-place fatigue analysis is required to prove that the fatigue life of the umbilical is 10 times the design life.
Selection of Sea-State Data
The wave scatter diagram describes the sea-state environment for the umbilical in service. It is not practical to run a fatigue analysis with all of the sea states described in awave scatter diagram. Hence, the usual methodology is to group a number of sea states together and represent these “joint sea states” with one significant wave height and wave period. The values of the wave height and wave period are chosen to be conservative. This methodology results in the reduction of the wave scatter diagram to a “manageable” number of sea states (say, about 20 to 50). This enables the analysis to be carried out in a reasonable amount of time. It is also very important to accurately consider the percentage of time that the umbilical is expected to be affected by these different sea states.
Analysis of Finite Element Static Model
A finite element static analysis is carried out for a model representing the steel tube umbilical. The static solution is used as a starting point for a timedomain or frequency-domain dynamic FEA.
Umbilical Fatigue Analysis Calculations
Fatigue damage in an umbilical is the product of three types of stress. These are axial (sA), bending (sB), and friction stress (sF). The equations defining these stress terms are as follows:
where SDT: standard deviation of tension; A: steel cross-sectional area of the umbilical; E: Young’s modulus; R: outer radius of the critical steel tube; SDk: standard deviation of curvature. The critical steel tube is the tube in the umbilical that experiences the greatest stress. This is usually the tube with the largest cross-sectional area
and furthest from the centerline of the umbilical. The friction stress experienced by the critical tube is the minimum of the sliding friction stress (sFS) and the bending friction stress (sFB). This is based on the theory that the tube experiences bending friction stress until a point is reached when the tube slips in relation to its conduit. At this point the tube experiences sliding friction stress..png){kind=link}
where m: friction coefficient; FC: contact force between the helical steel tube (defined below); At: cross-sectional area of the critical steel tube within the umbilical; RL: layer radius of the tube (this being the distance from the center of the umbilical to the center of the critical steel tube). where T: mean tension; f: tube lay angle (this being the angle at which the tubes lie relative to the umbilical neutral axis); EItube: individual tube bending stiffness; LP: tube pitch length/2.
References
[1] R.C. Swanson, V.S. Rao, C.G. Langner, G. Venkataraman, Metal Tube Umbilicals- Deepwater and Dynamic Considerations, OTC 7713, Offshore Technology 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, 010). 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, p://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 Conference 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 Nonlinear Time Domain Methods, OTC 16631, Offshore Technology Conference, Houston, Texas, 2004.
