SUBSEA PRODUCTION TECHNOLOGY: Umbilicals: The life support behind critical subsea systems

Umbilical lines are used extensively in the offshore oil and gas industry to convey control and production treatment fluids from the platform to the subsea wellhead. As we move into more marginal field developments and ever increasing water depths, the emphasis is increasingly on remote subsea production systems. Umbilicals are at the heart of the subsea production system. - Multiflex umbilical under test in NEL's dynamic flex-fatigue rig.

Typically, umbilicals have an expected service life of around 20 years. For platform operators, it is vital that umbilicals withstand the harsh environmental conditions to which they are subjected, in addition to the demanding requirements of carrying what are often highly aggressive control fluids. The umbilical is seen as one of the most critical components of the complex subsea system and is the life support for such systems.

The National Engineering Laboratory (NEL) (East Kilbride, UK) has been active for many years in the assessment of both static and dynamic umbilicals and has used its expertise to develop accelerated life testing techniques specifically aimed at verifying whether umbilical lines can deliver the performance demanded by operators. Testing, designed to evaluate thermoplastic lined umbilical hose chemical compatibility, effectively compresses the hose "aging" process into a shorter time period.

The test method involves subjecting hose samples to a cyclic pressure (typically 350 bar range) at elevated temperatures, while exposing them to the fluids that they will carry in service. These tests can run for several weeks or months, during which time the integrity of the hose liner (and if required the outer reinforcing layer) is monitored at regular intervals. Key mechanical properties of the hose material samples are measured periodically throughout the tests to determine whether any deterioration has occurred as a result of chemical attack or some other degradation mechanism. These mechanical tests are augmented by chemical analysis of the test fluids and scanning electron microscopy (SEM) examination of the materials to ascertain its condition and allow comparison with a reference sample.

NEL is regularly commissioned to carry out this type of testing by various operators, hose manufactures (such as Duco and Oceaneering Multiflex), and chemical suppliers. It is seen as a valuable tool which can predict how a particular umbilical/chemical combination will perform in service. It can be used for selecting products for use in a new installation or to evaluate the effects of change in chemicals in an umbilical already deployed in the field.

A further umbilicals research initiative has been established recently via the Postgraduate Training Partnership (PTP) which NEL runs in association with University of Strathclyde in Scotland. Under this scheme, a three-year research project on thermoplastic-lined umbilicals has been launched. This project will focus on evaluating the effect of the diffusion process between the fluid and hose liner upon the mechanical properties of the line material, validating the Arrhenius equation based on the fundamental process within the line material and ultimately developing an improved chemical compatibility test.

Flexing of lines

Chemical compatibility concerns, however, are only one of the many issues that confront the designers and operators of umbilical lines. Flexing of the lines, as we move to more dynamic applications as a result of deepwater developments, is at the root of another potential failure mode. Aware of this issue, NEL has developed the capability to perform full-scale dynamic flexural fatigue testing on umbilicals (or flexible risers) to assess their durability.

The test rig has the ability to apply reverse bending to an umbilical while simultaneously subjecting it to tensions of up to 500 kN (about 50 tons). This facility is currently being successfully used to evaluate the umbilicals destined for use in the Girassol development offshore West Africa.

The offshore oil and gas industry will continue to move into deeper waters. This will result in even greater demands of such a critical item as subsea umbilicals. A greater understanding of the principals involved as a result of pressure, temperature, chemical, and dynamic stresses will allow engineers to face the extensive challenge of designing and developing umbilical systems for ultra-deepwater applications to be found in new deepwater developments around the world. NEL's umbilicals test facilities include:

• Chemical compatibility

1. Cyclic pressure system (up to 690 bar)
2. Four 8,000-liter temperature controlled water tanks (tests up to 80° C)
3. Two explosion rated ovens (tests up to 120° C)
4. 44 separate pressure lines for individual test fluids
5. Hazardous chemical handling facilities
6. Material test facilities include tensile and hardness testers.

• Mechanical assessment

1. Reverse bending Flex Fatigue Rig (tensions of up to 500 kN)
2. Tensile testing (loads up to 30 MN)
3. 160 sq meters "strong floor" for one-off test configurations (loads up to 10 MN).