SUBSEA UMBILICAL — HEATING AND COOLING CYCLES OPTIMISED TO EXTEND SERVICE LIFE

SUMMARY

We supported our client by carrying out complex thermal analyses on production control umbilicals, designed for the Moho Nord Project. Our client wished to understand the effects of the venting of a particular gas through a casing vent line at both low and high temperature and how this might affect the other sensitive components of the umbilical. The results pinpointed some adverse effects when venting for extended periods. We developed an optimum heating and cooling duty cycle driving the operational envelope of the umbilical to mitigate these effects.

SITUATION

We worked with our client, a globally renowned, manufacturer of subsea power cables and umbilicals for the Oil & Gas and Renewable industries. We supported the team carrying out complex thermal analyses on production control umbilicals designed for the Moho Nord project off the West coast of Africa. Total’s Moho Nord project is a deepwater exploration and production development and is the largest oil project ever undertaken in the Republic of the Congo. Our client approached us because of our extensive experience and capability in carrying out transient thermal analyses.

CHALLENGE

For this particular project, the client required the venting of a particular gas through a casing vent line at both low (-50 degrees Celsius) and high temperature (85 degrees Celsius). They were interested in understanding how this high and low-temperature venting would affect the temperature of other sensitive components of the umbilical. This understanding, through the use of transient thermal assessments, would indicate potential failure points and areas susceptible to thermal ratcheting.  The risk of thermal ratcheting could then be mitigated by venting for certain durations and at specific intervals during operation; we would test these during the modelling work.

SOLUTION

For both high and low temperature venting an initial steady state thermal assessment was carried out to obtain the umbilical operating temperature without the venting temperatures. We used these thermal profiles as the initial temperature distribution for the transient analyses. For the high and low-temperature venting, the casing line temperature was rapidly increased to 85 degrees Celsius and to -50 degrees Celsius respectively. The temperatures were held there for a specified period and then ramped down to 34 degrees Celsius for hot venting and up to 19 degrees Celsius for cold venting and allowed to cool and heat respectively for a given time period. This heating and cooling were repeated nine times to assess the propensity for thermal ratcheting which could increase the vulnerability of the umbilical to the effects of thermally induced fatigue. The results pinpointed particular hoses that were adversely affected by venting for long periods and allowed for an optimum heating and cooling duty cycle to be developed. All analyses were carried out in 2D using ANSYS APDL input decks to automate post-processing procedures.

BENEFITS

• More reliable performance and hence decreased risk.
• Longer service life and hence reduced cost.
• Analysis inputs and outputs driven by ANSYS APDL scripts to increase the speed and efficiency of post-processing and reporting