AHMS has an important role in FPSO design assumptions versu


The relationship between Didier L’Hostis, a naval


architect from TOTAL, and MARIN goes back a long


way – probably getting on for nearly 20 years. Didier explains why TOTAL


decided to support the Monitas Joint Industry


Project and he outlines the steps TOTAL takes to ensure the long-term structural integrity of its FPSOs.


D


idier was involved in many JIPs and used results from MARIN’s deep-water basin on many


projects, even before the start of the FPSO JIP Week, he emphasises. For the last seven years, Didier is well known in his role as chairman of the steering committee for Monitas 1 & 2.


“Naturally we do inspection and mainte- nance on a continuous basis to prevent corrosion and keep the FPSO classification up-to-date. We carry out regular inspections to see if any degradation is occurring.” Typically, water ballast tanks are inspected annually and cargo tanks are inspected once every five years but when an FPSO gets older than 15 years, the frequency increases to once every 2.5 years. “We also have an annual underwater inspection by divers.”


But obviously as Didier points out, an FPSO is an extremely complex structure to design and to fabricate with numerous details so it is not possible to monitor and check every single detail during the design phase. “Therefore we have to work using assump- tions and then take any necessary repairs or action.”


Didier L’Hostis 6 report


This is really where the Monitas JIP comes in, he says. “When we are in the design phase, there are a number of criteria we take into account and we make several assumptions including how we are going to operate the facility.” “But as we all know,” he quips, “there are always surprises when any facility starts operating!” TOTAL and MARIN started the original Monitas JIP in 2006 and it was eventually supported by more than 15 companies. Essentially, the project was aiming to develop an Advisory Hull Monitoring System (AHMS) for FPSOs. The original Monitas idea came when TOTAL commissioned MARIN to generate a specification for a monitoring system on the basis of various sensors for stress monitoring on the hull, he adds, incorporating


wave measurement and motion recording data. “The idea then came that makes Monitas unique in the industry to date!”


Real value “Once we have crunched the numbers from the measurements, we could then compare the results with the design values. But the real value is that this can be done on the spot. If all the number crunching has taken place and then been pre-programmed so the software tools can be available on board, this is very interesting. It is possible to look at regular intervals on line and view exactly what is happening with your installation. We now know what to measure, how to measure and crucially, how to extract the real essence from these measurements.”


The aim in the next few years is that TOTAL has AHMS on all of its new FPSOs. Currently, it has installed AHMS on board the USAN and CLOV FPSO. Further systems are currently being prepared for the INPEX Ichthys FPSO and Total E&P Congo Moho Nord FPU. The pilot AHMS following on from Monitas was installed on the USAN in Nigeria but this FPSO has recently left the TOTAL operation group. The company has around 2.5 years of data however. The second AHMS is in- stalled on the CLOV FPSO that is due start operations this summer.


Didier is looking forward to the time when TOTAL’s facilities all have an AHMS on board. “We will try to take all the information available on the loading side of the equation into account when doing our fatigue analysis: metocean (mainly wave and swells), FPSO draughts, tank contents etc. and use it in our analysis. We use it to compare with the original loading distributions that were assumed during the design phase. We can assess consequences in terms of accumulated fatigue damage and therefore, expected residual fatigue life for the unit. And if it has significantly deviated from predictions, we need to understand why and make an informed decision with this data in hand.


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