Operators have been installing deepwater risers in the Gulf of Mexico and off the coasts of West Africa and Brazil for over 10 years now.
Steve Hatton, riser specialist, 2H Offshore, has been involved in providing these critical structures from the beginning and, despite participating in many prominent jointindustry research projects, he freely admits that he still has much to learn.
“The more you understand about risers, the more you realise what you do not know,” he says. “They are fatigue driven and their structural responses are highly complex. In my opinion, riser design remains one of the most interesting and challenging areas of offshore engineering there is.”
Riser failure is simply not an option. As well as causing massive health, safety and environmental issues, failure inevitably results in lost production and a costly replacement exercise. For these reasons, the industry has tended to adopt a conservative approach to riser design.
Operators were slow to embrace the notionof riser monitoring, as Hatton explains: “Monitoring equipment used to be expensive and unreliable. Also, you often had to strip any protective coatings applied to the riser to fit the sensors, and you could not discount the possibility of the equipment actually degrading the response of the structure. In addition there is the chance of the monitoring system hindering installation of the riser, so you can see why people tried to avoid following this route.”
So what has changed to cause the interest in riser monitoring among operators over the last couple of years? According to Hatton, a series of things: advances in monitoring technology that have reduced the cost and improved the reliability of the available systems; demonstration projects that have proved monitoring is capable of highlighting genuine operational threats; concerns over the severity of the hurricanes in the Gulf of Mexico; and, not least, the current industry focus on risk-based integrity management.
Instrumentation is obviously a prime ingredient of any monitoring system – you must have robust, reliable and accurate sensors capable of measuring the motion of the riser or the strain induced in it. But just as important is knowing where to locate the sensors and how to interpret the mass of data they generate.
“A multidisciplinary approach is essential,” says Hatton. “As well as having knowledge of the instrumentation, you need skills in data sampling, transmission and storage, and signal processing. You also require a deep understanding of riser structural responses, plus experience of riser design engineering, construction, installation and operation.
One of the fundamental advantages of riser monitoring is that it reveals whether the assumptions made during the riser design process were valid. This check is easily carried out by comparing the responses measured at selected places on the structure with the predictions from the original riser analysis. The operator will hope to see a negative ‘delta’ (actual results lower than predicted) to confirm the validity of the design.
“Giving the operator confidence that his riser was up to the task was the first aim of most riser monitoring exercises,” says Hatton. “The information also provided valuable input to the design of the next structure. But what operators really want these days is information that enables them to manage their immediate operations and maintenance more effectively – they are seeking input to risk-based integrity management programmes. For this application, you really need to know the peak stresses in the riser and the fatigue life at the most critical locations along its length.
“Unfortunately, predetermining these critical locations is not easy, as they are affected by the installation process and may shift over time with operational changes and vessel/platform movement. In any case, you might not have access to the critical regions owing to the existence of riser guides, rollers, etc. If you add the fact that the stress gradients on a riser can often be large, you can see it is not hard to miss the areas of peak stress on a structure, even if you install a high density of gauges in the potential problem areas.”
2H has overcome this challenge by a sophisticated back analysis process in which response data measured at a series of discrete locations along the riser is fed into an analysis package to compute the global structural response of the riser. It is then possible to calculate the fatigue properties of the riser along its entire length and to highlight the precise points where problems are most likely to develop.
2H’s monitoring know-how, experience and development record made it a strong candidate to support BP when the operator decided in 2005 to set up an integrity management system to cover all its risers in the Gulf of Mexico, thought to be the first exercise of this kind in the world.
The riser integrity management process follows established risk-based principles and starts with the identification of potential failure modes and threats. Each of these is assessed in terms of the probability of it occurring and its consequences. From this assessment, the operator assigns an overall criticality to each potential failure event. The operator then estimates how much confidence there is in the assessment by, for example, judging how well the failure mode is understood, how predictable it is and how effective the chosen riser monitoring techniques are at detecting the threat. By combining the criticality analysis with the confidence ratings, the operator is able to formulate a riskbased inspection and monitoring plan.
Hatton stresses the value of starting the risk assessment process early. “Ideally, you need to carry out the risk assessment during the design and construction phase. Engineering input and information about the inevitable construction-led design changes are readily available then, and the people involved have not yet moved on to the next project.”
Riser monitoring has come a long way since 1994, the year 2H installed its first system; this is due in large part to the many technical advances made in electronics, data management and signal processing. But it has also been important, certainly in Hatton’s opinion, to integrate knowledge in these areas with expertise in riser analysis and engineering.
“The effort we have made to bring these skills together at 2H has paid dividends, not only in commercial terms but also from the viewpoint of improving the industry’s understanding of these complex structures,” concludes Hatton. “Having been consumed by this subject for the past 12 years, I am probably a little biased; however, I believe that riser design and monitoring provide the key to field developments in ultradeep water and, therefore, will have an impact on the future of the offshore oil industry itself.”