The compelling case for drilling riser monitoring

April 1, 2011

Pulse Structural Monitoring, a recent offshoot from 2H Offshore, is gearing up post Macondo to provide what it claims is the most comprehensive drilling riser monitoring service on the market, as Bill Clewes reports.
Despite its environmental impact, the blows to people's livelihoods, the government rhetoric and, above all, the loss of life, the Macondo disaster has never really looked like putting an end to deepwater drilling. There is a strong sense that the world needs oil and gas too badly for that.

But nobody wants to go where BP has been this past 12 months – arguably, close to ruin with its reputation severely shredded. One might therefore expect the industry to proceed with considerable caution, certainly in the Gulf of Mexico, as the pace of deepwater drilling picks up. It would be surprising if casing design, cementing procedures and blowout preventer inspection and testing were not top of most drilling project agendas. That said, attention is likely to be paid to every aspect of the drilling process, from the rig right down to the reservoir.

Though the Deepwater Horizon's drilling riser seems to have played no direct part in the Macondo incident, there are some in the industry who have been arguing for years that more needs to be done to monitor the behaviour of drilling risers, the crucial link between the surface and the seabed, especially given the industry's move into ever deeper waters.

The arguments sound compelling. Drilling risers are incredibly dynamic and complex structures with elaborate, flexible interfaces on the rig and at the wellhead. They are subject to a range of sometimes extreme environmental and operational forces. They are prone to vortex-induced vibration (VIV) and accumulate fatigue in several other ways besides. They are manhandled relatively regularly for connection and disconnection. And, finally, by the nature of their design, they provide the sole barrier, a single steel wall, between the high-pressure well fluids and the outside world.

Early resistance
This is not a revelation, of course. The industry has long appreciated the critical nature of these structures. Yet, it has been slow to embrace the idea of riser monitoring, as advocated particularly strongly by riser specialist 2H Offshore.

Richard Kluth, managing director of Pulse Structural Monitoring, which was spun out of 2H last year to concentrate on developing the company's business interests in this area, is clear about why drilling riser monitoring has encountered the resistance that it has. ‘Some of the early systems were expensive, impractical, insufficiently robust and unreliable,' Kluth concedes. ‘Further, the data generated often stayed as data; it was not always turned into the sort of information that could immediately improve the lives of either the driller or the field operator. There was a perception that riser monitoring was R&D, and expensive R&D at that.'

Kluth's assessment is refreshingly candid, when coming from someone who is trying to sell riser monitoring systems. He can afford to be so because his company has been making the case for simplicity over sophistication for years, all the time attempting to overcome the practical and technical failings that have blighted the market. Significantly, others have now also joined the battle. A genuinely competitive market for riser monitoring systems is developing, with some of the industry's big names, such as Schlumberger, Fugro, Kongsberg and Sonardyne, seeking a share of the action.

Even though Pulse's parent company 2H can justifiably claim to be the pioneer of riser monitoring (the company began installing monitoring devices on risers of different kinds as long ago as 1998), Kluth is far from being complacent and knows it will take a concerted effort to stay ahead of serious players like these.

‘To win over the drilling community to the advantages of riser monitoring and to stay ahead of the competition, you need to offer the whole package. As well as the monitoring hardware, you have to be able to provide a system design and installation service, and also the analysis and visualization software necessary to turn the data into meaningful information presented in ways that suit different client's needs,' he says.

Among the criteria for success, according to Kluth, are simple, accurate, robust and well-packaged sensors. They need to be easy to install at critical points on the riser and certainly must not interfere with its running and retrieval or normal drilling operations; if they do, the chances are they will be destroyed before they even enter the water. Given the harsh surroundings on the rig and in service, there also needs to be a degree of redundancy built into the system, something that takes considerable practical experience to engineer effectively.

When it comes to the generated data, the first problem is getting it back to the surface. The early monitoring systems generally employed autonomous sensors that had to be returned to the surface to download their data, which is still fine today if the interest is in long-term performance, notably changes in fatigue life, or if the object is to validate design assumptions for the benefit of future projects.

Physical retrieval can be avoided by using acoustic modems to transfer data or ROVs with suitable stab connectors to engage the sensors. However, if the aim is to have a constant, reliable data stream back to the rig, the best solution at the moment is to hard wire the sensors. Looking ahead, Kluth sees this solution being replaced by data telemetry through the structure of the riser itself.

It is possible to measure a wide range of data related to the performance of the riser these days (see panel above), which raises the question of how to make sense of it all. ‘The data is of little use without the processing capability to provide users with meaningful parameters that can be compared with key operational limits,' explains Kluth. ‘Ideally, the results of this process will be displayed on a single screen, with a simple traffic light system to indicate the status of the riser within in its overall operating envelope. In addition, there will be audible alarms to indicate when key parameters are about to exceed preset limits. It needs to be easy for the operator to respond quickly and decisively when something begins to happen that potentially compromises the integrity of the drilling riser.'

Modular system

The criteria just outlined have been centre stage during the lengthy development of what Pulse calls its DrillAssure service offering, a modular system to aid the integrity management of drilling risers. At the centre of the offering is a real-time drilling operations module. This uses vessel position and motion data, metocean readings, upper and lower flex-joint angles and tension in the region of the lower marine riser package (LMRP) to provide the optimum position and heading of the drilling rig (a warning is provided if the rig is in danger of drifting off position) plus information with which to control the riser tensioning system. Most importantly, it provides a constant picture of where the riser is sitting in its defined operating window. This is an invaluable aid to making prudent connect and disconnect decisions, which are key to both maximising drilling uptime and ensuring the safety of the drilling operations.

Pulse has supplied numerous modules like this. Clients include BP, for monitoring completion risers in the Thunder Horse field, in the Gulf of Mexico, and Murphy, for monitoring the drilling riser aboard its Azurite FDPSO vessel, working offshore the Republic of Congo.

ATP Oil & Gas's Titan drilling and production spar, which is supporting the Telemark Hub development in the Gulf of Mexico, also benefits from having a Pulse real-time monitoring system. In this case, it is installed on a top-tensioned production riser primarily to quantify the fatigue damage caused by wake-induced oscillations (the result of the strong currents) and to check the integrity of the riser after severe environmental events.

One of Pulse's most recent clients is a drilling contractor working offshore East Africa, where the currents are among the strongest in the world. In addition to providing real-time operating information, a further aim here is to monitor the buildup of fatigue damage to the riser.

As part of the same DrillAssure service, and before drilling starts, Pulse offers to assess a given drilling riser's fitness for purpose and define its operating window.

This is bread and butter to the parent company 2H Offshore, which uses established analytical techniques to calculate drilling rig operating limits under static, dynamic and hang-off conditions, as well as the extent of driftoff that can be tolerated in given ocean environments.

Given that VIV is such an important issue for deepwater risers of all kinds, there is a DrillAssure module available to monitor the extent of the effect and the associated fatigue damage to the riser. This uses a combination of motion sensors on the main part of the riser and angular rate and inclination sensors at the upper and lower flex joints. As well as determining whether the VIV is within or outside the acceptable limits, this sort of monitoring can provide information about the effectiveness of anti-VIV measures as well as valuable input to riser inspection and maintenance programs.

And the possibilities do not end there. There is a separate module to help drilling contactors with riser joint inventory management. Brazilian drilling contractor Petroserv used this particularly successfully when it recently increased the number of rigs it operates from one to four. Its objective was to make best use of its riser inventory and avoid rig downtime caused by too many joints needing inspection at the same time.

Conductor studies
Then there is a module to monitor the riser and the LMRP when being towed beneath the rig between wells; the objective here is to help optimize transit speeds. Finally, there is a module that focuses on the integrity of the conductor. This reflects increased concern in the industry over fatigue-induced weld failures and problems with wellhead connectors, as Hugh Howells, co-founder and principal director of Pulse parent company 2H Offshore, explains. ‘Not only does the conductor have to withstand the immense loads imposed by the drilling riser, especially in deep water, but BOPs and LMRPs have become taller and heavier. Also, wells are often much longer nowadays, which means protracted drilling periods when the conductor is under considerable strain.'

Data for studies of this kind come from movement and strain sensors attached to the riser, the LMRP and the conductor itself. In the last case, retrofitting sensors using magnetic attachments has proved surprisingly successful. Pulse has undertaken work of this kind for Statoil in the North Sea to estimate the fatigue damage to conductors caused by drilling operations and thence their time to failure. This information has been used to calibrate analyses performed on nearby conductors in similar seabed conditions in an effort to determine whether they will withstand the forces imposed on them by heavy workover operations using risers and BOPs that are more substantial than those used to drill the wells originally.

A similar project for an operator in the Atlantic to the west of Shetland used data collected from new drilling operations to help determine whether conductors in suspended appraisal wells would withstand the forces imposed by re-entry and conversion of the wells to full producers.

Howells has been involved in riser analysis, engineering and monitoring for most of his working life and has extensive experience of the whole business from the technical, operational and regulatory standpoints. He points out that there is no reason to believe that monitoring the Macondo drilling riser would have helped to avoid the disaster last year. But, in his opinion, the intense focus that deepwater drilling is now under means that almost any deviations from operational excellence are likely to have serious repercussions for drillers and operators.

‘Drilling riser monitoring offers lots of advantages, particularly at a time like this,' he says. ‘There are two issues: in the first place, demonstrating the intent to monitor the drilling riser is likely to have a positive effect on the process of securing permits for drilling in deep water. Then, once drilling commences, operating safely within a defined operating window without adopting an unduly conservative approach will only be possible if one clearly understands the state of the riser: how it is responding to the operational and environmental forces it is being subjected to.'

Although Howells obviously wishes to promote a technology that, for commercial reasons, 2H and Pulse are keen to see more widely adopted, it is difficult not to be swayed by his analogy of driving an expensive, fast car along a busy road without any idea of how one of the principal dynamic elements essential to a safe journey is performing. Pulse certainly believes that it can provide that performance monitoring capability as far as the drilling riser is concerned. The presence of some serious competitors poses a challenge, of course, but tends to suggest the company is on the right lines. It will be interesting to see not just how the technology but also the business of drilling riser monitoring develops in the post-Macondo era. OE



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