The safety of drilling operations, especially in deepwater, has come in for massive scrutiny following the deepwater horizon disaster in the Gulf of Mexico.
Here, we take a look at a unique product and service package designed to monitor drilling risers in an effort to ensure their operational integrity.
Drilling risers are incredibly dynamic and complex structures that are subject to a range of sometimes-extreme environmental and operational forces. Yet, despite their critical nature and the serious consequences of a riser failure, there has been a degree of resistance to the idea of monitoring them.
Richard Kluth, managing director of Pulse Structural Monitoring, explains why: “Unfortunately, some of the early monitoring systems were expensive, poorly integrated, impractical and insufficiently robust. Further, the data generated did not always translate into the sort of information that immediately improved the life of the driller. There was a perception that riser monitoring was research and development, and expensive research and development at that.
“To win the drilling community over to the advantages of riser monitoring, you need to offer a comprehensive service. As well as the monitoring hardware, you have to be able to provide a system design and installation capability, and the analysis and visualisation software necessary to turn the data into meaningful, well-presented information,” says Kluth.
According to him, simple, accurate, robust and well-packaged sensors are the first essential. They need to be easy to install at critical points on the riser and must not interfere with normal drilling operations; if they do, the chances are they will be destroyed before they even enter the water.
It is possible to acquire a wide range of data related to the performance of a riser, which raises the question of how to make sense of it all. “The data is of little use without the processing ability to provide users with meaningful parameters they can compare with key operational limits,” says Kluth. “Ideally, the results of this process will be displayed on a single easy-to-view screen (a riser dashboard), with a simple traffic-light system to indicate the status of the riser within its overall operating envelope. 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 riser.”
Modular integrity management system
The issues just outlined were central during the development of Pulse’s DrillASSURE service offering, a modular system to aid integrity management of drilling risers. At the heart of the offering is a real-time drilling operations advisory module called DrillADVISE.
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, and information with which to control the riser tensioning system. Most importantly, it provides a constant picture of where the riser lies within its defined operating window. This is an invaluable aid to making prudent connect and disconnect decisions, which are key to maximising drilling uptime and ensuring the safety of 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 floating drilling, production, storage and offloading vessel, which is working offshore of the Republic of Congo.
The DrillASSURE offering also includes DrillWINDOW, a software module to assess a given drilling riser’s fitness for purpose and define its operating window. As with the other DrillASSURE software modules, this was developed in conjunction with 2H and can be utilised for planning, during operations and for post-analysis. This is bread and butter to the company, which uses established analytical techniques to calculate drilling rig operating limits under static, dynamic and hang-off conditions, as well as the extent of drift-off that can be tolerated in given ocean environments.
Given that vortex-induced vibration (VIV) is such an important issue for deepwater risers of all kinds, there is a DrillASSURE module designed specifically 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 in or outside the acceptable limits, this sort of monitoring provides information about the effectiveness of anti-VIV measures and valuable input to riser inspection and maintenance programmes.
The possibilities do not end there. There is a separate module called DrillJOINT to help drilling contractors with riser-joint inventory management and another, DrillTRANSIT, to monitor the riser and the LMRP when they are being towed beneath the rig between wells; the objective is to help optimise transit speeds.
Finally, there is an important module, DrillFATIGUE, 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 Kluth explains. “Not only does the conductor have to withstand the immense loads imposed by the drilling riser, especially in deep water, but blowout preventers (BOP) and LMRPs have become taller and heavier. Also, wells are often much deeper nowadays, which means protracted drilling periods when the conductor is under considerable strain.”
Data for these studies come from movement and strain sensors attached to the riser, the LMRP and the conductor itself. 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 thereby 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 their ability to withstand the forces imposed on them during workovers using risers and BOPs that are more substantial than those originally used to drill the wells.
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 to full producers.
“DrillASSURE offers lots of advantages,” Kluth says. “Operating safely within a defined operating window without adopting an unduly conservative approach is only possible if one clearly understands how the riser is responding to the operational and environmental forces it is being subjected to.”
It is difficult not to be swayed by Kluth’s graphic 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 ability as far as the drilling riser is concerned.
Commercial success
DrillASSURE has become an increasingly important part of Pulse’s business, as operators have begun to appreciate the benefits it provides. Most recently, Pulse has supplied services to a major operator in the Caspian Sea with conductor integrity issues. BG Group has also joined the growing list of customers: its requirement is to monitor a riser used to drill shallow water wells in the Norwegian sector of the North Sea. In shallow water, any motion of the rig results in large angular deflections of the riser; this, in combination with the possibility of wave patterns matching the natural frequency of the riser, raises concerns that the fatigue life of the structure could be dramatically reduced.