Riser monitoring system addresses impact of complex currents

The Riser Monitoring and Information System (RiMIS) developed by Oslo-based Robit Technology provides on-line information on the behavior of marine risers, contributing both to the optimization of drilling operations and safety. The system is particularly valuable in deepwater, and though it has been developed in a drilling context, a recent application is for monitoring production risers on a deepwater Gulf of Mexico field.

The riser is a critical element in determining the operational envelope for drilling operations, and the deeper the waters involved, the greater the environmental forces to which it is liable to be exposed. In two of the foremost deepwater provinces, the Gulf of Mexico and Brazil, currents are especially strong. Increasing depth is also associated with more a complex current profile, with currents of different strengths moving in different directions at different depths. These will determine the riser shape, which may vary from bow-shaped in a simple current profile to S-shaped in three dimensions in a complex current profile.

Monitor parameters

RiMIS makes it possible to monitor critical parameters such as riser angle and shape, vortex induced vibration (VIV), and fatigue development, according to Morten Eriksen, Director of R&D/business development.

If the angle at the bottom of the riser where it connects with the BOP is too great, the drillstring will cause wear on the inside of the riser and on the BOP components. Similarly, if the angle at the top of the riser is too great, the internal wall of the riser can suffer substantial wear from contact with the drillstring, and there is a risk that the stroke may be damaged.

Complex current patterns may also lead to the creation and shedding of vortices around the riser. These can lead to standing vibrations in the riser, the VIV phenomenon, which both accelerate fatigue and increase the drag from the riser, making it more difficult for the rig to maintain station, Eriksen says. RiMIS makes it possible to monitor the behavior of the riser in these and other respects, so that its position can be adjusted if critical angles are exceeded. In extreme circumstances the riser can be disconnected from the well, and the fatigue condition is known at all times.

The system consists of a number of sensors installed along the riser, which collect data and send it, either via cable or hydro-acoustic means, to the topsides. Robit, part of the CorrOcean group, has developed a range of sensors which measure both environmental loads and structural response. Among these are the Dacos Dynic sensor which uses accelerometers in combination with rate of rotation sensors to measure dynamic angles and the motion of the riser. It also has exclusive rights to use the Hysens strain sensor for measuring external stresses which has been developed by Norsk Hydro. For riser monitoring a strain bracelet with four Hysens sensors placed at 90 deg intervals around it is normally used. The data is analyzed using a software system made up of four modules:

• VIV detection and classification
• Fatigue, including calculation of the cumulative fatigue for the whole riser
• Riser shape - the global riser shape can be estimated in both x and y directions, and a 3D model of the riser can be generated
• Current - the current profile, including speed and direction of the water column, can be estimated using information on the physical attributes of the riser, the location of the rig in relation to the BOP, and the riser shape.

Information on the position of the riser can also be used as a backup position reference, for example when the GPS link is lost, as sometimes happens, Eriksen says.

Test results

The system has been tested and refined at a number of exploration wells drilled in UK and Norwegian waters. The first was a well on BP's Schiehallion prospect in 370 meters water depth west of the Shetlands. This was followed by three wells drilled in the V ring Basin under the Norwegian Deep-Water Program - BP's Nyk High (1,274 meters), Statoil's Vema Dome (1,238 meters) and Shell's Helland Hansen (684 meters).

For these purposes, data was gathered by battery-operated sensing systems with data loggers, which were retrieved when the riser was recovered. On the basis of the data thus gathered, Robit has developed its analytical software. In the case of VIV, Eriksen says, it has developed an algorithm which shows whether there is vibration in a riser, and if so, its amplitude, frequency, and modal shape. Oil companies that have acquired RiMIS are able to use such data to calibrate their own VIV prediction models.

Compared with existing methods such as using remotely operated vehicles (ROV) to read eyeball meters, RiMIS offers a fast and efficient system collecting a wide array of date at a high level of accuracy.

This summer, Robit supplied angle and stroke sensors to Saipem for riser monitoring on the semisubmersible Scarabeo 5 during the drilling of the Gjallar Ridge well in a water depth of 1,352 meters, the deepest so far in Norwegian waters. A Dacos Dynic sensor system for monitoring the bottom riser angle has been acquired by Brasdrill for use on the Ocean Yatzy semisubmersible during deepwater drilling off Brazil.

Robit has also supplied a RiMIS system for measuring bending moments in a workover riser to Statoil for use on its Statfjord satellite fields. The system is also appropriate for long-term riser monitoring, and as such has been supplied to Chevron for use on production risers on the Genesis spar platform in 792 meters water depth in the Gulf of Mexico.

RiMIS has grown out of response monitoring technology which Robit began developing in the early 1990s when it was contracted by Phillips to monitor a number of jacket structures at the Ekofisk complex to verify the design basis in the face of subsidence and wave reflections around the Ekofisk Tank.

In the mid 1990s, the company provided Conoco with monitoring equipment used during the towout of the Heidrun tension-leg platform tethers. While calculations made with mathematical models indicated that as much as 30% of the tethers' fatigue budget would be spent on the tow-out, response monitoring during the tow allowed direction to be optimized, thus minimizing fatigue, and indicated that for the majority of the tethers, fatigue spending was less than 2%. This finding made it possible to eliminate the base-line inspection of the tethers, leading to a significant cost saving.

The technology is finding applications in other areas than risers, and instrumentation has been supplied to Statoil for measuring dynamic stresses in the hull structure of the Norne production ship, which is installed in the harsh environment of the Norwegian Sea.