Subsea/Surface Systems

J-lay tower that can be used for a variety of offshore installations.

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J-lay tower gimbals to change direction Radoil has introduced a new multi-purpose

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The Radoil Multipurpose Tower offers the advantage of gimballing 360°.
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J-lay tower that can be used for a variety of offshore installations. Benton Baugh, President of Radoil, said the primary advantage of this system over conventional J-lay towers is that the multi-purpose tower will gimbal any direction up to 15 degrees, where as a conventional tower will only pivot. Without the gimbal a pipelay barge is forced to follow the pipeline track. In heavy winds or currents this may be a difficult proposition. Baugh said this was the case with the first system they installed. The pipelay vessel could not hold station going straight into the pipeline route. He said that without the gimbal capability the crew would have been forced to abandon the pipeline in 3,000-ft water depth. Once the pipeline was laid it connected to a steel catenary riser (SCR) linking it to a floating production facility. The ability to spin the vessel around 180 degrees allowed for an easy hand off to the SCR. Baugh said a conventional J-lay is basically horizontal pipeline equipment turned to the vertical attitude. He said the Radoil design is more of an integrated system designed specifically for use offshore. It weighs around 10% of a conventional system and has more of the capabilities of a drilling rig than a J-lay barge.

Riser-fuses detect fatigue progress

Ian Cummins, Marine and Structures Engineer with Transocean Sedco Forex, speaking at the recent SPE conference said an ongoing deepwater riser management HAZOP study, involving Transocean Sedco Forex and Marathon Oil Co. has examined, among other things, riser inspection scheduling and the associated riser monitoring.

Riser welds are routinely inspected every six months. Cummins said, a reassessment of this interval was required because of concerns with drilling in harsher environments and the cost of these inspections. In areas such as the West of Shetland the riser may be subjected to vortex induced vibrations (VIV) which might require more frequent inspections. However, on the other hand if the removal of the buoyancy modules for inspection of the welds located in the middle of the deepwater riser joints is not required on a frequent basis, significant savings in cost and time can be made.

Guidelines for the inspection frequency of the riser welds based on fatigue damage accumulation in each riser joint and the severity of the operating conditions are impractical and difficult to implement, Cummins said especially for riser joints currently in use. Instead, he suggested a "fitness-for-purpose" approach. The basic premise is that welded steel structures invariably contain defects either as a result of the manufacturing process or initiated during service at a local stress concentration factor such as a weld. The cracks grow during the service life of the component. Ensuring that the cracks never reach a critical size prevents failure. Fitness for purpose assumes that after a riser weld inspection has been undertaken a crack remains present in the riser weld. The predicted wave and VIV loading is then applied to the riser and the time taken for the crack to reach an unstable size is determined. This time period represents the maximum period of time until another inspection is required. Once the next inspection is undertaken the clock is set back to zero and the process is repeated, the only difference being the predicted riser loading may change if the rig location changes.

The advantage of this approach, Cummins emphasized, is that continuous calculation and logging of the accumulated fatigue damage is not required. Based on case studies in the extreme environment of the West of Shetland and the less harsh deepwater Gulf of Mexico, he said that the fatigue life of the riser joint welds varied from 5 to 100 years of service. However, there are currently no measured current profiles to undertake accurate VIV assessments in the Gulf of Mexico.

For the predicted approach the most common source of inaccuracy is the loading prediction, Cummins said. A direct indication of the fatigue loading or a fatigue monitoring system is the ideal solution and would complement the theoretical inspection scheduling. Strain gauges could be used for a fatigue monitoring system. These are beneficial on a short-term basis for fatigue prediction verification, but continuous monitoring is difficult because it requires an uninterrupted electrical supply and continuous computational analysis of cyclic strains. The ideal approach is a fatigue monitoring system based on fatigue fuses.

These fuses consist of flat elongated legs of the same material as the riser bonded to the riser pipe so that the legs experience the same loading history as the riser joint. Each leg includes a notch designed to produce a stress concentration factor that varies in intensity. Applying the same history to all the legs results in the development of different stress concentrations so each leg has a different fatigue life. The fatigue life of each leg represents a percentage of the riser joints fatigue life. Once the fatigue life in each leg has been reached, the leg breaks. Cummins said the system provides a means of calibrating the predicted riser inspection scheduling.

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