Pipeline Vibration Dampers (PVDs) have been utilized to mitigate wind induced vibrations for above-ground pipelines in the Arctic Circle for decades. Although such pipeline vibrations are relatively small, the accumulation of vibration cycles can cause excessive fatigue at pipeline joints. This paper considers the application of PVDs to help suppress vortex induced vibrations (VIV) of subsea pipelines, and presents an analytical method for estimating PVD weight, damping, and tuning frequency to provide optimal vibration reduction. The paper also reviews some of the fundamental VIV concepts and provides an overview of the PVD behavior and equations of motion. Akin to the above ground implementation, each PVD may have to be self-contained in a small pressure vessel for the subsea application in order to avoid hydrodynamic interactions with the surrounding sea water and ensure the PVD mechanical performance. The PVD devices may be used exclusively for VIV mitigation, or they may be used in combination with helical strakes to help suppress VIV. PVDs can be pre-installed on the pipeline on-shore or offshore, or they may be post-installed offshore after the pipeline is laid on the seabed. Hence, if they are post-installed, PVDs may be used as intervention devices to improve the VIV performance of existing pipelines. This paper analytically shows that significant vibration reductions in subsea pipelines can be achieved by utilizing PVDs with a relatively small amount of added mass to the pipeline. Experimental test data will be needed to validate the estimated vibration reductions.
Subsea pipeline routes are generally planned to minimize the number of long spans. Suspended spans in subsea pipelines are typically constructed in ways to mitigate the irregularities in seabed bathymetry. A subsea pipeline span may be subjected to motions due to currents that produce a phenomenon commonly referred to as VIV. The motions could result in high cycle fatigue damage that may potentially reduce the effective life of the pipeline. This paper is concerned with pipelines operating in relatively deep waters (depths greater than about 500 ft) which are not affected by surface wave effects.
Reducing the length of a pipeline span may accomplish sufficient mitigation of the VIV effects provided that the natural frequency of the pipeline span is away from the shedding frequency of the VIV so that no resonant vibration can take place. Pipeline span length reduction may be accomplished by re-routing the line along a path associated with shorter spans or by supporting the span using various methods. However, these span length reduction attempts may be impractical or prohibitively expensive in some cases.
Helical strakes are widely used to mitigate VIV as they can generally accomplish significant pipeline response reductions with no requirements for pipeline span shortening. Due to the many uncertainties involved in VIV response prediction, the engineering methodologies currently available tend to be conservative and decisions for the provision of strakes over significant extensions of pipeline lengths are not unusual. This increases project costs for procurement, fabrication, inspection, transportation, and installation, with the associated impact on project schedule and risk.
One important factor inherent in pipeline VIV response is the amount of damping of the system. Typically, the pipelines have small amounts of damping because (a) the damping capacity of the pipeline itself (structural damping) is quite limited, (b) the pipeline is in contact with the soil only at the span ends which significantly limits the soil contribution to the damping of the system, and (c) under the effects of VIV, usually referred to as "lock-in," there is no hydrodynamic damping available.