A pluck and decay, structural damping test was conducted on a 1/3 scale steel riser section in air, in an attempt to quantify the damping coefficients of single pipe and multi-pipe riser systems, and to understand the damping mechanisms involved. The test rig consisted of a vertically cantilevered pipe, fixed at the base and with provisions to add inner pipes to turn into concentric multi-pipe configurations. The results of this test program are probably the first large scale damping data applicable to steel riser systems.
When designing and analyzing steel risers for deep or shallow water, the assumption for structural damping is a critical input and can have a significant effect on dynamic response and particularly fatigue performance resulting from VIV (vortex-induced vibration), Figure 1. A typical value of 0.3% critical damping is often assumed, probably based on experience in the aircraft industry, but its applicability to steel riser systems is often questioned. The problem is compounded when pipe-in-pipe arrangements are used wherein the response of the inner pipes and their interaction with the annulus fluids can generate additional damping mechanisms and influence the overall damping behavior. The paper describes a large scale test conducted on behalf of BP by 2H Offshore at the structural laboratory of COPPE in Rio de Janeiro, Brazil. The test was conducted specifically for the shallow water BP Shah Deniz project but the results have wider implications on the analysis and design of other shallow and deep water risers.
The damping test riser set up is illustrated in Figure 2, and a site photograph is shown in Figure 3. Two 18m long pipes, of 6–5/8 in. and 3 in. OD, were used in the test to model a section of the Shah Deniz pipe-in-pipe riser system to an approximate geometrical scale of 1/3. The bottom of the outer 6 in. pipe was rigidly fixed to the ground via a structural base, forming a vertical cantilever with the top end allowed to move freely. To make up different test riser configurations, concentric inner pipes were introduced into, and centralised within, the 6 in. pipe to form a multiple-pipe test riser. The spacing and radial gap of the centralisers can be varied, and the pipe annulus could be filled with different fluids, to study their effects on the overall structural damping. In addition, the 3 in. pipe could be put under tension or compression against the outer 6 in. pipe to represent the conditions of pre-tensioned or thermally expanded inner pipe in a real riser system. During a test, the top of the riser was pulled to the side and released, allowing the riser to oscillate. The motions at the top and middle of the riser were monitored over a period of time to record the amplitude decay, from which the structural damping coefficient of the test riser could be determined. The test riser was designed to accommodate a third 1 in. inner pipe, and tests including the 1 in. pipe were conducted, but these results have not been included in this paper.
