Fatigue analysis of steel catenary risers at the touchdown point
Feng, Zi Li
Date of Issue2012
School of Civil and Environmental Engineering
In deepwater exploration, Steel Catenary Risers (SCRs) are extensively employed in riser systems. Static and dynamic bending stresses are the largest at the touchdown point (TDP) where the SCR first touches the seabed, and thus fatigue damage is most significant at this location. Unfortunately, challenges encountered in fatigue design of SCRs at the TDP are prominent, such as significant uncertainties arising from the seabed-riser interaction, prediction of vortex induced vibration, impact from vessel motions and time-dependent development of trench, among others. The objective of this dissertation is to improve the understanding of some critical factors profoundly influencing fatigue behavior of SCRs at the TDP and consequently to improve fatigue design confidence. The work conducted in this thesis is divided into three stages. In the first stage, random variables which exclusively affect the uncertainty of damage at the TDP are first systematically identified and subsequently analyzed using reliability-based approaches. Seabed uncertainties which are represented by three parameters of a seabed-riser interaction model are found to be the most sensitive. However, these special uncertainties are not considered in the recommended generic design safety factor. In order to maintain the safety level recommended by design codes, a new design factor is required and is quantified to be appreciably larger by using a proposed method. Due to second-order wave forces, low frequency (LF) vessel motions can occur, dramatically changing the location of TDP. However, considering LF motions in fatigue analysis is time consuming and they are not encountered in the reliability analysis. Moreover, influence of LF motions on fatigue performance of SCRs at the TDP is not widely investigated in the literature. Hence, they are studied in the second stage. It is found that LF motions can render the trench profile broader and shallower. LF motions are beneficial as they can shift the location of the TDP, thus spreading the damage along the riser. Nevertheless, fatigue damage is increased due to the resultant bimodal stresses. A strategy is proposed so as to efficiently incorporate the LF motions in the fatigue analysis. Scour can occur around underwater pipes when subjected to waves and currents. Scour profiles are deemed to influence the fatigue damage of SCRs at the TDP. However, there is so far no research devoted to this problem while previous studies are merely for submarine rigid pipelines. Thus in the third stage, laboratory experiments were conducted to investigate the scour under a periodically vibrating catenary riser. The scour profiles under a stationary and a vibrating catenary riser are discovered to be remarkably different. The maximum scour depth increases with both vibration amplitude and frequency, with the frequency being the more prominent factor. Subsequently, a measured three-dimensional scour profile was incorporated in the numerical simulation. The maximum fatigue damage is found to be diminished while the damage curve around the TDP is erratic, compared to the scenario with a flat seabed.
DRNTU::Engineering::Civil engineering::Structures and design