Frans Kopp

In-Line Motion of Subsea Pipeline Span Models Experiencing Vortex-Shedding

Subsea pipeline spans, when experiencing bottom ocean currents, are prone to vortex-induced vibration (VIV). Experiments and computational fluid dynamics (CFD) are conducted to evaluate the effects of the pipe stiffness on its first mode in-line VIV motion, primarily in the reduced velocity range from approximately 1.0 to 4.0. Experimental results also indicated that there was obliqueness in motion trajectories, which could have impacts on VIV design of the free spans. The main findings of this investigation are presented in this paper.Copyright

Statistical Characterization of Slacking and Snap Loading During Offshore Lifting and Lowering in a Wave Environment

During offshore lifting operations, appropriate characterization of the dynamic forces is critical to ensuring that the proper lifting equipment has been selected and designed. When a load is supported by a wire rope, or any member that will not support compression, there is a possibility of the rope becoming slack during the lift. The resulting shock load when the rope becomes taut again can produce significant impact loads in the lifting equipment, which is a situation preferably avoided. Thus, of particular importance is the reasonably accurate estimate of the probability of occurrence of sudden loading events. During offshore lifting operations, this situation can occur under the following circumstances: 1) During a transfer of a weighted load between two vessels in an offshore wave environment. 2) Lowering a load through the water surface, particularly in a wave environment. 3) Lowering a load through the water column with significant motions of the support at the surface. This paper deals with the first two situations. First, a model to predict the probability of slacking the rope while lowering a load through the water surface in a wave environment will be presented. Secondly, some commentary will be made about the possible magnitude of the shock loads that can occur during vessel to vessel transfers and lowering through the water surface. Finally, a brief comparison will be made between these models and others that have been proposed within the industry, some of which have been incorporated in design standards and guidelines.Copyright

Loading History Effects for Deepwater S-Lay of Pipelines

For economic reasons S-Lay is often preferred to J-Lay. However in very deep water S-Lay requires a high curvature of the stinger to achieve the required close-to-vertical departure angle. This can lead to plastic deformations of the pipe. The high top tension increases the plastic deformations in two ways: firstly it adds an overall tensile component to the strains, thereby increasing the strains at the 12 o’clock position. Secondly it increases the strain concentrations which arise due to discontinuous support of the pipe on the stinger. Typically the pipe is guided over a series of roller beds. The high top tension tends to straighten the spans between the roller beds. To accommodate this (so that the pipe can still follow the stinger), higher curvatures are required at the roller beds. Analytical and numerical solutions are provided to quantify this effect. The analytical solution is fully developed for an elastic-perfectly-plastic pipe, but can also be applied for other material models provided that: (i) the moment-curvature relation for the pipe under tension is known, and (ii) no cyclic plastic ratchetting occurs due to repeated bending of the pipe over the roller beds and straightening in the spans between roller beds. Agreement between the analytical and numerical (finite element) results is excellent, if the proper loading history is used in the numerical simulation. Otherwise the level of strain concentration can be overpredicted.Copyright