In this paper results of flexible riser model tests will be discussed and compared with results of time domain simulations for regular and irregular wave conditions with current.
The model tests were performed at the Maritime Research Institute Netherlands (MARIN) with a large scale riser model instrumented to determine bending moments at a number of point along the riser. Motions of the riser were measured by means of underwater optical tracking systems. The riser top was attached to a mechanical oscillator programmed to carry out the motions of the attachment point of a tanker in irregular waves. Special attention was given to the correct phasing between riser motions and irregular waves. The calculations were performed with the MARIN riser analysis program DYNAFA.
Results of tests and calculations are compared for the riser top area. Correlation is obtained for both regular and irregular wave conditions. It is concluded that computations for irregular wave conditions provide valuable information in early design stage and can be used to extend the model test results.
During the last years it has been recognized that permanent floating production units can be a viable solution for marginal field development in hostile environments like the North Sea. Due to the crucial role of flexible risers as link between oil well and floater, stringent demands are put on the safety and reliability of flexible riser schemes designed for these extreme conditions.
Current riser design procedures are mostly based on theoretical prediction methods for maximum internal load levels and bending radii, using numerical methods to simulate the riser behavior. Design procedures generally include static geometry analyses in near and far position to determine the required pipe length. In addition to this, deterministic regular design wave analyses are performed for several wave height/period combinations and corresponding harmonic top displacements [1].
In recent years a variety of numerical tools has been developed generally based on finite element or finite difference solution schemes offering different options for analysis, see refs. [2] to [6]. An indication of the computing efficiency of several of these methods is presented in [7]. Although most of the programs have been verified against simple analytical and/or model test cases, very little data is available on correlation between computational results and realistic test data for extreme design conditions.
To assess the overall behavior of specific flexible riser systems and to provide measured data to enhance theoretical prediction methods BP International, London, commissioned the Maritime Research Institute Netherlands (MARIN), Wageningen, to carry out representative scale model tests on several flexible riser configurations exposed to extreme environmental conditions and large top displacements.
As part of a BP design study on permanently moored floating production system MARIN was requested to perform model tests for a tanker-based system, see Figure 1.
