Finite element analysis used to optimize design and calculate fatigue of peripheral-lines on Aker Drilling Risers clip connector

Abstract

Increasing prices- and high demand for hydrocarbons worldwide have encouraged oil companies to explore in deeper, more vulnerable- and hasher ocean environments. A consequence of working in such areas is that the requirements of the equipment are rising. Aker Drilling Risers, together with Institut Français du Pétrole, have taken all this into account, and are currently designing their second edition of the clip connector drilling riser. This newly developed riser system is based on a load-sharing-system, were both the peripheral-lines (P-lines) and the main pipe are in tension. With the development, Aker Drilling Risers are interested in a parameter study around the fatigue of P-lines.

This master’s thesis combines a global and a local Finite Element (FE) Analysis together with Miner’s rule to calculate the fatigue life. A local FE model of the connector calculates the hot spot stress in the p-line, and a global analysis calculates the tension variation in the riser pipe. The two analyses are coupled together to create a local stress histogram. It is then summed up by using Miner’s rule to calculate the fatigue life.

In the first chapter an introduction to the problem and specification around Aker’s drilling risers are given. Further the chosen procedure and the theoretical background are shown. This is primarily limited to DNVs main code for the area, DNV-RP-C203; fatigue design of offshore steel structures.

The main part is divided in two; first the local analysis in Abaqus is described and different parameters, such as, threads, design, pressure and friction are analyzed. The parametric optimized design is used to calculate a stress transfer function (STF). In the second part an existing analysis from the Gulf of Mexico is used to create the tension variation. The tension variation was connected to the local stress transfer function to create the local stress histogram and then summed up with Miner’s rule.

A discussion around the results and the procedure are done before the author concludes:With the optimized design, with the new elliptical radius on the box, the structure is well prepared to handle to cyclic stresses for the Gulf of Mexico. With this tension variation, it has a constant operating fatigue life of 740 years.