Evaluation of fatigue in single sided x – joint welds on Oseberg UWP

Abstract

This report assesses the challenges regarding fatigue calculations on the inside of a single sided weld on the Oseberg UWP. SN – fatigue and FM – fatigue calculations are performed on two different models to establish a foundation/basis for evaluation of the inside.

Model one is a beam model used to identify the most critical joint on the Oseberg UWP. Model two is a FE – model of the critical joint identified. A comparison of the fatigue results from the two models provided a good foundation/basis for evaluating the inside of the single sided weld.

The calculated fatigue life on the outside of the single sided weld is 447 years using FE - fatigue. With a DFF of 3,0 this correspond to a design life of 149 years. For the inside of the single sided weld with a DFF of 10,0 to have the same safety level as the outside, the inside fatigue life is calculated to be 1490 years which correspond to a design life of 149 years.

For a fatigue life of 1490 years the critical initial crack size on the inside is calculated to be:

ai = 6,35 mm ci = 63,5 mm

This crack size is larger than the smallest detectable crack size of 5,0 mm according to DNVGL – RP – C203 [4].

Following the procedure described in standard DNVGL – RP – C203 using a W3 – curve the calculated fatigue life is only 263 years, which differ significant from the FM – fatigue results. Therefore there is a belief that the procedure is too conservative and a parameter study based on the approach in this report is recommended for further work.