If you participate in the pension scheme, you accrue pension years. One year’s service is equivalent to one pension year if you work full-time. If you work part-time, one year's service results in a pension year in proportion to your part-time percentage for that year.
For example: If you work 15 years for Shell on a 100% (full-time) basis followed by another 10 years on 60% part-time basis, you will have 25 year's service. However, you have accrued less pension years, i.e. 15 + 0.60 x 10 = 21 pension years.
Your pension basis, i.e. the salary that forms the basis for the calculation of your pension, is always determined on the basis of 100% (full-time) employment and does therefore not change in the case of part-time employment.