Life extension via risk based management for tabular leg structure of mobile offshore production unit using finite element method / Emi Hafizzul Jamaluddin

A Mobile Offshore Production Unit (MOPU) is any type of portable structure that can
be reused when procuring oil and gas from the seabed. There are a few reasons why
MOPU was chosen over the conventional fixed offshore platforms, mainly due to its
ability to be installed and relocated in a short period, low installation and abandonment
cost, and providing adequate time for properly evaluating the new reservoirs. In certain
circumstances, MOPU may be required to remain at the same location for a longer
period beyond its designed life. This study explains the method of life extension
assessment to extend the useful life of the MOPU. A three-legged jack-up rig MOPU in
cruciform shape was designed, constructed, and commissioned based on the American
Bureau of Shipping (ABS) Classification requirement with a design life of 5 years. The
MOPU was constructed with three tabular legs with hull hanged at the tabular using
jacking-pin at the equal elevation. The MOPU was put in laid-up mode, nonoperational,
and clear of hydrocarbons after 10 years in operation. The MOPU is
intended to remain at the site for another 5 years or beyond before any plan is
developed. The legs are the main focus of the condition assessment for the life extension
of the MOPU. The method of condition assessment used is Non-destructive Tests
(NDT). No severe defects or cracks were recorded at the stress concentration points.
All NDT measurements were found within the acceptable ranges, which qualifies the
Fatigue Design Factor (FDF) to be reduced from 5 to 3 at the critical inspectable
location and 10 to 5 at the critical non-inspectable location for the new Fatigue Design
Life calculation. The maximum stress calculated is 343 MPa located at leg 2 at the
jacking pinhole. The result of the life extension assessment shows some locations have
exceeded the Design Fatigue Life (DFL), where the lowest fatigue life calculated is
10.3 years. A crack simulation was developed to assess the crack propagation at the
lowest fatigue life location where 0.4 years of crack propagation time was calculated
before the crack propagate to the next phase. Based on the crack propagation time, the
Risk-Based Management (RBM) method was deployed to develop an inspection
interval aimed to provide an early deterioration detection at the most susceptible
location to ascertain the Fitness For Purpose (FFP) of the MOPU for a life extension.