Extending the life of ageing rigs

Jacking trial in the yard for a jack upAgeing rigs can become a cause of worry for oil companies with the decay in metals, the environmental stress on the rigs and the lowering of efficient functioning. Here is a look at ways to combat such problems.

DNV regional offshore manager responsible for Middle East and India, Bijali MP said, “Majority of jack up rigs operating are 20 years or older. Most ageing jack-ups are designed and constructed according to Mobile Offshore Rules from 80-ties and 70-ties. Design life normally considered in the range of 20 years.

“If an owner is going for a new building, then the cost is quite high and delivery will be more than 18 months. Meanwhile, market demands for rigs increase and owners like to continue with their ongoing contracts. These reasons force the owners to go for refurbishment of their ageing rigs. Ageing rigs properly maintained and upgraded may be successfully operating much beyond their originally planned design life.”

The challenges on ageing rigs are related to:

- Fatigue properties/cracking for dynamically loaded parts to the rig structure

- Maintaining an efficiently operating corrosion protection system

- Structural degradation due to corrosion and thickness reduction, e.g. structural strength and fatigue

The onboard marine and jacking equipment are normally in a constant cycle of review, maintenance and renewal, and therefore rarely follow the age of the hull.

Whenever there is an issue in any of the equipment onboard, spare parts are available and are easy to maintain. Most of the rig owners follow some sort of preventive maintenance system for these equipments. Thus such equipment are not considered in this article.

DNV has developed Inservice Inspection programme (IIP) and Hull Integrity Management system (HIM) to assist the rig owners/managers/operators to maintain the structures in accordance to classification requirements.

This will help the owners/managers to plan the surveys according to class survey requirements. DNV’s IIP establishes a clear and concise inspection work scope and plan. All DNV class rigs have the IIP in place. Whenever the owner/manager’s team is carrying out an unscheduled inspection, results can be incorporated in HIM system.

DNV has developed IIP to ensure that critical areas are surveyed according to a proper schedule using appropriate techniques. The IIP shall be based on experience gained during the design and fabrication of the rig and shall be modified throughout the in-service period to account for survey experience.

When a rig is in the yard for refurbishment, the outcome of DNV in-service inspections and hull integrity management system reveals the rig’s structural conditions. Generally this will be part of the yard scope of work in addition to rig owner’s or the oil company’s additional requirements.

It is very important to control weight during modifications/conversions. Inspection and maintenance of the corrosion protection system is an essential part of the structural follow-up both within the anticipated design life and for ageing rigs. So, this also will be taken care during refurbishment of rigs.

“To summarise, clear knowledge of the condition of the rig, well-defined scope of work, equipment ordered well in advance and involvement of class at an early stage are success factors of a good refurbishment project” said Bijali.

This article originally featured in Oil Review Middle East Issue 6 2013

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