Oil and gas production in the North Sea has been running at full pace for more than thirty years. Many of the rigs, and cranes, in the area have a long life behind them. Speakers at the 12th Offshore Crane and Lifting Conference in Stavanger, Norway explained how a range of testing techniques have allowed them to extend the working life of their cranes, and to make the most effective use of maintenance resources.

Managing risk

In the autumn of 2004, ConocoPhillips worked with National Oilwell Varco to conduct a total reliability-centred maintenance (RCM) analysis of two typical types of crane, king post and slew bearing models, in the Norwegian sector of the North Sea. King post cranes feature a static mast, with the boom pendants attached to a slewing bearing at the top of the mast, and the boom pivot point and cab fixed to a platform that rotates around the mast. Slew bearing cranes are similar to onshore luffing jibs, with the cab, boom, counterweight and backmast mounted on a slewing ring at the top of the mast.

At the time, the firm was planning the dismantling of a large number of platforms on the Ekofisk field, and needed to ensure that the cranes were upgraded on the field were capable of taking the weight of heavy components during decommissioning. Jan Ketil Moberg, technical responsible engineer for lifting equipment for ConocoPhillips, Norway, told the conference how this maintenance programme had been performed.

The RCM process consists of three key steps. First, the engineers listed all the functions of the cranes, and their accompanying components in a hierarchy of main functions (lifting, for example), sub functions (winching), and components (winch). Next, they conducted a failure mode effect consequence analysis (FMECA) for each component. Finally, using the FMECA as a guide, they could then set appropriate maintenance activities for each component.

The ‘failure modes’ in an FMECA usually refer to component failures; for example, major failures such as a wire rope breaking or a less significant failure, such as a light bulb blowing. Only one failure mode is handled at a time, and an assumption is made that all other equipment is working properly and no maintenance has taken place. The failure rate for the component is estimated in three broad classes based on mean time between failure (MTBF) values, arrived at from consultation with the manufacturer and experienced engineers. Class A covers components with an MTBF of more than 12 years, class B components have an MTBF of between 3-12 years, and class C components have an MTBF of less than three years.

Next, the consequences of particular failure are assessed and numbered, with a focus on safety and operation, and split into five classes. These range from normal operation, but with irregularities that should be repaired within a reasonable time (category 1), through limited operation, loss of main function (the crane stopping), a possible fall of load or boom, to a definite fall of load or boom (category 5).

The engineers then create a table (see Figure 1), plotting the MTBF class against consequence category, and assign a criticality value to each section of the table. At the top left of the table (A1) will be types of failure that are expected to occur with an MTBF of more than 12 years, causing minor irregularities: these are given a low criticality value. At the bottom right of the table are types of failure that are expected to occur within three years, and to cause the boom or load to fall (C5): these are given a high criticality value.

Where failures have a low criticality value, they should be run to failure, and no risk reduction is needed. Where the criticality is medium, the cost of maintenance should be weighed against its benefit, and potential safety hazards considered; risk reduction should be discussed. Finally, where failure is highly critical, an improved maintenance strategy should be put in place, or the component replaced or modified. In this case, risk reduction is essential.

Each possible component failure is then plotted on this matrix, and a new maintenance strategy developed, based on the criticality value of each potential failure. In the example, more than 800 crane components were evaluated for each crane model. Potential failures of wire rope were given a high criticality value. The team extended their inspection regime for this component, and initiated a replacement programme. At the other end of the scale, faults with switches in the electrical system were one of the potential failures that were given a low criticality value: no action needed to be taken.

Organic maintenance

As a result of the RCM analysis, ConocoPhillips was able to identify gaps between the RCM programme and the manufacturer’s maintenance programme. Working with the manufacturer (National Oilwell), maintenance personnel and a verification body, it developed a new, expanded, maintenance programme, revised the spare part system with a focus on critical components, and created a fixed offshore schedule for the manufacturer’s crane specialists. It identified a need for continuous verification and adjusting of the maintenance programme based on experience gained as the programme took place.

Jim Bentley, chief engineer, pressure systems and non-destructive examination, for IMES, UK, expanded on the importance of risk-based examination regimes. The traditional approach has been based on prescriptive test and inspection. Bentley explained that, while this approach has generally served the industry well, it tends to manage assets on a day to day, week to week basis. It doesn’t, in his view, adapt to the changing state of equipment.

Bentley identified a series of flaws with standard, prescriptive, testing programmes. Overload testing potentially does more harm than good, reducing the safety of the crane or its components by putting them under stresses that they would not otherwise face. Visual inspection, although a very powerful technique, is subject to human error and defects can be missed or misinterpreted.

Formerly with the Royal Navy, Bentley treated the audience to a piece of salty naval wisdom: “When you’re in a swamp, up to your arse in alligators, the thing to remember is that you’re there to drain the swamp.” That is to say, prescriptive tests that stick to a routine of performing the same test every time encourage engineers to focus only on present circumstances (the alligators). The type of reliability centred programme he and the other speakers proposed, focusses on a long term, fixed objective (draining the swamp), and asks how best to make use of maintenance resources to achieve this goal.

He explained how the approach he suggested would work in a feedback loop. Engineers would start with data from an initial inspection of the equipment, and from its known history: its use, damage it had suffered, know problems from similar cranes, or previous failures. This would guide the development of a maintenance programme that he called “organic”. As the programme developed, new data would be considered, and data mining techniques used to look for trends or indications of future problems. These would then be used to modify the maintenance programme, all with the aim of ensuring the cranes’ long-term reliability.

Gap analysis

Rasmus Bjerkan, of National Oilwell Varco, explained a number of different approaches to testing and maintenance, which are flexible, and respond to changing circumstances. Like Bentley and Moberg, he stressed the importance of an RCM risk based methodology, based on an FMECA analysis of failure probability and risk.

He also looked at gap testing. This is a systematic evaluation intended to clarify the gap between the crane, as designed and documented, and defined requirements and standards. Typical requirements against which the crane would be compared could be regulations, standards, and specifications.

The advantage of conducting a gap analysis is that it accounts not only for changes in regulations (such as the new European standard, EN 13852-1), but also for changes in the owners’
planned use of the crane, or the conditions available for servicing. These could include modifying the crane to allow for personnel lifting, changes to the manufacturers’ recommendations, new technology and experience, or to account for components that are out of production.

A gap analysis form lists requirements, and compares the crane and its components to these. Each requirement is either marked ‘OK’, or a deviation is listed. The deviation is then detailed, and remedial action suggested. As an example, Bjerkan considered a crane that the owner planned to use for personnel lifting, but which had not been designed for this. The crane was compared to EN 13852-1 Clause 5.8, ‘Lifting of personnel’. The gap analysis found that the crane deviated from the standard, which required the following items, not fitted to the crane: AOPS/MOPS (automatic/manual overload protection system) interlocking functions; redundant braking systems; secondary power supply/emergency drive system; and safety related controls. With the analysis complete, the remedial actions could be identified. In this example, the crane’s winches and braking systems needed to be modified, an emergency drive system needed to be implemented or upgraded, and specific dedicated controls needed to be fitted.

Condition and lifetime

Next, Bjerkan looked at a condition and rest-of-life analysis. The idea of this analysis is to consider the crane’s current condition, and estimate its potential lifetime. Four factors are considered here. First, the safety lifetime: that is, can safety be maintained at approximately its original level? Next, the reliability lifetime: can operational reliability be maintained to the original level or at an acceptable level in relation to the planned use of the crane? After this, the economic lifetime is considered: what will be the overall cost of maintenance and purchase? Finally, the technological lifetime is considered: how long will it be before components are no longer available? The overall lifetime of the crane will be limited by whichever of these is shortest.

The lifetime of the crane can be limited by many factors. Examples include, fatigue and wear, both measured in operating hours, or corrosion and aging, measured in years since the crane was built. Outdated technology or availability of spare parts, measured in the number of years for which these will before the components, will also limit the lifetime of the crane, as will damage to the crane.

Coming ashore

By performing a gap analysis, it is possible to compare the crane to its planned use and current regulations, and make modifications as needed. A condition and lifetime analysis can assess the potential lifetime of the crane. An FMECA can identify potential causes of failure, and guide engineers in developing an RCM, which focuses maintenance where it is most needed and will do the most good.

By combining these techniques, it was possible for these engineers to make the most efficient use of their resources, and extend the life of their equipment. Can engineers onshore learn lessons from this? At a time when the market is peaking, and many fleet owners are considering upgrading their fleet, can these analyses help them to more effectively plan what should be sold, what should be kept, and what should be repaired?


The Ekofisk field in the Norwegian sector of the North Sea. ConocoPhilips implemented a reliability centred maintenance programme here, to ensure that aging cranes would be able to lift heavy modules during decommissioning. Ekofisk field