It takes careful planning and proactive policies.
by Murray Wilson
April 24, 2018

The offshore industry faces two main challenges: maximizing production within the limits of the reservoir and minimizing operational costs while maintaining the safety of the platform. Pumps form one of the main groups of equipment that influence the outcome of both challenges, and they require expert knowledge to ensure continued reliability and performance.

Some experts argue that in each case, industry engineering expertise and commercial innovation are required to deliver these goals. Furthermore, the capital expenditure to improve reliability is often far outweighed by the costs incurred by an unexpected failure and the subsequent costs of lost production. By taking a proactive approach and involving an expert maintenance provider, platform operators can deliver significant benefits to the business in the long term.

Improving Performance

In the years following commissioning, the actual duty requirements of production pumps are likely to change. Production rates may start to decline after the initial plateau period, or the connection of additional wells may mean that potential production is being limited by the processing trains that were designed for lower volumes.

precision engineeringImage 1. Precision engineering is required to prolong reliability and effectiveness. (Images courtesy of Sulzer)

Equipment being pushed to operate significantly outside of its original design envelope can cause operating problems that impact reliability and affect platform production. This increases maintenance costs, since operators and equipment specialists are required to overhaul plants more frequently.

Ultimately, the goal is to improve reliability and efficiency while reducing downtime and energy consumption, at the same time as satisfying American Petroleum Institute (API), Atmosphere Explosibles (ATEX) and many other engineering standards. It can be achieved through the implementation of preventative maintenance techniques and the adoption of the latest engineering designs for pumps.

Water injection pumps, seawater lift pumps, crude oil offloading pumps and fire suppression systems all require individual designs to deliver the best efficiency and productivity. They also need specialist maintenance routines that will prolong reliability and effectiveness.

Proactive Maintenance

A proactive maintenance regime is crucial to identifying potential issues before they develop into problems. However, this requires knowledgeable and experienced personnel to carry out the in-situ platform maintenance, and these skills take time to perfect.

The time required for this process can be greatly reduced by instigating a training program prepared by experts in equipment maintenance who can pass on their collective knowledge in a structured and efficient manner.

In terms of through-life maintenance cost, preventative action is almost always less costly than corrective action. Two of the most prominent symptoms that occur prior to failure in mechanical and electrical equipment are increasing vibration and rising operational temperature.

Regular trending and analysis of radial and axial vibration signatures and thermographic/visual inspections of bearings, coils and electrical connections can prove invaluable. The latest developments in operational monitoring can be applied to existing assets and then used to determine the optimal point at which planned maintenance should be conducted.

OEM expertiseImage 2. OEMs are an excellent source of engineering expertise.

Understanding Cavitation

Cavitation, which is most commonly seen on the pump impeller, is caused by a pressure difference, either on the pump body or the impeller. A sudden pressure drop in the fluid causes the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where the local pressure is greater than saturation pressure, the vapor bubbles abruptly collapse, creating a shockwave that, over time, can cause significant damage to the impeller vanes or pump housing.

In most cases, it is better to prevent cavitation rather than trying to reduce the effects on the pumping equipment. This is normally achieved by increasing the suction head, lowering the fluid temperature and decreasing the net positive suction head required (NPSHr).

For situations where cavitation is unavoidable, or the pumping system suffers from internal recirculation or excessive turbulence, it may be necessary to review the pump design or minimize the potential for damage using a bespoke coating system.

The offshore production environment exposes pumps to harsh conditions, and the abrasive nature of the fluids being pumped in certain processes on board can ultimately result in reduced efficiency and decreased pump performance.

Produced water re-injection pumps that are employed to force water back into the oil field and maintain the reservoir pressure needed to lift the oil to the surface are often subjected to high levels of abrasion. This is the result of sand carryover from upstream filtration where there has been a process upset or where filtration systems are not adequate. Pumps that are used to transfer these fluids can experience significant levels of erosion, especially in areas with high flow velocities. The entrained sand particles act as an abrasive and higher working pressures only serve to compound the issue.

Pump manufacturers will aim to minimize flow velocities throughout the pump or design it so the flow velocities through close-running clearances are as low as practically possible within the duty for which the product has been designed. Under these circumstances, one of the most effective solutions is the use of specialist protective coatings, which can be used to protect selected areas in the pump.

Premature Failure

Choose the most appropriate materials, equipment and application procedures, otherwise the coating may degrade and fail prematurely. The processes and specifications used by experts for applying coatings are essential to delivering a durable and reliable product. To illustrate importance of these procedures, consider the process of installing and removing an impeller. Often, the impeller is heated to allow it to be installed or removed. This can cause inappropriate coatings to be damaged during a routine maintenance operation.