Dropped blocks: OMV gets to root causes, zeros in on prevention
Equipment audits, crew competence, strict enforcement of management systems among mitigation measures
By Stefan Wirth and Adrian Wolf, OMV
Over the past years, OMV E&P has experienced several incidents in its drilling and workover activities related to dropped blocks. The root causes are all related to improper maintenance and wrong use of the hoisting system related to insufficient crew competence. OMV is determined to reduce the reoccurrence of such high-potential incidents to ALARP.
The equipment of primary focus is the drawworks system with its main and auxiliary brake systems. Examinations have been conducted to identify major risk scenarios and practicable mitigation measures.
While a drawworks always consists of the drum that handles the drill line, nowadays there are systems available that are very different. In general, one can distinguish drawworks by how their drive and the main and auxiliary brake systems are realized. Table 1 gives an overview of common possible combinations and how they developed over time.
Mechanical drive systems are rarely found on rigs contracted by OMV and are therefore not discussed here in detail. On the other hand, DC-driven units are very common. Because power on rigs is now typically generated via synchronous generators, these units require SCR cubicles to work. These cubicles are mostly set up to deliver power in one direction only, from the AC side to the DC side. Therefore, an auxiliary brake is required.
AC drives are a modern development but still do not represent the majority of units in the field. They can come in two ways: with or without the possibility of feeding back energy into the rig net. The first option does not require a separate auxiliary brake as these units can be switched into generator mode, thus reducing the block speed.
The main brakes are always friction brakes. They are able to hold the load steady and deliver high braking forces in emergency cases. It is, however, not desirable to use them for all braking action because of the excessive amount of heat created in the brakes at higher velocities and the associated material loss due to wear.
The band brake system uses two steel bands with screwed-on brake pads that grip around the brake drums on each side of the main drum. As both bands need to be attached to an equalizer beam to achieve equal braking forces on both sides of the drum, they are not independent of each other, and failure of one band will compromise the function of the other. Braking force is delivered mechanically from a brake lever to the bands.
The disk brake on the other side uses a series of brake calipers (most commonly four per disk) distributed around the front side of the braking disks. The system is similar to disk brakes found in cars; however, the amount of calipers and the size are different. The benefit of these brakes is that they use emergency calipers that close automatically in case power is lost. The brake operates hydraulically and therefore requires a hydraulic power pack.
Parkersburg (or hydrodynamic) brakes develop braking force by shearing water molecules though small gaps between the stationary and the rotating part, thus reducing speed. As the shear effect is dependent on the relative speed between these two parts, this type of brake yields very low braking forces at low speeds and can therefore not bring the load to a complete stop. On the other side, it efficiently transforms the involved potential and kinetic energies into heat. Parkersburg brakes are most commonly found on workover rigs and rarely on smaller drilling rigs.
Elmagco (or electromagnetic) brakes operate by eddy currents that are induced in the rotor due to a magnetic field borne in DC stator coils and the rotor movement. Again, this brake delivers braking force only when there is movement. It is not suitable for keeping the load stationary. Due to its design, this auxiliary brake is easier to control and can handle the highest braking energies.
Dropped block incidents
The following briefly describes incidents that occurred on OMV-contracted rigs. Mitigation measures will be discussed in the following section.
Deficient power pack for disk brake
A dropped block incident occurred due to a complete failure of the disk brake system. Investigation showed that the power pack was not adequately maintained and did not deliver sufficient hydraulic pressure. Due to this, the emergency calipers never opened fully, and for a prolonged time the brake pads inside of these calipers were in constant contact with the moving brake disk. This resulted in excessive wear and heat.
The disc overheated, resulting in heat cracks and partial expansion of the steel. This partial expansion resulted in a reduced surface area where the brake pads were in contact with the disk, leading to even more heat and ultimately to glazing, reducing the friction coefficient dramatically. This ultimately resulted in the dropped block. Figure 1 shows a picture of the disk and the described effects. The incident was a result of improper maintenance, both on the power pack and the brake itself.
Operation out of design limits
In another case, a misunderstanding led to a dropped block. While tripping in, the Elmagco brake experienced a bearing failure. Consequently, operations were stopped, and the Elmagco was removed for repair. As the arrival of the replacement Elmagco was scheduled for one week later, OMV and its rig contractor jointly searched for a way to continue operations. After discussions and agreement in the field, it was decided to continue with tripping in using the rig’s DC motors in generator mode. However, the rig was not configured to allow for a backflow of electrical energy from the DC to the AC side. This is possible to achieve but is basically never utilized in oilfield applications; thus, the drawworks motor could not work in braking mode and take out energy from the descending hoisting system.
When the driller realized that the motors were not limiting the lowering velocity, it was already of a magnitude that his attempt to stop the block with the disk brake failed, resulting in the block hitting the rig floor before the brake could reduce the drum speed to zero. Missing management of change was a predominant factor for this incident.
PLC program error
Programmable logic controllers (PLC) are used on modern rigs to partly automate work processes, such as the lowering of the block and brake action. To do so, the PLC receives a series of input parameters (e.g., block position, drum rpms, hookload) and uses them to produce commands for different motors, pumps, etc. The clear aim for such controllers is to enhance safety by limiting block speeds, anti-collision, etc. For this block incident, however, a program code deficiency led the PLC to do exactly the opposite. Although the programming was extensively tested during rig commissioning, one piece of coding remained: In the case of a short circuit on one of the PLC input sensors (a temperature sensor, in this case) when the block is in a very low position (below the floor saver limit), the PLC would give an output signal to the drawworks motors, ordering them to move full speed in a downward direction. As this command put full power on the motors, the slightly worn disk brakes were not able to hold against the motors, and the block slowly descended into the workfloor. Insufficient quality control of the PLC program, along with insufficient maintenance, was a major factor for this incident.
Although no one was injured in any of these incidents and they resulted only in equipment damage, the potential for serious injuries or worse is clear.
By analyzing the above incidents in more detail, many lessons can be learned. The following is an overview of measures that have been implemented at OMV to reduce the risk of similar incidents.
The measures in regard to equipment are straightforward and well known in the industry: use of adequate equipment from notable manufacturers, rigorous maintenance following all OEM and international standards requirements. Repairs are to be conducted in accordance with OEM instructions and by certified personnel. New safety-related equipment shall undergo a commissioning test.
The example of the defective PLC coding is a good example illustrating that there will always be residual risk associated with equipment: Even with rigorous commissioning process, QA/QC controls by the manufacturers and implementation of safety barriers, a chance for failure remains. It is important to keep that in mind and to reach for ALARP.
To ensure ALARP in regard to the equipment, it is recommended to both contractors and operators to establish an audit initiative that revisits all units in adequate intervals and checks on equipment condition and certifications.
Crew competence is – even on the most modern and heavily automated rigs – of paramount importance. A driller with many years of experience on rigs and a certain feel for “his” rig is able to determine changes to the brake systems just by sounds and physical feedback. A knowledgeable mechanic will not only conduct regular equipment checks but will also be able to see and measure indications for degradation or misuse of equipment.
Contractors must have a detailed understanding of training requirements for their rig crews and the individual positions. Some equipment require special training in regard to use and maintenance, and that should be reflected in a training matrix. Additionally, it is important to understand the value of experience of key personnel and avoid overly rapid development of personnel or frequent crew changes.
Operators are advised to develop their own competency requirements and enforce them. Although they need to take a wider approach than contractors, they are able to set up boundaries that will limit the risks significantly.
With the development of management systems in recent years, the industry has taken a leap forward in achieving safer work environments. While the documents are just paper, adequate management systems that are lived and enforced are of great value as they analyze the involved risks, determine mitigation measures, define the boundaries of safe work and ensure adequate oversight.
A major deficiency encountered with several contractors is that, although they possess detailed and adequate management systems, they are not fully implemented in the field, especially in second and third world countries. It remains a major challenge for both contractors and operators to ensure that management systems are adhered to on all operations worldwide. This calls for operators to make clear demands and ensure they are met through audit and review. Operators must understand this comes with additional costs for contractors.
Ultimately, it requires fair partnerships, where safety comes first.
This article is based on a presentation at the 2013 IADC Drilling HSE Europe Conference, 25-26 September, Amsterdam.