Anchoring Failure Can Cause Major Accidents

The Petroleum Safety Authority Norway is concerned that incidents involving anchoring systems could cause major accidents. The report entitled "Anchoring of facilities on the Norwegian shelf" shows that such incidents are caused both by failure to organize work and responsibility, deficient training of involved personnel and failure of technical equipment.

In other words, it is not an isolated cause but rather a number of factors that form the background for anchoring system incidents in recent years.

This is illustrated by a summary compiled by the PSA. In this respect, last year's accident involving the Ocean Vanguard was the most serious incident to occur in many years.

Greater focus
The PSA feels that training and organizational factors must receive more attention in the future. On the technical side, the number of uncontrolled chain deployments must be limited. Ruptures of chains, shackles and fiber lines must be avoided, and the necessary holding power for the anchor must be safeguarded. Assumptions made in the anchoring analysis must be observed when the facility is in operation. Better quantification of the risk entailed by the loss of anchor lines is also necessary. The PSA has not identified a need for amendments to the regulations.

Raising standards and responsibility
The PSA would point out that many of the incidents have occurred in connection with operations. The facility has been connected to the well - or has been alongside another facility. Even though the anchoring system is designed to withstand a line break, this is still an undesirable incident. The PSA sees a need for a general raising of standards in the industry.

The equipment on the facility is the shipowner's responsibility, and this is followed up through the AoC scheme. The site-specific evaluations are the operator's responsibility, and these are followed up through the consent system. The shipowners themselves have initiated work under the direction of the "Operations Committee for Drilling Contractors" (DUBE), and have proposed measures aimed at reducing the number of incidents.

Deployment of chain from anchor winches
According to the regulations, each anchor winch must have two independent brake systems engaged at all times. Both brake systems have failed on those occasions when the chain deployed in an uncontrolled manner. It has been impossible to determine how often each individual brake system fails. However, the high number of incidents involving failure of both brake systems with resulting chain deployment indicates that the failure rate is extensive.

We believe that several of the incidents probably would not have occurred if the industry had a good system for exchanging experience - and if the crew had good enough knowledge about how anchor winches and brakes function. The PSA believes that the shipowners' maintenance of the anchor winches should receive more attention.

Several fatigue ruptures have occurred on anchor chains. The cause of the fatigue has been bending stress. It is reasonable to assume that the bending stress has occurred at the sprockets. The PSA believes that this may be reason enough to consider whether the design of the sprockets is serviceable.

Chains that are more than 20 years old are still in use. Therefore, the inspections and repairs conducted in connection with recertification are essential in ensuring that the chain meets the applicable quality requirements for the anchoring line. The shipowner must know the history of each individual line (traceability) in order to ensure a successful recertification.

In the event of deficient traceability, the PSA requires that 20 percent of the chain links are inspected in connection with recertification. On the other hand, if one knows the history of each individual line, inspection of just five percent of the chain links is sufficient.

There are just as many ruptures in shackles as in chain links, and both types of ruptures result in the same consequence - unintentional drift. Since the number of shackles is small compared to the number of chain links, each individual shackle has a higher rupture frequency than the chain links. The PSA is of the opinion that special attention should be given to the selection of shackles, as well as in connection with assessments of the condition of the shackles.

Fiber rope
In some cases, use of fiber rope in the anchor lines may be advantageous in both safety and operational terms. On the other hand, fiber rope has proven to be very vulnerable to mechanical strain, e.g. when in contact with wire. The PSA believes that activities carried out within the anchor pattern must be facilitated so as to take better account of the vulnerability of the fiber lines.

Anchor holding power
The anchor holding power on mobile facilities must be stipulated more precisely than has been the practice so far. For a drilling facility that is drilling an exploration well, a limited dragging of the anchor will not necessarily entail major consequences. On the other hand, experience has shown that dragging an anchor can lead to ruptures of the neighboring line. More and more often, drilling facilities are anchored in areas with many subsea facilities.

There is a need to increase the anchors' test load (test tension) on more mobile facilities. Even with good knowledge of the soil conditions, it may be difficult to achieve a good foundation solution based on conventional anchors (drag anchors). In some cases, consideration should be given to selecting a different form of anchoring. If one accepts dragging anchors, the PSA believes that this must take place together with an evaluation of the consequence this will have for the tension on other lines - as well as the possibility of damage to subsea facilities.

One must expect concurrence between the anchoring analyses that are carried out and the practical anchoring work on the facility. Good anchoring analyses will not enhance safety if they are merely an academic exercise.

The risk analyses we have considered have not addressed anchoring in detail. Several failure modes have not been identified or analyzed, nor have the analyses been used as a basis for reducing risk. There is a need for improvements here. As such, the report is an initial attempt to describe how one can analyze the likelihood anchoring system failure.

The verification in Section 57 of the Facilities Regulations can help ensure that the anchoring analyses provide the desired level of safety. The verification must address all aspects associated with the anchoring, and the anchor holding power must thus also be part of the verification.

For More Information on the Offshore Rig Fleet:
RigLogix can provide the information that you need about the offshore rig fleet, whether you need utilization and industry trends or detailed reports on future rig contracts. Subscribing to RigLogix will allow you to access dozens of prebuilt reports and build your own custom reports using hundreds of available data columns. For more information about a RigLogix subscription, visit

Our Privacy Pledge

Most Popular Articles

From the Career Center
Jobs that may interest you
Staff Engineer / Scientist
Expertise: Environmental Engineer
Location: Los Angeles, CA
Staff Engineer / Scientist
Expertise: Environmental Engineer
Location: Irvine, CA
Health Scientist / Toxicologist
Expertise: Environmental Engineer
Location: Somerset, NJ
search for more jobs

Brent Crude Oil : $51.78/BBL 0.77%
Light Crude Oil : $50.85/BBL 0.83%
Natural Gas : $2.99/MMBtu 4.77%
Updated in last 24 hours