SIL and ballast systems

Working on floating oil and gas facilities, one question keeps popping up about ballast systems. Should they have SIL requirements, and what should in this case the requirements be? When seeking to establish requirements for such systems, several issues are uncovered. First of all, current designs of ballast systems are very robust due to evolution of designs and requirements in shipping over a long time. Further, the problem is much more complex than collecting a few well-defined failure modes with random error data leading to a given situation, as typically seen in may process industry type problem descriptions. This complexity depends on a number of factors, and some of them are specific to each ship or installation, such as location, ship traffic density or operating practices of personnel onboard. Therefore, any quantitative estimates of “error probabilities” contributing to an expected return frequency of critical events concerning the system will have significant uncertainties associated with them.

ship-storm1

A ballast system is used to maintain the stability of a ship or a floating hull structure under varying cargo loading conditions and in various sea conditions and ship drafts. Water is kept in tanks dispersed around the hull structure, and can be pumped in or out, or transfered between tanks, to maintain stability. Errors in ballasting operations can lead to loss of stability, which in the worst consequence means a sunken ship. The ballasting operation is normally a semi manual operation where a marine operator would use a loading computer to guide decisions about ballasting, and manually give commands to a computer based control system on where to transfer water into or out of a particular ballast tank. Because this is such a critical safety system it is a natural question to ask: “what are the performance requirements?”.

Ballast systems have been part of shipping for hundreds of years. Requirements for ballast systems are thus set in the classification rules of ship classification societies, such as Lloyd’s Register, DNV GL or ABS. These requirements are typically presecriptive in nature and focus on robustness and avoidance of common cause failures in the technology. Maritime classification societies do not refer to safety integrity levels but rely on other means of ensuring safey operation and reliability. Society has accepted this practice for years, for very diverse vessels ranging from oil tankers to passenger cruise ships.

In oil and gas operations, the use of safety integrity levels to establish performance requirements for instrumented safety functions is the norm, and standards such as IEC 61508 are used as the point of reference. The Norwegian Oil and Gas Association has made a guideline that is normally applied for installations in Norwegian waters, which offers a simplification of requirements setting based on “typical performance”. This guideline can be freely downloaded from this web page. This guideline states that for “start of ballasting for rig re-establishment”, ths system should conform to a SIL 1 requirement. The “system” is described as consisting of a ballast control node, 2 x 100% pumps and three ballast valves. In appendix A.12 of the guideline a description of this “sub-function” is given with a calculation of achievable performance.

It may be argued that this functional description is somewhat artificial because the ballast system on a production installation is normally operated more or less continously. The function is defined for a single ballast tank/compartment, irrespective of the number of tanks and the necessary load balancing for re-establishing stability. The Guideline 070 approach is based on “typical performance” of the safety system as it is defined, and is not linked directly to the required risk reduction provided from the system. Multiple approaches may be taken to assign safety integrity levels based on risk analysis, see for example IEC 61508. One such method that is particularly common in the process industries and the oil and gas industry is “layers or protection analysis”, or LOPA for short. In this type of study, multiple initating events can contribute to one hazard situation, for example “sunken ship due to loss of stability”. Multiple barriers or “independent protection layers” can be credited for reducing the risk of this hazard being realized. In order to use a risk based method for setting the integrity requirement, it is necessary to define what is an acceptable frequency of this event occurring. Let us say for the sake of the discussion that it is acceptable that the mean time between each “sunken ship due to loss of stability” is 1 million years. How can we reason about this to establish requirements for the ballast system? The functional requirement is that we should “be able to shift ballast loading to re-establish stability before condition is made unrecoverable”. In order to start analyzing this situation, we need to estimate how often we will have a condition that can lead to such an unrecoverable situation if not correctly managed. Let us consider three such “initiating events”:

  • Loading operator error during routine ballasting (human error)
  • Damage to hull due to external impact
  • Error in load computer calculations

Both of these situations depend on a number of factors. The probability that the loading operator will perform an erronous situation depends on stress levels, competence/training and management factors. A throrough analysis using “human reliability analysis” can be performed, or a more simplified approach may be taken. We may, for example, make the assumption that the average operator makes 1 error without noticing immediately every 100 years (this is an assumption – must be validated if used).

Damage to hull due to external impact would depend on the ship traffic density in the area, if there is a difficult political condition (war, etc.), or if you are operating in arctic environments where ice impact is likely (think Titanic). Again, you may do extensive analysis to establish such data, or make some assumptions based on expert judgment. For example, we may assume a penetrating ship collition every 100 years on average.

What about erros in load computer calculations? Do the operators trust the load computer blindly, or do they perform sanity checks? How was the load computer programmed? Is the software mature? Is the loading condition unusual? Many questions may be asked here as well. For the sake of this example, let us assume there is no contribution from the loading computer.

We are then looking at an average initiating event frequency of 0.1 for human errors and 0.01 for hull damage.

Then we should think about what our options for avoiding the accidental scenario are, given that one of the initiating events have already occurred. As “rig re-establishment” depends on the operator performing some action on the ballast system, key to such barriers is making the operator aware of the situation. One natural way to do this would be to install an alarm for indicating a dangerous ballast condition, and train the operator to respond. What is the reliability of this as a protection layer? The ballast function itself is what we are trying to set the integrity requirement for, and any response of the operator requires this system to work. Simply notifying the operator is thus necessary but not enough for us. In case the ballast system fails when the operator tries to rectify the situation, the big question is, does the operator have a second option? Such options may be a redundant ballast system, not using the same components to avoid common cause failure. In most situations the dynamics will be slow enough to permit manual operation of pumps and valves from local control panels. This is a redundant option if the operator is trained for it. If the alarm does not use the same components as the function itself, we have an independent protection layer. The reliability of this, put together with the required response of a well-trained operator cannot be credited as better than a 90% success rate in a critical situation (ref. IEC 61511, for example).

So, based on this super-simplified analysis, are we achiving our required MTTF of 1 million years?

Events per year: 0.02.

Failure in IPL: Alarm + operator response using local control panels: 0.1.

OK, se we are achieving an MTTF of:

1/(0.02 x 0.1) = 500 years.

This is pretty far from where we said we should be. First of all, this would require our ballast system to operate with better than SIL 4 performance (which is completely unrealistic), and furthermore, it includes the same operator again performing manual actions. Of course, considering how many ships are floating at sea and how few of them are sinking, this is probably a quite unrealistic picture of the real risk. Using super-simple tools for adressing complex accidental scenarios is probably not the best solution. For example, the hull penetration scenario itself has lots of complexity – penetrating a single compartment will not threaten global stability. Furthermore, the personnel will have time to analyze and act on the situation before it develops into an unrecoverable loss of stability – but the reliability of them doing so depends on a lot on their training, competence and the installation’s leadership.

The take-away points from this short discussion are three:

  • Performance of ballast systems on ships is very good due to long history and robust designs
  • Setting performance requirements based on risk analysis requires a more in-depth view of the contributing factors (initators and barriers)
  • Uncertainty in quantiative measures is very high in part due to complexity and installation specific factors, aiming for “generally accepted” technical standards is a good starting point.

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