P-36 Accident - Lessons learnt Considerations with respect to DNV classification rules and systematics By: Svein Flogeland, DNV Head of section, classed units in operation Presentation at NPD / Petrobras seminar, Stavanger 2002-04-30 Slide 1
Background Mobile Offshore units classified in DNV: 73 Column-stabilised drilling/support units 16 Ship-shaped drilling units 25 Oil Production and/or Storage units(of which 5 Column-stabilised) 9 Floating production units (of which 4 columnstabilised) designed and built to DNV Rules, class not maintained in-service Slide 2
Background DNV conducted verification of P-36 Inquiry Commission work This work has given detailed insight in contributing and possible underlying factors Root causes seem to be mainly of operational nature Underlying design issues may have played a role DNV has reviewed current Rules to check how such issues are addressed Review has mainly addressed rules for production units but bearing on drilling units has also been considered Slide 3
Design issues Arrangement - hydrocarbon tanks in marine zones Risk assessment for non-standard design solutions System interfaces Use of vital common systems on floating units e.g. cooling, ballast, firewater Prioritised alarm system design Overall risk analysis Slide 4
Slide 5 Area Classification Several standards accepted by the industry as well as by DNV (API, IEC, MODU Code, IP) Requirements vary between standards Important to exercise sound engineering judgement when choosing standard understand consequences such as e.g. requirements to ventilation, protection of equipment, alarms and marking arising out of using chosen standard use the chosen standard consistently and to document hazardous area classification on drawings, ref. DNV-OS-A101 sec. 4 A202
Area Classification DNV has no technically sound reason or experience to prioritise among accepted standards DNV considers to emphasise aspects related to choosing and using code for area classification further in the Rules Slide 6
Risk assessment non-standard design Location of emergency drain tank inside loadbearing structure allowable according to DNV rules under certain circumstances (implicit in DNV-OS-A101 Sec7) Area classification will give rise to requirements for ventilation, ESD and signboard marking (DNV-OS- A101 Sec4 F104) Slide 7
Risk assessment non-standard design DNV would regard this issue a non-standard design solution requiring risk assessment (ref DNV-OS-A101 Sec 2 C103) and evaluation versus established acceptance criteria Risk analysis such as Hazop, FMEA etc. would be complement to engineering standards and classification rules Slide 8
System interfaces Interfaces between maritime/process and topsides/hull of major importance for floating oil production units Rules and standards focus technical requirements on individual systems Interfaces are normally described only in general terms as they will vary from unit to unit Important to establish safety strategies and area classification plan for the interfaces and give them attention as required by criticality and possible failure consequences Slide 9
Combination of vital common systems Important systems like these shall normally be separated, ref. MODU Code 9.4.15, DNV-OS- D301 Sec3 B306 and NORSOK S-001 10.7.2 Rules and standards used in the offshore industry allow for combination of systems based on risk assessment (principle of equivalent safety) Slide 10
Combination of vital common systems Combination of sea water and fire water systems not uncommon on offshore units/platforms Combined sea water and fire water systems may lead to conflict of safety issues, and thus complex decision processes in emergency situations, e.g. maintaining firewater supply versus watertight integrity by shut-in of supply valves/pumps DNV will continue practice of equivalent safety but will focus even more on corresponding risk assessment and documentation of equivalent safety Slide 11
Prioritised alarms The combined alarm system on P-36 gave rise to unmanageable alarm flood when the emergency developed On this background, one could argue that maritime and process alarms should be separated on floating production units alarms should be prioritised according to possible consequences to aid people interpreting the alarm signals in emergency situations Slide 12
Prioritised alarms DNV will address relevant organisations such as IACS and IMO to discuss possible code requirements for separation and prioritisation of alarms Slide 13
Overall risk analysis Governing rules and standards may capture specific systems but can not cover all possible system combinations and events Risk analysis of complex production units is a useful and necessary tool Sound engineering judgement and imagination w.r.t possible failure modes required Previous accidents and lessons learnt always need to be brought along as part of imaginative process Slide 14
Overall risk analysis DNV-OS-A101 contain requirement for risk analysis of complex and non-standard applications, as well as guidelines for Formal Safety Assessment Detailed guidelines for risk analysis are available from several other sources like API, ISO and NORSOK Slide 15
Conclusions Existing DNV classification rules contain technical provisions and requirements addressing the underlying design issues that possibly contributed to the chain of events that lead to the loss of P-36 DNV will not introduce major new technical requirements for floating oil production units following the investigation report, but consider: possible requirement for separation of alarm signals to emphasise existing rule text regarding area classification and non-standard design applications Slide 16
Conclusions The real challenges appear to be to define scope and extent of risk and consequence analyses, and to engage designer and controller in comprehensive what-if processes Knowledge and experience must go hand in hand with constructive imagination Slide 17
Conclusions Drilling units could be subject to similar events as P- 36 if hydrocarbon tanks are located in the structure (welltest, tempstore) If so, rules and regulations governing such use must be applied, and resulting hazards and accident scenarios must be duefully considered Slide 18