_____________________
DP Conference
Houston October 21-22, 1997 Session 7 Page 1
Marine Technology Society
Dynamic Positioning Conference
21 - 22, October, 1997
Session 7
Power Plant
Design Review - Fit for Purpose
By: Roger Cornes
Wavespec Ltd.,(Maldon, U.K.)
Session Planner
Pete Fougere:
Transocean (Houston) R. Cornes Dynamic Positioning Committee Marine Technology Society
____________________________________________________________________________________________
DP Conference
Houston October 21-22, 1997 Session 7 Page 2
Design Review - Fit for Purpose
R.G.Cornes, Wavespec Limited
Maldon, UK
1.0
Introduction
Offshore exploration and production calls for extensive use of offshore vessels of all types and
sizes. These vessels are essentially floating platforms supporting activities associated with the
exploration, development and maintenance of oil and gas fields. The safety aspects of operations
such as drilling, well stimulation and those involving the deployment of subsea divers are
extremely sensitive to station keeping capability of the vessels employed. The consequences of
uncontrolled excursions are potentially catastrophic in terms of loss of life and damage to
equipment and environmental damage. Project losses in terms of delays and capital expenditure
have also to be taken into consideration.
In the early days, particularly in the North Sea, when there was little regulation concerning
offshore vessels, a number of serious incidents occurred involving DP vessels. This lead to the
necessity for the charterer to impose on the vessels owners DP audits and trials to determine
fitness for purpose before a DP vessel was taken on hire. Modifications and limitations were
recommended by the auditors where deficiencies were found and this had the effect of reducing
the number of incidents. Today regulation and the continued need for audits or design reviews
have continued the downward trend in the number of incidents with the incidents that do occur
being of much less seriousness. However, from time to time, new technology and equipment
introduced perhaps before being fully proven can provoke a major incident. Consequently, in
spite of the regulations in force today, there is still a need for technical audits or design reviews to
ensure a vessel has a capable and safe DP system and is fit for purpose. Wavespec Limited
personnel have carried out DP audits and trials worldwide but most of this work has been gained
in the North Sea environs. Wavespec Limiteds believe is that should one deficiency be
uncovered during an audit which could potentially result in a serious incident, then the audit is
fully justified.
So what does constitute a capable and safe DP system? Quite simply it is one in which any single
failure cannot result in a loss of station keeping ability. For example, all thrusters taking their
power supplies from a single switchboard is unacceptable on the basis that a short circuit on the
bus bars of that switchboard would have disastrous consequences. It could be argued that the
risk of such an occurrence is extremely small, but it is one which could nevertheless happen and
cause a blackout resulting in a catastrophic event. Prudent operators take the view that no matter
how slight the probability of a failure, it will happen and happen at the worst possible moment.
This leads to the concept of redundancy. In the example above, the switchboard would be split
into two sections coupled by a bus tie breaker with half of the thrusters powered from one section R. Cornes Dynamic Positioning Committee Marine Technology Society
____________________________________________________________________________________________
DP Conference
Houston October 21-22, 1997 Session 7 Page 3
and half from the other section. The thrusters would be arranged such that, on loss of one section
of switchboard, only half of the alongships and only half of the athwartships thrust would be lost.
In order not to lose position, the vessel would be limited to 50% capability on thrust in each
direction.
2.0
Consequence Classes and Equipment Classes
The concept of redundancy leads further to the question of how much redundancy is required in
the DP system. The location in which a DP vessel is allowed to work and the scope of the work it
is going to carry out should be governed by the amount of redundancy the vessel has in its DP
system. This has been addressed by the NMD and IMO and has led to the introduction of
Consequence Classes and Equipment Classes.
The Norwegian Maritime Directorate (NMD) grouped the consequence of failure into four
classes: Consequence Class 0, which are operations where loss of position keeping capability is
not considered to endanger human life or cause damage; Consequence Class 1 operations which
are operations where damage or pollution of small consequence may occur in case of failure of the
positioning capability; Consequence Class 2 operations which are operations where failure of the
positioning capability may cause pollution or damage with large economic consequence, or
personnel injury; and Consequence Class 3 operations, which are operations where loss of
position keeping capability will probably cause loss of life, severe pollution and damage with
major economic consequences.
IMO defines the vessel equipment classes by their worst case failure modes. For Equipment Class
1, loss of position may occur in the event of a single fault. For Equipment Class 2, a loss of
position is not to occur in the event of a single fault in any active component or system.
Normally, static components such as manual valves and piping systems are not considered to fail
provided they can be shown to be adequately protected from damage and reliability is proven.
Single failure criteria include any active component or system, e.g. generators, thrusters,
switchboards, remote controlled valves, etc., together with any normally static component (cables,
pipelines, manual valves, etc.) that cannot be shown to have adequate protection from damage or
have proven reliability.
For Equipment Class 3, the single failure modes include those in Equipment Class 2 plus that in
which any normally static component is assumed to fail. Additionally, all components in any one
watertight compartment are assumed to fail due to the effects of fire or flooding and all
components in any one fire subdivision are assumed to fail due to the effects of fire or possibly
flooding.
A summary of the vessel requirements to suit these Classes is given at the end of this
paper.
The design of a vessels DP system complying with Equipment Class 3 would therefore have a
power system divided into two or more systems so that failure of one will have no effect on the
other(s). The power generation system will have a minimum of two enginerooms separated by an R. Cornes Dynamic Positioning Committee Marine Technology Society
____________________________________________________________________________________________
DP Conference
Houston October 21-22, 1997 Session 7 Page 4
A60 bulkhead. In the case of a two engineroom system, half of the generating capacity would be
located in one engineroom and the other half in the other engineroom. The switchboard room
would similarly be split into two rooms with half of the switchboard located in one room and half
in the other room. The sections of bus bars would be coupled by two bus tiebreakers one located
in each section of switchboard. The supplies to the thrusters would be configured such that only
half of the thrust capability in both alongships and athwartships direction is lost should a section
of switchboard fails. Thrusters would be located in compartments such that those located in a
single compartment would not be supplied from both sections of switchboard. With the effect of
fire being considered, a backup DP control station would be located in a separate compartment to
that in which the main control station is located. Cabling to items of redundant equipment would
not be run through the same compartment but be run segregated such that a cable blow out or a
fire would not affect both units.
It follows from these Consequence Classes and Equipment Classes that a Consequence Class 0
operation can be carried out by the equivalent of an Equipment Class 1 vessel with little or no
redundancy or, indeed, a vessel with more redundancy, but a Consequence Class 3 operation can
only be carried out by the equivalent of an Equipment Class 3 vessel with considerable
redundancy.
Prior to the audit taking place, it is established from the Charterer of the vessel what
Consequence Class operation will be carried out and hence the "Equipment Class" which will be
required in the vessel. If the vessel is to be operated on full DP and an unintentional loss of
position can be tolerated, then DP to Class 1 should be chosen. On the other hand, some clients
will require Class 2, but require a Class 3 operation for critical operations. The trade off between
cost and benefit should be considered when an "Equipment Class 3" vessel is requested.
3.0
Auditing of DP Vessels:
Wavespec carries out formal FMEAs (Failure Modes and Effects Analyses) for new buildings,
conversions and existing vessels to