There are international standards for the design and operation of DP vessels. The main reference standards have been established by the International Maritime Organisation (IMO) and by the major Classification Societies.
In addition there are various international industry standards that have been developed over the years, the principal sources of which are the International Marine Contractors Association (IMCA) for equipment and technical standards and the Nautical Institute for DP operator training standards.
Further guidance for accepted industry operating standards are contained in the following documents which should be read in conjunction with this Section.
IMCA Guidelines for the Design and Operations of Dynamically Positioned Vessels
IMCA Guidelines for the Safe Operations of Dynamically Positioned Offshore Supply Vessels
IMCA Guidelines for Diving Operations from Vessels Operating in Dynamically Positioned Mode.
Documentation for DP Vessels
IMO MSC Circ. 645 (1994)
IMCA Circulars M103 (1999) and M117 (1996)
Any DP vessel/barge contracted by, or on behalf of, R G Howell shall have a structured and documented Safety Management System (SMS) enabling Company personnel to effectively implement the Company health, safety and environmental policies. Two of the key elements of success are the competence of personnel and the identification of risk.
Audits and Assurance
Vessel Safety Management Systems require inspections and audits, and any vessel chartered by, or on behalf of, R G Howell (RGH), shall be subject to inspection and audit. The first stage of this process is for the Operator of the vessel to supply RGH with basic information on their SMS and the levels of inspection.
All DP vessels must be approved by RGH marine Assurance Department prior to commencing work at any RGH worksite.
It is recommended that vessel Owners are members of the IMCA so that they can be kept advised of any industry standards and, in particular, the networking of DP incidents and statistics. The following documents are required for review of a DP vessel, the five bullet points in bold are always required:
- Copy of the latest valid independent DP FMEA
- Confirmation that no major changes to DP systems have been carried out since the FMEA, or if so, details
- Copy of the current valid independent DP annual trials
Copy of the valid independent marine safety audit report. (Preferably
IMCA/UKOOA or similar standard)
- • Copy of the marine crew qualifications and experience (ie CV-type document, particularly with reference to experience on the proposed vessel)
- • If possible a brief matrix of crew courses attended
- • Record of any DP failures in the last 6 months and the actions to prevent recurrence.
Depending on the amount of information available for review and the findings, a visit to thevessel may be necessary.
If a vessel visit is necessary, information will be gathered by:
• Review the minutes of safety meetings for any accidents and in particular any recurring actions that have not been addressed, assess the trends in ccidents/incidents
• Brief and informal discussions with officers and crew to determine the level of afety culture
• Review the Permit to Work System, risk assessments and their control
• Review the effectiveness of the application of the Safety Management System
• Review the planned maintenance system in place, reasonably up to date and spot heck being followed.
• Review the Company’s Marine and DP Operations Manual/Procedures
It is useful to know the type of workscope that the vessel is to be carrying out, particularly critical operations in order to decide if any limitations on the vessels use should be recommended. As the review progresses it may be necessary to request additional documents or information to clarify certain points.
After the above review process the Marine Assurance Department will advise those concerned and the vessel operators whether the vessel is acceptable and if any restrictions on operations areto be imposed. This advice will be for the specific workscope.
DP Classification (IMO)
International Maritime Organisation (IMO) has defined three equipment classes:
Equipment Class 1
Loss of position may occur in the event of a single fault.
Equipment Class 2
Loss of position should not occur from a single fault of an active component or system such asgenerators, thrusters, switchboards, remote controlled valves, etc. but may occur after failure of a static component such as cables, pipes, manual valves, etc.
Equipment Class 3
Loss of position should not occur from a single failure including a completely burnt fire sbdivision or flooded watertight compartment.
A single fault includes a single inadvertent act by any person onboard the DP vessel.
Classification Society DP Rules
All major Classification Societies have developed their own rules for DP vessels based on the IMO guidelines. The major classification societies are the American Bureau of Shipping (ABS); Lloyds Register of Shipping (LRS); Det Norske Veritas (DNV)
General Policy on DP Operations
RGH has adopted the policy that all DP vessels operating at their offshore locations shall have the following:
• Valid DP Class notation from one of the main Classification Societies, three of hich are referred to above.
• Flag State Verification and Acceptance.
It is RGH’s policy that when operating on DP the following will apply:
- Supply Vessel’s will have a minimum of Class 1 with the long term objective that ll support vessels will be Class 2.
- Survey and ROV vessels will have a minimum of Class 2 when operating within th 500m-zone and in areas where there is any possibility of contact with or damage o equipment. Outside the 500m zone Class 1 can be accepted.
- Diving, Flotel, Pipelay, Fracturation and Construction vessels will have aminimum of DP Class 2.
- Drilling vessels have a minimum of Class 3.
It is RGH long-term objective that all vessels operating on DP at offshore installations will be a minimum of IMO DP Class 2 or equivalent classification.
World wide there are a considerable amount of supply and survey vessels operating on DP with no equipment classification where there is no recognized standard for the installation and operation of the DP equipment. With retrofits on older vessels, the installations generally do not have adequate propulsion or thruster systems, and on occasions the DP control system is not type approved. These vessels may not be using competent and trained personnel and are a serious importation of risk to an offshore installation. Their use requires careful consideration.
Enhanced Joystick Facility
On newer PSV tonnage these non-classed systems can be called ‘Enhanced Joystick Facility’ where the ship handler can use the input of a reference system, such as Fanbeam , DGPS or RADius, to the joystick to maintain the vessel’s position or move the vessel to a position as a result of command input by the ship handler. This facility is basically a degraded DP system and should only be considered as an aid to ship handling and only be used with limitations agreed by RGH.
The weakness of these systems may include:
1 Levels of equipment and redundancy on board.
2 Reduced capability on weather side working
3 Limited capability for working within anchor patterns
4 Limited use of setup, entry and operational check lists
5 Manning levels and experience of operators.
Weather conditions, suitability of the position reference system, knowledge and skill of the ship handler are to be taken into account prior to engaging enhanced joystick control. It must only be used where it would be possible to manoeuvre the vessel in joystick control and where the ship handler could remove the vessel from hazards, in event of failure of the system using normal manual control.
Authority and Responsibility
Nothing in this section shall supersede the spirit or letter of any legislation that covers theauthority and responsibility of individual’s onboard DP vessels or representatives of wners/operators ashore.
Vessels owned, operated or chartered by, or on behalf of, RGH, will comply with the latest revision of International Marine Contractors Association (IMCA) guidelines for the design and operation of DP vessels, and will be audited in line with the latest revision of United Kingdom Offshore Operators Association (UKOOA) guidelines for auditing vessels with DP systems. Where conflict exists between legislation and/or these guidelines, the more rigorous requirement will be used in order to safely and efficiently complete the workscope. Vessels operating on DP shall also comply with International Maritime Organisation guidelines published June 1996.
Irrespective of the sea area in which a DP vessel is working, it is a fundamental necessity for the Owner/Operator of the vessel to define the responsibilities and authority of the various key members of the crew.
A fully operational DP system shall reliably keep a vessel in position and working such that the maximum excursion from vessel motions and position control system accuracy shall be equal to, or less than, half the critical excursion for the work being carried out (>50% redundancy).
Safe Working Limits
Safe working limits shall be determined for each geographical location and type of task to be performed. These limits must consider every possible failure mode, and the likely time to restore the DP system and/or move clear of an area such that a safe situation exists. Particular attention should be paid to the following scenarios:
• Failure of any section of the power generation, distribution and control systems;
• Blackout situation and the vessel movement against time to restore power while operating within the 500-m zone or anchor catenaries or engaged in crucial subsea or urface activities;
• Time to disconnect any equipment such as hoses from an installation or make safe an nderwater worksite and recover divers to a safe position.
“A Safe Situation” means where the work has or could immediately cease and there are noserious consequences.
“Safe Working Limits” are the environmental limits that would make critical excursion from a single fault very unlikely either through adequate control and power remaining; environmental loads are small and the time to reach a safe position is short; existing circumstances and conditions position loss do not have any serious consequences.
Generally vessels would not operate on DP closer than 10 metres proximity from another vessel or installation extremity such as an overhang unless under exceptional circumstances.
Redundancy of Equipment
DP vessels requiring to operate within the 500-m zone of an installation or within anchorcatenaries or engaged in crucial subsea or surface activities must have procedures, which reflect their station keeping and control capability and consequence of possible loss of equipment and position.
Levels of redundancy shall be addressed during pre-mobilisation hazard analysis.
It is essential that all key DP personnel know the consequences to be expected from various failures as the levels of sophistication and redundancy of vessels vary considerably. To assist operators with worst case failures for Class 2 and Class 3 equipment, the DP control system for vessels built after 1st July 1994 should have a continuous analysis function checking that, in terms of thruster and power, the vessel can maintain station following the worst case failure mode.
Failure Modes and Effects Analysis (FMEA)
Documentation on the reliability and availability of the DP system shall be available in the form of an FMEA. The FMEA should be updated after any major changes to plant or equipment, it should be a live document held on board the vessel and the core crew should be familiar with its contents. The purpose of an FMEA is to give a description of the various failure modes of the equipment:
1 Functional breakdown of the DP system or subsystem into blocks with each block described.;
2 Description of each physically and functionally independent item and the associated failure mode;
3 Description of the effect of each failure mode on other items within the system, and on the DP system as a whole;
4 The crew to understand the consequences of any failure should use the FMEA as a reference document.
Capability Plots (Environmental Footprint)
The maximum continuous operational station-keeping capability for the DP system shall be calculated for the following cases:
1 All thrusters operational with maximum effective thrust
2 All thrusters, except the most effective thrusters, operational with maximum effective thrust
3. The maximum number of thrusters and/or power units remaining operational after the worst single failure
4 The equivalent loading on all thrusters in the intact condition shall be calculated to correspond with Item 3.
This information shall be presented in the form of a polar plot. For each condition, several currentspeeds shall be considered, e.g. 0, 1, 2, 3 knots, etc. Current to be coincident with wind and
associated wave loads to be from a fully developed sea.
This theoretical exercise is to provide plots which should be verified during the vessel’s operational life, in the form of ‘footprint’ plots which are to be retained onboard for reference.
All vessels operating on DP should have their own current operational procedures provided by the:
• Control system manufacturer for the operation of the system
• Vessel owner for the current operating practices of the owner
These procedures shall cover all the work for which the vessel is designed or likely to be used and shall include, but not be limited to, the following:
• Capability plots and verification footprints
• Working profiles and capabilities of equipment
• DP alerts and emergency responses
• Responsibilities and communications
• Manning and watch keeping procedures
• Trials and checklist procedures
• Reporting and recording procedures;
• FMEA (current)
All systems trials are to be documented and recorded.
New DP vessels are required to undertake proving trials in addition to, and after, testing and commissioning trials. The DP system shall be proven in all normal modes of operation. When all normal modes are functioning correctly, failure modes shall be simulated and the results of such tests documented. Various performances shall be demonstrated for both the intact and various failed conditions, the outcomes should be documented.
DP systems trials shall be completed annually. The basis, outcome and results of these should be fully documented with results made available for review. A copy of the trials report should be kept onboard.
The trials shall be designed to fully test all power generation, distribution and control systems and be witnessed by a competent independent third party.
Other DP Trials
Other DP systems trials shall be completed whenever major defects are discovered, following major repair and/or up-grade or when a DP related incident has occurred.
Checklists & Documentation
All vessels operating on DP shall have comprehensive system of checklists and documentation for the safe operation on DP. The IMCA “A Guide to DP-Related Documentation for DP Vessels” should be read in conjunction with this Paragraph. However, as a minimum these shall cover the following:
- • Mobilisation checks for new charter;
- • Checklist for completion prior to setting up in DP
- • Checklists for:
a) field arrival
c) Status check/watch keeping hand over
- • Project/task specific checklist for completion prior to starting, for example, heavy lifts, running drill strings, pipe laying, launch/recovery of bell or ROV, diving or any other activity requiring DP
- • Incident recording and reporting
Where the DP and engine control stations are located separately, then each shall require separate checklists. The DP system shall be fully technically documented and current. In addition to the vendors’ data it is essential that there is an overall document, which brings together the equipment, describes the interfaces between various components and identifies the actual make, model and any modifications made to the standard supply. Drawings shall identify all the main components, their location on the vessel and their cable and/or pipe routes. Repairs, modifications and maintenance call out of service engineers shall be recorded, as shall intermittent faults and failures. A comprehensive picture of the DP reliability shall be continuously assessed and made available to RGH.
DP Manning and Training
DP Manning (Bridge & Engine Room)
When any vessel is operating on DP within the 500-m safety zone of an installation, or anchor catenaries or engaged in crucial subsea or surface activities, RGH’s minimum requirement is that there shall be two (2) DP competent and qualified operators on the bridge and on watch at all times. They will man the DP console alternatively, being relieved at regular intervals. [At least one of the operators shall hold a Class 1/Full DP qualification; the other may hold a limited/Class 2 qualification provided he/she has sufficient hours, is considered a competent shiphandler and has the experience to be able to operate unsupervised for the operations envisaged]
On-duty DP operators should not be multi-tasked where safety may be affected.
On drilling, construction or diving vessels, or on other vessels where there are extended periods on DP and the Master has other responsibilities, at least two people (in addition to the Master), capable of assuming the role of DP Operator, must be provided for each watch. On any vessel, the DP Operators will, after a specified period of watch keeping duty, have an equivalent period off-duty.
No DP Operator will have a period of duty exceeding 12 hours.
When any vessel is operating on DP and within the 500-m zone of any installation, anchor catenaries or engaged in crucial subsea or surface activities, the vessel’s engine room will be manned by an appropriately qualified person at all times.
Additionally, on drilling, construction and diving support vessels etc. at least two competent and qualified Engineering Officers are to be on duty at all times. There must be onboard one technician capable of minor fault-finding and maintenance of the DP system.
DP Operator Training
A fundamental aspect of safe and successful DP operations is the training and experience of staff including DP operators, engineers and maintenance technicians. The provision of external qualifications and evidence of competency does not in itself guarantee the people are ‘fit for purpose’ and it is necessary that each vessel identify training requirements for its crew.
Owners are directed to consult the following:
• IMO document – International Convention on Standards of Training, Certification and Watch keeping of Seafarers 1995;
• IMO Marine Safety Circular 738 and quoted in M103 – “Training and Experience of
Key DP Personnel”
• IMCA guidelines on “The Training and Experience of Key DP Personnel”.
When two vessels are operating on DP and working alongside one another the competency of both crews should be the same. In other words, if one vessel is operating to DP Class 2 and the other to Class 1 both crews shall be Class 2 trained even if the equipment they are using is Class 1. If this is not the case the Class 1 vessel should revert to Poscon/Joystick control.
Levels of training and experience shall be addressed during any pre-mobilization-hazard-risk assessment for each work scope. In general, they shall follow the IMCA guidelines.
Responsibilities and Accountabilities
The vessel’s DP operations manual shall define as a minimum the responsibilities and accountabilities of the key following personnel:
Operational personnel such as drilling, diving or project superintendent
• Chief Engineer
• DP Operators,
• Watch keeping Engineers and DP Technicians
Position reference systems provide essential positioning information required by the DP control system. It is RGH policy that two [independent] position reference systems are active and on-line at all times and when operating Class 2 and 3 a third [independent] system shall be on-line or available for immediate selection. The number and types of position references required should depend upon the nature of the work to be performed as well as the environment in which they are required to operate. Care is needed to determine whether redundancy is completely provided by duplication of similar sensors.
Position reference systems should not be subject to common failure modes and, in particular, should be separately powered.
The overriding principle is that safe working limits are to be determined and agreed with RGH and the installation operator. Safeguards against ‘all identifiable risk’ shall be included within the risk-specific procedures together with the type of position reference system. Hazard analysis shall include the following:
• Taut wires – failures shall not give a constant signal when the vessel is moving.
Fouled wire, faulty sensors or inadequate weight should be addressed. Positions of subsea infrastructures that may be damaged or fouled by taut wire weights shall be considered.
• The possibility of single-point failure onshore to DGPS positioning systems
• Acoustic – limits of performance, subsea obstructions shall be addressed. Sensors tethered to the vessel should be avoided where practicable. Deployment of more than one transducer does not make the system redundant if it is still subject to a common mode failure, e.g. one transducer, one transceiver, the thruster noise, thermal layering, etc
• Short-range radio signals – failures from loss of line-of-sight communication
• Pseudo signals
Differential Global Positioning System (DGPS)
Loss of satellite strength and signal whilst operating within the 500m-zone and in particular when close alongside an installation, or anchor catenaries or engaged in crucial subsea or surface activities. Interference on the signal from the installation. Reliability of differential correction systems.
Differential Absolute GPS application (DARPS
Some DP operations require the positioning of a vessel relative to a moving structure. An example of this is the operation of a DP shuttle tanker loading via a bow loading hose from the stern of an FPSO.
DARPS 102 is a position reference system based on dual-frequency GPS. The receiver allows for real-time compensation of errors introduced by high ionospheric activity. Data from dual frequency reference stations will nearly remove the influence of ionospheric effects.
Fan Beam Laser
System that locks onto a single target and/or a number of targets on the structure, from which position must be maintained. Light pulses are sent and received so that range and bearing can be measured. Positioning of reflector on installation, dirty reflectors, fan beam being affected by high visibility markings on installation crews PPE and obstructions.
Failures may not give a constant signal when the vessel is moving. Limits on wire angle, fouled wire, faulty sensors or inadequate weight should be addressed. Positions of sub sea assets that may be damaged or fouled by taut wire weights shall be considered. Not suitable for deep water application.
Limits of performance, sub sea obstructions shall be addressed. Sensors tethered to the vessel should be avoided where practicable. Deployment of more than one transducer does not make the system redundant if it is still subject to a common mode failure, e.g. one transducer, one transceiver, the thrusters noise, thermal layering, etc
Failures from loss of line-of-sight communication; Loss of power on mobile unit
Radius Radar System
RADius was developed for applications that need a relative positioning system. The system detects and measures accurate range and bearing to a transponder(s).
Positioning of reflector on installation, dirty reflectors, and obstructions.
DP Consol Alarm
Visual and audible alarm should there be a deterioration of the position reference signal.
DP Alert Levels.
There are three DP alert levels:
Normal operational status (green light): adequate equipment is online to meet the required performance within the declared safe working limits.
Degraded DP operational status (Yellow Alert); in general it is the condition where one or more items of redundant DP equipment has failed, safe working limits are being exceeded or an excursion of heading or position is likely but the excursion will not be critical.
DP emergency status (Red Alert); is where there is loss of position, or position loss is inevitable.
For a vessel operating on task-specific procedures, the responses to the DP alert signals shall behall be documented.
For a vessel operating on task-specific procedures, the responses to the DP alert signals shall be documented. The following would be the minimum expected onboard drilling, diving or flotel units:
• Green – full DP diving, drilling or
flotel operations can be undertaken
• Yellow – divers shall return to the bell immediately and obtain a seal; air divers shall return to the surface, and decision made by responsible parties whether or not to abandon the dive. Drilling activities suspended. Flotel gangway closed to personnel crossing
• Red – if not already done so the divers shall return to the bell and obtain a seal, and air divers return to the surface; the bell shall be recovered to the surface after due consideration of the hazards involved; the DP Operator shall use all means available to maintain position until the divers are sealed in the bell and the bell is clear of obstructions. Overside equipment to be released or recovered. Drilling equipment to be disconnected. Flotel gangway disconnected.
The response to each status level will depend on the type of operation being carried out.
However, the Master/Officer in charge must move the vessel clear of the 500-m zone should any equipment failure occur which results in operating on DP without 100% redundancy in the required systems as defined in the relevant paragraphs above. With any failure, the DP operation must be suspended and the vessel moved outside the 500-meter zone until the cause of the failure has been identified and the correct remedial action taken.
Due regard shall be paid to any indications of impending weather changes, in particular sudden wind shifts and/or gusting, to ensure that timely action is taken to reduce the possibility of loss of position. Prior to commencing operations every effort should be made to assess any known unusual currents that may occur as a result of changing weather conditions.
• Obtaining regular and frequent weather forecasts for the area of operations.
• Seeking information by radio from other units in the vicinity about prevailing weather conditions in their areas;
• Use of experience to assess the prevailing conditions and likely trends;
• The presentation of environmental information measured by the DP system and any trends in conditions which it can provide;
• The use of onboard meteorological instruments.
Operations Close to Obstructions
Particular care must be exercised when operating on DP in close proximity to fixed objects such as platforms, mooring buoys, etc. The use of anchors and tugs shall always be avoided unless the DP system is particularly equipped and proven for operating with anchor lines. These options are not preferred and shall only be approved after having been fully considered and all other possible alternatives rejected. It is not normally necessary or desirable to use backup moorings alongside a structure. The value of a visual reference from the DP control position to provide an early additional indication of vessel movement is substantial, and the DP vessel heading shall be selected to make this feasible whenever possible.
Close to Other Vessels
When operating on DP close to another vessel, which may or may not be on DP, a vessel is potentially subject to several forms of interference. These include thruster wash (which may affect both hulls and taut wires), acoustic and radio position reference signal interference and intermittent shelter from wind and sea. These factors shall be considered when planning such operations and due allowance made for them. This may take the form of assuming less accurate position-keeping than would normally be expected. Co-ordination and choice of position reference sensors and frequencies, and careful choice of the relative positions of the vessels, is essential.
Comprehensive procedures shall be established and approved by the Marine Consultancy Branch prior to the commencement of the operation. Working close to vessels not on DP, eg. cargo handling from supply vessels, shall be avoided wherever possible especially if divers are deployed.
Towing and Pipelaying
DP shall only be used for these operations when the DP software has been modified/to recognize the additional external forces that such operations can place on the system.
Working with Weather-Vaning Installations/vessels
DP operations are not permitted when working with weather-vaning installations such as turret moored FPSO/installations, off take tankers etc. where there is the possibility for thevessel/installation to swing onto the DP vessel
DP operations at an anchored semi submersible drilling unit should be disco the cargo work to take place.
Supply vessel loading and unloading operations may be conducted simultaneously with diving operations provided that:
- • The wash from the supply vessel does not impinge on the station keeping ability of the DP vessel/barge
- • Containerised/packaged cargo: – there is a safe horizontal separation between the overside lift and the divers work area;
- • Tubular cargo – the diving activity is out with the envelope of a 90-degree cone. The apex of the cone to be sited beneath the crane hook and the cone radius will be no less than 1.3 times the water depth (IMCA D007). The apex of the cone shall be that point in the cranes traverse which is nearest to the diver’s work area.
- • Dive vessel Master, diving supervisor and RGH representative agree.
Pre Set Up on DP
Prior to establishing position keeping on DP at the worksite, the vessel will carry out a series of pre-set-up checks to ensure that all power generation, distribution and control system are fully operational.
These checks will be completed out-with the 500-m safety zone of an installation prior to commencing operations on DP within that zone.
In the case of a vessel which has been operating on DP immediately prior to entering the 500-m safety zone, the completion of such a checklist will not be required prior to entering the zone.
Vessels entering the 500-m zone must advise the installation OIM of the operational status of their vessel by confirming that they have completed their DP checklist. They must also complete the installation “Entry to the 500m zone” checklist along with the installation and receive all relevant information prior to receiving permission to enter the zone.
The closest proximity to an offshore installation is to be established for all operational circumstances.
Generally closest approach should not be less than 10 metres from the installation extremity such as an overhang.
Under exceptional circumstances when this is not achievable the vessel may move closer provided the following conditions are to be applied:
• Captain on the bridge
• Allowable only in “force off” or “drift off” situations
• Station keeping accuracy to be within +/- 1.5 metres of desired position
• Task Based Risk Assessment/Job Safety Analysis completed and recorded.
• DP hardware fully functional, including thrusters, main propulsion, position reference systems, environmental and vessel sensors.
• Duty DP operators must be dedicated to DP duties are not to multi-tasked.
• Approval of RGH Representative prior to ‘closing in’.
• As soon as reasonably practicable vessel returns to >10m envelope.
Within a mooring spread, the DP vessel/barge shall have a minimum 180° safe sector for exit. In the case of a diving vessel if the bell is below the catenary then it shall not be closer than 50m to any mooring line, the safe sector shall be on recovery of the diving bell and the guide weight above the mooring catenary. At all positions within that ‘safe exit sector’ the deepest part of the vessel/barge hull, or hull penetration where applicable, shall be at least 10m above the mooring catenary. In all cases where close approach is required, scale plan and profile drawings shall be provided to ensure that:
• There are no surface obstructions such as offshore Installation platform extremities, lifeboats, gantry systems, etc. The lateral and roll movements of the vessel/barge shall always be taken into account
• There are no subsurface obstructions.
Diving Support Vessels
All DP Diving Support Vessels (DSVs) shall, be minimum DP Class 2 or DP Class 3 and operating in that mode. In addition to the requirements outlined in Section 1 above, DSVs will also be:
• Designed and operated in accordance with relevant legislation and industry practices
• Free of any known failure mode which shall prevent the safe recovery of divers or cause a ‘red alert’
• The determination of safe working limits shall consider the time necessary for the divers to return to the bell and, bearing in mind that position loss is always possible, the likely speed of position loss and the increased position excursion after the worst known failure condition.
Note: Safe working limits will vary depending on location and water depth; lower limits shall apply when divers are working inside a jacket than when they are working in open water.
It is recommended that vessel owners are members of the IMCA so that they can be kept advised of any industry standards and, in particular, the networking of DP incidents and statistics.
Reference should be made to IMCA publication Guidelines for the Design and Operation of Dynamically Positioned Vessels.
For every vessel/barge and project there shall be a pre-mobilisation risk assessment/Hazard Identification (HAZID) study to identify and evaluate the risks which are not included in generic and task-specific procedures.
The maximum operating parameters, regardless of wind or sea direction, shall be established for each workscope.
When diving operations are to be undertaken in the vicinity of anchor mooring lines, the accuracy of their position and excursion is to be determined.
If it is necessary to dive within a mooring system, the catenary and the touchdown points of the mooring lines are to be confirmed by the use of a Remotely Operated Vehicle (ROV).
All diving operations within a mooring pattern will be conducted under a Permit to Work System issued by the unit within whose mooring pattern the diving operations are to take place. The should include detail of:
- • Mooring Line Diagram
A diagram showing the aspect, length, catenary and touchdown points of all relevant legs of the mooring pattern is to be provided at the DP console for reference by the DP Operators.
• Tension Variations
The Installation shall note the variations in tension in the mooring lines due to prevailing environmental forces and estimate the probable variation in catenaries in which the operations are to take place.
Means of communication between the DP vessel and the Installation. Each shall update the other at regular intervals as to the progress of their individual operations and any proposed actions which may impinge on the safe conduct of the other’s operations, eg overside work, construction or ROV operations etc.
• Emergency Response and Recovery Vessel
Emergency Response and Recovery Vessel cover. The Installation standby vessel is to be briefed as to the proposed diving operations.
• Tension Monitoring
Tension Monitoring throughout the period that diving operations are being conducted within the mooring pattern of an Installation. Any change in the tensions beyond the prevailing variations is to be immediately transmitted to the diving vessel.
The horizontal clearance between suspended mooring lines, the diving bell wire, diving bell and bell guide weight shall, be maximised and shall, whenever possible, be no less than 50m.
The above limitation need not apply when operating procedures ensure that the diving bell wire, diving bell, diving bell guide weight, and diver maintain a clearance of at least 10m vertically above the mooring catenary.
In the event that it is necessary to reduce the horizontal clearance between the bell/bell wire and the mooring line catenary below 50m, the following shall be adhered to:
• The mooring line must be marked in such a way that its location, relevant to the diving bell wire, diving bell and bell guide weight, can be determined at any time.
Methods of marking can include ROV, transponders and other beacons.
• Consideration must be given to the prevailing environmental forces. The prevailing environmental forces shall be such that in the event of total loss of power the vessel will drift clear of the mooring catenary. In any event, if the power output requirement to maintain station exceeds 40% on any one thruster, operations are to be suspended.
• When twin bell systems are utilised, both bells must not be deployed simultaneously within the 50m envelope.
During the time that the bell and divers are deployed, the Installation will take no action that could cause the position, catenary or touchdown point of the Installation’s mooring lines to alter, whether by tensioning the mooring lines, altering the position of the Installation, or adjusting the ballast of the Installation.
In the event that a unit reports an unexpected variation in tension of its mooring lines, diving operations will be suspended immediately. Diving operations will not recommence until such time that the reason for the change in tension has been determined and the catenary, etc. of the mooring line confirmed.
The Installation from which the mooring lines are deployed shall be monitored at all times by the DP vessel. The position shall be monitored by visual, radar, or other electronic means.
In the event that the Installation requires to make an adjustment to her mooring pattern, whether in tension on the mooring lines, position or aspect of the mooring lines, diving operations will be suspended. The adjustment to the mooring pattern/lines will not commence until the diving vessel has confirmed that diving operations have ceased and the bell has been recovered.