well stimulation on offshore guidelines

BRITISH PETROLEUM GUIDELINES
FOR WELL STIMULATION VESSELS

9 Well Stimulation Vessels
9.1 Design Philosophy
All DP well stimulation vessels shall be a minimum of DP Class 2 or DP Class 3 and operating in that mode. It is recognised that at present there are not many DP Class 2 well stimulation vessels but some are being converted so proposed vessels will be assessed by Marlog on a case-by-case basis.
In addition to the information given in Paragraphs 1 and 2, a well stimulation vessel shall meet the following criteria:
The safe working limits shall additionally be determined by the limits of heading capability from failure of thrusters furthest from the process umbilical in beam environmental conditions.
Reference should be made to IMCA publication 'Guidelines for the Design and Operation of Dynamically Positioned Vessels'.
• No known failure shall cause a position loss which could result in contact with another vessel, mooring line or offshore structure, or cause tension to be applied to the process umbilical, or cause the uncontrolled release of chemicals.
• The determination of safe working limits shall consider the consequences of a sudden mechanical failure so that such a failure does not risk:.
– Injury to personnel working on or near the process umbilical
– The vessel coming into contact with any nearby structure or vessel

RELEVANT REFERENCES
2.2 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:
'Safe Working Limits' are the environmental limits that would make a 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.

• 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 500m zone.
• Time to recover any equipment or divers from the sea to a safe position .
• Divers working within a jacket.
• Water depth.
Note: 'A Safe Situation' means where the work has or could immediately cease and there are no serious consequences.

2.3 Redundancy of Equipment
All DP vessels used to support 'diving, drilling, flotel, pipelay operations' must be Class 2 (refer to Paragraph 1.3).
Class 1 DP vessels requiring to operate within the 500m zone of Installations must have procedures which reflect the possible loss of equipment and position. They will be required to operate in a 'blow-off sector' and will not be allowed to approach closer than 100m to the Installation. ('Blow-off sector' means a position where if all power on the vessel is lost, then the vessel will drift clear of the Installation.)
Required levels of redundancy shall be addressed during pre-mobilisation hazard analysis and evidenced within project documentation.
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 1 July 1994 should have consequence analysis function checking that, in terms of thruster and power, the vessel can maintain station following a worst case failure mode.