Subsea Production Systems generally incorporate a template or satellite structure which has the following primary functions
location and verticality of wells,
support of subsea components and maintenance equipment,
limitation of deflections between components of the system,
protection of subsea equipment
The first three of these requirements are normally driven by system parameters, such as number of wells, method of maintenance or method of flowline pull-in and connection. Protection of the system, however, depends largely on the local environment and can have far-reaching consequences on the layout, installation and maintenance of the subsea system
In particular, if a subsea system is installed in a fishing zone, it must be decided at an early stage which of the follouring solutions is to be chosen:
a structure which will protect equipment by snagging any trawl gear on an outer bumper frame, thus preventing snagging or snarling of equipment in the template the trawl gear is, of course, usually destroyed,
a structure which will deflect trawl boards and nets without damage to either subsea equipment or fishing gear
This chapter discusses options for alternative forms of protection, and describes and compares various structures already installed or taken to detail design as examples of the different philosophies
It is general practice to protect equipment on a template or satellite to some degree, m order to limit damage from typically, the following sources
fishing gear,
dropped objects,
anchor snags
Because the wells on a subsea production system will be protected by subsea safety values, it is unlikely that damage from these causes could result m hydrocarbon spillage. Damage could, however, result in the following
inability to re-enter or kill the well,
intervention required to replace damaged equipment,
additional inspection requirements and removal of tangled nets
It is interesting to note that the NPD Regulations now require that all subsea installations m the Norvegian Sector of the North Sea be designed so that fishing gear will not be harmed. This requirement may not apply to fishing exclusion zones, which could be requested on the grounds of
low fishing activity m the area,
proximity to permanent platforms
No such requirements yet exist in the UK sector
There are various trawling techniques commonly used in the North Sea, and among these are
white-fish trawling,
industrial trawling,
beam trawling
Since the white-fish trawl arrangement can apply the largest snag load to a subsea structure, only this form of trawl will be described Trawlers also deploy anchors which can cause similar loadings to trawl gear snag loads.
White-fish trawling (Fig 1)
A white-fish trawl consists of a net with a mouth up to 25 m wide and 9 m high. The net is connected to the two towing warps by a ground line with bobbins and a headline with floats.
