RIASSUNTO
Abstract
This paper reviews the design of subsea developments in relation to possible external hazards. The hazards considered are fishing gear impact, dropped objects and anchor lines. Alternative protection measures are discussed and suggestions are made for the most suitable design approaches over a range of practical situations.
The emphasis in this work is on the application of risk analysis methods to produce 'fit-for purpose' designs - avoiding unwarranted costs from over- conservative approaches.
Introduction
There is an increasing emphasis on minimising capital costs for subsea installations. By selecting a structural design basis which suits the particular situation it will be possible to minimise weight and hence fabrication and installation costs. The objective of the engineer will generally be to produce designs which are adequately robust and functionally suitable where total capital plus operational costs are minimised.
The main external hazards to be considered are fishing gear, dropped objects and anchor lines.
FISHING GEAR
* General
The possible consequences of a strike on a subsea installation depend on the shape of the structure, the characteristics of the fishing gear and on the fishing vessel displacement/speed. The structure may be designed as 'overtrawlable' - with sloping sides and smooth profile - in which case the fishing gear will likely ride over the obstacle without snagging. However, there will be some local impact load. Additionally, the gear may become snagged.
There are thus two types of load which the structure has to resist. The first is the local impact load, which is governed by the energy of the impacting trawl shoe, beam etc. The second, which would result from snagging, is a static load which could be up to the breaking load of the warp. The size of the load depends on the fishing gear.
* Types of fishing (Fig 1)
Only seabed fishing methods are of interest and the following are some of the more common methods.
Seine net. A large open net is hauled along the seabed and vessels often operate in pairs.
Trawlers. In this method the net is heavily weighted and held open by trawl 'doors' or by Otter boards.
Beam trawlers. The net is held open by a long beam, weighted at each end with trawl shoes.
* Salient Parameters
The possible loading on the subsea structure depends on the weight of the fishing gear, vessel speed and on the strength of the tow warp.
Weight of seabed gear: The mass of the trawl shoe, beam, etc. will influence the local impact energy.
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