Steel reinforced flexible pipe has increasingly been utilised in offshore applications since the 1970s. It is principally used as part of the overall production and transport system, where there are specific requirement to accommodate large relative motions, to provide high corrosion resistance, and to reduce installation time and cost. Given its desirable mechanical properties, such as high dampening coefficients and high resistance to dynamic loads, the use of flexible pipe offers fixed and floating production designersmore options to achieve greater cost effectiveness.
With the development of smaller, more marginal fields have come significant advances in the use of light-weight, unmanned fixed structures as well a variety of floating or tethered surface vessels. In the majority of current field developments, flexible pipe acts as a key subsystem providing the primary connecting links between subsea sell the surface facility as wells as a motion compensation between the surface safety valves and the main process equipment(see fig. 1). For deeper water applications, flexible pipe is increasingly utilized to provide desirable catenaries and low surface weight; to assist the fluid transport system to accommodate undulating seabed conditions; and to permit remote connection to subsea production equipment.
With such an expansion in utilisation - there are than 1200 km of flexible pipe in service worldwide - also comes an increasing need to monitor the behaviour of flexible pipe and to conduct periodic inspections. Despite the popular myth of being a ‘Black Box Technology’, flexible pipe performance can be measured and assessed to permit evaluation of overall integrity.
When considering how to evaluate pipe integrity, one must first establish what constitutes the generic term flexible pipe. The American Petroleum Institute (1) has defined flexible pipe as being'.. a composite of layered materials which form a pressure containing conduit. The pipe structure allows large deflections without a significant(resultant) increase in bending stresses?. The pipe constructions is made up from alternating layers of polymeric (thermoplastic and elastomeric) material, together with steel armour (both carbon and stainless) textiles, and fabrics. The resultant composite construction causes the pipe structure to display a heterogeneous structural behaviour. Stress distribution, a fairly well recognized phenomenon in homogeneous structures, is to non-specialists very little understood.
To further confuse the non-specialist, assessment of individual layer stresses can be difficult to measure and even if known generally offers only minimal guidance towards (Fig. 1, 2 and 3 are available in full paper) assessing overall pipe behaviour.
Accomodating overall stresses is very important and represent the art of the design engineer. The tolerance of stresses will vary significant depending an whether the pipe is intended for static or dynamic application, and wheter the pipe is of the ‘bonded’ or ‘non-bonded’ type construction (see figs. 2 and 3)
Bonded pipes are those whereby component materials are applied in alternating layers using adhesives and chemical agents.