Code Conflicts for High Pressure Flowlines and Steel Catenary Risers

Abstract

Abstract There is a growing concern among pipeline engineers in the offshore industry that the existing pipeline codes lead to overly conservative designs for the class of high pressure flowlines that transport unprocessed well fluids from subsea wells or subsea manifolds to the first downstream processing facility, usually located on a nearby platform. Of greatest interest are flowlines and steel catenary risers whose designs are dominated by the well shut in pressure, a loading which acts on a flowline only in rare events such as when several subsea valves fail simultaneously. Significant savings in materials, welding, and installation costs can be achieved, together with adequate safety, by changing to a limit state design based on the actual burst strength of pipe. Upgrades to both lay vessels and production platforms to handle the weight of thick walled flowlines and risers can be minized with this design approach. This paper presents rationale for future code changes and describes attempts under way to make these changes. Appeal is made to the offshore industry to create a large data base of pipe burst tests. These data data be shared by various code committees to develop less conservative design formulas for subsea flowlines and risers. The ultimate aim is to provide a cost effective and reliable design providing a uniform factor of safety independent of pipe size, wall thickness, and grade. Introduction Design rules in the existing ASME and API pipeline codes and their counterparts in the U. S. federal regulations generally result in overly conservative designs for high pressure subsea flowlines and steel catenary risers that are connected between subsea production wells and their host platforms, as illustrated in Figure 1. The platform may be a fixed platform, a compliant tower, a tension leg platform, a floating production system, or a SPAR, etc., and the subsea facility may be a single satellite well or a subsea multi-well manifold The typical design outcome is a thick-wall pipe with low D/t ratio, having a burst safety factor between 2 and 4, will be demonstrated by the examples presented below. This compares with burst safety factors of most existing pipelines that fall between about 1.5 and 2.5. Consequently, unnecessarily large steel and welding cost penalties must be paid for flowlines to a deepwater field development project, and major upgrades to layvessels and platform structures may be required to handle the increased weight of the thick-wall flowlines. This is particularly critical for floating platforms where added buoyancy is very expensive. The main f8ctor leading to this conservatism is the Barlow formula, which was introduced into pipeline design practice in the 1 930's and which loosely relates the maximum hoop stress with the initial yield stress.