Richard Verley

CARISIMA: A Catenary Riser/Soil Interaction Model for Global Riser Analysis

Steel or titanium catenary type risers may provide cost effective alternatives to flexible risers, particularly for deeper water (>300 meters). Existing analysis tools for riser response have proven reliable for most types of risers. However, in the area where catenary risers are resting on the seafloor, existing models for riser/soil contact are too simplified to capture the complexity of the interaction. Some of the key findings of earlier work are that the fatigue life predictions at the touchdown area are sensitive to the FEM modelling, fatigue accumulation procedures and, in particular, the soil model. Hence, an important step forward in improving riser analysis tools is to improve the riser/soil interaction model. This issue was the key objective of the CARISIMA JIP. This paper gives an overview of the main activities in the CARISIMA project.Copyright

Pipeline-Trawl Interaction: Effect of Trawl Clump Weights

This paper deals with clump weights, a new type of component in bottom trawl gear that needs to be considered in design and reassessment of pipelines. In recent years it has become popular to use two trawl bags, a so-called twin trawl, to increase the swept area and, thus, the efficiency of the trawler. In addition to the trawl boards at each side of the trawl bags, a heavy weight, a clump weight, is used between the bags to keep them close to the seabed. Clump weights can have a mass up to 9 tonnes, and the largest ones are shaped as spheres or cylinders inside a steel frame. Tests performed have revealed that the loads on pipelines from clump weights may exceed the corresponding loads from trawl boards. Thus, interaction with clump weights may govern the design and qualification testing of coating, and the design related to global pipeline response. This paper discusses loads from clump weights. Updated design approaches including pull-over load estimates are presented. The background in terms of small scale test results is also shown for some cases. The conclusion is that clump weights may govern the trawl design of pipelines, especially for trawl gear impact and pull-over.Copyright

Assessment of Long Axial Corrosion Defects

Severe Microbiologically Induced Corrosion (MIC) has been discovered in several offshore pipelines, and in particular in water injection pipelines. The corrosion is characterised by long axial channels around the 6 o’clock position. The channelling corrosion generally shows an irregular form which influences the quality of the ultrasonic inspection results; both due to the irregular form itself, and because it affects the ability to clean the pipeline prior to the inspection. The common practice is to produce a feature list reporting the deepest point of a metal loss defect and associate this with a long defect length, typical equal to the pipe joint length for channelling corrosion. Calculating the pressure capacity based on the reported defect depth and length will give a too conservative result for each defect. At the same time; a pipeline with severe channelling corrosion will have many pipe sections subject to an invariant loading condition, and potential structural failure may occur in connection with the lowest structural resistance among the pipe sections. This system effect is a statistical effect that will result in a lower pressure capacity. A procedure to establish the remaining wall thickness profile, a river bottom profile (RBP), based on detailed wall thickness and stand off data from inspections has been developed. A total probability of failure (PoF) for the pipeline given a design pressure is estimated. In case this total PoF exceeds the allowable PoF level, a procedure to establish a pressure adjustment factor, γs , has been developed. This paper outlines the procedure to establish RBPs along the pipeline based on detailed inspection results together with the methodology to calculate the total PoF, including system effects, and the pressure adjustment factor, thus coming up with a pressure capacity at the time of the inspection. The paper also reflects on how consecutive inspections can be used to estimate the yearly corrosion rate between the inspections.© 2010 ASME

Deep Water Remote Pipeline Repair Using Welded Sleeve Technique

A technique for sub-sea pipeline repair using remote welding without the use of diverse has been developed. The technique involves the installation of an oversized pipe segment (sleeve) over the join by threading the pipe ends through the sleeve and performing a sub-sea fillet weld between the pipe and sleeve on each end. Current traditional welding techniques use pipe end butt to butt welding performed with the aid of divers in a habitat and are therefore depth limited. This technique is fully remote controlled and has been proven by simulating conditions in a hyperbaric chamber to work down to at least 2500 msw represented by 250 bar. This paper describes the technique, the structural considerations, and the equipment currently being built to perform this task, which will be in test in the second half of 2004 with sub-sea trials to follow. This paper compliments the paper IPC04-0630 “Diverless underwater GMA welding for pipeline repair using a fillet welded sleeve” [2], in which the welding procedural development is described. When compared with traditional welding or mechanical options (in particular for large pipes), the technique provides a much cheaper repair option.© 2004 ASME