Aker (United Kingdom)

Abstract A routine inline inspection pigging operation revealed a number of dent anomalies in an onshore pipeline transporting refined products. A detailed fitness for service assessment was subsequently performed by a leading engineering consultant company using the best industry practices, concluding that the dent defects with varying level of ovality in the order of 5% presented no immediate threat to the normal service under projected pressure cycles. The plain dent defect features were classified into a non-urgent category well down the scheduled repair list, requiring no priority intervention or replacement programme except further monitoring inspections and operating pressure control before a repair was planned. However, the dent defects in the proximity of highway crossings predicted to have longer remnant fatigue life failed unexpectedly within a much shorter time span, leading to costly emergency replacement and disruptive system downtime. This disparity has raised a question about the likely failure cause, uncertainty and reliability of the defect assessment. To this end, one anomaly was re-examined and its study is presented in this paper on how a fitness for service assessment for such dent anomalies can be improved using FEA modelling and consideration of some of the lessons learnt from similar failures and assessment experiences. It demonstrates that certain dynamic loading and additional fatigue promoting factors commonly ignored are of importance for fatigue life assessment.

Abstract Monohull based production platforms are the most widelyused floating production systems worldwide. They are proven as a reliable workhorse and have provided the industry with a commercially attractive platform for producing large and small offshore oil developments in most major offshore producing regions. The ship-shaped monohull offers low cost "real estate" for production facilities, and its integrated storage and offloading feature eliminates the oil export pipeline. To date an FPSO * has not been employed in the US Gulf of Mexico because of the extensive infrastructure of existing host platforms and pipelines on the continental shelf. However, in recent OCS lease sales, operators have acquired vast, undeveloped acreage that is increasingly remote from this infrastructure. Beyond the shelf, the potential for a low capital cost platform and elimination of, what can be a cripplingly expensive oil export trunkline, has many operators considering FPSOs to develop these remote, deepwater prospects. This paper examines and discusses major technical and regulatory issues that the industry (regulators, operators, contractors) must collectively address prior to introducing the first FPSO into the US Gulf of Mexico. * Abbreviations are explained at the end of the paper. Introduction The monohull FPSO was first used to produce the small Castellon field in the Mediterranean Sea in 1977. The system produced 20,000 bopd from a single subsea well tied-back to a turret moored converted tanker in 350 feet of water. This FPSO was the genesis of what has become the most widely employed floating production system worldwide. Over 60% of the worlds floating production platforms in use today are either converted from tankers or are purpose built monohulls. From 1977 to 1993, 24 FPSOs were installed, 17 of which were conversions. They were mostly used to develop small oil fields in areas remote from pipeline infrastructure. Large integrated oil storage capacity permited periodic shuttling of parcels of produced crude in shuttle tankers to a sales point, while enabling uninterrupted production and eliminating the need for an oil pipeline. These FPSOs proved to be reliable workhorses, that gave operators a commercial option to exploit marginal, remote oil fields. As confidence in these systems grew, along with technological advances in subsea wells and flexible risers, operators began to expand their application to progressively larger developments in increasingly deeper waters and harsher environments. Since 1994, 36 FPSOs have been installed. Of these, 15 are in the demanding North Sea and Atlantic Frontier Provinces. Petrobras has converted several VLCCs for use as FPSOs in the Campos basin in water depths upto 6000 feet. Table 1 captures the progression of FPSOs from its humble beginnings at Castellon, to the most technologically advanced FPSO recently installed in Statoils Asgard field in the Norwegian North Sea. Despite its widespread and burgeoning use, only one FPSO has been employed to develop an offshore field in US waters. That was Exxon's Offshore Storage and Treating Unit (OS&T) that operated in the Santa Barbara Channel in 500 feet of water from 1981 to 1995. A monohull FPS has never been used in the US Gulf of Mexico, one of the most prolific offshore oil and gas producing regions in the world.

Abstract As the offshore industry looks beyond water depths of 3,000 ft for future oil and gas production, the industry standard steel catenary spread mooring (CSM) system used on all floating production systems installed to date becomes increasingly inefficient and costly, An alternative to the CSM is the Taut Leg Spread Mooring (TLSM) system with its characteristic short scope legs and vertically loaded anchors. In water depths greater than 3,000 ft, it has been shown that TLSMS comprised of synthetic mooring lines have performance advantages over systems utilizing steel wire rope and demonstrated potential for significant cost reductions. Early studies investigated the use of aramid fiber rope due to its high strength, lower in-water weight and lower axial stiffness as compared to ropes made from steel. Later studies indicated that the material properties of polyester fiber, primarily it's lower Young's modulus, made polyester mooring lines more suitable for use in TLSMS2'S. As the TLSM knowledge base expanded, aramid rope construction evolved. Earlier efforts to match the characteristics of steel wire rope have given way to new generation 'soft' aramid constructions. This paper compares the mooring performance and total installed cost of a "soft? aram id TLSM to a typical polyester configuration. Both systems were designed for use in the Gulf of Mexico (GOM) with an FPS based on the Aker P45. Introduction This study was performed with the objective of investigating and comparing two taut leg spread mooring systems comprised of differing types of synthetic rope. The two types of synthetic mooring line constructions investigated were:Whitehill 36-strand aramid rope.Marlow Superline parallel-strand polyester rope. Mooring systems using both constructions were designed for the new-build Aker F'45 Semi-submersible Drilling and Production platform (FDPU), The first P45 is currently being constructed for Norsk Hydro for their Njord field development. The vessel's displacement will be 49,600 tons when completed; production facilities will be capable of 70,000 BOPD of crude throughput. The particulars of the Aker P45 are presented in the Appendix to this document. The study this paper is based upon has been planned and organized such that it would be a suitable extension to work previously presented13 A deep water (3,000 ft) GOM location was chosen as the field location for this work. Three environmental design scenarios were investigated for this site. They are:Maximum Wave Hurricane Event with associated wind and currentMaximum Loop Current Event with associated wave and windMaximum Wave Case -5 year return The Maximum Wave Hurricane condition noted above was the controlling case and, therefore, was used in the design effort. The goal of this study is to evaluate whether mooring lines constructed of aramid fiber are cost and performance competitive with polyester for deep water taut leg spread moorings. As part of the scope of the study, installation costs and schedules have been developed for each system. Preliminary component specifications were developed to a level of detail where costs could be estimated for each mooring case.