An API task group has developed the process for assessment of existing platforms to determine fitness for purpose; this has been released as a draft supplement to API RP 2A. The process is prescriptive, with criteria based upon consequence of failure in terms of life safety and environmental impact. Platforms are assessed according to either design basis check or analysis. Two levels of analysis may be used, with increasing complexity and decreasing conservatism design level and ultimate strength. Design level analysis is similar to that used in new platform design, while ultimate strength analysis attempts to provide an unbiased estimate of platform capacity. This paper describes the draft API assessment process and associated acceptance criteria. The criteria are based on over forty years of successful operations, field experience, and detailed investigation of platform failures and survivals in past hurricanes, notably hurricane Andrew. The draft process is currently being tested through a joint industry project, with over 20 participating operators and contractor. BACKGROUND In 1992, an API task group was established with the objective of developing guidelines for assessment (i.e., demonstrate fitness for purpose) of existing platforms. The impetus for developing assessment guidelines was the evolution of platform design practice over the past forty years, resulting in new platform design standards which are considerably more stringent than those used earlier. Concerns were raised by the Minerals Management Service (MMS) regarding the adequacy of older structures, prompted also by the expiration of initial operating permits and by the occurrence of significant environmental events such as hurricane Juan (1985) and the Loma Prieta earthquake (1989). Hurricane Andrew (1992) provided further justification for establishing assessment guidelines, as well as a large amount of reliable information useful in this regard. An API task group, chaired by Kris Digre of Shell, was directed to develop a supplement to the API document, "Recommended practice for Planning, Designing and Constructing Fixed Offshore Platforms - Working Stress Design" (RP 2A - WSD) 20th edition. The supplement was to provide guidance on assessment of fried offshore platforms located in US waters, addressing metwean, earthquake, and ice loading. The task group effort was divided among seven supporting work groups:assessment process,condition assessment,loading,structural,foundations,operational/mitigation measures, andacceptance criteria. Considerable interaction was required between the groups, particularly in establishing criteria. The task group completed the work within one year and issued a draft supplement prior to the International Workshop on Reassessment and Re-qualification of Offshore Production Structures held in New Orleans in December of 1993 [1]. Discussion of the draft supplement was a principal topic at the workshop, and the recommended prescriptive assessment procedure was widely endorsed [1]. The purpose of this paper is to describe the assessment process developed by the API task group and contained in the draft supplement to RP 2A, to discuss the evolution of this assessment process, and to provide justification for the specified acceptance criteria. Discussion will be restricted to the metocean assessment process and criteria; ice and earthquake loading assessments are addressed in [2].

No abstract

Gulf of Mexico deep water oil fields present challenges to the offshore industry. Large fields require heavy drilling and production topsides which favor inshore deck installation, hook-up and commissioning in order to reduce time, costs and risks associated with carrying out such operations offshore. Harsh environments require that the motions of the structure be small to allow the use of dry trees and SCRs. A new system, called the DPS 2001, has been developed, which is a deep draft semisubmersible with a retractable heave plate. The system combines the advantages of a semisubmersible with the operational motion advantages of a truss spartype floater. Minimal motions of the DPS 2001 allow the use of dry trees with drilling/workover capability. The truss/heave plate is in a retracted configuration during fabrication and towing; this allows the deck to be stalled and commissioned inshore. The truss/heave plate is lowered and locked at a deep draft after the DPS 2001 is towed to the installation site. Because of the heave plate, the DPS 2001's motions are significantly less than that of conventional semisubmersibles and ship-type hulls. INTRODUCTION The DPS 200101 is a floating drilling and production platform designed specifically for oil and gas development in deep water harsh environments. Three different versions of the DPS 2001system are currently being developed by CSO Aker:DPS 2001-4: a fourcolumn version of the system for 8,000 - 10,000 st topsides,DPS 2001-3: a three-column version of the system for 5,000 - 8,000 st topsides, andDPS 2001-5: a five-column version of the system for use with a topsides up to 40,000 st. A patent is pending for the DPS 2001. The DPS 2001-5was presented at the 2001 DOT conference. The DPS 2001-3will be presented in the near future. The DPS 2001-4is the focus of the present paper. The DPS 2001-4is shown in Fig 1. The upper hull consists of horizontal pontoons and four corner columns and provides buoyancy. The lower hull consists of a heave plate and a truss that supports the heave plate. The heave plate provides large added mass and significantly reduces the platform's motions. The DPS 2001-4is anchored to the sea bottom with a spread mooring system consisting of chains, steel strand or polyester ropes and suction anchors. The upper hull is similar to a typical semisubmersible hull. The lower truss and heave plate unit is similar to the truss structure employed on truss spars. The system is an integration of proven components and embodies the best features of both a semisubmersible and a truss spar. The hull and the truss/heave plate unit can be fabricated in a number of yards in Asia or Europe. Upon completing the hull and truss/heave plate, it will be drytransported to the GOM. The hull and truss/heave plate will be in a retracted configuration during fabrication and transportation (Fig 2).

Currently API provides two sets of guidelines for the design of offshore piles in compression. The first is a working stress design approach (WSD), while the second and more recent is a reliability-based approach that uses load and resistance factor design (LRFD). In recent years, as the offshore industry has moved into deeper water, suction caisson foundations have evolved as alternatives to piles. Presently only the WSD design approach is available for design of deepwater suction caissons. This paper presents some preliminary estimates of the reliability (probability of failure) of suction caisson foundations for deepwater applications. It is an initial step in an overall program intended to provide a similar reliability-based design approach as that currently existing for piles. This paper examines several different aspects of foundation reliability for mooring systems applicable for a range of deepwater structures. The potential impact of spatial soil variability on the overall foundation reliability also is examined. The preliminary results from this study suggest that a factor of safety of about 2.0 against the 100-year design load produces comparable levels of reliability as those currently obtained for compression piles using API design procedures. Although the results from this study are preliminary, they help to focus future work on key uncertainties and provide significant insights into potential foundation performance. Introduction As the offshore industry has moved into deeper waters, several aspects of foundation design have changed. One primary difference is that foundations are no longer loaded primarily in compression. For structural concepts such as Tension Leg Platforms (TLPs), Spars, and monohulls, the primary foundation loads have changed from compression to either uplift or combined uplift and lateral loading. In addition, suction caisson technology has been evolving slowly, initially starting with North Sea applications such as Snorre and Heidrun and then being used in the Gulf of Mexico (GoM) for projects such as the Hoover-Diana floating system. The fundamental mechanisms for providing foundation resistance with a suction caisson are different from the mechanisms provided by piles. Also, because of the high costs associated with extending shallow water site investigation techniques to deepwater, new methods for collecting soils data have been proposed. Some of the fundamental issues associated with site investigations have changed for deepwater because the anchorage foundations for Spars and monohulls are spread over much greater distances (kilometers) instead of, as is typically the case for shallow water foundations, a hundred or so meters. Despite these fundamental changes, engineers still are faced with the challenge of selecting and designing foundations for deepwater applications that will have a comparable level of reliability as those designed for shallow water, at the lowest possible cost. To address this problem, a deepwater technology project was initiated at BP Amoco to use a reliability-based approach to:make consistent cost and reliability comparisons between piles and suction caissons, as well as establish a framework for assessing future foundation types such as vertically loaded anchors, andestablish and define the steps required to develop a robust reliability-based design approach (e.g., using Load and Resistance Factor Design) for deepwater foundations.

The objective of this ongoing API sponsored research study is to develop design criteria for selection of winds, waves, and currents in the Gulf of Mexico to be used in design of offshore structures. A probabilistic methodology is developed that leads to the joint probability distribution of hurricane induced winds, waves and currents for a generic site. Both analytical and statistical (simulation) procedures are used to generate the joint probability information, where the statistical input is based on the historical hurricane information in the Gulf. Three different methods are compared for selection of the design combination criteria:ignoring the platform response and simply focusing on the joint probability of the environmental events,aiming for a target return period for the platform response, andaiming for a target reliability for the platform. For fixed platforms, the results are shown to be similar based on the three selection methods; this conclusion may not be valid for other climate areas. The project Technical Advisory Committee's recommendation for fixed platforms is a combination of the 100-year maximum wave height, the 100-year wind, and 54% of the 100-year current or roughly a 2.0 knot current. This, on the average, leads to an annual safety index of 3.0 for the platform. Background and Introduction Selection of the storm (or hurricane) environmental conditions for design of a platform is an important part of the design procedure. This selection process involves two distinct steps:selection of an appropriate return period, andadopting a criteria for selection of waves, winds, and currents. Both the return period and the design combination criteria affect the strength of a platform and thus directly impact the reliability of the design. The generally accepted design practice for fixed platforms in the Gulf of Mexico (GOM) is to use a wave height with a 1% annual probability of exceedance, i.e., the l00-year return period. The design combination criteria, however, are not as well established. The present API recommended practice (RP2A, 18th Ed.) suggests using the specified recurrence interval (e.g., 100-year) wave height, and the currents and winds likely to coexist with the design wave. The objective of this ongoing study, which is funded by the API, is to quantify the environmental combination criterion for design of fixed platforms in the Gulf of Mexico. The criterion should account for the joint probability of occurrence of winds, waves, and currents. This requires a storm hindcasting model that can predict the magnitudes and directions of winds, waves, and currents as a storm passes by the site. Early in the project, it was decided that such a combination criterion should be platform dependent, i.e., the design criterion for fixed platforms should not be the same as that for compliant platforms such as TLPs or FPSs. The reason is that fixed platforms are inherently more sensitive to waves than winds and currents. Compliant platforms, are less susceptible to waves because of their dynamic motion.