Design and Analysis of West Seno Floating Structures

Abstract

Abstract This paper describes the metocaen data, design and analysis of the West Seno TLP, the mooring system design for the Floating Production Unit, and the coupled TLP and the drilling tender assist vessel. The paper is divided into the following sections:The collection of metocean data, extrapolation, and application to design. Particular emphasis is given to current data.The model test program in a deep water basin facility with capability of generating steady current, with all the risers and tendons modeled. Application of test results to design.Method of analysis and validation of analytical tools and procedures. The analysis was based on fully coupled time domain simulations with all the risers and tendons modeled.FPU mooring system design and analysis.Fully coupled drilling tender vessel and TLP mooring system design and simulations. Metocean Wind design criteria was developed from 16 years of wind data recorded at the Sepinggan airport, site measurements carried out by the Institute of Technology Bandung, wind measurements by the Navy in the Makassar Strait, and wind and wave measurements from the drilling vessels and fixed installation. There were no specific wave measurements, the wave data were determined from the wind by hindcasting. The wave growth in each direction was determined by the water depth, the wind speed, the duration of an event and the directional spreading. The hindcast modeling was verified by applying the same procedure to one of the wind and wave measurements from an offshore platform in the area. The current conditions in the strait were of prime concern, both for their effect on the floating facilities as well as on the sub-surface elements such as tendons, well conductors and the steel catenary risers for intrafield and export oil and gas pipelines. To determine the design current data, a long term current measurement program was initiated lasting 13 months with data collection intervals in four phases. The data were obtained from six instruments mounted on a subsurface buoy mooring. These consisted of a near surface ADCP, three current meters, temperature/depth recorder, and a pressure sensor. There is a persistent southward flow through the strait from the Pacific Ocean on the sorth through the Java Sea to the Indian Ocean on the south, which are referred to as the Indonesian Through Flow (ITF) (Figure 1). The flow, however, is complicated by the geometry of the strait, which at the Seno site produces an eddy and a flow that is intermittently counter to the southward flow. The strongest currents in the region flow northward, parallel to the bathymetric (depth) contours. Bottom currents were oriented perpendicular to the depth contours, running back-and-forth through the slope canyon. Most of the current variability was identified as currents that change slowly over periods of weeks or months, which appear to be linked to larger scale of the ITF. The extreme current measured during the campaign is presented in Figure 2.