SS:Spar Technology: Measurement, Hindcast and Prediction of Holstein Spar Motions in Extreme Seas

Abstract

Abstract Both environment and response measurements have been recorded continuously from the Holstein spar since 2004. The paper takes a close look at some of the key events. During this time at least one major hurricane passed very close to the platform. Actual data recording an individual wave height in excess of 80 ft. is included. Holstein is the largest truss spar and the first to utilize mechanical riser tensioners, rather than buoyancy can riser tensioners. The added stiffness, which the mechanical tensioners provide, moves the heave natural period closer to resonance with extreme sea states. The measured responses are compared to hindcast predictions. Predictions are also made to potential environments using a post Rita hindcast, which exceeds the wave environment used in the original design. Real data from extreme storms is very rare. When the actual response can be captured and compared to design predictions, valuable insight can be gained into the design assumptions and methodologies. The result can determine how much conservatism is in the original design and can also have an impact on future spar designs. Introduction The Holstein truss spar was constructed between 2002 and 2004, as described by Perryman [2004] and installed on location in Southern Green Canyon Block 645 during April and May of 2004. First oil was obtained on Dec. 9, 2004. The largest truss spar constructed to date, Holstein has a draft of 692 ft. and is moored via 16 chain/wire/chain semi-taut lines attached to 18 ft. diameter suction piles in 4344 ft. of water. The produced oil and gas exit the platform via two steel catenary risers, a 24 inch diameter oil riser and a 16 inch diameter gas riser. Holstein also has 15 vertical combination drilling and production risers supporting dry trees and tensioned via ram type mechanical tensioners decribed by Perryman, et al, [2005]. The stiffness provided by these tensioners lowers the natural period in heave to 18.2 - 20 seconds, close to the peak spectral period of extreme sea states of 13.5-15.5 secs. A question is raised whether the waves in such sea states could cause the spar to heave excessively and damage the riser system. In order to find out, measurements of sea states and motions are required. BMT-Scientific Marine Services was retained to provide an integrated marine monitoring system, (IMMS) to record the winds, waves and currents, as well the spar motions and the mooring line and riser tensions, as described by Irani, et al, [2007]. Draft and ballast tank levels are also recorded. In 2006 an independent remote monitoring system, (IRMS) was also added by BMT to record motions and winds if power was interrupted to the IMMS, as described by von Aschwege, et al, [2007]. In the five hurricane seasons since the platform was installed, there have been four major hurricanes in the northern Gulf of Mexico, which have had significant wave heights equal to or greater than the 100-year. significant wave height used during the design. The data from Rita and Ike have been post processed by BMT, as part of their role as Marine Monitoring Contractor for BP, to reveal the magnitude of the environment and the response of the spar. Rita and Ike have passed near Holstein, where individual waves in excess of 80 ft. have been recorded. Rita caused a nearby platform to capsize. Waves of this magnitude raise the question whether some of the original design loadings may have been exceeded. To answer this question, a study was commissioned with Technip, the original designer of the spar to try to predict spar motions by using a hindcast sea state of hurricane Rita.