Sonar Surveys For Pipeline Inspection Show Extent Of Pipeline Displacement And Seafloor Instability Following Hurricane Ivan

Abstract This paper summarizes activities and primary findings of the post-Hurricane Ivan survey activities to assess storm effects on BP's oil and gas export lines in the portion of the Gulf of Mexico impacted by the hurricane. Like other operators, BP was challenged by shortages of survey resources and the need for quick answers to questions regarding pipeline integrity. BP mobilized multiple survey vessels including a shallow water draft vessel for near shore work, two vessels for investigations in intermediate water depths up to 1100 ft (335 m) and a two-boat deep tow kit for surveys of deepwater pipelines. For land approaches in very shallow water, divers were mobilized to probe the pipeline position to assure it was in position, intact and had adequate depth of cover. Per regulatory requirements, a high-resolution (500 kHz) sonar survey was performed at all pipeline crossings and subsea tie-ins in water depths between 200 ft and 500 ft. Initial results of the survey effort revealed that the Main Pass Oil Gathering pipeline underwent horizontal translation in three areas, but was still intact as demonstrated by sonar surveys. The Destin gas pipeline system was in place. This presentation shows examples of sonar data gathered along the pipeline routes and discusses possible mechanisms for pipeline displacement. Initial Observations Shortly after passage of Hurricane Ivan, BP mobilized survey vessels to track our shallow water (less than 500 ft water depth) pipelines within the hurricane swath (Fig. 1). The pipelines were initially investigated by running a single survey line parallel to and offset about 25 m to the as-built locations. The sidescan sonar data were reviewed for evidence of pipeline displacements resulting from the conditions created by Hurricane Ivan. Pipeline exposures in water depths less than 200 ft were mapped as well as any lineations representing anchor scars or significantly displaced foreign pipelines. In areas where the pipelines were buried, magnetometer data were collected by running transects perpendicular to the pipeline positions. Areas where fresh mud slides impacted the pipelines were mapped and scheduled for more detailed investigations. Following collection of these shallow water data, a two-boat deep-tow survey was performed along the deepwater portion of our export system within the hurricane corridor. In addition, a high-resolution (500 kHz) sonar survey was performed to provide detailed inspection of valves and crossings along our pipelines in the 200 ft to 500 ft water depth range. This was done in compliance with Minerals Management Service Notice to Lessees No. 2004-G18 (MMS 2004). MPOG Displacements The Main Pass Oil Gathering (MPOG) system consists of five segments, primarily in the Main Pass protraction area. Segment I is an 18-in. line running from the oil gathering platform in MP225 westward to MP69 near the Mississippi River delta and is the main portion of the system. Initial survey results showed displacements in three areas: MP144 Platform Area The largest displacement was noted in a portion of the MPOG line focused on OCS block MP144.