Oseberg B Jacket - Damage Assessment and Repair After Submarine Collision
Authors: Sveen Dagfinn 
Published: December 2007

Summary
This paper describes a submarine collision with a jacket platform and thesubsequent damage assessment, safety evaluation, and repair. Astrain-monitoring system on the structure gave very good data for analyticalverification of the elasto/plastic response at impact. Advancedprogressive-collapse analysis was applied to assess structural safety inprogressive-collapse analysis was applied to assess structural safety indamaged and brace-removed conditions. This collapse analysis reveals thegenerally high reserve strength in jacket structures. Habitat welding replacedabout 26 m of the 31-m-long brace in a complicated repair job.

Introduction
Norsk Hydro A/S is the operator of the Oseberg field in Blocks 30/6 and 30/9on the Norwegian Shelf. The field center consists of a combined drilling, wellhead, and riser jacket platform (Platform B) and a concrete productionplatform (Platform A). The field production platform (Platform A). The fieldcame into production in Dec. 1988 and is the first field that Norsk Hydrooperates. When a West German submarine collided with Platform B during hookupin March 1988 and damaged a major brace, the company immediately organized aspecial project team to assess damage and safety project team to assess damageand safety consequences and to repair the platform before the field center cameinto production.

The Incident
At 1:25 p.m. on March 6, 1988, the people on board Oseberg field's PlatformB felt a strong shaking. The jacket that serves as a riser and wellheadplatform on the Oseberg field center was in the hookup phase. The field'sproduction platform (Platform A) was not yet installed. Adjacent to Platform Bwas the semisubmersible living-quarters platform Poly Confidence, which wasanchored and Poly Confidence, which was anchored and connected to Platform B bya bridge (Fig. 1). Hookup activities on the platform were immediately stoppedand the people evacuated. No reason for the shaking of the platform wasobserved above the sea surface, platform was observed above the sea surface, yet bubbles were coming up from the sea. It was speculated that either anearthquake had occurred or a submarine had collided with the platform. At 2:17p.m. a submarine surfaced on the starboardaft side of Poly Confidence. The bowof the submarine had extensive damages and the front of the tower had a smalldent, confirming that a submarine had crashed into the substructure of PlatformB. The vessel's crew members were confused and could not immediately give anyclear picture of what had happened. picture of what had happened. The submarinebelonged to the West German Navy, was about 46 m long, and had a displacementof 500 Mg. It had been cruising at roughly 30 m water depth at a speed of 16.7km/h 180 degrees north to south.

Actions After the Incident
The urgent matter was now to determine the extent of damage to the platformand the possible reduction in structural integrity that possible reduction instructural integrity that could affect the ongoing completion work. That samenight, the jacket was inspected by a remotely operated vehicle (ROV), allowingus to estimate the main damages. We concluded that the submarine had enteredinto the eight-legged jacket from the north broad side and collided on theinside with a diagonal bracing on the west short side (Fig. 2). The diagonalbracing (with a diameter of 1200 mm, a thickness of 35 mm, and a length of 31m) thus suffered the major damage, a large local dent, and overall deformation. In addition to the deformation at the point of contact, the brace had beenexposed to yielding at the upper end close to or in the node. This wasconcluded from the overall deformation and from cracks on the painted surfacein the upper node stub area. With this ROV inspection it was not possible todetermine whether cracking had possible to determine whether cracking hadoccurred in the node welds. Other damages, such as small surface dents andscratches, were insignificant. From this first damage assessment we concludedthat the structural capacity was probably reduced as a result of the braceprobably reduced as a result of the brace deformation and possibly weakenednodes. It was now urgent to resume the hookup work on the platform. After arough, conservative evaluation, we determined that the platform was safe forwave heights less than 15 m. Accordingly, normal working activities wereresumed 2 days after the incident. Two days after that, we concluded by use oflinear analysis that the limiting wave height could be increased to 20 m. Thismeant that the work could continue as normal with a very low probability ofweather interruptions.

Inspection and Damage Assessment
After we established that hookup activities could continue as normal, a morethorough investigation of damages and safety consequences was planned. Thefirst activity was an inspection with divers. With knowledge from the ROVinspection, a detailed inspection program was formulated.
JPT
P. 1421