MODU Marine Safety: Structural Inspection and Readiness Surveys
Authors: Cole M.W. , Marucci T.F. , Taft D.G. 
Published: December 2007

Several years ago, Exxon instituted a survey of mobile offshore drilling units (MODU's) under contract to the corporation to evaluate structural integrity and readiness to respond properly to marine emergencies. This paper briefly describes results of the inspections and our on-going marine safety program. Industry activity is also highlighted.

Introduction

In recent years, the offshore drilling industry has experienced three major marine losses involving high loss of life -- the Alexander Kielland, Ocean Ranger, and Glomar Java Sea.

Alexander Kielland.

The semisubmersible Alexander Kielland was lost because of a structural failure. Results of the official Norwegian Ministry of Local Government and Labour inquiry indicated that the initial structural failure occurred because of improper welding detail at a hydrophone -- high stresses at this location propagated fatigue cracks that eventually caused the member to fail during a storm. This first failure then led to the failure of other members and loss of the rig. Of the 212 men on board, 123 were lost.

Ocean Ranger.

The semisubmersible Ocean Ranger was lost during a severe winter storm because of a stability problem. Results of official inquiries by the U.S. Coast Guard (USCG) and the Canadian Royal Commission indicated that the loss was caused by a ballast-control system failure. All 84 men on board were lost when personnel were unable to abandon the rig safely in heavy seas and cold weather following the ballast-system breakdown.

Glomar Java Sea.

A recent USCG report concluded that shifting cargo and flooding through unsecured openings (not a structural failure, as had been reported initially) contributed to the loss of stability and eventual capsizing of the drillship Glomar Java Sea during Typhoon Lex in 1983. One lifeboat was apparently launched, but it was never located. All 81 men on board were lost.

Although the specific cause of each loss was different, these disasters all occurred at night in severe weather, and emergency procedures and on-board lifesaving systems were not effective. Exxon considers the first line of defense against a major marine disaster to be the structural integrity of the rig. In the event personnel must abandon the rig and helicopter support is not available, however, equipment and personnel training must be adequate to carry, out this operation successfully.

Surveys of rigs contracted to Exxon around the world, including Australia, Southeast Asia, the Gulf of Mexico, eastern Canada, and the North Sea, indicated that structural integrity and readiness varied widely from rig to rig. After careful evaluation of survey results, it was found necessary to implement a marine safety program to ensure both structural integrity and readiness to respond to marine emergencies successfully. This program consists of structural inspections and marine safety surveys. Results described below highlight observations made during past surveys and summarizes Exxon's future plans.

Structural Inspection Surveys

Structural inspections were conducted to assess the structure of each rig. The inspection program for semisubmersibles and jackups included nondestructive testing (NDT) of approximately 25% of the highest-stressed structural connections above and below the water-line, 100% of the crane pedestals, and 10% of such other miscellaneous attachments as fairleads, anodes, ladders, and hydrophone brackets. Drillship inspections focused on the hull, cranes, and miscellaneous attachments. If significant cracking was found, the inspection program was expanded. We believe a 25% sample of selected major connections provides a suitable basis for evaluating the integrity of a rig.

Results.

The inspection program was implemented on 19 rigs-13 semisubmersibles, 5 jackups, and 1 drillship. The number of cracks found at major structural connections varied from a minimum of 20/rig to more than 300/rig. Some 40% of the cracks were shallow and were eliminated by light grinding, while the remaining cracks extended deeper and required weld repair.

Semisubmersibles.

Cracks were found in one or more major structural areas of every semisubmersible surveyed; however, most cracks were located in the lower hull area, which usually cannot be inspected unless the rig is at shallow draft. Inspection results for the lower hull of 13 rigs are shown in Table 1. Rigs are identified by number in Col. 1, and the typical operating environment is shown in Col. 2. Each crack has been tabulated in the center of Table 1 according to surface length. The totals at the bottom of each column represent the number of cracks in that crack-length range and the corresponding percentage of cracks in that crack range based on the total number of cracks located during the survey. Cracks less than 4 in. [100 mm] in length were usually shallow, and many disappeared after light grinding. Cracks longer than 4 in. [100 mm] were generally deeper and most were repaired by welding. In general, the deeper, longer cracks have greater potential for rapid growth to through-thickness cracks that can affect member strength and rig integrity and stability. Consequently, the number of cracks longer than 4 in. [100 mm] were of particular interest.

Results of the inspections shown in Table 1 indicate the following.Cracks were found in rigs operating in both mild and severe environmental areas as well as in new rigs that had not seen previous drilling service.Twenty percent of the cracks at major structural connections were longer than 4 in. [100 mm]. Eight rigs had cracks that had grown to more than 12 in. [300 mm] in length, and six rigs had more than one crack longer than 12 in. [300 mm].Four rigs contained cracks that penetrated lower structural members and allowed water to enter lower hull compartments. [Note: laboratory analysis confirmed that these cracks were service (fatigue) enhanced.]Approximately 90% of the cracks longer than 4 in. [100 mm] were repaired by welding.

Cracks were also found in the upper hull, around crane pedestals, and around attachments such as fairleads and hydrophone brackets however, the most significant defects were located in the lower hull area.

JPT

P. 1437^