Evaluation of Fatigue Considerations in the Design of Framed Offshore Structures
Authors: Kallaby J. , Price J.B. 
Published: December 2007

This paper evaluates the implications of fatigue criteria on platform design using recent experiences with the Maui Platform A, offshore New Zealand. The importance of realistic fatigue criteria is noted and various methods to analyze fatigue are examined. The impact of fatigue on the weight and cost of a platform is discussed.

Introduction

Offshore structures installed in the hostile North Sea can be subjected to fatigue loading far exceeding that experienced in the Gulf of Mexico. The China Sea and offshore Australia and New Zealand are other areas that impose a fatigue premium. Several studies have dealt with analysis and design of fatigue, focusing on the estimation of stress-concentration factors at the nodes of frame structures.

This study evaluates the significant elements of fatigue design using recent experiences with the Maui Platform A, installed offshore New Zealand. It evaluates the importance of realistic fatigue criteria and examines various methods for analysis. A comparison of stress-concentration factors found using several proposed equations with a finite-element analysis is presented. Weight and cost implications are discussed.

Maui Platform A

The Maui Platform A (Fig. 1) currently installed in 354 ft of water about 23 miles offshore Taranaki, North Island, New Zealand, is composed of a 432-ft tower, topped by three deck levels for the development drilling, production, and processing of 660 MMscf/D of gas and 20,000 production, and processing of 660 MMscf/D of gas and 20,000 B/D of condensate through 12 wells.

The four-legged tower is 70 x 160 ft at the top and 160 x 160 ft at the base, with battered broad sides and vertical narrow sides (Fig. 2). Two legs are 22 ft in diameter, narrowing to 6 ft at the top. These legs served as pontoons to float the tower on a 5,200-mile journey from the manufacturing site in Japan. The other two legs are 6 ft in diameter, increasing to 16 ft in the bottom panel. The tower is anchored to the sea bed with panel. The tower is anchored to the sea bed with twenty-eight 48-in.-diameter steel piles penetrating 240 ft.

The tower float-out weight is 14,020 kips. Maximum deck loading is 24,000 kips. A 100,000-kips mass is used to calculate the dynamic properties of the platform. Steel specifications required normal DIN steel with 50-ksi yield strength. To enhance the resistance of the structure to fatigue, improved DIN steel with special impact and through-thickness properties was specified at the joints. Approximately 2,500 kips of this special steel was required. Further discussions of the tower are given in Refs. 1 and 2.

Every joint in the structure was analyzed for fatigue. Cyclic stress for the in-place platform was computed using wave forces determined by Morison's equation and Stokes' fifth order or Airy wave theory. A mass coefficient of 1.5 was used for members 10 ft or less in diameter. This coefficient was increased to 2.0 for members greater than 10 ft in diameter. A drag coefficient of 0.5 and 4 in. of marine growth on the diameter of members below the water line were used. The drag coefficient was increased to 3. 0 to consider wave slam. The effect of the towing on fatigue life was considered also.

JPT

P. 357