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Spectral Fatigue Analysis Methodology in Assessing Ageing Jacket Platform Offshore Malaysia - Presented during First Asia-Pacific Conference on Offshore System ( April 23-26, 2001 )

Abstract

Most of the oil platforms located offshore Malaysia was installed in the late seventies and early eighties. Despite their ageing status, these platforms continue to produce an economically viable amount of oil for local and overseas consumption. In order to ensure that these platforms are operationally safe, the relevant authorities has embarked on a structural reassessment program for all these platforms. This program covers structural integrity analyses of the jacket, structural reliability analysis and risk based inspection schedule. This paper details the design methodology adopted for the spectral fatigue analysis for the jacket structures. Spectral approach is a technique capable of relating, in a statistical manner, cause and effect due to randomly occurring phenomena in a linear system. This paper discusses the preparation of computer/mass model and analyses procedure. Environmental load consideration represented an important aspect of the analyses. The frequency selection, dynamic response analysis, selection of stress concentration factor (SCF), and fatigue life calculation is also discussed.

Introduction

More than 100 platforms have been identified by the relevant authorities in the integrity assessment program. Most of these platforms are located offshore Sabah, Sarawak and Terengganu in a water depth range of 30m to 70m. The main aim of this assessment program is to determine the safety and operability of the platform for continued utilization. This paper details the design methodology adopted for the spectral fatigue analysis for the jacket structures as part of the structural integrity assessment program. The design methodology was based on the use of SACS suites of software (Engineering Dynamics Inc, 1998).

Methodology

Spectral fatigue analysis is the most comprehensive of fatigue analysis procedure and is the one which best represents the random nature of the wave environment. It builds upon established principles for general spectral response analyses and extends these for application to fatigue assessments in the marine environment. It has been applied to a wide range of offshore structures of various types.

The method uses the wave scatter diagram directly to represent the long term statistics of seastates and uses the wave spectrum to represent the range of frequencies present in a random seastate. The effect of wave frequency on wave loading and structural response is explicitly accounted for through the determination and use of hot spot stress transfer functions for evaluating response statistics in each random seastate. The spectral response analysis technique is only applicable to linear systems. A summary of the analyses procedure is shown in Figure 1 (MMC Oil & Gas, 2000).

Computer Model

The model used for this analyses is the same as the one adopted for inplace, but with the necessary alterations. Since the jacket appurtenances are quite well defined already under the inplace conditions, no additional alterations are performed. It is however verified that the simulation for these jacket appurtenances exhibit equivalent true mass characteristics as well including the entrapped and displaced conditions. The peak marine growth profile is used for this analysis. The water depth is assumed to be at the mean sea level. The structural mass inclusive of the added mass i.e. both displaced and entrapped is generated by the SEASTATE routine of the SACS program. The hydrodynamic modeling is consistent with the inplace conditions. No associated current, wind and gravity loads is considered for this analysis.

The topsides modeling will ensure that the distribution of the structural mass is ideally represented. The topside mass is applied at the major key joints.

Structural Framing

A three dimensional computer model of the structure is prepared reflecting its inplace condition. This structural model includes all framing members represented correctly with its cross sectional properties including the sectional variations along with the appropriate lengths, joint eccentricities and the end fixities. All framing members will be considered to be spanning between the face of the members. Flooded members include jacket legs, risers, conductors and caissons.

The integrated deck is modeled in detail for this analysis. This model also includes the extremities of the deck and also the critical component/ equipment locations.

Appurtenances

In general, all the jacket appurtenances that are required to withstand the inplace loading conditions is accurately covered in the computer model with proper releases, such that their hydrodynamic characteristics are truly represented. The appurtenances falling under this category include:

a) Riser Guard
b) Boatlanding
c) Conductors
d) Risers

Conductors

The conductors above mudline are modeled as structural members with a cross sectional property based on actual size as per as-built drawings. These conductors are self standing and their weight is not transferred to any part of the structure. These conductor pipes are connected to the structure at each of the horizontal framing levels through dummy members connections. These connections simulate the effect of the guide, releasing these sections axially and at the same time transferring the lateral loadings to the adjacent framing. The conductor pipes continue into the soil for substantial depths and therefore these are modeled in as piles. The depth of penetration will be taken as the setting depth of the conductors based on the as-built drawings. Such an idealization permits the conductors to the global conductor lateral loads and also permits the guide framing to be designed to withstand the appropriate loading.

Marine Growth

Marine growth profile used for design is as per the environmental data given by the relevant authorities.

Risers

Risers are modeled for their contribution to the environmental loads. The weight of the risers will be calculated and input at the location of the hanger flange elevation. The risers will be set off the face of the jacket and connected to the structure at the defined elevations through the stand-off sections in order to ensure that the jacket and the framing members are subjected to the appropriate load.

Splash Zone Protection

The jacket legs and the diagonal framing members that are subjected to corrosion will be modeled as structural members with reduced diameter and thicknesses to reflect the decayed state. However for the purposes of wave loading the specified diameters will be overridden to reflect the initial conditions. The splash zone area depends on individual platform as defined in as-built drawings.

Jacket Walkway

The top jacket horizontal framing members support a walkway ail around on the exterior and also in the region of the conductors. In order to account for the additional environmental load contributions from the walkway grating and handrails, the diameter of the supporting members are factored up with the width of the contributory grating and also the number of horizontal rails of the handrail. This revised diameter is reflected in the form of a member diameter over ride. The weight of the jacket walkway framing is input as joint load at the appropriate nodes.

Conductor Guide Framing

The connecting members of the conductor guide framing are provided with an override on the member diameter in order to represent the correct environmental loading on the plated connections. These members are assumed to be weightless and the weight of the conductor guides is separately assessed and input as joint loading at the appropriate nodes.

Anodes

The environmental loading on anodes is included by applying a factor of 5% on the global coefficient of drag values. The weight of anodes is input as joint loading at the appropriate nodes.