Gunnar Solland

Assessment of Capacity of Grouted Connections in Piled Jacket Structures

The design of the grouted connections in jacket structures has until recently been based on a control of capacity with respect to axial force in the pile while the effect of the bending moment has been neglected, ref. ISO 19902 (2007). In later design of grouted connections it is observed that the moments in the piles can contribute to more than two times larger stress in the pile than that due to axial load only. A significant moment can hardly be transferred from the pile to the sleeve without sliding of the steel against the grout. This sliding is considered to increase with increasing diameter of the pile. Therefore it is difficult to develop design criteria based on small scale testing. The contact pressure between grout and steel will lead to compressive and tensile stresses in the grout. This requires design criteria for compressive stress and tensile stress in the grout.ISO 19902 do not require fatigue assessment of grouted connections subjected to wave loading. This was based on a review and assessment of jackets present in the Lloyds database at the time the design formulation was developed. It was assessed that fatigue assessment was not required as long as design was performed with respect to the Ultimate Limit State. However, it is now judged that fatigue is important for jackets with significant dynamic load that exceeds the axial and bending moment from permanent loads.A review of design standards for grouted connections in jacket structures has shown that there is a need for more relevant test data and revisions of these standards such as ISO 19902 in order to assure reliable design with respect to all potential failure modes.Copyright

Extreme Storm Wave Histories for Cyclic Check of Offshore Structures

Offshore platform resistance to cyclic storm actions is addressed. In order to achieve the best economy of the structure especially when assessing existing structures, the ultimate capacity of the structure is utilized. This means that parts of the structure may be loaded into the non-linear range and consequently the load-carrying resistance of the structure against future load cycles may be reduced. In such cases it is required to carry out a check of the cyclic capacity of the structure. Such checks are required in the ISO 19902 code for Fixed Steel Offshore Structures. The paper presents a proposal for how a load history for cyclic checks can be established. The method is in line with what is included in the NORSOK N-006 standard on “Assessment of structural integrity for existing load-bearing structures”. The load-history for the waves in the design storm may be expressed as ratio of the dimensioning wave. The ratio will be different for check of failure modes where the entire storm will be relevant such as crack growth, compared to failure modes like buckling where only the remaining waves after the dimensioning wave need to be accounted for. Using simple order statistics and simulation, the statistics for the ith (Hi ), i = 1, 2, 3, 4 etc. highest wave in the storm is studied in some detail, assuming that the maximum wave (H1 ) is equal to an extreme wave obtained by a code requirement. Environmental contours for the pair (H1 ,H2 ) are established by Inverse FORM for design conditions. Further, the long term statistics for load effects that are expressed as a function of H1 , .., H4 , i.e. L = f(H1 , .., H4 ), are determined. The R-year value LR for the load effect L is determined by structural reliability techniques, and the most probable combination (design point) (H1 *, .., H4 *) for L = LR is determined. The design point values Hi *, as well as the design point value for the significant wave height, are determined for different load effects, and their characteristics for different types of load effects are discussed. The paper gives advice also on how to establish the magnitude for the remaining waves in the storm.Copyright

On Safety and Reliability for Platforms That are Unmanned During Severe Storms

The paper addresses safety and reliability issues for platforms where an unmanning strategy is premised.The standard NORSOK N-006 recommends how to deal with the specific aspects that engineers meet when assessing existing structures, including life extension. A possible mitigating measure for structures that do not meet today’s structural requirements for environmental loads is to unman the platform during storms. The basis for the unmanning criteria in this standard is that the safety for personnel on a platform that needs to be unmanned during storms is consistent with the safety for personnel on platforms that satisfy structural requirements for manned platforms.The prevailing metocean conditions at a North Sea location is modeled by a storm statistics approach. The capacity waves according to the codes checks are calculated for a jacket structure and the limiting metocean conditions that comply with the acceptance criterion are established. The expected frequency of unmanning events is determined, and the issue of forecast uncertainty discussed.The annual maximum wave height distribution for the location is compared with the corresponding distribution that applies when the platform is manned, i.e. for metocean conditions that do not trigger unmanning. The probability of failure for important limit states is calculated on condition that no unmanning is required, and for a platform that satisfies the requirements for manned platforms. The most likely realizations of sea state variables and extreme wave cycle are determined for the different cases.Parts of the structure may be loaded into the non-linear range and consequently the load-carrying resistance of the structure against future load cycles may be reduced. In such cases it is required to carry out a check of the cyclic capacity of the structure. The statistics for the second highest wave during storm conditions is investigated for unmanning scenarios and for a platform that satisfies the requirements for manned platforms.It is normally acknowledged that the structural failure probability associated with normal statistical variations is considerably less than the failures that are due to gross errors. The difference in risk due to gross errors between platforms that are operated as unmanned during storms compared to the gross error risk level for manned platforms is discussed.Copyright

Safety Format and Acceptance Criteria for Analysis of Marine Structures Using Non-Linear Methods

This paper deals with how structural integrity can be documented by use of non-linear analysis methods. The focus is on structures that are exposed to extreme environmental loads. In order to prove the structural integrity for aging offshore installation, non-linear analysis methods are finding increased applications. The codes ISO 19902, API RP 2A and Norsok N-006 give recommendations for use of non-linear methods for capacity checks of existing structures. This paper discusses the different safety formats given in these codes. Also presented is how practical acceptance criteria that have to be applied in conjunction with such analyses can be formulated. The topics that are dealt with are issues that the analyst performing such analyses needs to address like: definition of failure, representation of joints, issues on cyclic loading, instability of cross sections and member buckling. In cases of novel design solutions or special structural details, non-linear analysis methods may also find application when planning new structures.Copyright

Extreme Weather Warning Criteria

The paper addresses determination of the weather criteria that should apply for platforms that are assumed to be unmanned during storms. The methodology ensures that the safety of personnel on these platforms is at least as good as for personnel on platforms that satisfy current structural requirements for manned platforms. A probability based procedure is defined to achieve this goal. In this paper the requirements in the NORSOK standards are used as the reference standard for the structural reliability. The forecasted environmental conditions that require unmanning are determined by a stepwise procedure: First, adopting the principles of the structural code, we determine the maximum environmental action (described by crest elevation Clim ) the structure can resist. If Clim is lower than the crest corresponding to the requirements for manned structures, unmanning criteria are established. By integration over all sea states below the threshold significant wave height Hs,thr , the required Hs,thr is determined such that the probability that the annual maximum crest exceeds Clim , complies with the acceptance criterion. The forecasted environmental conditions at which shut-down and unmanning shall be completed are for Hs ≥ Hs,thr . The target probability for the governing failure mode is distributed between the directional sectors such that the frequency of unmanning the platform is minimized. Because forecasted wave directions are uncertain, the decision process is made more robust by a smoothing technique that retains the exceedance probabilities for the directional sectors.Copyright

New Standard for Assessment of Structural Integrity for Existing Load-Bearing Structures-Norsok N-006

An increasing number of platforms in the Norwegian continental shelf are reaching their design life. For various reasons these platforms will require an assessment of their structural integrity. When performing these assessments the engineer is faced with tasks where little guidance is found in design standards, for several reasons. The two most important being: 1) The analyses that is performed in a typical assessment of existing structure is often applying very advanced techniques and methodology that seldom is used in design of new structures, as the cost of doing advanced analysis is relatively low compared to replacement of an existing structure, but relatively high compared to moderate additions of e.g. steel in the design of a new structure. 2) Design standards are based on theories, methods and experience for structures in a given design life (e.g. fatigue design and corrosion protection design). When this design life is extended, sound methods for ensuring that the structures are still sufficient safe is needed. Such methods will normally be “condition based design”, where inspection, maintenance and repairs are included in the assessment in integrated way. Such methods are not given in normal design standards. For these reasons a new NORSOK standard is developed that gives recommendation on how to deal with the specific aspects that engineers meet when performing assessments of structures in general, but also specifically for assessment for life extension. The standard is named “Standard for Assessment of Structural Integrity for Existing Load-bearing Structures” and is issued as a NORSOK standard and given the number N-006 [1]. The topics that are covered in the standard include: Shut down and unmanning criteria for platforms not meeting ordinary requirements, specific issues for determination of ultimate capacities by use of non-linear methods, cyclic capacity checks, fatigue life extension, requirements to in-service inspection etc. The paper describes the background and the content of the new standard and it presents examples of recommendations given. The role of the new standard in the Norwegian regulatory system is shown.Copyright

New Data on the Capacity of X-Joints Under Tension and Implications for Codes

This paper reviews available static strength data and presents results of finite element analyses on first crack loads and ultimate loads of X-joints in tension. A critique of existing guidance for such joints is given. An examination of hot spot stress for such joints is presented, together with new capacity formulations based on test data. The new formulations are verified with reference to new data from a finite element analysis. The new capacity formulations will be of interest to regulatory authorities, to designers of new offshore installations and to engineers carrying out assessments of existing structures. It is also expected that the formulations will be considered by code drafting committees, e.g. for API RP2A, ISO 19902 and NORSOK, during code revisions. The paper demonstrates that present guidance is unduly conservative in two respects: (1) high γ joints (i.e. thin-walled chords) in the range 0.7 ≤ β ≤ 0.9 joints (i.e. moderately high brace/chord diameter ratios), and (2) joints with β = 1.0 having low γ. However, it is shown that present guidance may be optimistic for low γ joints with β < 0.9. The new capacity formulations proposed in this paper correct these deficiencies. As one example, the new formulations give an increase of 60% in capacity compared to existing guidance for a joint with β = 1.0 and γ = 10, not untypical of many joints in service. In the near term, the paper may be most appreciated by those involved with structural integrity assessment studies. There have been some recent examples where existing guidance has indicated that some primary structural joints are under-strength. This has prompted extensive numerical work to prove the adequacy of the joints. A worst case scenario would be the implementation of unnecessary offshore strengthening work.Copyright