|Purpose of issue||Rev||Date of issue||Technical author||Policy contributions||Technical editor|
|Issued||1||September 09||M Birkinshaw||A Stacey|
M Birkinshaw - OSD4.2
Duty holders should be able to demonstrate that the degradation and deterioration of structures is kept within acceptable limits with no impairment of overall integrity. This is likely to involve a combination of:
It should be noted that, although this relates primarily to ongoing degradation processes, account also needs to be taken of the effects of discrete damage events (e.g. due to dropped objects, collisions, etc) that may exacerbate deterioration.
The standard set by this policy is compatible with the International Standards Series for Offshore Structures and there should be no adverse impact on new installations designed to this standard or equivalent, nor on existing structures using the processes and procedures described in this standard.
Damage and deterioration can be the result of inadequate design and construction, operational accidents (dropped objects, boat impact for example), ageing processes or a combination. The age profile of North Sea installations show that many are now outwith their notional design life and, hence, may be susceptible to increasing and possibly rapid deterioration. The threat is that the deterioration, from whatever cause, impairs structural integrity to a level whereby safety of the installation is threatened.
There are no precise methods to quantify risk, and sound engineering design and construction principles are required to keep this risk to a low level. Managing the risk by use of techniques and methods described in the ISO Standards for offshore Structures is used to ensure that no sudden, catastrophic failure is encountered.
There are a number of examples of damage and deterioration, emanating from all the above mentioned causes, resulting in incidents of severed members, fatigue failures, the need for strengthening, or staffing/operational restrictions.
The level of integrity has traditionally been set through prescribed Regulation in the North Sea, by an industry standard (API) for the USA, and Class Rules for mobiles and floating installations. These practices are currently being harmonized through ISO to a set of appropriate criteria, similar to those in place in Norway (NPD Regulations and guidance, NORSOK etc.). This technical policy is based on the ISO principles.
The most significant degradation mechanisms for offshore structures are under design, fatigue, corrosion, and impact resulting in fracture. Fatigue of an inadequately designed and constructed detail was a major factor in the Alexander Kielland disaster in 1980 in which 123 people lost their lives.
More recently, cracking has occurred on a number of installations and this has led to the complete severance of members in a few cases. Corrosion is an ever-present hazard in a marine environment. Although it has not led to any major failures in the North Sea to date, it has been strongly implicated in a number of incidents in the shipping industry.
The general principles to be applied are in ISO 13822 (Assessment of Existing Structures) and 19900 (Offshore Structures – General Requirements)
ISO 19902 for Steel, ISO 19903 for Concrete
ISO 19904 for Floating Installations, and
19905 –1 for Jack Ups
Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 (DCR), Regulations 4, 5(1)(a) and (e), 6, 8 and 9
Offshore Installations (Safety Case) Regulations 2005 (SCR), Regulations 12(1)(d) and 19
The Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995 (PFEER), Regulations 4 and 5
Remedial actions will depend on the degree of lack of conformance with the relevant ISO Standard and original design intent. However of utmost importance is that the discovery of deterioration and degradation be undertaken in a systematic way within a well-formed structural integrity management system. Remedial actions may take the form of:
It is expected that protective measures enshrined in good engineering practice will be provided during initial design and construction, as well as monitoring carried out during the life cycle. A number of possible control measures are listed below:
This section contains references that are relevant concerning the subject matter of this technical policy e.g. standards, and a list of background notes giving guidance on specific topics but this is not exhaustive:
This policy emphasises a management approach to activities associated with maintaining integrity. This should ensure a life cycle approach to damage and deterioration which is important in understanding the integrity and history of degradation and repair.
Areas of particular consideration and are known to accelerate degradation are:
Good Practice in initial design and construction include:
It is expected that combinations of these measures will be used for each installation. Close attention should be given to cases where reliance is placed solely on either initial measures or on in-service inspection, especially the latter as this measure becomes increasingly important for ageing installations.
The ability of a repair to restore the integrity of a fatigue damaged component is a necessary requirement in maintaining the overall integrity of an offshore structure. Several different repair methods can be used offshore. They involve weld repair, structural modification or the use of strengthening techniques. Methods used include:
Much of this information is taken from GASCET (Guidance for the topic assessment of the major accident hazard aspects of safety cases).
For welded joints in offshore structures, the fatigue life N3 is defined as the point at which a through-thickness crack forms. However, actual failure will occur when the load bearing capacity of the remaining ligament is insufficient for the applied load and this is designated N4. At this stage load shedding will take place and the applied loads will be transferred to neighbouring components. The reliance on FMD (flooded member detection) in maintenance strategies for offshore installations requires that the inspection interval is such that N4 is not exceeded. The available information indicates that the remaining fatigue life on penetration of the wall thickness may be rather limited. It is therefore important that due consideration is given in the development of the structural integrity management plan to the possibility of total member failure occurring after penetration of the wall and of the consequences to structural integrity.
Most offshore structures are constructed from weldable medium strength steels [usually grade 50D], for which codes and standards exist, eg BS 7191. Welding procedures are now well developed for the medium strength steels used offshore and are well codified, eg EEMUA 158 and AWS D1.1. More recently, newer higher strength steels, with a better strength to weight ratio, are increasingly being used. However, in general there is less test data available to support the design equations and the duty holder should ensure that sufficient and reliable data are available to enable a structural integrity assessment with an appropriate level of confidence. Inspection at the fabrication stage is recognised as a major part of the reliability aspect of performance standards and there is a need for this to be well documented for proper life cycle efficiency.
Structural inspection is a key factor in providing data for the management of structural integrity. For North Sea structures on the UKCS, preparation of an inspection plan is a requirement of DCR Regulation 8. This requires that the duty holder ensures that suitable arrangements are in place for maintaining the integrity of the installation, through periodic assessments and carrying out any remedial work in the event of damage or deterioration. The inspection programme includes:
A set of default inspection requirements is included, with prescriptive survey periods for cases where an inspection plan has not been produced. The inspection planning methodology should demonstrate an understanding of the significance of the analytical information requirement and the inspection strategy implementation.
Normal underwater inspection programmes include a condition survey of the anodes, the extent of marine growth and corrosion potential monitoring of areas of the jacket structure. Through this, anodes can be identified and subsequently replaced to ensure an adequate level of cathodic protection is provided for the life of the structure.