Learnings From Strain Measurements on an In-Field Conductor and Wellhead System

In recent years due to use of drilling risers with larger and heavier BOP/LMRP stacks, fatigue loading on subsea wellheads has increased, which poses potential restrictions on the duration of drilling operations. In order to track wellhead and conductor fatigue capacity consumption to support safe drilling operations a range of methods have been applied: • Analytical riser model and measured environmental data; • BOP motion measurement and transfer functions; • Strain gauge data. Strain gauge monitoring is considered the most accurate method for measuring fatigue capacity consumption. To compare the three approaches and establish recommendations for an optimal approach and method to establish fatigue accumulation of the wellhead, a monitoring data set is obtained on a well offshore West of Shetland. This paper presents an analysis of measured strain, motions and analytical predictions with the objective of better understanding the accuracy, limitations, or conservatism in each of the three methods defined above. Of the various parameters that affect the accuracy of the fatigue damage estimates, the paper identifies that the selection of analytical conductor-soil model is critical to narrowing the gap between fatigue life predictions from the different approaches. The work presented here presents the influence of alternative approaches to model conductor-soil interaction than the traditionally used API soil model. Overall, the paper presents the monitoring equipment and analytical methodology to advance the accuracy of wellhead fatigue damage measurements.

Fatigue Strength Assessment of a Structure Considering Corrosion Wastage and Corrosion Fatigue

It is known that the fatigue strength decreases in corrosive environment and many experiments were carried out to comprehend the decrease in fatigue strength in corrosive environment. In order to comprehend the actual state, a cycle speed of fatigue test loads should correspond to a wave frequency. Therefore, an experiment in the long life region is practically difficult, then the corrosion fatigue data available for the life assessment of the structure is quite limited. In this study, the fatigue strength of the welded joints in long life service was evaluated according to the calculations of corrosion fatigue crack propagation subjected to the random loadings which followed an exponential distribution. In the crack propagation calculations, the progress of corrosion wastage from the plate surface and the resultant stress increase were considered simultaneously. In the high stress and the short life region, the decrease in fatigue strength due to the accelerated crack propagation in corrosive environment was dominant because the progress of corrosion wastage was little. On the other hand, in the low stress and the long life region, the decrease in fatigue strength became dull as longer the fatigue life because the corrosion fatigue crack propagation was suppressed by the corrosion wastage, but after that the fatigue strength showed the precipitous decrease due to the increase in stress resulted by the progress of corrosion wastage.

Analysis of Yielding and Buckling Strength of a Minimal Ballast Water Crude Oil Tanker

The minimal ballast water crude oil tanker has the design of a trapezoidal-shape inclined bilge entirely instead of the regular U-shaped tanker, also known as the trapezoidal tanker, which reduces the amount of the ballast water. This type of ship does not only reduce the cost on ballast water management, but also is beneficial to the environment for reducing the risk of water pollution. Since it is a new design, there are no applicable specifications for the assessment of structural strength at present. In order to find out characteristics of the yielding and buckling strength of this type of ship, the strength of a conventional tanker and a trapezoidal tanker are calculated by a finite element method and then compared with a variety of cases. It can be seen that the trapezoidal tanker has lots of advantages in strength and lighting weight.

Nonlinear Wave Crest Distribution on a Vertical Breakwater

The paper addresses the problem of deriving the nonlinear, up to the second order, crest wave height probability distribution in front of a vertical wall under the assumption of finite spectral bandwidth, finite water depth and long-crested waves. The distribution is derived by relying on the Quasi-Deterministic representation of the free surface elevation in front of the vertical wall. The theoretical results are compared against experimental data obtained by utilizing a compressive sensing algorithm for reconstructing the free surface elevation in front of the wall. The reconstruction is pursued by starting from recorded wave pressure time histories obtained by utilizing a row of pressure transducers located at various levels. The comparison shows that there is an excellent agreement between the proposed distribution and the experimental data and confirm the deviation of the crest height distribution from the Rayleigh one.

Numerical Investigation of Ultimate Strength of Stiffened Plates With Various Cross-Section Forms

There is an increasing interest in the lightweight design of ship and offshore structures, more specifically, choosing aluminum alloys or other lightweight high-performance materials to build structure components and ship equipments. Due to its better mechanical properties and easy assembly nature, extruded aluminum alloy stiffened plates are widely used in hull structures. When the load on the hull reaches a certain level during sailing, partial or overall instability of stiffened plate makes significant contribution in an event of collapse of the hull structure. It is very necessary to investigate the ultimate strength of aluminum alloy stiffened plate to ensure the ultimate bearing capacity of large aluminum alloy hull structure. Most of studies of the ultimate strength of stiffened plates deal with stiffened plates with T–shaped stiffeners. Stiffeners of other shapes have seldom been explored. In this research, the ultimate strength of six different cross–section aluminum alloy stiffened plates and one steel stiffened plate was studied based on the non–linear finite element analysis (FEA). Taking into account stiffness, weight and other issues, the new cross–section aluminum stiffener has finally been concluded for replacing the original steel stiffener in upper deck of a warship.

Risk Analysis and Management of Moored Ships in Ports

The risks associated with mooring of ships are a major concern for port and maritime authorities. Sea waves and extreme weather conditions can lead to excessive movements of vessels and mooring loads affecting the safety of ships, cargo, passengers, crew or port infrastructures. Normally, port activities such as ships’ approach manoeuvres and loading/unloading operations, are conditioned or suspended based solely on weather or wave forecasts, causing large economic losses. Nevertheless, it has been shown that some of the most hazardous events with moored ships happen on days with mild sea and wind conditions, being the culprit long waves and resonance phenomena. Bad weather conditions can be managed with an appropriate or reinforced mooring arrangement. A correct risk assessment must be based on the movements of the ship and on the mooring loads, taking into account all the moored ship’s system. In this paper, the development of a forecast and warning system based on the assessment of risks associated with moored ships in port areas, SWAMS ALERT, is detailed. This modular system can be scaled and adapted to any port, providing decision-makers with accurate and complete information on the behaviour of moored ships, movements and mooring loads, allowing a better planning and integrated management of port areas.

Short-Term Extreme Motions of a Spar Floating Wind Turbine Estimated Through a 1:30 At-Sea Experiment

The present paper deals with the estimation of the short-term extreme motions of a spar floating wind turbine in parked rotor conditions, through a 1:30 at-sea experiment, carried out at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy). Thanks to some favorable local environmental conditions of the site, several wind-generated sea states with relatively low significant wave height (Hs < 0.50 m) have been collected during the experiment. These sea states are scale models of ocean storms, which are relevant hydrodynamic design conditions for the spar platform. The 30-minutes extreme values of the model structure motions have been estimated for all the six degrees of freedom, using the Weibull Tail Method (WTM), and the results obtained are presented in the paper. Such estimations are 1:30 scale models of the 3-hours extreme values of the spar motions in parked rotor conditions and may be directly used for design purposes.

Pipeline Fracture due to Compression-Tension Loading Caused by Foreign Object Impact

In areas frequented by fishing vessels, trawl equipment or anchors may interfere with pipelines and cause damage through impact, potential hooking, and ensuing release of the pipeline. This load sequence of denting followed by global bending and springback results in a complex stress and strain history. Experiments have shown that fracture in an impacted pipe typically arises along the bottom of the dent, where the material suffers high compressive strains in the impact and hooking phase, and a rapid change to tension during the rebound phase. High compressive strains may reduce the strain to failure significantly for a succeeding tensile phase. A common trait of ductile damage models is to account for damage through nucleation, growth and coalescence of voids, which traditionally is thought to occur during tension. In this study, an uncoupled phenomenological Cockcroft-Latham-type fracture model accounting for anisotropic damage is used. The fracture model is implemented in the explicit finite element programme IMPETUS Afea Solver, and calibrated using material tests. Simulations show that the proposed fracture model is able to account for the observed behaviour.

An Assessment of Load and Response for Horizontal Slamming Loads From Model Scale Experiments

Steep breaking waves can result in high impact loads on offshore structures, and several model test campaigns have been conducted to assess the effect of horizontal wave slamming. High loads have been measured, and they can be challenging to withstand without significant deformation. For wave slamming problems it is common to estimate the characteristic slamming load and assume that this will give an equivalent characteristic response. One challenge related to the slamming load is that it has a large variability in load level, the duration of the load and the shape of the overall load pulse. This variability can have a large impact on the estimated response to the characteristic load, causing a similar or larger variability in response. Due to the sensitivity to the structural response, it may be difficult to interpret large amounts of such data to arrive at a relevant design load without making overly conservative assumptions. This paper investigates the sensitivity of the structural response to assumptions made in the material modelling and how the short term variability is affected if we instead of load use response indicators such as plastic strain and max deformation to arrive at a characteristic load. For this purpose, a simplified dynamic response model is created, and the recorded wave impact events can then be evaluated based on the predicted structural response from the simplified model. It was found that the structural response is sensitive to the structural configuration. The assumed material behavior and hydro-elastoplastic effects were identified to greatly affect the structural response. A reasonable approach to arrive at the q-annual response seems to be to first estimate the q-annual extreme slamming load, and then run the structural analysis on several of the measured slamming time series with the estimated q-annual extreme pressure.

Simulation-Based Tracking of UOE Pipe Yield Strength Considering Various Thickness-to-Diameter Ratios

The plastic forming processes involved in the production of UOE pipes alter significantly the yield strength of the original steel plate. Numerous studies indicated that the work hardening and Bauschinger effect are the main factors influencing the alteration of the yield strength. Moreover, apart from the forming process itself, the flattening executed on strips sampled from the formed pipe appears to have also nonnegligible effect on the final yield strength that is used as quality index of the formed pipe. Therefore, this study tracks the yield strength of UOE pipe made of API-X70 steel with various thickness-to-diameter ratios by FE-simulation of the forming and flattening processes so as to identify the factors influencing the yield strength of the UOE pipe. The results show that the flattening process constitutes a critical phase in which steel experiences large loss of its tensile yield strength.